Introduction to Borehole Geophysics
Borehole geophysics is the science of recording and analyzing measurements of physical properties made in wells or test
holes. Probes that measure different properties are lowered into the borehole to collect continuous or point data that is
graphically displayed as a geophysical log. Multiple logs typically are collected to take advantage of their synergistic nature--much
more can be learned by the analysis of a suite of logs as a group than by the analysis of the same logs individually. Borehole
geophysics is used in ground-water and environmental investigations to obtain information on well construction, rock lithology
and fractures, permeability and porosity, and water quality. The geophysical logging system consists of probes, cable and draw works, power and processing modules, and data recording units. State-of-the-art
logging systems are controlled by a computer and can collect multiple logs with one pass of the probe.
Borehole-geophysical logging can provide a wealth of information that is critical in gaining a better understanding of
subsurface conditions needed for ground-water and environmental studies. Geophysical logs provide unbiased continuous and
in-situ data and generally sample a larger volume than drilling samples.
- Delineation of hydrogeologic units
The different hydrogeologic units found in the subsurface display a wide range of capabilities
to store and transmit ground water and contaminants. Borehole-geophysical logging provides a highly efficient means to determine
the character and thickness of the different geologic materials penetrated by wells and test holes. This information is essential
for proper placement of casing and screens in water-supply wells and for characterizing and remediating ground-water contamination.
- Definition of ground-water quality
The quality of ground water is highly variable and ground-water contamination may be caused
by man-made or natural sources. Integration of borehole-geophysics logging with water-quality sampling provides a more complete
picture, whether the objective is to develop a water-supply well or remediate a contaminated aquifer.
- Determination of well construction and conditions
Wells are the access points to the ground-water system, and knowledge of their construction and condition are important
whether they are being used for ground-water supply, monitoring, or remediation. The location and condition of casing and
screen can be rapidly evaluated with geophysical logging.
To access to a water filled borehole on the site of investigation a geophysical logging tool will allow
to correlate your surface data to reach a higher degree of accuracy in your interpretation of the geophysical data. Detailed
information on subsurface conditions is essential for the development and management of ground-water resources and the characterization
and remediation of contaminated sites. Borehole geophysical techniques provide a highly efficient means for the collection
of such information. Recent advances in methods and equipment have greatly increased the ability of geoscientists to obtain
subsurface information in ground-water investigations through borehole geophysical techniques
WELL-LOGGING
The
full potential of a logging program cannot be realized until the logging measurements are interpreted. Log interpretation
should start at the time of data acquisition and should continue as an iterative process through-out the project.
WELL EVALUATION:
- The reason for running a Well Log is to help locate an Oil or Gas bearing formation.
Other reasons may be related to defining bed thickness of Coal or other minerals and defining aquifers for production of water.
- In order to analyze a formation, three types of logs are available. The types are;
Lithology, Resistivity, and Porosity.
When a well logging unit arrives on location, a geologist or engineer approaches
the evaluation task in the following manner:
- The first step is to run a “base log” , normally a resistivity log, which
is used to correlate formations with previous logs ran in the local area. This base log helps establish the structural position
of the well.
- Information from the base log that indicates lithology or formation type is examined
to determine which zones have sufficient porosity and permeability to be of most interest for production. The resistivity
anomalies are then further evaluated.
- No definite conclusions can be made at this point regarding the commercial value
of the well. Further information on porosity must be obtained in order to make a quantitative evaluation.
- The porosity logs only “respond to variation in porosity”. Therefore
logging engineers are advised that the porosities calculated from tool response may be subject to correction after further
evaluation.
- When Lithology, resistivity, and porosity logs are available then the analyst has
sufficient information to proceed with a numerical analysis of porosity and saturation. This data combined with other
geological information provides the basis for determination of the commercial value of a well.
Introduction:-
Well logging involves measuring the physical properties of surrounding rocks with a sensor located in a borehole. The
record of the measurement as a function of depth is called well log.
Geophysical well-logging methods include mechanical methods, passive and a number of active electrical methods (including
self-potentential, Resistivity, induction, induced polarization), several nuclear methods(natural gamma ray detection and
observation from induced nuclear reactions), acoustic logging and measurement of magnetic and thermal properties.
