WhittakerA.GeologicaEvaluation-LithologyMineralogy.pdf

The On-line Mud Logging Handbook Alun Whittaker
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The On-line
The On-line
Mud Logging
Mud Logging
Handbook
Handbook
by Alun Whittaker
by Alun Whittaker
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Geological Evaluation
Geological Evaluation
- Lithology & Mineralogy
- Lithology & Mineralogy
Aegis Group
244 Ohio Street
Vallejo, CA 94590-5051
USA
mudlogman@yahoo.com
mudlogman@yahoo.com

Geological Evaluation – Lithology & Mineralogy
How many times have you sat a a conference table to discuss a geological or engineering problem and, while boxes and discs of other well
data, logs and samples, sit by, the discussion has resolved around three or four mud logs, laid out along the table or magnet-ed side-by-side
on the white board.
Historically, the mud log became rotary drilling's replacement for the older the driller's log as the composite day-by-day, and foot-by-foot
record of drilling events, progress, and – most significant – formations penetrated. This is still true, no other log contains the density of data:
more parameters, of more kinds, over longer intervals of the well, than the mud log.
But the geological data on the mud log is not just there for completeness. It also serves, in combination with drilling data, rate of penetration,
and so on, to add interpretation and significance meaning to the gas and oil traces recorded on the log.
Finally, the mud logger is almost certainly the only scientifically trained person available at the well site, all day, every day (24-7, in the
trendy new argot), If a careful, accurate, reproducible chemical or mineralogical test or measurement is required, then who else should be
doing it?
Of course, we have to remember the Golden Rules of Mud Logging (see Chapter 1) and ask whether the need for measurements to be
made on fresh samples, and the value of real-time results outweigh the cost of keeping the equipment at the well site, and time and effort
taken away from other tasks.
Sampling, curation, testing, and evaluation tasks for well cuttings include:
✔ Routine sampling programs and protocols
✔ Sample catching
✔ Sample handling and curation
✔ Mineralogical tests and analyses
✔ Clay maturity and compaction tests for geo-pressure evaluation
✔ Petro-physical tests & analyses (core analysis)
✔ Drilling fluid and tracer chemical tests

Table of Contents
Catch It and You Keep It..................................................................................................................................................................................9
Sampling Protocol.........................................................................................................................................................................................10
Table of Organization..................................................................................................................................................................................10
Documented Procedures.............................................................................................................................................................................11
Number of Sample Sets.........................................................................................................................................................................12
Sample Distribution................................................................................................................................................................................12
Sample Intervals.....................................................................................................................................................................................12
Sample Curation.....................................................................................................................................................................................13
Sealed Untouched.............................................................................................................................................................................13
Unwashed.........................................................................................................................................................................................14
Washed and Dried.............................................................................................................................................................................14
Lag and Circulation................................................................................................................................................................................15
Sample History Log................................................................................................................................................................................15
Standards...............................................................................................................................................................................................16
Cutting Sampling...........................................................................................................................................................................................17
Annular Recovery.......................................................................................................................................................................................17
Sample Catching.........................................................................................................................................................................................24
Cuttings Recovery Problems........................................................................................................................................................................43
Viscosity Additives.......................................................................................................................................................................................43
Solvents and Lubricants..............................................................................................................................................................................43
Lost Circulation Material..............................................................................................................................................................................44
Oil-based Mud.............................................................................................................................................................................................46
Core Sample Handling..................................................................................................................................................................................46
Catching & Sampling...................................................................................................................................................................................47
Core Handling Prep List..............................................................................................................................................................................49
For Recovery and Shipping....................................................................................................................................................................49
For Analysis and Sampling.....................................................................................................................................................................49
For Slabbing & Plugging.........................................................................................................................................................................50

Petro-physical Analysis................................................................................................................................................................................57
Core Analysis .............................................................................................................................................................................................57
Core Plug Porosity Measurement...........................................................................................................................................................58
Boyle's Law.......................................................................................................................................................................................60
Core Plug Permeability Measurement....................................................................................................................................................60
D’Arcy Equation.................................................................................................................................................................................61
Core Plug Saturations Measurement......................................................................................................................................................62
Cuttings Porosity.........................................................................................................................................................................................63
Praxis Measurements.............................................................................................................................................................................64
P-K Analyzer...........................................................................................................................................................................................65
pNMR Principals.....................................................................................................................................................................................66
Cuttings Bulk Density..................................................................................................................................................................................75
Clay Wettability...........................................................................................................................................................................................81
Cation Exchange Capacity..........................................................................................................................................................................81
Basic Method..........................................................................................................................................................................................82
Improved Method...................................................................................................................................................................................84
Mineral Identification.....................................................................................................................................................................................85
Calcimetry...................................................................................................................................................................................................87
Mud Tracer Tests...........................................................................................................................................................................................91
Ending It All....................................................................................................................................................................................................92
Next.................................................................................................................................................................................................................93

Didn't find what you needed here? Sorry.
Why not go back to the Chapter Summaries, and fine a better place to start, or use the Index to search for the subject you need.
List of Figures & Tables
Figure 1: Depending on mud flow rate and rheology, the mud flow may be in plug, laminar or turbulent flow. ................................................18
Figure 2: In laminar and turbulent flow, annular velocity is lowest adjacent to the drill string and outer bore hole wall and highest mid-way
between. .........................................................................................................................................................................................................19
Figure 3: Experimental results of return of large (), medium (), and small () cuttings from a bore hole, under different conditions of mud
viscosity and annular velocity. The velocity profile in drilling mud allows slippage of cuttings toward the slower inner and outer portions of the
flow. Rotation of the drill string modifies this effect by hurling cutting backward the high velocity region. .......................................................20
Figure 4: Formation boundaries will be indicated correctly by the rate of penetration log. Due to slippage and mixing in the mud stream, the
total hydrocarbons curve and cuttings descriptions may show differences in both the depth and nature of the formation boundary. ..............22
Figure 5: The recommended mud log format includes two lithology tracks. One shows rock types and proportions exactly as seen the
cuttings, The other shows the mud logger's geological interpretation..............................................................................................................23
Figure 6: Two sources of debris entering the bore hole from near or immediately above bottom, A: Spalling is a stress relief process creating
fairly large, relatively similar sized, blocky fragments. B: Consistent, or transitory under-balance caused by low mud density, or swabbing,
produces flaky, lenticular fragments in a range from very large to slightly larger than drill cuttings..................................................................24
Figure 7: Two sources of debris entering the open hole section at any time, and from locations above bottom. A: With excessive annular
velocity, turbulent flow causes erosion of the bore hole wall. B: If a portion of the drill string in open hole is held in compression, then the
pipe may flex causing the external upsets to impact and damage the bore hole wall......................................................................................25
Figure 8: The mud log worksheet is the place for a detailed sample tally: lag sample interval and catching times, and descriptions (courtesy
of EXLOG, Inc.) ..............................................................................................................................................................................................27
Figure 9: In Air or gas-based drilling, samples of dust or rock-flour are taken from a by-pass of the blooie lineBlooi......................................29

Figure 10: Samples of cuttings and mud must be taken no less often than every fifteen minutes while drilling. Sample processing requires
the packing of unwashed, and rinsed-and-dried samples. Cuttings and mud samples are sieved and blended for lithological, oil and gas
evaluation (See Chapter 8 for details).............................................................................................................................................................31
Figure 11: And for those of you who are too lazy to zoom --............................................................................................................................32
Figure 12: A table of standard sieve mesh sizes manufactures in the US, UK and other nations (nearest equivalent French, Canadian and
German standard metric sizes) commonly used in cuttings evaluation. The ASTM number 8, number 80, and number 170 (or equivalents
highlighted in the table) are the minimum necessary for mud log sample preparation and evaluation.............................................................33
Figure 12 (continued): A table of standard sieve mesh sizes manufactures in the US, UK and other nations (nearest equivalent French,
Canadian and German standard metric sizes) commonly used in cuttings evaluation. The ASTM number 8, number 80, and number 170 (or
equivalents highlighted in the table) are the minimum necessary for mud log sample preparation and evaluation..........................................34
Figure 12 (continued): A table of standard sieve mesh sizes manufactures in the US, UK and other nations (nearest equivalent French,
Canadian and German standard metric sizes) commonly used in cuttings evaluation. The ASTM number 8, number 80, and number 170 (or
equivalents highlighted in the table) are the minimum necessary for mud log sample preparation and evaluation..........................................35
Figure 13: This table provides a comparison of common sieve mesh and screen sizes, with typical sedimentary grains as graded by the
Wentworth grain size classification. (see Wentworth, 1922, or Wiki – it) and remember that well cuttings contain more than a single grains,
and so are larger..............................................................................................................................................................................................36
Figure 13 (continued): This table provides a comparison of common sieve mesh and screen sizes, with typical sedimentary grains as graded
by the Wentworth grain size classification. .....................................................................................................................................................37
Figure 14: The geological sample processing area of a modern standard mud logging unit: sieves, sink, blender, micro-gas analyzer,
microscope and ultraviolet inspection chamber. (Illustration courtesy of EXLOG, Inc.). ..................................................................................39
Figure 15: A traditional mud logging cutting lithology log.................................................................................................................................41
Figure 16: A modern mud log, in addition to a cuttings lithology track, includes interpreted lithology, and lithological descriptions tracks.......42
Figure 17: A Conventional Core is Recovered Piece by Piece from the Inner Core Barrel Hanging Over the Rig Floor...................................48
Figure 18: If the core is jammed, it may need to be laid down on the .............................................................................................................50
Figure 9: Cutting a core is expensive in rig time and materials. damaged, broken or disorganized core fragment may have greatly reduced
value. Boxing, labeling and packaging of the core, is very important. Attach tags (A) to the core to indicate where and why material has been
removed. Loose debris (B) should be bagged and appropriately placed in the core box.................................................................................51

