WELL LOGS Interpreting Geophysical Well LogsProf. Dr. Hassan Z. Harraz
Historical Aspect-Schlumberger brothers, Conrad and Marcel, are credited withinventing electrical well-logs.- On September 5, 1927, the first “well-log” was created in asmall village named Pechelbroon in France.- In 1931, the first SP (spontaneous potential) log wasrecorded. Discovered when the galvanometer began “wiggling”even though no current was being applied.-The SP effect was produced naturally by the borehole mud atthe boundaries of permeable beds. By simultaneouslyrecording SP and resistivity, loggers could distinguish betweenpermeable oil-bearing beds and impermeable nonproducingbeds.
a) Gamma Ray• The gamma ray measures the natural radioactivity of the rocks, and does not measure any hydrocarbon or water present within the rocks.• Shales: radioactive potassium is a common component, and because of their cation exchange capacity, uranium and thorium are often absorbed as well.• Therefore, very often shales will display high gamma ray responses, while sandstones and limestone will typically show lower responses.
• The scale for GR is in API (American Petroleum Institute) and runs from 0-125 units. There are often 10 divisions in a GR log, so each division represents 12.5 units.• Typical distinction between between a sandstone/limestone and shale occurs between 50-60 units.• Often, very clean sandstones or carbonates will display values within the 20 units range.
b) SP (Spontaneous Potential)• The SP log records the electric potential between an electrode pulled up a hole and a reference electrode at the surface.• This potenital exists because of the electrochemical differences between the waters within the formation and the drilling mud.• The potenital is measured in millivolts on a relative scale only since the absolute value depends on the properties of the drilling mud.
• In shaly sections, the maximum SP response to the right can be used to define a “shale line”.• Deflections of the SP log from this line indicates zones of permeable lithologies with interstitial fluids containing salinities differing from the drilling fluid.• SP logs are good indicators of lithology where sandstones are permeable and water saturated.• However, if the lithologies are filled with fresh water, the SP can become suppressed or even reversed. Also, they are poor in areas where the permeabilities are very low, sandstones are tighly cemented or the interval is completely bitumen saturated (ie- oil sands).
c) Resistivity (Induction)• Resistivity logs record the resistance of interstitial fluids to the flow of an electric current, either transmitted directly to the rock through an electrode, or magnetically induced deeper into the formation from the hole.• Therefore, the measure the ability of rocks to conduct electrical currents and are scaled in units of ohm-meters.• On most modern logs, there will be three curves, each measuring the resistance of section to the flow of electricity.
• Porous formations filled with salt water (which is very common) have very low resistivities (often only ranging from 1-10 ohms-meter).• Formations that contain oil/gas generally have much higher resisitivities (often ranging from 10- 500 ohms-meter).• With regards to the three lines, the one we are most interested in is the one marked “deep”. This is because this curve looks into the formation at a depth of six meters (or greater), thereby representing the portion of the formation most unlikely undisturbed by the drilling process.• One must be careful of “extremely” high values, as they will often represent zones of either anhydrite or other non-porous intervals.
d) Sonic• Sonic logs (or acoustic) measure the porosity of the rock. Hence, they measure the travel time of an elastic wave through a formation (measured in ∆T- microseconds per meter).• Intervals containing greater pore space will result in greater travel time and vice versa for non-porous sections.• Must be used in combination with other logs, particularly gamma rays and resistivity, thereby allowing one to better understand the reservoir petrophysics.
e) Density/Neutron• Density logs measure the bulk electron density of the formation, and is measured in kilograms per cubic meter (gm/cm3 or kg/m3).• Thus, the density tool emits gamma radiation which is scattered back to a detector in amounts proportional to the electron density of the formation. The higher the gamma ray reflected, the greater the porosity of the rock.• Electron density is directly related to the density of the formation (except in evaporates) and amount of density of interstitial fluids.• Helpful in distinguishing lithologies, especially between dolomite (2.85 kg/m3) and limestone (2.71 kg/m3).