Reservoir Parameters:-
The
main physical parameters needed to evaluate a reservoir, are its porosity, hydrocarbon saturation, permeable bed thickness,
and permeability.
Of the formation parameters needed to evaluate a reservoir, Resistivity is of particular importance in groundwater prospecting.Resistivity
measurements are used to deduce formation Resistivity in the uninvaded formation. Resistivity measurements are also used to
obtain values of water saturation.
Lithology identification::-
A logging
tool that could measure lithology and produce a “lithology Log” would be a valuable tool! When software is applied
to multiple logs in a well defined area, methods have been demonstrated that give lithological representations.
Unfortunately,
no tools exist that can measure lithology directly. One of the first logging measurements ever recorded, Spontaneous Potential,
or SP provides information that infers lithology. In addition, SP can infer permeability. It is possible to perform
Lithology identification using multiple logs
Methods applied:-
Of the several logs used to determine reservoir parameters, the ones used in my groundwater prospecting work were
1) Resistivity log
2) S.P log.
3) Gamma log
Resistivity logging:-
Introduction:-
The physical properties of rocks and minerals measured in electrical logging are principally Resistivity and self-potential
(SP). In practices Resistivity and SP logs are generally recorded as adjacent curves. Because most electrical measurements
can be made only where the hole has not been cased, logs are commonly run over different parts of borehole at different times.
Resistivity Concepts:
Resistivity can be defined as the degree to which a substance resists the flow of electric current.
Resistance from Ohms Law relates to current and voltage as follows:
R = V/I
Where: R = Resistance
V = Voltage
I = Current
The most simple galvanic measurement is Resistance. A Resistance log is
performed by connecting one electrode to the surface (ground) and another electrode to a downhole tool that is immersed in
borehole fluid. Applying a voltage and measuring current allows calculation of resistance. This type of log is called
a single-point resistance log. If both electrodes are placed on the tool then a differential resistance log is produced.
Multiple-electrode arrays extend the depth of investigation. A better representation
of True Formation resistivity (Rt) is obtained. Formation Resistivity can be measured when four electrodes are used. Two electrodes
– one on the surface and one downhole on the tool are used to generate an electrical current in the formations in and
around the electrodes. The surface electrode is referred to as B and the downhole electrode as A. The voltage measured between
two points referred to and M and N is then calculated as follows:
VMN = R x I/4p x
((1/rAM-1/rAN)-(1/rBM-1/rBN))
The Resistivity (R) (in a homogenous medium) is determined by:
R = G x V / I
The apparent Resistivity (Ra) (in a heterogeneous medium) is determined by:
Ra = G x V / I
Where:
G = Geometric array facto
Calculation of Geometric Factor (G):
Normal array:
G = 4 p x (1/rAM
– 1/rAN – 1/rBM + 1/rBN) -1
Lateral array:
G = 4 p x (1/rAM
– 1/rAN) -1
Resistivity is a “physical property” and is independent of size
and shape.
Resistivity, (R) is expressed in units of ohm-meter2 / meter –
abbreviated ohm-meters or ohms. Conductivity is the reciprocal of resistivity.
Conductivity = 1 / R
Conductivity is frequently expressed in units of micro-mhos/cm.
Conductivity in micro-mhos/cm = 10000/R
Where Resistivity ( R ) is in units of ohms – meter2 / meter
(ohm-meters).
Also, conductivity is expressed in units of milli-mhos per meter or simply milli-mhos. Another unit is
millisiemens.
Conduction in liquids is controlled by ion flow. Ions are created when
sodium chloride (or NaCl equivalent) are present in drilling and formation waters. The higher the sodium chloride concentration
the higher the conductivity and lower the resistivity. Ion flow is controlled by fluid viscosity and therefore temperature
affects the flow of ions and conductivity. Resistivity is affected by temperature. As temperature increases, conductivity
increases and resistivity decreases.
Determination of Rw:
Before any interpretation of resistivity data can take place, Rw must be known.
As mentioned previously, the value of Rw is affected by temperature. If a water
sample is taken and Rw is measured, it is equally important to note the temperature of the sample.