Figure 20: Bagged samples of core debris can replace missing or inferior cutting samples for the cored interval...........................................52
Figure 21: Sealed and canned core plugs are take for petro-physical core analysis: porosity, permeability, and fluid saturations. Canned
drilling mud samples taken during coring provide reference water and gas analyses......................................................................................53
Figure 22: The mud logging crew should create a core description and report, independent of any operator’s geologist, and append it to the
bottom of the mud log......................................................................................................................................................................................54
Figure 23: The mercury pump pycnometer has a stainless steel chamber into which mercury is pumped to enclose a core sample. The
difference in volume of mercury displaced from the chamber at atmospheric pressure, and at a higher pressure can be used to determine
the volume of effective pore space and the porosity. ......................................................................................................................................58
Figure 24: In the air permeameter test, compressed air flows at measured flow rate, pressure and temperature and pressure through a core
plug of measured length and diameter. ...........................................................................................................................................................61
Figure 25: On the mud log itself only brief graphical and text notations of core interval and recovery are made. ...........................................62
Figure 26: On the mud log core report pigtail (a supplementary report added at the bottom of the completed mud log, see Figure 22), more
detailed core descriptions, drawings are presented with, if available, plots of core analysis data. ..................................................................63
Figure 27: The Praxis ® Pulsed Nuclear Magnetic Resonance (pNMR) analyzer used pulsed magnetic fields from a large uniform electro-
magnet to measure the quantity and distribution of hydrogen-containing fluids a small sample of cuttings.....................................................64
Figure 28: The Exploration Logging (EXLOG) P-K® system uses a Praxis ® pulsed Nuclear Magnetic Resonance (pNMR) analyzer coupled
to a personal computer to handle the repetitive measurements and calculations, to determine free and bound fluid in the porosity of cuttings.
........................................................................................................................................................................................................................65
Figure 29: In water, the nucleus of the hydrogen atom, a positively charged proton, behaves as a tiny spinning magnet. Normally, the axes of
spin of the protons are randomly oriented – they are said to be relaxed. ........................................................................................................67
Figure 30: If the hydrogen nuclei are subjected to a strong magnetic field, then the axes of spin of the protons become aligned in orientation
with the lines of magnetic force in the field......................................................................................................................................................67
Figure 31: If the imposed magnetic filed is removed then, after the characteristic relaxation time, the protons return to their previous relaxed
orientation........................................................................................................................................................................................................68
Figure 32: A pulse of radio frequency energy can also cause the spin axes of the magnetically aligned hydrogen nuclei to become scattered,
and randomly oriented.....................................................................................................................................................................................68

Figure 33: After the radio frequency pulse is completed, the hydrogen nuclei, once again, begin to re-align themselves back into alignment
with the magnetic field,.each proton emitting a pulse of radio frequency energy. ...........................................................................................69
Figure 34: The rate at which the protons re-align, and give up energy is also representative of the characteristic relaxation time of the
physical state in of the fluid containing the hydrogen nuclei. In a sedimentary rock, this may be: free water, irreducible water, or interlayer
water................................................................................................................................................................................................................69
Figure 35: In a porous sedimentary rock, the physical environment of the hydrogen nuclei controls the distribution of relaxation times.........71
Figure 36: The P-K® Log is a reliable and inexpensive indicator of porosity available while drilling limited only by the quality of sample
available and the skill of the mud logger in selecting it. ..................................................................................................................................74
Figure 37: Shales and clay rocks have a chemically complex mineral matrix that continues to change with temperature and depth of burial.
Loss of chemically-bound water from mineral matrix in the pore space, results in an increase of both the matrix density, and the porosity,
without necessarily changing the bulk density. ...............................................................................................................................................75
Figure 38: Shale bulk density increases with depth of burial as de-watering occurs, porosity declines and clay mineral density increases.
Breaks in this uniform trend may result from chemical and mineralogical changes in the rock, but may also indicate geo-pressuring (see
Figure 39)........................................................................................................................................................................................................76
Figure 39: Breaks in this uniform trend may indicate failure of de-watering and retention of fluids in the clay rock, accompanied by
abnormally high geo-pressure (see Figure 38). Determination of which interpretation is correct requires additional real time information
commonly available in the mud logging unit (see Chapter 12).........................................................................................................................77
Figure 40: The liquid density gradient is used to determine bulk density of shales and other cuttings. Using calibration beads a column
height-versus-density calibration chart is established. ....................................................................................................................................79
Figure 41: The Shale Data Log is useful in source bed and geo-pressure formation evaluation and in drilling fluid management. .................80
Figure 42: The Warne staining procedure is used to identify components of mixed carbonates (see below, for an explanation of the reagents)
........................................................................................................................................................................................................................86
Figure 43: In the Bernard Calcimeter a measured sample of cuttings reacts fully with dilute Hydrochloric Acid to produce carbon dioxide. the
volume of Carbon Dioxide produced is measured and converted to the percentage of Calcium Carbonate in the sample. ............................88
Figure 44: The results of calorimetry: percentage calcite, dolomite and total carbonate is plotted in track 5a of a standard mud log adjacent to
the other detailed geological data and evaluations. ........................................................................................................................................90

Geological Evaluation – Lithology & Mineralogy
Catch It and You Keep It
Despite sophisticated remote and down-hole measurement technologies, rock
samples derived from drilling and coring remain the most important source of
data to the geo-scientist. Along with Rate of Penetration, and the Lag Time, these samples, and the direct observations made on them,
supply the benchmarks against which gas, and all other analyses can be standardized.
The catching, processing, description, and curation of material for geological and geochemical analyses, cuttings sampling is at least as
important a job function for the mud loggers as gas analysis.
However, in order to understand this effort, and its results, it is important to also understand the means by which they are taken, and the
environment through which they are transported back to surface. It is also necessary to maintain standards and quality control in the
collection, preparation and handling of these samples.
During and after drilling, several types of rock sample may be taken from the well bore:
✔ Drill cuttings,
✔ Bottom-hole or conventional cores (and samples extracted from them),
✔ Sidewall cores and samples,
✔ Opportunistic samples that are not routinely or systematically taken, but which can become available as a result of drilling activities.
For example: extremely large and undamaged cavings, formation debris picked up by junk baskets and stabilizers (see Chapter 4),
and so on.
In addition to rock samples, it is also possible to obtain various types of fluids from the bore hole during and after drilling. These include:
✔ Samples of drilling fluids, that may contain dissolved and dispersed fractions of minerals, volatiles, formation waters and their
dissolved anions and cations,
✔ Formation fluid samples taken in small volumes using down-hole samplers or wire-line well testing tools.
Ask a really old friend to tell you about the
Ask a really old friend to tell you about the Catch It and
Catch It and
You Keep
You Keep sketch from the National Lampoon Radio Hour
sketch from the National Lampoon Radio Hour

✔ Larger volumes of recovered fluids from temporary completion and flow tests of the completed wells.
For example: Open-hole, (OHT), or drill-stem tests (DST).
The composition of formation fluids (oil, and water) in situ is of course in equilibrium with the solid (minerals, and solid hydrocarbons) and
gaseous phases in the reservoir and will be modified by translation to surface temperature and pressure. Small samples, or early flow
samples will also be modified by contamination with drilling fluids and bore-hole debris including rock cuttings and cavings and fluids from
elsewhere in the well.
Sampling Protocol
To perform any useful work on each one of these sample types: rock, formation gases, liquids or drilling fluid, requires an understanding of
the nature of the others and of their relationships. Even so, there remain a large number of unknowns that must be considered when
designing a sampling program. Drill cuttings and core samples can be used reliably only when you have an adequate knowledge of their
nature and history, and have exercised standardization and strict quality control over the catching and preparation processes.
Table of Organization
Prior to commencement of the drilling program a table of organization should be established, agreed upon and documented. Pre-planning is
required to ensure that, at each stage of the well, the appropriate sample processing equipment and materials is available at the well site
and that enough, but not too many, qualified personnel are on hand to handle them.
It is also necessary to assign, agree and document the duties and responsibilities of all personnel involved in the program:
✔ Who is in charge?
✔ Who supervises?
✔ Who takes decisions?
✔ Who performs each task: processing samples, describing samples, writing and delivering daily reports, and so on?
✔ Who handles logistics, such as ordering supplies and materials?
✔ Who is to receive samples?
✔ When, where and what kind?
✔ Who is responsible for dispatching and shipping samples?