• Neutron Logs measure the amounts of hydrogen present in the water atoms of a rock, and can be used to measure porosity. This is done by bombarding the the formation with neutrons, and determing how many become “captured” by the hydrogen nuclei.• Because shales have high amounts of water, the neutron log will read quite high porosities- thus it must be used in conjunction with GR logs.• However, porosities recorded in shale-free sections are a reasonable estimate of the pore spaces that could produce water.
• It is very common to see both neutron and density logs recorded on the same section, and are often shown as an overlay on a common scale (calibrated for either sandstones or limestone’s).• This overlay allows for better opportunity of distinguishing lithologies and making better estimates of the true porosity.* When natural gas is present, there becomes a big spread (or crossing) of the two logs, known as the “gas effect”.
f) Caliper• Caliper Logs record the diameter of the hole. It is very useful in relaying information about the quality of the hole and hence reliability of the other logs.• An example includes a large hole where dissolution, caving or falling of the rock wall occurred, leading to errors in other log responses.• Most caliper logs are run with GR logs and typically will remain constant throughout.
WELL LOG (The Bore Hole Image)Interpreting Geophysical Well Logs Prof. Dr. Hassan Z. Harraz
What is well LoggingWell log is a continuous record of measurement made in bore hole respond tovariation in some physical properties of rocks through which the bore hole is drilled.Traditionally Logs are display on girded papers shown in figure.Now a days the log may be taken as films, images, and in digital format.
HISTORY 1912 Conrad Schlumberger give the idea of using electrical measurements to map subsurface rock bodies. in 1919 Conrad Schlumberger and his brother Marcel begin work on well logs. The first electrical resistivity well log was taken in France, in 1927. The instrument which was use for this purpose is called SONDE, the sond was stopped at periodic intervals in bore hole and the and resistivity was plotted on graph paper. In 1929 the electrical resistivity logs are introduce on commercial scale in Venezuela, USA and Russia For correlation and identification of Hydrocarbon bearing strata. The photographic – film recorder was developed in 1936 the curves were SN,LN AND LAT The dip meter log were developed in 1930 The Gamma Ray and Neutron Log were begin in 1941
LOGGING UNITS• Logging service companies utilize a variety of logging units, depending on the location (onshore or offshore) and requirements of the logging run. Each unit will contain the following components:• logging cable• winch to raise and lower the cable in the well• self-contained 120-volt AC generator• set of surface control panels• set of downhole tools (sondes and cartridges)• digital recording system
TYPICAL WIRELINE SKID UNITWelaco Unit at Ormat’s Puna Geothermal Venture in Hawaii
TYPES OF LOGS• Geophysical Logs • Production Logging – Resistivity – Pressure – Porosity – Temperature – Spinner – Gamma Ray – Fluid Density – Dip Meter – Borehole Imaging • Well Inspection – Sonic – Other – Caliper – Electro-magnetic – Ultrasonic – RA Tracer – Video
LOG INTERPRETATION OBJECTIVES• The objective of log interpretation depends very much on the user. Quantitative analysis of well logs provides the analyst with values for a variety of primary parameters, such as:• porosity• water saturation, fluid type (oil/gas/water)• lithology• permeability• From these, many corollary parameters can be derived by integration (and other means) to arrive at values for:• hydrocarbons-in-place• reserves (the recoverable fraction of hydrocarbons in-place)• mapping reservoir parameters• But not all users of wireline logs have quantitative analysis as their objective. Many of them are more concerned with the geological and geophysical aspects. These users are interested in interpretation for:• well-to-well correlation• facies analysis• regional structural and sedimentary history• In quantitative log analysis, the objective is to define• the type of reservoir (lithology)• its storage capacity (porosity)• its hydrocarbon type and content (saturation)• its producibility (permeability)
POROSITY LOGS• Neutron tool – Neutron source – High energy neutrons are slowed down by hydrogen atoms in water (or oil) and detected by tool – Porosity is function rock type and slow neutron count• Density tool – Gamma ray source – Electrons reflect gamma rays back to detector in tool – Electrons in formation proportional to density – Porosity is function of rock type and density• Sonic tool – Measures speed of sound in formation – Porosity slows sound – Porosity is function of rock type and measured speed of sound
