This document provides an overview of well log interpretation. It discusses how well logs are used to answer key questions about hydrocarbon-bearing formations like location, quantity, and producibility. The interpretation process involves identifying permeable zones using logs like SP and GR, then using resistivity and porosity logs to locate zones with hydrocarbons. Formations are further evaluated to determine porosity, fluid saturations, and other properties through techniques like density-neutron crossplots, environmental corrections, and determining formation temperature based on geothermal gradient. The goal is to locate potential producing zones and estimate hydrocarbon quantities and recoverability.
WELL LOG : Types of Logs, The Bore Hole Image, Interpreting Geophysical Well Logs, applications, Production logs, Well Log Classification and Cataloging
WELL LOG : Types of Logs, The Bore Hole Image, Interpreting Geophysical Well Logs, applications, Production logs, Well Log Classification and Cataloging
A small presentation about wireline logs, showing their function or the technology that they use.
Ruhr-Universität Bochum, Petroleum Geology II, Winter Semester 2013/2014.
Introduction
Petrophysic of the rocks
It is the study of the physical and chemical properties of the rocks related to the pores and fluid distribution
Porosity, is ratio between volume of void to the total voids of the rock.
Permeability, is ability of a porous material to allow fluids to pass through it.
Electric, most of the sedimentary rocks don’t have conductivity.
Radiation, clay rocks have 40K, radiate alpha ray.
Hardness, it depends on the cementing material and thickness of the sediments.
WELL LOGGING
The systematic recording of rock properties and it’s fluid contents in wells being drilled or produced to obtain various petrophysical parameters and characteristics of down hole sequences (G.E Archie 1950).
The measurement versus depth or time, or both, of one or more physical properties in a well.
These methods are particularly good when surface outcrops are not available, but a direct sample of the rock is needed to be sure of the lithology.
A wide range of physical parameters can be measured.
In some cases, the measurements are not direct, it require interpretation by analogy or by correlating values between two or more logs run in the same hole.
Provide information on lithology, boundaries of formations and stratigraphic correlation.
Determine Porosity, Permeability, water, oil and gas saturation.
Reservoir modeling and Structural studies… etc.
Types of Well Logging
Logs can be classified into several types under different category
Permeability and lithology Logs
Gamma Ray log
Self Potential [SP] log
Caliber log
Porosity Logs
Density log
Sonic log
Neutron log
Electrical Logs
Resistivity Log
For contact : omerupto3@gmail.com
Advanced logging evaluation gas reservoir of Levantine basinFabio Brambilla
Experience gained in recent activity in the Levantine basin has allowed for the development of a formation evaluation strategy for accurate gas reservoirs description in this region. The proposed evaluation approach considers operational issues of deep water wells, challenging borehole conditions (high salinity mud, deep invasion) and other geological features of these clastic reservoirs and their fluids. Our case study highlights benefits of the integrated evaluation of new laterolog resistivity data together with 2D NMR inversion results optimized for a gas bearing reservoir. Furthermore borehole imaging logs are included in our evaluation approach. The recently developed multi laterolog tool has an advantage of four multiple depths of investigation. It provides a detailed high 1ft vertical resolution radial resistivity profile overcoming the deep invasion often present in these reservoirs. The NMR acquired in gas oriented acquisition mode exploits the multi-frequency capability of the logging device. Combined together multiple G•TE and multiple TW experiments contribute to robust determination of the T1 and T2 reservoir fluid properties. This acquisition sequence allows for continuous hydrocarbon typing applying the T1/T2 vs T2 2D maps method, which is practical for these reservoirs given the T1 contrast between gas and other fluids. Consequently we are able to perform accurate HI corrections and therefore improve the estimates of NMR permeability and saturations. Further in the workflow we compare NMR and Stoneley wave permeability’s and assess in details their differences. The geological study performed with the combination of simultaneously acquired ultrasonic and resistivity borehole images provides additional insight into the reservoir architectures, taking advantage during the analysis of the different logging responses of the petrophysical factors to acoustic and resistivity investigation for a detailed delineation of the productive beds. The advantages of this integrated approach are illustrated with field data examples.
Well log analysis for reservoir characterization aapg wikiBRIKAT Abdelghani
Well log is one of the most fundamental methods for reservoir characterization, in oil and gas industry, it is an essential method for geoscientist to acquire more knowledge about the condition below the surface by using physical properties of rocks.