✔ Who is to receive reports?
✔ When, where and how many copies?
✔ How are they delivered: by: mail, fax, e-mail, encoded, and so on?
The answers to questions is used to construct a table of organization and responsibilities. When agreed, and signed-off be the necessary
authorities, this should be circulated to all concerned, and posted in rig offices, and the mud logging unit.
Documented Procedures
Procedures for sampling, sample processing, packaging and shipping should all be agreed and documented. Some are discussed below
and there are several others documented by oil companies, geological services companies, professional societies, and state agencies (for
examples, see AAPG, Colorado School of Mines, and the Oklahoma Geological Survey in the Bibliography).
In reality, the importance lies not in which procedure you choose to follow but that there is agreement, understanding and enforcement of a
standard procedure to be utilized by everyone involved in the sampling program.
Types of Sample
At the well site, there are two basic sample types:
✔ Samples for immediate inspection, and
✔ Samples to be preserved for later, off-site investigations or trade.
There is only one rule for samples to be used in preparing the mud log: they should be caught as often as time permits, and at as many
locations as necessary to obtain a complete and proper evaluation of formation drilled.
Samples for later evaluation fall under several classes and require detailed specification in the logging instructions:
✔ Number of Sample Sets
✔ Sample Distribution
✔ Sample Intervals
✔ Sample Curation
✔ Lag and Circulation
✔ Sample History Log

✔ Standards
Number of Sample Sets
You must specify how many sets of each type of sample are required. If you are going to require unusually large number of sets then be
prepared to sacrifice on the sample interval you can reasonably expect, or you must ask the mud logging company to provide additional
personnel, materials, and equipment (extra sample driers for example) to accommodate the load.
Asking rig crew members to help out with sample catching is a bad idea. Floor hands or roustabouts have neither the skill nor the motivation
to catch good quality, representative samples. Even worse, at times when sampling is most critical, for example in deeper, faster drilling
sections, they are likely to be called away by other duties on the rig.
Sample Distribution
For each sample set, instruction must be given for:
✔ Method of shipping.
✔ Destination
✔ Distribution schedule (weekly, monthly, or hold until the end of drilling)
✔ Special packing,shipping document content, labeling or shipment reporting requirements
For example, depending on security considerations, there may be extreme limits placed on what information should disclosed (depth
interval, sample type, or even box number) on the outside of shipping containers,, or accompanying documents.
Sample Intervals
You must specify the sample interval for each type of sample and for each section of the well. Commonly, the sample interval is increased
as the well is deepened. Remember that there has to be a compromise between number of sample sets, sample intervals for each set, and
mud logger’s time availability.
For each sample set, you must specify whether you require interval or incremental samples to be collected (see Chapter 1). If incremental
samples are required, then you should provide instructions to the drilling contractor to cooperate with the mud logger in assembling or
installing some form of continuous sample catching equipment.
You must also provide instructions on how to handle sampling during special circumstances to prevent gaps in the sample set. The most
common such circumstance is while coring. Samples cut by the slim, diamond-tipped core bit are very sparse and uninformative. It is a good

idea, to catch the best available samples while coring. After a core has been successfully recovered, the mud logger can take already
broken fragments from appropriate depths in the boxed core or, but only if necessary, break off small chips with a geological hammer.
Sample Curation
There are several different types of sample that may be processed at the well site. The mud logger needs specific instructions on each of
them.
Sealed Untouched
Samples required for geochemistry, palynology, or micro- paleontology are often required to be completely untouched and sealed at
the well site to avoid risk of contamination. They are never washed, not even rinsed, but may be splashed with a biocide to prevent
biological decomposition. They may be canned, or sealed in glass jars or heavy weight plastic bags.
For the non-geologists among us, micro-paleontology is the study of microscopic fossils. Microfossils, such as Forams
For the non-geologists among us, micro-paleontology is the study of microscopic fossils. Microfossils, such as Forams
and Diatoms, whose fossil remains are composed primarily of Calcium Carbonate, Silicate or Phosphate, are much
and Diatoms, whose fossil remains are composed primarily of Calcium Carbonate, Silicate or Phosphate, are much
more recognizable, more widely represented, and therefore more useful than macro-fossils in dating and correlation in
more recognizable, more widely represented, and therefore more useful than macro-fossils in dating and correlation in
petroleum exploration.
petroleum exploration.
Strictly speaking, Palynology is the study of pollen and spores, living and fossil, but the field is commonly broadened
Strictly speaking, Palynology is the study of pollen and spores, living and fossil, but the field is commonly broadened
to include other similar
to include other similar palynomorphs
palynomorphs. These are other microfossils of plant and animal origin that are microscopic in
. These are other microfossils of plant and animal origin that are microscopic in
size (5 µm to 500µm), and resistant to most forms of decay. Common examples include:
size (5 µm to 500µm), and resistant to most forms of decay. Common examples include:
* Carophytes and Chrysopytes: Calcium Carbonate and silicate skeletal structures from fresh water, blue-green algae.
* Carophytes and Chrysopytes: Calcium Carbonate and silicate skeletal structures from fresh water, blue-green algae.
* Coccoliths: Calcite remains of marine plankton.
* Coccoliths: Calcite remains of marine plankton.
* Conodonts and Scolecodonts: Calcium Phosphate dental structures of annelid worms.
* Conodonts and Scolecodonts: Calcium Phosphate dental structures of annelid worms.
* Diatoms and Radiolarians: Silica remains of single-celled, marine and fresh water organisms.
* Diatoms and Radiolarians: Silica remains of single-celled, marine and fresh water organisms.
* Foraminifera: Calcium Carbonate remains of single-celled organisms which have a calcareous test with a chitinous
* Foraminifera: Calcium Carbonate remains of single-celled organisms which have a calcareous test with a chitinous
inner lining.
inner lining.
* Ostracoda: Chitinous and calcareous structures from fresh water and marine crustaceans.
* Ostracoda: Chitinous and calcareous structures from fresh water and marine crustaceans.

In other cases, sealed, untouched samples may needed to support palynology, geo-chemistry and other off-site research projects.
For example, to provide background measurements of mud properties, every ten sealed cuttings samples are accompanied by a
single sealed drilling fluid sample.
Unwashed
Samples for trade, with neighboring exploration companies, government agencies, and for archiving are bagged unwashed in cloth
or plastic sample sacks. Although unwashed, some geologists want to have these samples lightly rinsed of drilling mud prior to
bagging. If recovered from an oil-based drilling mud, it may be desirable to lightly rinse the sample in an organic solvent — enough
to remove the coating of mud but not to wash away any legitimate formation oil staining. After rinsing, the bagged samples may be
hung in a sheltered location to air dry. Your instructions should be explicit as to your exact requirements.
To save well-site labor and shipping costs, sometimes a single, extra-large, unwashed sample set is caught and sent to a sub-
contractor for processing, splitting, and re-bagging into several sets. To ensure that everyone understands how much sample you
need for all your needs, it is a good rule to supply bags of an appropriate size for your requirements, and then require that they be
filled. Just like pouring cocktails, its easier to fill it to the top, than to know when to say when.
Washed and Dried
For routine sample examination at the well site or later it is usual to wash and dry the samples. If you are drilling through a clean,
granular sediment this is simple, but if there are soft, unconsolidated clays or fissile shales are being penetrated, or are exposed in
the bore hole wall, problems arise.
At the well site, the geologist or mud logger is able to see massive amounts of clay and shale cavings arriving with bottom hole
cuttings. This material can then be removed with a coarse (8-mesh) sieve before looking at the sample.
Similarly, if the formation actually being drilled consists almost entirely of soft, unconsolidated and otherwise featureless clay
(commonly described on the rig as Gumbo), then this can be completely washed out so that the small amount of clastic material
can be extracted and viewed.
Alternatively, this clastic material can be found at the de-sander or de-silter outlet, if they are running. From these various samples,
the mud logger can assemble a representative sample from which to make a valid description for the mud log or daily report.
Now, what about the single washed and dried sample, that someone away from the well site may look at and use to judge the
formation. It may contain:
✔ Cavings, making up more than half it’s volume. Hopefully, these can be recognized, but if the geologist looking at them has no
well-site experience they may not be.