GAMMA RAY LOG• Gamma ray detector measures natural radioactivity of formation• Mostly due to Potassium in Shale – Shale has porosity but no permeability• Uranium and Thorium – Less common sources natural radioactivity – Detected by more sophisticated tools that measure gamma ray energy• Run with other tools to correlate logs
GAMMA RAY LOG• Gamma Rays are high-energy electromagnetic waves which are emitted by atomic nuclei as a form of radiation• Gamma ray log is measurement of natural radioactivity in formation verses depth.• It measures the radiation emitting from naturally occurring U, Th, and K.• It is also known as shale log.• GR log reflects shale or clay content.• Clean formations have low radioactivity level.• Correlation between wells,• Determination of bed boundaries,• Evaluation of shale content within a formation,• Mineral analysis,• Depth control for log tie-ins, side-wall coring, or perforating.• Particularly useful for defining shale beds when the sp is featureless• GR log can be run in both open and cased hole
Spontaneous Potential Log (SP)• The spontaneous potential (SP) curve records the naturally occurring electrical potential (voltage) produced by the interaction of formation connate water, conductive drilling fluid, and shale• The SP curve reflects a difference in the electrical potential between a movable electrode in the borehole and a fixed reference electrode at the surface• Though the SP is used primarily as a lithology indicator and as a correlation tool, it has other uses as well: – permeability indicator, – shale volume indicator – porosity indicator, and – measurement of Rw (hence formation water salinity).
Neutron Logging• The Neutron Log is primarily used to evaluate formation porosity, but the fact that it is really just a hydrogen detector should always be kept in mind• It is used to detect gas in certain situations, exploiting the lower hydrogen density, or hydrogen index• The Neutron Log can be summarized as the continuous measurement of the induced radiation produced by the bombardment of that formation with a neutron source contained in the logging tool which sources emit fast neutrons that are eventually slowed by collisions with hydrogen atoms until they are captured (think of a billiard ball metaphor where the similar size of the particles is a factor). The capture results in the emission of a secondary gamma ray; some tools, especially older ones, detect the capture gamma ray (neutron-gamma log). Other tools detect intermediate (epithermal) neutrons or slow (thermal) neutrons (both referred to as neutron-neutron logs). Modern neutron tools most commonly count thermal neutrons with an He-3 type detector.
The Density Log• The formation density log is a porosity log that measures electron density of a formation• Dense formations absorb many gamma rays, while low-density formations absorb fewer. Thus, high-count rates at the detectors indicate low-density formations, whereas low count rates at the detectors indicate high-density formations.• Therefore, scattered gamma rays reaching the detector is an indication of formation Density. Scale and units:The most frequently used scales are a range of 2.0 to 3.0 gm/cc or 1.95to 2.95 gm/cc across two tracks.A density derived porosity curve is sometimes present in tracks #2 and#3 along with the bulk density (rb) and correction (Dr) curves. Track #1contains a gamma ray log and caliper.
RESISTVITY LOGS• Measure bulk resistivity of formation• Laterlog – The original well log – Electrodes direct current into formation to ground electrodes on surface• Induction – Magnetic field induces current in formation – Used with low conductivity well fluids• Porosity can be calculated if water salinity is known• Oil or gas saturation can be calculated if porosity and water salinity are known
Resistivity Log• Basics about the Resistivity:• Resistivity measures the electric properties of the formation,• Resistivity is measured as, R in W per m,• Resistivity is the inverse of conductivity,• The ability to conduct electric current depends upon: • The Volume of water, • The Temperature of the formation, • The Salinity of the formation The Resistivity Log: Resistivity logs measure the ability of rocks to conduct electrical current and are scaled in units of ohm- meters. The Usage: Resistivity logs are electric logs which are used to: Determine Hydrocarbon versus Water-bearing zones, Indicate Permeable zones, Determine Resisitivity Porosity.