Effect of Petrophysical Parameters on Water Saturation in Carbonate FormationIJERA Editor
Assessment of petrophysical parameters is very essential for reservoir engineers. Three techniques can be used to
predict reservoir properties: well logging, well testing, and core analysis. Cementation factors and saturation
exponents are crucial for calculation, and their values pose a pronounced effect on water saturation estimation. In
this study, a sensitivity analysis was performed to investigate the influence of cementation factor and saturation
exponent variation, as it applies to logs and core analysis, for use in water saturation estimates. Measurements of
water saturation resulting from these variations showed a maximum spread difference of around fifteen percent.
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A paper by Peter Cockcroft and John Owens about sensitivity of log analysis to oil and gas reserves estimations. Presented to to the Society of Core Analysts in 1990 by John Owens
Reservoir Characterization from Abnormal Pressure in Parts of Eleme, Southea...Scientific Review SR
Pressure in Geophysics is mostly explained in terms of hydrostatics. It is a three dimensional stress
state in which the magnitude of stress is the same in all directions. The pressure of a fluid is said to be “abnormal
pressure” if it is greater or lower than normal. Normal pressure is regarded as the rate of increase of formation
density where the pore pressure remains hydrostatic. The determination of zones of abnormal pressure was done
using geophysical well log method in the Eleme area. Sonic log and density log formed the porosity log and
consequently the porosity data. The logs were interpreted and plotted against depth. The trends were analysed for
wells and abnormal pressure. Overpressure was determined in between particular depths. For the two wells used,
it is found between 2185m and 2785m for well A and 1805m to 2525m for well B. Abnornally high pressure
zones have density of formation greater than 1.07kg/cm
3
. They also have pressure gradients exceeding
hydrostatic pressure gradients of 0.433psi/ft to 0.435psi/ft for fresh and brackish water with less than 20000ppm
of salt and 0.465psi/ft for salt water with about 80000ppm salt content. The determined abnormal pressure can be
taken as a guide in the Eleme area of Nigeria when oil wells are to be dr illed
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Hydrocarbon reservoir has been delineated and their boundaries mapped using direct indicators from 3-D seismic and well log data from an oil field in Nembe creek, Niger Delta region. Well log signatures were employed to identify hydrocarbon bearing sands. Well to seismic correlation revealed that these reservoirs tied with direct hydrocarbon indicators on the seismic section. The results of the interpreted well logs revealed that the hydrocarbon interval in the area occurs between 6450ft to 6533ft for well A, 6449ft to 6537ft for well B and 6629ft to 6704ft for well C; which were delineated using the resistivity, water saturation and gamma ray logs. Cross plot analysis was carried out to validate the sensitivity of the rock attributes to reservoir saturation condition. Analysis of the extracted seismic attribute slices revealed HD5000 as hydrocarbon bearing reservoir.
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Effects of shale volume distribution on the elastic properties of reserviors ...DR. RICHMOND IDEOZU
Shale volume (Vsh) estimation has been carried out on three selected reservoirs (Nan.1, Nan.2, and Nan.4) distributed across four wells (01, 03, 06, and 12) in Nantin Field, using petrophysical analysis and reservoir modeling techniques with a view to understanding the reservoir elastic properties. Materials utilized for this research work include: Well Log data (Gamma Ray Log, Resistivity Log, Sonic Log, Density Log, Neutron porosity log), and a 3-D Seismic volume were used for the study. Sand and shale were the prevalent lithologies in Nantin Field. Nan. 1 reservoir was thickest in Nantin well 12 (29.7ft), Nantin 2 reservoir was thickest in Nantin Well 12 (30.9ft) while Nantin 4 reservoir was thickest in Well 3 (72ft). Correlation well panel across the Field showed that Nantin 4 reservoir, was thicker than Nan 1 and Nan 2 Reservoir respectively. Normal and synthetic Faults were also mapped, the trapping system in the field includes anticlines in association with fault closures. The thicknesses and lateral extents of these reservoirs were delineated into three zones (1, 2, and 3) which were modeled appropriately. Petrophysical and some elasticity parameters such as Poisson ratio (PR), Acoustic Impedance (AI), and Reflectivity Coefficient (RC) were evaluated for the wells. The results from elasticity evaluation showed a high Poisson Ratio of 0.40 in Nantin 2 reservoir of Well 12 based on high shale volume distribution of 0.70 indicating high stress level and possible boundary to hydraulic fracture. The lowest Poisson Ratio was evaluated in Nantin reservoir of Well 1 with lowest shale volume of 0.18 which indicates weak zones and may not constrain a fracturing job. Results from Acoustic impedance showed a high AI value of 7994.3 in Nan 2 Reservoir compared to Nan.1 which has the least AI value of 7447.3 because of low shale volume. A higher Reflectivity Coefficient of 0.01 was recorded in Nan.2 reservoir indicating bright spot while a lower RC of -0.00023 was recorded in Nan.4 Reservoir indicating dim spot. Hydrocarbon volume estimate of the three reservoirs showed 163mmstb in Nan.1 reservoir, 169mmstb, in Nantin 2 reservoir and 115mmstb in Nan. 4 Reservoir. The reservoirs encountered were faulted and laterally extensive. Nantin 2 reservoir was more prolific with a STOIIP of 169 mmstb compared to Nan. 1 with a STOIP of 163 mmstb and Nantin.4 with a STOIP of 115 mmstb, because of its good petrophysical values, facies quality and low shale volume distributions.