✔ Mostly soft clay that, after heated drying will turn into a hard clay brick, that has few surface features and is almost useless as a
sample for visual examination or description, or
✔ Only the fine, clastic material representing less than 10% of the drilled formation, because more than 90%, the soft clay, has
been washed away.
There is no simple solution to this problem (other than to always look at the mud log sample, and read its formation descriptions
before starting to examine an archived sample set).
However, the sampling protocol must be specific about where to obtain samples for this sample set, and how rigorously to wash it
before drying.
Lag and Circulation
Although not specific to sampling, the logging and sampling instructions should specify how lag time tests are to be run and reported.
It should also explain circumstances when the mud logger is authorized to halt drilling and ask for a circulation of Bottoms Up. These
instructions should include:
✔ In what depth interval, or geological horizon this is applicable. You may want to ignore most drilling breaks for much of the well, but to
be very strict about circulating events when drilling one or two potentially productive horizons.
✔ How great a drilling break (proportionate increase in rate of penetration) signals the start of a drilling break.
✔ How many meters (or feet) must be drilled at this new rate to confirm the drilling break.
✔ What to do after drilling is halted. For example:
✔ Circulate until a bottom hole sample is recovered to surface,
✔ Continue circulation and call for orders,
✔ Complete only one circulation, and the pull out and prepare to begin, coring,
✔ and so on.
Sample History Log
You should supply a form, or at least specify a standard format, for a record (or log) to kept in the mud logging unit of all sample sets,
shipping container descriptions, contents, shipping dates, and destinations. This log should also list any special circumstances explaining

contamination, poor sample quality, or absence of samples from any set.
Along with responsibility for the samples, this log should eventually
be transferred from the well site to the sample and core repository.
Records should be consist of sample numbers, volumes and types
taken, their mode of storage and destination of distribution. At the
repository, the log can be updated with records of samples
removed and returned, volumes of material used up or damaged in
analytical procedures, and so on. Cuttings samples are a delicate
and limited resource. In the past, irreplaceable samples have been mislaid, contaminated or even thrown out due to the lack of adequate
record keeping.
Standards
The sampling instruction should be accompanied by copies of (or instructions on where to obtain copies of) standard bulletins and charts for:
✔ Descriptive terms and abbreviations (with necessary code names or terms if these are needed for open telecommunications),
✔ Rock type classifications,
✔ Grain size, shape and sorting, and
✔ Natural and fluorescent color charts.
Based upon these documented procedures, there can be total honesty and trust between all members of the geological team:
✔ Mud loggers should not be asked to collect larger volumes of sample than is physically possible (while keeping up with their other
tasks).
✔ Mud loggers who are asked to do so should explain that this cannot be done and propose alternatives: such as reducing the
sampling requirements, using sample catchers, or a sample splitting service, and so on.
✔ Sample catchers, if used, should be mud logger trainees, or geological technicians. Using unskilled, untrained casual labor or
members of the rig crew as sample catchers is not recommended as the quality and regularity of sampling will strongly the reflect the
workers interest and respect for the work being performed!
✔ Mud loggers should not accept impossible instructions without comment only to try to fulfill them by supplying inadequate, poor
quality or false samples. This serves no one!
Note to mud loggers: this sample log is primarily for the operating
Note to mud loggers: this sample log is primarily for the operating
company’s shipping and logistics people. I suggest that in addition to this
company’s shipping and logistics people. I suggest that in addition to this
log, you should report all of the same information on the Mud Log
log, you should report all of the same information on the Mud Log
Worksheet.
Worksheet.

Cutting Sampling
The recovery of cuttings from the bore hole is a more complicated situation than gas recovery. With good mud condition, gas peaks can be
sampled with sharp boundaries that can correlated with similar breaks in rate of penetration and confirming a formation boundary. Bed
boundaries defined by drill cuttings samples are never as sharp as this and may falsely appear to extend over tens or even hundreds of
meters.
Annular Recovery
So far we have considered the recovery of low density gas dispersed in viscous mud. The mud is only required to carry, suspend, and
release the gas and, by its viscosity, to prevent upward migration and mixing of the gas. This is a relatively uncomplicated condition
compared to that which occurs when mixtures of light and heavy, large and small, drill cuttings are carried in drilling mud.
Any flowing fluid will have a carrying capacity for solid particles that is governed by the density, viscosity, and velocity of the fluid. According
to the values of these, there will be a maximum limit set on the total load of solid material that may be carried, and to the mass of any
individual particle that can be lifted by the fluid. Of course, even at a constant fluid velocity, the particle load is neither constant nor uniform.
At any time, the total load will consist of some particles being lifted, and others being carried or deposited at random from the fluid.
When drilling fluid flows vertically, up through the well annulus, these same conditions apply but there are further considerations. For
example, in the annulus, we are concerned only with vertical flow in which the settling tendency of the solid particles (well cuttings) under
gravity is acting in the opposite sense to the flow direction. Fortunately, mud is a thixotropic (gel-forming) fluid that, when flow ceases, gels
to form a colloidal suspension, having a much greater viscosity, or gel strength, and hence carrying capacity to prevent particle settling.
However, in long annular sections, we can reasonably expect there to be some difference in upward velocity, causing slippage, or sorting of
cutting by mass. Some of the heavier, denser, and larger cuttings will settle in the annulus, arriving later at surface so that sharp bed
boundaries will appear transitional.
There may also be some sorting on the basis of cuttings shape. For example, flatter, tabular or disc-shaped particles, such as fissile shale
cuttings, will present more resistance to settling than equi-dimensional, sub-spherical cuttings, such as sandstone cuttings or single grains.
The annular mud flow may exist in one of three different flow regimes depending upon the annular diameter, velocity and mud rheology.
These are (see Figure 1):
✔ Plug Flow: a rare condition, usually seen in slow moving, extremely viscous fluids.
✔ Laminar Flow: the ideal condition for drilling mud, resulting in minimal pressure losses and bore-hole erosion.
✔ Turbulent Flow: a less desirable condition for drilling mud. It has greater carrying, and hole cleaning capacity (favored for pumping

cement into casing) but requiring increased energy expenditure to overcome circulating pressure losses.
Figure 1: Depending on mud flow rate and rheology, the mud flow may be in plug, laminar or turbulent flow.
For cuttings recovery, the significance of flow regime is in the velocity distribution across the flow path (see Figure 2). In laminar and
turbulent flow, the mud travels at a range of velocities across the annulus. Maximum velocity is achieved at the middle of the annulus, equi-
distant between the drill string and the bore hole wall. Following the same rules as vertical settling, the solid particles carried in the flow will
tend to slip across the annulus from higher to lower velocity regions in the flow, inducing vertical mixing of cuttings.

Figure 2: In laminar and turbulent flow, annular velocity is lowest adjacent to the drill string and outer bore hole wall and
highest mid-way between.
Further complication is added by the rotation of the inner wall of the annulus: the drill string. Friction at the pipe wall induces a radial flow
component to the drilling fluid and a centripetal force that sweeps cuttings back toward the high velocity region in the center of the annulus.
This enhances the random mixing of material. Solid particles carried in the flow will tend to slip across the annulus from higher to lower
velocity levels in the flow inducing a further degree of vertical mixing of cuttings (see Figure 2).
The net effect of these factors is to limit the value of cuttings as a means of determining formation boundaries with any degree of precision.
It also means that the lag time predicted by the carbide lag time test represents only the minimum time necessary for cuttings to reach
surface traveling with the maximum velocity mud region. Any slippage of cuttings will contribute to extending the range of cuttings lag time
between the minimum and a longer, maximum value.