Acoustic Log• Acoustic tools measure the speed of sound waves in subsurface formations. While the acoustic log can be used to determine porosity in consolidated formations, it is also valuable in other applications, such as:• Indicating lithology (using the ratio of compressional velocity over shear velocity),• Determining integrated travel time (an important tool for seismic/wellbore correlation),• Correlation with other wells• Detecting fractures and evaluating secondary porosity,• Evaluating cement bonds between casing, and formation,• Detecting over-pressure,• Determining mechanical properties (in combination with the density log), and• Determining acoustic impedance (in combination with the density log). Prof. Dr. H. Z. Harraz
DIP METER AND BOREHOLE IMAGING• Dip Meter – Four or six arms with few buttons measure small scale resistivity – Wellbore inclination and orientation – Map bedding planes of sedimentary formations• Imaging Tools – Resistivity imaging tools • FMI - Schlumberger, EMI – Halliburton • Pads with many buttons map small scale resistivity – Ultrasonic imaging tools • USIT – Schlumberger, CAST – Halliburton • Spinning ultrasonic transducer measures I.D. and sonic impedance – Borehole image • Dip and orientation of fractures • Structure and stress of formation – Borehole breakout – Drilling induced fractures
OTHER GEOPHYSICAL• LOGS Mineral identification – Pulsed neutron source stimulates gamma ray emissions – Tool measures energy spectrum of returning gamma rays – Percentage of elements (silica, calcium, etc.)• Magnetic resonance – Detects free water – Determine permeability
GEOTHERMAL APPLICATIONS• Geophysical tools designed for sedimentary formations – Algorithms for sandstone, shale, limestone, dolomite – Special algorithms required for crystalline rock• Resistivity tool is sufficient to quantify porosity when water salinity is known• Sonic tool puts seismic surveys on depth• Density tool calibrates gravity surveys• Formation imaging tools map fractures and quantify stress regime• Neutron and density tools can identify lithology, – if samples are available to create correlations – if there is variation in rock type
PRODUCTION LOGS• Very useful in geothermal wells• Can be run with simple or sophisticated equipment• Temperature surveys are essential for exploration work• Pressure & Temperature surveys are more useful for well testing and production
TEMPERATURE LOGS• Most important parameter in geothermal wells• Thermocouple wire – easiest for shallow holes• RTD – most accurate• Mechanical tool – Only option for deep hot wells 10 years ago• Electronic surface readout tool in thermal flask – Requires high temperature wireline• Electronic memory tool in thermal flask – State of the art – Slick line or braided cable• Fiber Optics – Instantaneous temperature profile of entire wellbore – Good for measuring transients• High temperature electronics – Not yet commercial
TEMPERATURE PROFILE SURFACE CONDUCTIVE GRADIENTDEPTH HYDROTHERMAL SYSTEM OUTFLOW ZONE UPFLOW TEMPERATURE REVERSAL CONDUCTIVE HEAT SOU TEMPERATURE
PRESSURE LOG• Second most important reservoir parameter – pressure drives flow – producing drawdown indicates reservoir productivity (or injection buildup) – drawdown curves analyzed to determine reservoir permeability• Water level, easily measured – used in hydrology but less useful in geothermal systems – dependant on wellbore temperature and gas or steam pressure above water• Mechanical pressure tool – common ten years ago• Capillary tubing filled with nitrogen or helium – reservoir pressure is measured at surface – good for long term reservoir pressure monitoring of hot wells• Electronic surface readout tool in thermal flask – requires high temperature wireline• Electronic memory tool in thermal flask – state of the art – slick line or braided cable