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2. BASIC WELL LOG INTERPRETATION
3.1 INTRODUCTION
The continuous recording of a geophysical parameter along a borehole produces a geophysical
well log. The value of the measurement is plotted continuously against depth in the well. Well
logging plays a central role in the successful development of a hydrocarbon reservoir. Its
measurements occupy a position of central importance in the life of a well, between two
milestones: the surface seismic survey, which has influenced the decision for the well location,
and the production testing. The traditional role of wireline logging has been limited to
participation primarily in two general domains: formation evaluation and completion
evaluation.
The goals of formation evaluation can be summarized by a statement of four questions of
primary interest in the production of hydrocarbons:
Are there any hydrocarbons, and if so are they oil or gas?
First, it is necessary to identify or infer the presence of hydrocarbons in formations traversed
by the wellbore.
Where are the hydrocarbons?
The depth of formations, which contain accumulations of hydrocarbons, must be identified.
How much hydrocarbon is contained in the formation?
An initial approach is to quantify the fractional volume available for hydrocarbon
in the formation. This quantity, porosity, is of utmost importance. A second aspect is to
quantify the hydrocarbon fraction of the fluids within the rock matrix. The third concerns the
areal extent of the bed, or geological body, which contains the hydrocarbon. This last item falls
largely beyond the range of traditional well logging.
How producible are the hydrocarbons?
In fact, all the questions really come down to just this one practical concern. Unfortunately, it
is the most difficult to answer from inferred formation properties. The most important input is
a determination of permeability. Many empirical methods are used to extract this parameter
from log measurements with varying degrees of success. Another key factor is oil viscosity,
often loosely referred to by its weight, as in heavy or light oil.
Formation evaluation is essentially performed on a well-by-well basis. A number of
measurement devices and interpretation techniques have been developed. They provide,
3. principally, values of porosity, shaliness and hydrocarbon saturation, as a function of depth,
using the knowledge of local geology and fluid properties that is accumulated as a reservoir is
developed. Because of the wide variety of subsurface geological formations, many different
logging tools are needed to give the best possible combination of measurements for the rock
type anticipated. Despite the availability of this rather large number of devices, each providing
complementary information, the final answers derived are mainly three: the location of oil-
bearing and gas-bearing formations, an estimate of their producibility, and an assessment of the
quantity of hydrocarbon in place in the reservoir.
3.2 APPLICATIONS
In the most straightforward application, the purpose of well logging is to provide measurements,
which can be related to the volume fraction and type of hydrocarbon present in porous formations.
Measurement techniques are used from three broad disciplines: electrical, nuclear, and acoustic.
Usually a measurement is sensitive either to the properties of the rock or to the pore-filling fluid.
Uses of well logging in petroleum engineering. (Adapted from Pickett)
Logging applications for petroleum engineering
Rock typing
Identification of geological environment
Reservoir fluid contact location
Fracture detection
Estimate of hydrocarbon in place
Estimate of recoverable hydrocarbon
Determination of water salinity
Reservoir pressure determination
Porosity/pore size distribution determination
Water flood feasibility
Reservoir quality mapping
Interzone fluid communication probability
Reservoir fluid movement monitoring
4. 3.3 Well Log Interpretation: Finding the Hydrocarbon
The three most important questions to be answered by wellsite interpretation are:
1. Does the formation contain hydrocarbons, and if so at what depth and are they
Oil or gas?