Figure 3: Experimental results of return of large (), medium (), and small () cuttings from a bore hole, under
different conditions of mud viscosity and annular velocity. The velocity profile in drilling mud allows slippage of cuttings
toward the slower inner and outer portions of the flow. Rotation of the drill string modifies this effect by hurling cutting
backward the high velocity region.
In mud logging there are two methods to handling this problem. One method is to add colored, solid material to the calcium carbide, lag time
test bomb. When the acetylene gas show arrives, the logger or geologist should go out to the shale shaker and begin collecting and rinsing
small samples of cuttings. A record is kept of the amount of colored particles collected in the sample. Usually, a long time can go by with just

a few colored particles appearing on the shale shaker but, in most cases the majority of particles can appear in the first 10 to 15 minutes. If
not, there is some argument for improving the mud condition.
The disadvantage of this method is the time it is not convenient, or safe, for the mud logger to spent so much time waiting around at the
shale shaker collecting samples, and away from his gas analyzers and other sensors. This is a process that can be neither avoided nor
speeded-up. There is also some doubt about the shape, size and density distribution of the material that can be packaged into the test bomb
(with confidence that it can pass successfully through the filters. valves, jet nozzles and other down-hole assemblies in a modern drill string).
If the material used is not representative of real cuttings, the test result also may not be representative.
Another solution is to use careful logging practice to monitor cutting recovery rate. Whenever a formation change is suggested by a change
in rate of penetration (a drilling break, or reverse break) or an increase total hydrocarbon concentration, you should prepare to search the
cuttings samples for confirming evidence:
✔ After the minimum lag time, go to the shale shaker and collect a cuttings sample.
✔ Whether or not a lithology change is detected, go back again and collect further small samples ten, twenty and thirty minutes later.
✔ If no change is detected, the event may be a false alarm (although you should investigate to determine a believable cause — neither
rate of penetration nor total hydrocarbons change markedly without either a natural change in the formation, or a man-made change
on the rig, or in the bore hole).
✔ If a change is detected in the cuttings, you must first investigate, describe and report the occurrence.
✔ Next, by comparing the appearance of the three (or more) samples you have taken, you can gain some insight into the relationship
between carbide and cuttings lag times (see Figure 4).
The mud logger is faced with a quandary as a result of this annular slippage. He knows that the type and boundaries between the formations
penetrated are not reflected in the samples he is preparing:
✔ If he plots a graphical lithology track on the mud log reflecting just the composition of the samples, then he is intentionally plotting
what he knows to be wrong, but
✔ If he plots a graphical lithology track on the mud log reflecting what he has concluded from the samples, and other logging data and
observations, then he runs the risk that someone, away from the well site, may inspect the samples and conclude that mud logger to
be incompetent.

Figure 4: Formation boundaries will be indicated correctly by the rate of penetration log. Due to slippage and mixing in the
mud stream, the total hydrocarbons curve and cuttings descriptions may show differences in both the depth and
nature of the formation boundary.

There are two possible solutions to this:
✔ The Gulf Coast Lithology log is a convention based on the (reasonable, in that region) assumption that most of a sedimentary
section consists of (slow drilling) Shale, and (fast drilling (Sandstone):
✔ On the mud log, there is a second ROP track (identical to the first, but scaled from 0-100%, see Figure 4).
✔ The mud logger then plots Shale symbols (- - -) to the left of this second ROP trace, and Sandstone symbols (. . .) to the
right.
✔ The result is a lithology curve that consistently uses the samples to show the rock types present, and the rate of penetration to
show the proportions.
✔ The recommended method (as used in the examples in this book, see Figure 5 and Chapter 11) is to plot two lithology logs in
parallel on the mud log:
✔ The first is scaled fro 0-100%, and records the rock types actually seen in the samples, in the proportions actually determined.
✔ The second is similar to the free-hand interpreted lithology log drawn by a well-site geologist (any other field geologist). It
shows the geological column, exactly as the mud logger views it to be from based on all of his observations and
measurements.
Figure 5: The recommended mud log format includes two lithology tracks. One shows rock types and proportions exactly as seen the
cuttings, The other shows the mud logger's geological interpretation.

Sample Catching
On arrival at surface, even with an ideal drilling fluid and circulating system, drill cuttings are physically and chemically weathered, flushed
with, and expelled of fluids, and mixed with material drilled earlier, and later and with debris caved from elsewhere in the bore hole wall (see
Figure 6 and Figure 7). Obviously, cuttings carried by a liquid-based drilling fluid cannot be relied upon to provide accurate values of
discrete data items. However the material is too valuable and hard won to be simply dismissed as unreliable. It can be valuable where
qualitative evaluations alone are required or where extreme accuracy is not a necessity. Chemical analyses and physical tests must be
performed over a range of samples from depths nominally above to nominally below the required interval and the resulting uncertainty
accepted.
Figure 6: Two sources of debris entering the bore hole from near or immediately above bottom, A: Spalling is a stress
relief process creating fairly large, relatively similar sized, blocky fragments. B: Consistent, or transitory under-balance
caused by low mud density, or swabbing, produces flaky, lenticular fragments in a range from very large to slightly
larger than drill cuttings

Figure 7: Two sources of debris entering the open hole section at any time, and from locations above bottom. A: With
excessive annular velocity, turbulent flow causes erosion of the bore hole wall. B: If a portion of the drill string in open
hole is held in compression, then the pipe may flex causing the external upsets to impact and damage the bore hole
wall.
You should remember that even the best wire-line log measurements are subject to edge effects when crossing
You should remember that even the best wire-line log measurements are subject to edge effects when crossing
bed boundaries where there are markedly different properties above and below. These can cause depth errors in
bed boundaries where there are markedly different properties above and below. These can cause depth errors in
estimating the depth of the boundary or invalid measurements above and below. Another cause of depth errors
estimating the depth of the boundary or invalid measurements above and below. Another cause of depth errors
in wire-line logs is the continuous variation in stretch of the logging cable caused by friction between the logging
in wire-line logs is the continuous variation in stretch of the logging cable caused by friction between the logging
tool and bore hole wall. The first task when performing digital (both wire-line and mud) log analysis is to adjust
tool and bore hole wall. The first task when performing digital (both wire-line and mud) log analysis is to adjust
depths up and down the well to bring recognizable correlation points in to line on all logs.
depths up and down the well to bring recognizable correlation points in to line on all logs.

On the other hand, cuttings are an ideal source of samples if the purpose of the examination is to establish lithological information to fill in
boundaries and characteristics defined by other, more precise but less informative measurements.
For example, rate of penetration can very precisely locate a boundary in the formation but only a cuttings sample can tell us what rocks are
present above and below that boundary. In pressure evaluation, cuttings measurements may also be used to determine the trend of data
with depth and, when we have established that trend, they can indicate reversals or other abnormalities in the data. Selecting a mandatory
sample interval, for bagged and preserved samples, can be very important.
Common sample intervals are:
✔ The length of a joint or single of drill pipe (approximately 10 meters or 32 feet) from surface to the depth at which intermediate casing
is set, and
✔ Three to five meters (10 to 16 feet) for the deeper, more interesting and usually slower drilling formations.
A sample interval should be selected so that samples rarely need to be caught more often than every fifteen minutes. If you demand a
consistently shorter time interval then you are likely to discover that sample quality is declining, or other logging tasks are being neglected,
Using full-time sample catchers can allow more repetitive sampling, but remember that the quality of sample and the honesty of sampling at
un-supervised times will reflect the training and motivation of the people used.
A sample collection tally, should be maintained on the mud log work sheet (see Figure 8) showing:
✔ The most recently measured lag time (in Minutes or Pump Strokes)
✔ The current calculated annular velocity for drill collars in open hole (in Minutes or Strokes per 10 meters) to allow regular updating of
the measured lag time until the next carbide test.
✔ For each sample:
✔ Interval, top and bottom (meters)
✔ Interval finished drilling (strokes or
time) Sample Due Time at the
shale shaker (strokes plus lag
strokes or time plus lag time plus
pump-off time)
Late one cold, wet night, as a very young mud logger, I was resting in the warm logging unit
Late one cold, wet night, as a very young mud logger, I was resting in the warm logging unit
certain that the next sample due in another 40 minutes would be the same featureless gray
certain that the next sample due in another 40 minutes would be the same featureless gray
shale we’d been seeing for the past three days. At that moment, the driller came in and asked:
shale we’d been seeing for the past three days. At that moment, the driller came in and asked:
“WHAT'S THAT LUMPY RED STUFF COMING OVER THE SHAKER?”
“WHAT'S THAT LUMPY RED STUFF COMING OVER THE SHAKER?”
A desperate scramble to get my boots on, get out to the shaker, and scoop up what turned out
A desperate scramble to get my boots on, get out to the shaker, and scoop up what turned out
to be a pan full of dull gray shale taught me two powerful lessons: never neglect regular
to be a pan full of dull gray shale taught me two powerful lessons: never neglect regular
sampling, and never overestimate the sophistication of a driller’s sense of humor!
sampling, and never overestimate the sophistication of a driller’s sense of humor!

Samples for examination and description for the mud log should be collected on a regular basis of every fifteen minutes throughout the well
even if the mandatory sample interval requires sampling less often.
Figure 8: The mud log worksheet is the place for a detailed sample tally: lag sample interval and catching times, and descriptions (courtesy
of EXLOG, Inc.)