STATIC PRESSURE AND TEMPERATURE PROFILES PRESSURE TEMPERATURE 0 50 100 150 200 250 300 350 0 200 400 WATER LEVELDEPTH 600 STATIC PRESSURE STATIC TEMPERATURE 800 1000 1200
STATIC AND FLOWING PRESSURE AND TEMPERATURE PROFILES PRESSURE TEMPERATURE 0 50 100 150 200 250 300 350 0 200 FLOWING TEMPERATURE 400DEPTH STATIC 600 TEMPERATURE STATIC PRESSURE 800 FLASH DEPTH FLASH DEPTH PRESSURE DRAWDOWN 1000 FLOWING PRESSURE 1200
SPINNER LOG• Propeller measures flow in wellbore• Identifies production (or injection) zones• Calculate fluid velocity from series of up and down runs at different cable speeds
FLOWING SPINNER SURVEY Log down 100 fpm Log up 100 fpm SPINNER COUNTS -10 0 10 20 30 40 50 0 200 400DEPTH 600 FLASH DEPTH 800 1000 MAIN PRODUCTION ZONE 1200
TYPICAL SHALLOW WELL LOGGING UNIT From USGS website, nc.water.usgs.gov
TYPICAL SLICK LINE WINCH From BOP Controls Inc. website, bopcontrols.net
WELL INSPECTION LOGS• Sonic Cement Bond Log (Same tool as sonic porosity log) – Measures quality of cement on outside of casing – Difficulty with large geothermal well casing – Difficulty with micro-annulus caused by temperature and pressure changes• Caliper – Measures I.D. of casing – Detects corrosion, scale, washouts, parted casing• Electro-magnetic – Measures metal loss – Detects corrosion, holes and parted casing• Ultrasonic (same as imaging tool) – Measures I.D. and thickness of casing, and impedance of material behind casing – Detects corrosion, holes and cement• RA Tracer – Injects slug of iodine 131 into wellbore – Gamma ray detector measures radioactive slug – Detects leaks in casing and flow behind pipe• Video – Identify well problems – Requires very clear water
PRESSURE CONTROLShould be used there is any possibility of well flowingPack-off – Rubber cylinder tightens around wireline Grease out – Few hundred psi Low pressureLubricator – Length of pipe below pack-off – Necessary to run tool in pressurized wellBlow out preventor – Valve below lubricator that closes around wireline – Useful if pack-off fails or wireline gets stuck in pack-offGrease tubes for high pressure – Placed below pack-off – For thousands of psi Grease in High pressure – Grease pumped in high pressure end flows to low pressure
PRESSURE-TEMPERATURE- SPINNER TOOLS FOR SALE• MADDEN SYSTEMS (Odessa, TX) – Flasked surface readout and memory tools• KUSTER COMPANY (Long Beach, CA) – Mechanical tools – Flasked surface readout and memory toolsAnyone with a slickline or braided cable winch can run memory tools.
GEOPHYSICAL LOGGING TOOLS AND WIRELINE WINCHES FOR SALE• Companies that used to make tools and sell wireline systems went out of business in the 1990’s• Companies that sell systems now are on the internet
COMMERCIAL BOREHOLE LOGGING COMPANIES Geothermal Production LoggingThe Big Three • WELACO – Bakersfield CA• SCHLUMBERGER • PACIFIC PROCESS SYSTEMS – Bakersfield CA• HALLIBURTON • SCIENTIFIC PRODUCTION• BAKER ATLAS SERVICES – Houston TX • INSTRUMENT SERVICES INC. –Worldwide Geophysical, Production Ventura CA & Inspection Logging Pressure-Temperature-Spinner & some other servicesVideo Sell and service equipment• DOWNHOLE VIDEO – Oxnard CA Many other companies in Japan, New• many other companies Zealand, Philippines, Iceland, Kenya (KenGen), etc.