2. If so, what is the quantity present?
3. Are the hydrocarbons recoverable?
3.4 INTERPRETATION PROCEDURE
The basic logs, which are required for the adequate formation evaluation, are:
1. Permeable zone logs (SP, GR, Caliper)
2. Resistivity logs (MFSL, Shallow and Deep resistivity logs)
3. Porosity logs (Density, Neutron and Sonic).
Generally, the permeable zone logs are presented in track one, the resistivity logs are run in
track two and porosity logs on track three.
Using such a set of logs, a log interpreter has to solve the following problems,
(I). Where are the potential producing hydrocarbons zones?
(II). How much hydrocarbons (oil or gas) do they contain?
First step: The first step in the log interpretation is to locate the permeable zones. Scanning the
log in track one and it has a base line on the right, which is called the shale base line. This base
line indicates shale i.e., impermeable zones and swings to the left indicate clean zones- e.g.,
sand, limestone etc. The interpreter focuses his attention immediately on these permeable
zones.
Next step: To scan the resistivity logs in track 2 to see which of the zones of interest gives
high resistivity readings. High resistivities reflect either hydrocarbons in the pores or low
porosity.
Next step: Scan the porosity logs on the track 3 to see which of the zones have good porosity
against the high resistivity zones. Discard the tight formations. Select the interesting zones for
the formation evaluation.
5. 3.5 FORMATION EVALUATION
Determining Geothermal Gradient
The first step involved in determining temperature at a particular depth is to determine the
geothermal gradient (gG) of the region. Temperature increases with depth, and the temperature
gradient of a particular region depends upon the geologic, or tectonic, activity within that
region. The more activity, the higher the geothermal gradient. Geothermal gradients are
commonly expressed in degrees Fahrenheit per 100 m (°F/100m).
If the geothermal gradient of an area is not known, then it can be determined by chart or by
formula.
gG= (BHT- Tms/TD) x100
Where:
BHT = bottom hole temperature (from header)
TD = total depth (Depth-Logger from header)
Tms = mean surface temperature
Determining Formation Temperature (Tf)
Once the geothermal gradient (gG) has been established, it is possible to determine the
temperature for a particular depth. This is often referred to as formation temperature (Tf).
Where:
Tms = mean surface temperature
gG = geothermal gradient
D = depth at which temperature is desired
Environmental Corrections
In actual logging conditions, porosity (Ø) and the "true" resistivity of the uninvaded zone (Rt)
cannot be measured precisely for a variety of reasons. Factors affecting these responses may
include hole size, mud weight, bed thickness, depth of invasion, and other properties of the
logging environment and formation. Many of these effects have strong impacts on analysis and
must be corrected prior to evaluating the formation. Several types of corrections and the tools
for which these corrections are necessary are illustrated in table 3.1
6. Table 3.1: Required Environmental Corrections
Correcting Resistivity for Temperature
Resistivity decreases with increasing temperature, and therefore any value of Rmf and/or Rw
determined at one depth must be corrected for the appropriate formation temperature (Tf)
where those values will be used to calculate water saturation (Sw). It is vital that formation
water resistivity (Rw) be corrected for temperature. Failing to correct Rw to a higher
temperature will result in erroneously high values of water saturation (Sw). Therefore, it is
possible to calculate a hydrocarbon-bearing zone as a wet zone if the temperature correction is
not applied.
Correction may be applied through the use of a chart (GEN-5) or an equation
(Arp's equation).
Where:
R2 = resistivity value corrected for temperature
R1 = resistivity value at known reference temperature (T1)
T1 = known reference temperature
T2 = temperature to which resistivity is to be corrected
k = temperature constant
k = 6.77 when temperature is expressed in °F
k = 21.5 when temperature is expressed in °C
7. Density porosity
Formation bulk density (ρb) is the function of matrix density, porosity, and density of the fluid
in the pores (salt mud, fresh mud, or hydrocarbons). To determine density porosity, either by
chart or by calculation the matrix density and the type of fluid in the borehole must be known.
The formula for calculating the density porosity is:
Where;
ρma = matrix density of formation.
ρb = bulk density of the formation.
ρf = pore fluid density in the borehole.
Cross-Plot Porosity
There are a variety of methods--visual, mathematical, and graphical--used to determine the
cross-plot porosity of a formation. Porosity measurements taken from logs are rarely adequate
for use in calculating water saturation. There are two methods for the determination of
porosity:
1. Cross-Plot Porosity Equation
Where:
ΦD = density porosity
ΦN = neutron porosity
2. Cross- Plot Porosity from Chart
The proper Cross-Plot Porosity (CP) chart is determined first by tool type, and second by the
density of the drilling fluid.