Geothermal gradient:
Geothermal gradient is a measure of temperature increase with depth. Geothermal
gradients are normally 1.0 to 1.7 degrees per 100 feet. For example: If a well has a surface temperature of 75 degrees
F and bottom hole temperature is 175 degrees F at a depth of 10,000 feet, the geothermal gradient is 1.0 degrees per 100 feet.
Evaluation of a formation can be performed using a corrected Rw at formation
temperature.
Finally, the value of Rw @ temperature should be documented on the log heading.
Resistivity related to porosity:
The amount of water contained in a formation is directly related to porosity.
Porosity therefore affects formation resistivity. As the volume of water increases, the capacity for ions increases. More
ions mean more conductivity. Conductivity and Resistivity are inversely related as previously mentioned.
Formation resistivity is affected by three factors: Salt Concentration, Temperature,
Pore volume (porosity).
Formation Resistivity Factor (F) is a fundamental concept in log interpretation
and analysis.
Formation Resistivity Factor is a proportionality constant based on the ratio
of Ro to Rw.
The equation is:
F = Ro/Rw Known as
the Archie equation.
Ro is resistivity of a 100 percent water filled formation and Rw is resistivity
of the water.
Given Rw = .05,
If Ro = 5.0 then F = 100
If Ro = 1.25 then F = 25
If Ro = .55 then F = 11
Resistivity Factor (F) is related to Porosity (f) as follows:
F = a / fm
The constants (a) and (m) are related to lithology. Cementation factor (m) in
a cemented sandstone or a porous limestone is 2.0 and (a) is equal to 1.0.
Resulting in the equation:
F = 1 / f2
Porosity of 10 percent results in a Formation resistivity Factor of 100
Porosity of 20 percent results in a Formation resistivity Factor of 25
Porosity of 30 percent results in a Formation resistivity Factor of 11
These three Formation Resistivity factors are the same as calculated with
F = Ro/Rw above.
RESISTIVITY TOOLS:
Water saturation and hydrocarbon saturation affect formation resistivity.
The measurement of resistivity is therefore one of the most important measurements to be made in logging a well. A resistivity
tool is most useful if it measures two or more characteristics of formation resistivity. Resistivity measurements combined
with porosity measurements and estimations of permeability allow a complete analysis of a well to be performed.
ELECTRIC LOG (E-LOG):
As new resistivity methods are introduced, they will never make this fundamental method obsolete! Electrical
resistivity is popular because it is a simple, low cost and efficient method. It is without doubt the most practical,
cost-effective logging method available today.
Electrical resistivity is a fundamental geophysical method used in both surface and subsurface geophysics. The method is legendary among Geophysical methods for exploration, development and definition
of existing targets.
It is because we need to know the content
of the pore space
within the earth beneath us that the E-Log is so important!
APPLICATIONS OF ELECTRIC LOGGING
- In metal mining, the measurement of resistivity (inverse
of conductivity) is related to metallic/sulfide content of the rock materials and host rock formations. Quantitative information
is provided on relative concentration of mineralized zones including porphyry targets.
- In oil and gas well drilling,
resistivity is a measurement of the electro-chemical content of the pore space of the earth surrounding the well. Oil and gas to water saturation is determined by measuring resistivity.
- In water well drilling, resistivity
is a measurement of the electro-chemical content of the pore space of the earth surrounding the well. Water quantity and quality are determined by measuring resistivity.
- Resistivity is an indirect measurement
of Total Dissolved Solids (conductive ions) in a fluid.
Total Dissolved Solids (TDS) in
water is directly related to conductivity. Conductivity ( C ) is the inverse of resistivity (r).
r = 1/C and C = 1/r
Electric logs are run in wells containing
conductive fluid to provide an electrical connection to the surrounding formations.
Other Applications:
In the Coal mining industry, during exploration
or definition of bed boundaries and thickness, resistivity logging is used to mark bed boundaries and indicate quality of
coal.
In Coal Bed Methane exploration, an E-log helps in stratigraphic correlation of coal seams.
WHAT IS MEASURED:
Typical borehole logging systems provide
16 inch “short” Normal and 32 or 64 inch “Long” Normal resistivity measurements. Resistivity
measurements are frequently combined with Spontaneous Potential (SP) and/or Natural Gamma to provide a four or five curve log. This combination gives Lithology, resistivity and porosity. Porosity is derived from resistivity.