Important QuestionsIs the Well Producing at Its Potential?If It Is Not , Why Isn’t It?What is the Well Production Potential?Is It: the Well Production on Well TestORIs It: What Well Is Capable to Produce
Causes of Low / Production DisturbanceA- Non- Treatable Problems1- Low Formation KH2- Poor Relative permeability3- High GOR or WOR4- High ViscosityB- Treatable Problems1- Formation Problems ( Organic & Inorganic Precipitates, Stimulation Fluids, Clay Swelling, Mud Effects)2- Production Equipments Problems ( Cement & casing, Tubing, Artificial Lifts)
It is fine to Understand Types of Problems and Their Causes.But It Is More Important To Determine That A Problem Does Exist. Diagnosis of CausesA- Surface Data AnalysisB- Drilling ReportC- Workover, Completion and Stimulation Data
Well Log Classification and CatalogingWell Log Data Repository PWLS Class Repository Well Log Catalog Industry Data Company Data
Activities Enabled by PWLS Meta Data• Classify well logs• Classify well log channels• Query for well logs• Query for well log channels
Classify a Well Log / Channel / Parameter• well log well_log_service_class – by interpretation of well log header• channel company_channel_class – validate against dictionary• parameter company parameter spec. – validate against dictionary
Genericity of classification• original acquired data primarily co. data – company channel class – well log service class• computed data primarily industry data – well log curve class – well log tool class• processed data combined approach
Query by technology• goal: logs of a given technology• industry classification:well log tool class• company classification: well log service class• catalog: classification by well log service class• result: well log data
Query by channel attributes• goal: channels of a given object, property, function, ...• industry classification:well log curve class• company classification: company channel class class• catalog: classification by company channel class• result: well log data
Query by propery type• goal: channels of a given property type• industry classification: well log curve class• company classification: company channel class class• catalog: classification by company channel class• result: well log data
Parameter-Augmented Query• goal: well logs, subject to parameteric constraints e. g. total_depth > 33000 ft• industry classification: param spec (property type) e. g. Bottom_Depth• company classification: company parm spec e. g. BOTTOM_DEPTH• catalog: parametric classification e. g. BOTTOM_DEPTH=44000(m)• result: well log data
Existing Data Well Log Data Repository Dictionaryqueryengine Well Log Catalog 15:MDL : xxxxxxxxx 150:CDL : xxxxxxxxx 280:SLD : xxxxxxxxx 440:LDS : xxxxxxxxx
Queries Well Log Data Repository Where are my density logs? DictionaryWell Log Catalog 15:MDL : xxxxxxxxx 150:CDL : xxxxxxxxx 280:SLD : xxxxxxxxx 440:LDS : xxxxxxxxx
Existing Data PWLS... density ... Well Log Data Repository Industry Data Density : xxxxxxxxx Acoustic : xxxxxxxxx Dictionary Neutron : xxxxxxxxxqueryengine Well Log Catalog 15:MDL : xxxxxxxxx 150:CDL : xxxxxxxxx 280:SLD : xxxxxxxxx 440:LDS : xxxxxxxxx Company Data 15:MDL : xxxxxxxxx : Density 150:CDL : xxxxxxxxx : Density 280:SLD : xxxxxxxxx : Density 440:LDS : xxxxxxxxx : Density Prof. Dr. H. Z. Harraz
Textbook & ReferencesTextbook:1- Hill, A.D., 1990," Production Logging- Theoretical and Interpretive Elements", SPE Series, vol.14.2- Instructor Notes: Production Logging & Cased–Hole Logging in Vertical and Horizontal Wells).References:1- Schlumberger, 1987," Cased- Hole Log Interpretation: Principles / Applications", Schlumberger Ltd., Houston.2- Rollins, D.R., et al, 1995," Measurement While Drilling", SPE Series vol.40.