8. SONIC POROSITY
Sonic Tool Cross-Plot Charts
The "Sonic versus Bulk Density" and "Sonic versus Neutron Porosity" charts may be
interpolated and extrapolated in the same manner as the "Bulk Density versus Neutron
Porosity" charts. These charts may be used as an alternative to the neutron-density cross-plots,
or an additional method for providing more information on the possible lithology of a
formation.
Wyllie-Time Average Equation:
Consolidated and compacted sandstones:
Unconsolidated sands:
Where:
∆tlog = travel time from the log.
∆tma = formation matrix travel time.
∆tf = fluid travel time
Cp = compaction factor.
Determining Formation Water Resistivity (Rw) by the Inverse Archie Method:
Determining a value for formation water resistivity (Rw) from logs may not always provide
reliable results; however, in many cases logs provide the only means of determining Rw. Two
of the most common methods of determining Rw from logs are the inverse-Archie method and
the SP method. Another method of Rw determination is by means of Hingle plot.
INVERSE ARCHIE METHOD: Rwa
Where:
Rt = resistivity of the uninvaded zone
Φ = porosity
9. Sw Calculations:
Water saturation may now be calculated for those zones that appear to be hydrocarbon bearing.
The water saturation equation for clean formations is as follows:
Archie's Equation
Where:
Sw = water saturation
n = saturation exponent
a = tortuosity factor.
Φ= porosity.
m = cementation exponent.
Rt = formation resistivity
Rw = formation water resistivity
Among the most difficult variables to determine, but one which has a tremendous impact upon
calculated values of water saturation (Sw). Often best obtained from the customer, but can be
obtained from logs under ideal conditions. Other sources include measured formation water
samples (DST or SFT), produced water samples, or simply local reservoir history.
Moveable Hydrocarbon Index (MHI)
One way to investigate the moveability of hydrocarbons is to determine water saturation of the
flushed zone (Sxo). This is accomplished by substituting into the Archie equation those
parameters pertaining to the flushed zone.
Where:
Rmf = resistivity of mud filtrate.
Rxo = resistivity of flushed zone.
10. Once flushed zone water saturation (Sxo) is calculated, it may be compared with the value for
water saturation of the uninvaded zone (Sw) at the same depth to determine whether or not
hydrocarbons were moved from the flushed zone during invasion. If the value for Sxo is much
greater than the value for Sw, then hydrocarbons were likely moved during invasion, and the
reservoir will produce.
An easy way of quantifying this relationship is through the moveable hydrocarbon index
(MHI).
11. SHALYSAND INTERPRETATION
The presence of shale (i.e. clay minerals) in a reservoir can cause erroneous results for water
saturation and porosity derived from logs. These erroneous results are not limited to
sandstones, but also occur in limestones and dolomites.
Whenever shale is present in the formation, porosity tools like, (sonic and neutron) will record
too high porosity. The only exception to this is the density log. It will not record too high a
porosity if density of shale is equal to or greater than the reservoir’s matrix density. In addition,
the presence of shale in a formation will cause resistivity logs to record lower resistivity.
Calculation of Vshale:
The first step in the shalysand analysis is the calculation of volume of shale from a gamma ray
log. Volume of shale from gamma ray log is determined by the chart or by the following
formulas:
Where:
IGR = gamma ray index
GRlog = actual borehole-corrected GR response in zone of interest
GRmin = minimum borehole-corrected GR response against clean zones
GRmax = maximum borehole-corrected GR response against shale zones
Determining Effective Porosity (Φe):
The second step of shaly sand analysis is to determine the effective porosity of the formation
i.e. determining porosity of the formation if it did not contain clay minerals.
Effective Porosity from Neutron-Density Combinations:
Φn-corrected = Φn - (Vcl x Φnsh) For Neutron
Φd-corrected = Φd - (Vcl x Φdsh) For Density
12. These values of neutron and density porosity corrected for the presence of clays are then used
in the equations below to determine the effective porosity ( effective) of the formation of
interest.
Determining Water Saturation (Sw) :( Indonesian Equation)
There are many different equations by which water saturation (Sw) of a clay-bearing formation
may be calculated. However, the most suitable equation is the Indonesian Equation, which is as
follow
Where:
Rt = resistivity of uninvaded zone
Vcl = volume of clay
Φe = effective porosity
Rcl = resistivity of clay
Rw = resistivity of formation water