For water-based or oil-based drilling muds, the principle location for cuttings sampling is the shale shaker. Older shale shakers have two or
four screens side-by-side, each with the same mesh size, intended to remove only cuttings and larger material. Because of differences of
mud flow volume and direction of flow through the ditch, there will be sorting by particle size between the screens, with the coarsest material
accumulating on the screen nearest to the mud inlet, and the finest cavings predominating on the screen furthest from it.
More modern, double-deck shale shakers have a second, finer screen below, through which the mud must also pass and which is intended
to take out finer, recycled, weathered material, unconsolidated grains and other debris. If sampling is regular, there should be sufficient
material on the shale shaker screens to represent the entire sample interval.
After a sample is taken, with a fraction from each screen, the screens should be cleaned off to allow fresh material to begin to accumulate.
When drilling with very small-diameter bits or when coring, only a small volume of material will be recovered and, to avoid losing any, a
catching board or bucket may be placed beneath the lower end of each screen. If this is done, be very sure that they are completely cleaned
and rinsed out between samples (but at no other time! You must warn the derrickman, or shaker-tender about this).
There are several designs of automated sample catchers available to simplify and standardize the process. So far, none of these have
achieved wide acceptance.
Collecting samples of drill cutting from a gas-based drilling fluid system is a simpler though less rewarding task. A short by-pass line from the
blooie line allows a portion of the return gas flow, dust and fluids to be diverted into a sample trap (see Figure 9). Periodically, the by-pass
line is closed with a valve allowing the sample trap to be opened and the accumulated dust removed for examination.
Downstream of the Shale Shaker are other solids control devices that may not be used all of the time, but only when it is necessary to
remove fine abrasive debris or reduce the mud density by removal of fine suspended solid material. These include two sizes of hydroclones
— the de-sander and the de-silter. There also maybe a decanting mud centrifuge that is intended to remove the very finest material,
extremely fine, artificially ground silt created in the grading mill from the small amount of sand contaminating the Bentonite clay.
These devices, like the shale shaker, and any other operating solids control device should be sampled on a regular basis in a uniform
manner whenever they are running. It is worthwhile to sample the de-silter and centrifuge even though these extract only material that is far
finer than probable cuttings. They can be sampled from time to time, to establish a baseline of drilling fluid contaminants, and background
fine formation debris, so that you will have something against which to judge material found later in unconsolidated cuttings samples.
Are we seeing something new, or just more of the SOS (same old stuff) ?
Are we seeing something new, or just more of the SOS (same old stuff) ?

Figure 9: In Air or gas-based drilling, samples of dust or rock-flour are taken from a by-pass of the blooie line
When sampling:
✔ Collect cuttings from all shaker screens, de-sander, de-silter and centrifuge outlets.
✔ Use a pail or liter-sized (or quart-sized) jug to collect sufficient uniform material to fill all of the required sample containers. Routinely,
you will require enough for:
✔ Large unwashed, micro-paleontology, geochemistry or trade samples,
✔ Sieved, rinsed and dried, reference samples.
✔ Sieved, rinsed, washed and blended samples for lithological and hydrocarbon evaluation.
✔ Use a second jug to collect about a liter (quart) of fresh drilling mud.

Don't shovel cutting straight from a shaker screen it sample sacks. You may be putting a different, unrepresentative fraction of the sample
into each sample set.
A sufficient volume of sample, commonly a half liter (pint) each, must be packaged into each cloth sample sack, with minimal processing, to
serve as an untouched resource for future research by the operating company, partners, national geological surveys, or petroleum
ministries, and for trade with other oil companies. This sample should contain cuttings and material from all sources, the upper and lower
shaker screens, de-sander, and so on, although if the regular sample contains few cavings it is unnecessary to search them out and add
them to the sample. Beyond this point the sample should be minimally processed.
For purely geological applications, it is common to lightly rinse the sample with tap water to remove excess drilling fluid (or an organic
solvent, to remove oil-based mud, but not legitimate formation oil staining), pack the sample into a tagged cloth or woven paper sample sack
and hang the closed sacks to dry in a sheltered outdoor location. The tag should be clearly labeled with:
✔ Oil Company Name
✔ Well Name and Number
✔ Well Location
✔ Sample Depth Interval,
✔ From-
✔ To-
✔ Set Number (if multiple sample sets are being collected) Remember, a sample represents an interval not a single depth. The interval
must be specified on each sack, so that if the sample is handled alone (separated from the samples above and below) it is still
possible to know both the top and bottom depth. On extremely secret projects, it may be necessary to record some of this information
in a coded form.
If geochemical and other analytical procedures are planned, a better scheme is to seal the sample into a labeled can without any washing or
rinsing. For every ten canned rock samples, an eleventh can containing only fresh drilling fluid taken from the flow line can also be added to
provide an analytical baseline.
A smaller volume of sample (about 10 or 20 ml) must be minimally processed to provide an easily accessible and visible sample sequence
for future quick reference and correlation:
✔ Rinse the sample vigorously through 8-mesh, 80-mesh, and 170-mesh sieves in order to remove drilling fluid, hydrated rock debris
and rock flour.
✔ Set aside a fraction of each sieve’s content for later evaluation, and

✔ Dry a few grams of the 80-mesh fraction to be saved as a reference sample set.
Figure 10: Samples of cuttings and mud must be taken no less often than every fifteen minutes while drilling. Sample
processing requires the packing of unwashed, and rinsed-and-dried samples. Cuttings and mud samples are sieved
and blended for lithological, oil and gas evaluation (See Chapter 8 for details).

Figure 11: And for those of you who are too lazy to zoom --

Traditionally, washed-and-dried samples have been packed in manila paper clasp-top envelopes, and archived in boxes or drawers. More
recently, people has taken to using grip-top polyethylene bags. These are much less successful since the hard abrasive cuttings can cause
the bags to leak or even burst (particularly when an impatient mud logger tries to fill a bag with sample still hot from the drier) and the
precious sample can be lost. The best method of curation is in the form of clear, sub-divided, plastic sample trays (with locking lids) that can
each hold and display ten or more samples and can be microscopically examined without removing the sample. This is both convenient and
prevents progressive sample loss, or degradation of the sample or its container by constant removal and replacement.
Another innovative method sometimes used in hard rock drilling areas (where cuttings samples have a good, hard, attractive appearance,
and slow drill rates leave free time in the mud logging unit for handicrafts) is the preparation of a cuttings lithology log — cuttings are
attached to the self-adhesive surface of a depth scaled board and covered with a transparent film, producing a visual lithological log that can
be overlaid and correlated with other logs from mud logging and wire-line logging operation.
A final method is to mount several cuttings on a slide in balsam, and grind it to prepare a thin section record of the rocks penetrated.
Although lacking orientation, this nevertheless provides a detailed mineralogical and textural record. On the other hand it is very time
consuming in proportion to the information gained.
Sieve Aperture
(mm)
US ASTM E-11-81
Mesh Number
UK BS410
Mesh Number
Nearest Metric Equivalent
(mm)
6.300 ¼ inch 6.300
5.600 3½ 3 5.600
4.750 4 3½
4.000 5 4 4.000
Figure 12: A table of standard sieve mesh sizes manufactures in the US, UK and other nations (nearest equivalent
French, Canadian and German standard metric sizes) commonly used in cuttings evaluation. The ASTM number 8,
number 80, and number 170 (or equivalents highlighted in the table) are the minimum necessary for mud log sample
preparation and evaluation.

Sieve Aperture
(mm)
US ASTM E-11-81
Mesh Number
UK BS410
Mesh Number
(mm)
3.350 6 5 3.150
2.800 7 6
2.360
2.360 8
8 7
7 2.500
2.500
2.000 10 8 2.000
1.700 12 10 1.500
1.400 14 12 1.400
1.180 16 14 1.120
1.000 18 16 1.000
0.850 20 18 0.800
0.710 25 22 0.710
0.600 30 25
0.500 35 30 0.500
Figure 12 (continued): A table of standard sieve mesh sizes manufactures in the US, UK and other nations (nearest
equivalent French, Canadian and German standard metric sizes) commonly used in cuttings evaluation. The ASTM
number 8, number 80, and number 170 (or equivalents highlighted in the table) are the minimum necessary for mud log
sample preparation and evaluation.

Sieve Aperture
(mm)
US ASTM E-11-81
Mesh Number
UK BS410
Mesh Number
(mm)
0.425 40 36 0.400
0.355 45 44 0.355
0.300 50 52 0.315
0.250 60 60 0.250
0.212 70 72 0.200
0.180
0.180 80
80 85
85 0.180
0.180
0.150 100 100 0.140
0.125 120 120 0.125
0.106 140 150 0.100
0.090
0.090 170
170 170
170 0.090
0.090
0.075 200 200 0.071
Figure 12 (continued): A table of standard sieve mesh sizes manufactures in the US, UK and other nations (nearest
equivalent French, Canadian and German standard metric sizes) commonly used in cuttings evaluation. The ASTM
number 8, number 80, and number 170 (or equivalents highlighted in the table) are the minimum necessary for mud log
sample preparation and evaluation.