ADVANTAGES OF DIFFERENT SPACING:
The advantage of 64 inch spacing is that
resistivity is measured deeper into the formation. The advantage of 16 or 8 inch spacing is that thin beds are better
defined.
When invasion is important, It is necessary
to measure at least two or more horizontal depths using two or more electrode spacings in order to define zones that have
invasion and show permeability. Two resistivity curves can show relative “ability of formations to produce”. When
resistivies from two spacings are the same value, then it may be assumed that invasion has taken place and the pore space
contains the same fluid. The deeper region of the surrounding formation has been flushed by the drilling fluid.
PROGRAMMABLE:
The E-Log uses a bipolar alternating square
wave transmitting voltage. A programmable electric log gives added flexibility. Low frequencies are preferred.
High frequencies are affected by induced polarization . With programmable frequency, and programmable sample measurement it is possible to optimize for varied borehole conditions. A computer controls the transmitting frequency and measurement
window for precise timing. The measurements are stored in a file in the computer for log plotting, statistical plotting or
other graphic presentation. Log files may be used for later interpretation using advanced methods and may be used in combination
with data from other logs.
Monoelectrode configuration.
The concept of operation of the electric logging tool is as follows:
When two electrodes are placed in a oil or water filled well and voltage is
applied to them, a current will flow through the well fluid and formation fluids. If additional electrodes are placed
in the vicinity of the current producing electrodes, a voltage can be measured. The voltage measured is directly related
to the resistivity of the surrounding formation fluids. Electric logging tools generate an alternating current and measure
the resulting alternating voltage at measurement electrodes. The depth of measurement is directly related to the
spacing or separation between electrodes. The depth is approximately equal to ½ of the distance from the measure electrode
and the midpoint between the two current electrodes.
Different electrode configurations yield different depths of investigation.
The “normal” electrode configuration is as follows:
One current electrode (A) on the tool down-hole and the other current electrode
(B) located at the surface. Measurement electrodes (M) are spaced from the down-hole current electrode at 8 inches, 16 inches,
32 inches or 64 inches above the “A” electrode depending on tool design. The reference electrode (N) is on the
surface. The most common configuration is 16 inch (short normal) and 64 inch (long normal) spacing. This configuration
results in a shallow resistivity and deep resistivity measurement.
The “lateral” configuration uses a current electrode (A) down-hole on the upper part of the tool or on an electrode “bridle” and the other
current electrode (B) on the surface. Two lower electrodes (M) (N) measure the lateral voltage which is representative
of a much deeper formation resistivity. Lateral measurements can be from 72 inches to 18 feet or more depending on electrode
spacing and tool design. See AMN Lateral configuration.
The advantage of short spacing is better thin bed definition. The advantage
of longer spacing is a deeper measurement of true formation resistivity. Comparison of deep and shallow resistivity
give information about invasion. If shallow and deep resistivity are the same, no invasion has occurred. If there
is separation, the most probable reason is that invasion has occurred causing the shallow (invaded) and deep water resistivities
to differ.
The electric logging tool requires a fluid filled borehole in order to have
a complete electrical path.
In wells containing highly conductive drilling fluids, guard tools are
used. A focused guard tool offers the function of having a focused current path into the formation. Electrodes
surrounding the current electrode are used to focus the tool current outward into the surrounding formation and not allow
the current to travel through the conductive borehole fluid.
OTHER RESISTIVITY TOOLS:
Many specialized varieties of resistivity tools are available. The
micro-log, mini-log, FoRxo, Contact and others that measure resistivity of the borehole mud cake and flushed zone for example.
Recently added Electric and Induction tools can perform a synthetic 
aperture – measuring at a great many different depths into the surrounding formation. Such
tools give a more precise profile of resistivities surrounding the borehole
Normal Device Lateral
Device
The basic vmethods of Resistivity logging are similar to those used in surface Resistivity prospecting. A low frequency
alternating current is applied between the current electrodes and the potential is measured between two or more potential
electrodes. The record is then a plot of potential variation with depth.