Approximate Size
(mm)
US Mesh Number
or Screen Type
Particle Name
Approximate Size
(inch)
Greater than 256 ¼ inch Boulder Greater than 10.1
64 - 256 ¼ inch Cobble 2.5 – 10.1
32 - 64 ¼ inch
Pebble or
Very Coarse Gravel
1.26 – 2.5
16 - 32 ¼ inch
Pebble or
Coarse Gravel
0.63 – 1.26
8 - 16 ¼ inch
Pebble or
Medium Gravel
0.31 – 0.63
4 - 8 6
Pebble or
Fine Gravel
0.157 – 0.31
2 - 4 12
Granule or
Very Fine Gravel
0.079 – 0.157
1 - 2 20 Very Coarse Sand 0.039 – 0.079
Figure 13: This table provides a comparison of common sieve mesh and screen sizes, with typical sedimentary grains
as graded by the Wentworth (1922) grain size classification. Use these sizes to estimate whether grains from
unconsolidated sediments can be reliably caught by a sieve, but remember that well cuttings from consolidated
formations contain more than a single grain in each, and so are many times larger.

Approximate Size
(mm)
US Mesh Number
or Screen Type
Particle Name
Approximate Size
(inch)
0.5 - 1
0.5 - 1
40
40
Upper screen on a double-deck
Upper screen on a double-deck shale shaker
shale shaker
Screen on a single-deck
Screen on a single-deck shale shaker
shale shaker
Coarse Sand
Coarse Sand 0.02 – 0.039
0.02 – 0.039
0.25 – 0.5 70 Medium Sand 0.01 – 0.02
0.125 – 0.25 140 Fine Sand 0.0049 – 0.01
0.0625 – 0.125
0.0625 – 0.125
200
200
Lower screen on a double-deck
Lower screen on a double-deck shale shaker
shale shaker
Mud Engineer's API Sand Test Kit
Mud Engineer's API Sand Test Kit
Very Fine Sand
Very Fine Sand
Barite Mud Weight
Barite Mud Weight
Additive
Additive
0.0025 – 0.0049
0.0025 – 0.0049
0.0313 - 0.0625
0.0313 - 0.0625
400
400
De-sander Outlet
De-sander Outlet
Coarse Silt
Coarse Silt 0.0025 – 0.0049
0.0025 – 0.0049
0.0156 – 0.0313 Medium Silt 0.00155 – 0.0025
0.008 – 0.0156
0.008 – 0.0156 De-silter Outlet
De-silter Outlet Fine Silt
Fine Silt 0.000614 – 0.00125
0.000614 – 0.00125
0.003912 – 0.008
0.003912 – 0.008 Mud
Mud Centrifuge
Centrifuge Outlet
Outlet Very Fine Silt
Very Fine Silt 0.000315 – 0.000614
0.000315 – 0.000614
0.001 – 0.003912 Clay 0.000039 - 0.000315
Less than 0.001
Less than 0.001 Premium Grade Wyoming Bentonite
Premium Grade Wyoming Bentonite Colloid
Colloid Less than 0.000039
Less than 0.000039
Figure 13 (continued): This table provides a comparison of common sieve mesh and screen sizes, with typical
sedimentary grains as graded by the Wentworth grain size classification.

The remainder of the 8-mesh and 80-mesh sieve contents are processed, analyzed and tested as necessary to provide detailed worksheet
notes on formation identification, sample condition, contaminant content, and so on. There is no set procedure for this; the tests to be
performed depend upon the mineralogy and condition of the sample.
Material to be examined should include:
✔ Unwashed samples of cavings, cuttings and mud are examined under natural and ultraviolet light for oily sheen, petroleum odor, or
fluorescence (See Chapter 8).
✔ The 8-mesh sieve contents are usually cavings or spallings (see Figure 6), but they must not be discarded without examination.
They can be identified, described on the worksheet and (briefly) on the mud log, for the benefit of later investigators who may not
understand drilling, or the origin and significance of caving and spalling. A portion of this sample may also represent over-sized
cuttings produced by some PDC, or long milled-tooth hard rock bits.
✔ The 80-mesh sieve contents are the best source of fresh, representative cuttings from bottom. They may also contain smaller or
reworked cavings, and recycled cuttings from up-hole and even some un-dispersed mud additives.
✔ The 170-mesh sieve contains fine, detrital and
unconsolidated material from freshly cut formation For
example: the fine sand, silt fractions, secondary minerals
and micro-fossils from massive claystones and shales.
Some of the material may be recycled (perhaps, more
than once). Regular inspection allows identification of
the recycled material allowing later recognition when
fresher material of more interest is added.
✔ Cuttings and mud samples are agitated with fresh water
in the blender for a timed interval. Then the air space drawn off and analyzed for total hydrocarbons (see Chapter 5) to give a
measure of hydrocarbon mobility.
✔ After each blender test:
✔ Open the blender jar, sniff the head space, and look for any oil slick or rainbow on the surface of the water. If seen, test a sample
of the water for hydrocarbons (see Chapter 8).
✔ Pour the water and sediment from the blender jar through the 170-mesh sieve and rinse. This provides a supplement to the
regular 170-mesh sample, and is particularly useful if the sample contains a lot of soft, sticky, gumbo clay that is otherwise
difficult to wash out.
170-mesh sieves are easily torn if not handled properly and some mud logger
170-mesh sieves are easily torn if not handled properly and some mud logger
contractors may try to save money by not repalcing them in a timely
contractors may try to save money by not repalcing them in a timely
manner. This is not acceptable!
manner. This is not acceptable!
However, if a 170-mesh sieve is temporarily unavailable, then try rinsing the
However, if a 170-mesh sieve is temporarily unavailable, then try rinsing the
sample very lightly through an 80-mesh sieve , and then flip the sieve over,
sample very lightly through an 80-mesh sieve , and then flip the sieve over,
Material clinging to the bottom of the sieve may be used as a stand-in.
Material clinging to the bottom of the sieve may be used as a stand-in.

✔ If a pure sample of the clay or clay-sized material is required, then the material that passes with the water through the 170-mesh
sieve can be filtered using a filter paper, or the filter press from a drilling fluids test kit.
Figure 14: The geological sample processing area of a modern standard mud logging unit: sieves, sink, blender, micro-
gas analyzer, microscope and ultraviolet inspection chamber. (Illustration courtesy of EXLOG, Inc.).

Material from all of these fraction should be microscopically and chemically examined to provide a detailed description on the worksheet for:
✔ Lithological identification of the drilled cuttings, and
✔ A record of the:
✔ Cavings,
✔ Recycled material, and
✔ Solid drilling fluid contaminants contained in the sample.
This record, prepared from fresh sample, and with current well-site knowledge, is an essential reference for the future evaluation of the mud
log and of any other data gathered on this well.
A small fraction of significant material found in these samples, from at least the last 30 meters (100 feet) along with the rinsed and dried
reference sample set must be kept near the microscope. Remember, that in thick sedimentary successions, important changes may occur
gradually. Make sure that you have sufficient retrospective sample so that progressive changes of color, mineralogy, or texture do not take
you by surprise.
When training new mud logging geologists in sample evaluation, I have noticed one common weakness — a reticence to make any
statement at all until ready to deliver a complete evaluation. This is a commonly trait of well-qualified, but inexperienced geologists. It may
work when scoring points in the classroom, but it doesn't work in the filed. When working with the many different data, and sample sources
in the mud logging unit, the rules should be:
✔ Work with a pencil in your hand, and write down on the worksheet everything you observe as soon as you observe it.
✔ Don’t go looking for your expectations (trying to see what you know from the well prognosis, or have seen in a nearby well log). Look
at the sample, and note what you do see. Don’t start drawing conclusions until you are close to being finished.
✔ Don’t set out to identify the rock, your job is to describe it. Remember color, luster, hardness, break, texture. These are the most
recognizable features of the well cuttings, and so will be the most useful information you can put on the mud log to help future users.
✔ Write down on the worksheet everything you see. After you have viewed several samples you will have the perspective to refine
your opinions and condense your observations into a concise description that can be transferred to the mud log.
There is no space here for a detailed guide to well-site geology and microscope technique but these are clearly needed to complete the
discussion of well-site processing. They are of course necessary and, if your university education gave too much time to maps, cross-
sections, and field work, then I heartily recommend that you, or your employer, should invest in a class on how to use a binocular
microscope, and a routine laboratory test kit. It takes more than a little skill to use a microscope properly. When you first approach one,
almost everything you do intuitively – is wrong!