In a normal electrode configuration one current electrode (A) and one potential electrode (M) on the logging sonde are closely
spaced down hole [16” apart for the short normal and 64” apart for the long normal] and the other two electrodes
(B, N) are fixed near the top of the hole or a long distance away from the borehole. The apparent Resistivity ρa in homogeneous ground is given by-
ρa= (4πΔV/I)(1/AM-1/BM-1/AN+1/BN)-1
Because the distance AM is much smaller than any of the other 3 dimensions, this becomes-
ρa= (4πΔV/I) (AM)
The measured apparent Resistivity depends mainly on the resistivities of the beds in the vicinity of A
and M. Measurements will also be affected by the mud in the borehole and the penetration of the drilling fluid in the formation.
The Resistivity log of fig-1 is symmetrical with respect to beds where the Resistivity differs from that above and below.
Higher Resistivity beds appear thinner than their actual thickness whereas conductive beds appear thicker. The effective penetration
into the formation is about twice the electrode spacing and varies inversely with the diameter.
2)
Self-potential logging:-
Spontaneous potential is a measurement of the natural voltage that is created
from current produced in the earth because of electrochemical action. It is normally recorded in wells drilled with water.
Formations having permeability are invaded by mud filtrate from the drilling
mud. The result is electrochemical action that causes current flow in the formation. Shale formations have very low
or non-existent permeability and therefore no current flow and low spontaneous potential.
The SP curve is recorded in track 1 (left-hand track) of the well log. The intensity
of the Spontaneous potential is determined by the resistivity of the mud filtrate (Rmf) and the Formation water resistivity
(Rw).
SP is expressed as:
SP = -(60 + .133T) log10 (Rmf/Rw)
Where:
T = temperature
Rmf = Resistivity of the mud filtrate
Rw = resistivity of the formation water.
Since SP is not a zero based curve, it’s deflection is measured from a
“shale base line”.
Shale formations have little or no permeability and sandstone, limestone and
dolomite do have some degree of permeability, the SP is useful in detecting permeable beds, locating bed boundaries, determining
water resistivity, and shale indicator.
In formations containing hydrocarbons, SP is depressed because of the reduction
of conductive ions.
SP curves are calibrated using a fixed voltage calibrator.
Introduction:-
Self-potential (SP) anomalies are apparently generated by thermoelectric or thermo kinetic coupling processes. The thermal mechanism results from differential thermal
diffusion of ions in pore fluids and electrons with donor ions in the rock matrix. The ratio of voltage to temperature difference
ΔV/ΔT is known as thermo electrical coupling coefficient. The thermo kinetic coupling coefficient, Ek/ΔP depends on fluid flow , which may due to thermal as well as the pressure gradient
Working:-
The SP-Logging curve is a recording of SP value v/s depth of the difference between the potential of a movable electrode in
the borehole and the fixed potential of the surface electrode. The SP is generally recorded in track-1, usually in conjunction
with Resistivity surveys, but it can also be recorded with other logs.
Uses:-
The main uses of SP logging are locating boundaries between shales and porous beds such as sandstones, determining the cleanliness
of sands, correlating between wells, and determining formation water Resistivity.the shape of the SP curve is often characteristic
of particular depositional conditions and well to well correlation can be used to indicate thinning , pinching out , and dip
of formation.
In addition to its use in identifying shales and for correlating corresponding points from well to well, other stratigraphic
interpretation can be inferred from the SP curve. In a somewhat simplistic way the SP value can be read as the degree of shaliness
and as the inverse of the “energy” in the original depositional environment
In groundwater prospecting the SP logging is useful to
1)
Detect the permeable beds
2) Locate their boundaries and to permit correlation of such beds.
3) Determine values of formation water Resistivity
4) Give qualitative interpretation of bed shaliness.
Instrumentation:-
Equipment for SP logging is fundamentally very simple. A recording potentiometer or dc voltmeter with high input impedance
is connected across two non-polarizable electrodes. The potential recorded is usually between a moving down hole electrode
and a fixed electrode at the surface.
Gamma-ray logging:-
Because all rocks emit natural –gamma radiation ,a record of this constitutes a natural -gamma log.The
radiation originates from unstable isotopes of potassium ,uranium and thorium. In generally the natural gamma activity of
clayey formations is significantly higher than that of quartz sands and carbonate rocks .The most important application to
ground water hydrology is identification of lithology , particularly the clayey and shale bearing sediments ,which posses
the highest gamma intensity.