However, a few points, specific to preparing a mud log, are worth discussion here.
Traditionally lithological evaluation on the mud log consisted of a cuttings log (see Figure 15). The track is divided into ten sub-divisions. In
each, a lithology symbol was printed (usually a modified mechanical typewriter symbol) representing the composition of ten percent of the
sample.
Figure 15: A traditional mud logging cuttings lithology log
In some areas this is simply intended to be an honest representation of what was seen in the cuttings sample (excluding cavings and
contaminants) and, as we have discussed, it follows true formation lithology with some adjustment for slippage of bed boundaries.
In other mud logging units, at other places and times, a degree of interpretation was attempted; adjusting boundaries and percentages, and
so on. Sometimes this was based upon rules of thumb rather than geological training. Figure 4 contains (in the right-most lithology track) an
example of a so-called Gulf Coast Cutting Log. The log is based upon the assumption that the only rocks seen in the section are slow drilling

shales and faster drilling, unconsolidated sandstones. The lithology data on that mud log represents neither what the mud logger saw, nor
what the mud logger interpreted. It is just a slavish copy of the ROP curve. A partially interpreted cuttings lithology log can be confusing to a
later user who finds disagreement between the log and his own cuttings observations.
A modern mud log should contain three lithology tracks:
✔ The Cuttings Lithology track reporting exactly what is being seen in the cuttings samples being packaged for later reference (see
Figure 5).
✔ The Interpreted Lithology track representing the mud logger’s un- compromised opinion of what formations have been penetrated
based upon all observations (see Figure 16).
Figure 16: A modern mud log, in addition to a cuttings lithology track, includes interpreted lithology, and lithological descriptions tracks

✔ The Lithology Description track containing an abbreviated text description of the lithologies penetrated. Each description having been
compiled with the benefit of several separate detailed sample descriptions reported on the worksheet. The description may also
include brief notes on cavings and contaminants that are present in the unwashed sample but not shown in the cuttings and
interpreted lithology tracks.
Cuttings Recovery Problems
In addition to the appearance of cuttings, debris and poorly dispersed mud chemicals, there are also problems for geological evaluation of
cuttings resulting from drilling events and special drilling fluid treatments. These must be described on the worksheet ,and the mud log in
order to avoid later difficulties with stored samples and inexperienced observers. We need to discuss some of these problems.
Viscosity Additives
Mud additives can be a problem both in themselves and when poorly-mixed. For example, starch was routinely added to mud to thicken it.
Unfortunately, starch when kept in a warm, wet environment (such as a mud pit) has an unfortunate tendency to ferment. Today, Carboxy
Methyl Cellulose (CMC) is an alternative additive used to
improve the viscosity and filter cake building ability of the mud.
When mixed correctly it disperses fully, and simply thickens the
mud. When added too quickly and not properly dispersed, it
congeals into large, soft, translucent, sticky globs, a centimeter
or more in diameter. These globs arrive at the shale shaker in a
mass resembling a bad movie monster. Extricating cuttings from this gooey mess is an unpleasant and difficult task.
Solvents and Lubricants
When the mud logging crew arrives at a new rig, one of their first jobs should be to inventory the mud storage area including the oil and
water storage tanks:
✔ Check the oil for viscosity, and color in natural, ultraviolet light (see Chapter 8) and solvents.
✔ The water is tested for salinity, and the concentration of other ions the unit may be equipped to detect: sulfide, sulfate, carbonate,
and so on.
Using clear, seal-able sample trays or glass phials, you can prepare samples of all the mud additives: alone and dampened with a small
quantity of rig water, and with oil. Once a set like this has been prepared, it is a simple job to keep it fresh with the occasional replacement,
CMC is also a common additive in diet snacks and drinks. Working on the
CMC is also a common additive in diet snacks and drinks. Working on the
same principal, it swells inside you, and makes you feel full.
same principal, it swells inside you, and makes you feel full.

and updated with new additives. The set can be kept in the sample processing area for comparison with wet or dry samples when doubt
arises, or for familiarizing new mud logging crew members.
Mud additives can usually be removed easily from cuttings samples with vigorous washing. However, when they been have recognized but
cannot be removed, a brief notation of the additive’s abundance and appearance in the sample must be added to the sample description on
the mud log for the elucidation of later users.
Lost Circulation Material
Under normal circumstances, drilling fluid filters into porous and permeable formations under the pressure differential between well-bore
hydrostatic and formation fluid pressure. Mud filtrate flushes into the formation pore space leaving a filter cake of mud clays on the bore hole
wall. A thick, tough, impermeable filter cake rapidly forms and prevents further mud loss, and filtrate invasion.
Sometimes filter cake formation is not enough to seal the formation permeability and whole mud loss can take place. This is usually when:
✔ The porosity and permeability are on a very large scale. For example: extensively fractured or cavernous formations.
✔ The formation fluid pressure is extremely sub-normal, or the formation is dry, and contains no fluid at all.
✔ Excessive mud density, swabbing of the drill string, or high shut-in pressures during a well kick, or blowout cause an over-balance in
the well bore so great that fracturing is induced in the weakest formation exposed in the bore hole.
When lost circulation (or loss of returns) occurs, the fluid level falls in the bore hole, and hydrostatic pressure declines with the risk that well
control will be lost.
There are two common ways of addressing lost circulation:
✔ If the down-hole formations really are weak, sub-normally pressured, or dry, then there is no risk of losing well control. Drilling can
proceed using plain water as the drilling fluid. Most of the water is lost into formations down hole but the drill string is lubricated and
cooled, and the well bore remains clean. The well can be drilled this way to the depth at which casing can be set and circulation with
drilling mud re-established.
✔ If the need for pressure control makes it necessary to regain circulation before drilling ahead, new drilling mud is made containing
lost circulation material: light fibrous, flaked and granular materials added to the drilling fluid to plug coarse permeability, caverns and
fractures that cannot be sealed with normal mud filter cake.
When drilling with water and continuous loss of circulation, few if any cuttings are recovered to surface. Those that are recovered are only
the finest fragments and often contain material that has been re-cycled or re-worked in the bore hole. When this happens, an appropriate
annotation must be made on the worksheet and the mud log.

Probably the worst formation evaluation problems in water-based muds occur when lost circulation materials are used. Most of the materials
used are man-made and only cause problems by being troublesome to remove. They include:
✔ Cellophane flakes,
✔ Crushed walnut shells,
✔ Shredded leather,
✔ Horse hair,
✔ Mattress stuffing (yes from real mattresses, I swear I saw it!), and
✔ Wood flakes.
✔ If some process’s left overs are cheap, non-toxic, able to float and swell in water, then you are likely to find them bagged, marked up
a few thousand percent, and sold for lost circulation material.
Two particular types of lost circulation material can cause the worst problems (or embarrassment) when describing samples. These are
walnut shells, and mica.
Walnut shells are crushed and bagged in various grades. The finest grade of crushed walnut shells, commonly called Nut Plug®, is very
popular with drilling crews in rainy or muddy conditions. They will take a bag, and scatter the material on slick and slippery rig steps and
walkways. Ever when there is no lost circulation, a little of this material can find it’s way into the drilling mud and your samples. Everyone
knows what nut shells look like, and when they are thoroughly soaked in mud filtrate, they become soft, mushy and very easy to recognize.
Even if you can't say what they are, you can certainly say what the are not – well cuttings!
Unfortunately, when the nut shells are just a little wet, they remain brittle and angular, but they become translucent and pinkish in color. You
would not be the first observer to report detrital rose quartz fragments in the sample. To avoid this embarrassment, grip your quartz grain
with tweezers, and apply steady pressure with a metal probe or needle. If the needle slowly penetrates into, and then through the grain then
you know you are looking at a piece of walnut shell.
Another lost circulation material that is seriously troublesome in formation evaluation is mica which is added in the form of large flakes that,
at first, are much too large to be believable in a sedimentary rock. Of course, after circulation, the mica flakes are broken into small
fragments. The sericite-like fine mica can dust the surface of cuttings and look like a secondary mineral or micaceous cement, and can
cause errors in correlation or environmental conclusions.
Other kinds of LCM are only a problem because of their high concentration in the mud compared to the cuttings volume and the difficulty of
removing them. These materials cause time consuming problems in sample catching and washing. Fortunately, the material floats, and can
be removed by panning a sieve full of cuttings in a tub of clean water. The LCM quickly floats away on the surface of the water.

WhittakerA.GeologicaEvaluation-LithologyMineralogy.pdf

WhittakerA.GeologicaEvaluation-LithologyMineralogy.pdf

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