Gamma rays are bursts of electromagnetic waves which are emitted spontaneously by some radioactive elements.
Nearly all of the gamma radiation encountered in the earth is emitted by radioactive potassium isotope of atomic weight 40
and the radioactive elements of the uranium and the thorium series.
In passing through matter gamma rays experience successive Compton-scattering collisions with the atoms of the formation ,
losing energy with each collision.
Gamma rays are gradually absorbed and their energies degraded as they pass through formations. The amount
of absorption varies with formation density.
The gamma ray log response , after correction for borehole casing, etc, is proportional to the weight concentration
of the radioactive material in the formation . Also it is assumed that the density variations are due to porosity and ordinary
lithology changes and not to the presence of elements of high atomic numbers whch would change the absorptive characteristics.
Considering a formation series containing chiefly one specific radioactive mineral, the gamma ray log reading at a given level
will be:-
GR=ρ1V1A1/ρb
Where ρ1 is the density of the radioactive mineral
V1 is the bulk volume fraction of the mineral
ρ1V1/ρb is the concentration by weight
of the mineral
A1 is the proportionality factor corresponding to the radioactivity of the mineral
Equipment:-
The gamma ray sonde consists of a detector and an amplifier.In sediments the gamma ray log reflects mainly shale content because
the radioactive elements tend to concentrate in clays and sands.Volcanic ash , granite wash , formation waters that contain
radioactive salts, potash ,and uranium ores may may cause γ-ray responding to shale and clay is generally correlateable
with the SP log.
The interface between adjacent barren and radioactive beds is located fairly accurately at half the maximum deflection when
the beds are thicker than 1m . For thinner beds the bed centre is taken as the peak deflection.Because the gamma ray log generally
defines formation interfaces sharply in both open and cased holes, it is often run with other logs and production tools so
that one can correlate the cased and open-hole logs and relate other logs to specific formations with greater certainty .
The gamma ray log is used quantitatively to indicate shale percentage and to grade uranium deposits . It is the only logging
tool used routinely in the mineral industry.
Data Acquisition
Area: KASNAU
Location: MATA SUKH
Well No.: M04
Total Depth: 100.6m
Total Depth Logged: 97.0m
Resistivity of Mud Cake: 2.1W-m
Temperature: 37oC = 98.6OF.
Date: 17-12-205.
Data Processing
The data acquired is in the form of graph, from which the following correction and calculation are done.
i)
Borehole Correction (N64/RM) for 64” Normal (N64) reading.
ii) Borehole
Correction (N16/RM) for 16” Normal (N16) reading.
iii) Estimation
of Formation Temperature (OC).
iv) Resistivity
of Water (Rw).
v) Resistivity
of Mud Cake (Rm).
vi) Resistivity
of Mud Filtrate (Rmf).
vii) Total Dissolved Solid
(TDS).
viii) Formation Factor (F).
DEPTH
(in m) |
Temp
oC |
N16
m |
N64
m |
Rm
W- m |
Rmf
W-m |
N16
Rm |
N64
Rm |
N16
Rm
Corr |
N64
Rm
Corr |
N16
Corr |
N64
Corr |
F= N16
Rmf |
RW=N64
F
W-m |
TDS
NaCl
Equi. |
|
34-39 |
76.2 |
4.0 |
2.6 |
2.6 |
1.95 |
1.53 |
1.00 |
1.53 |
1.00 |
3.97 |
2.6 |
2.03 |
1.3 |
4200 |
|
42-46 |
76.7 |
4.0 |
2.8 |
2.6 |
1.95 |
1.53 |
1.07 |
1.53 |
1.07 |
3.97 |
2.78 |
2.03 |
1.4 |
3900 |
|
48-56 |
77.0 |
3.2 |
1.4 |
2.5 |
1.87 |
1.28 |
0.56 |
1.28 |
0.56 |
3.2 |
1.4 |
1.71 |
0.81 |
6900 |
|
61-90 |
78.2 |
4.0 |
1.6 |
2.45 |
1.83 |
1.63 |
0.65 |
1.63 |
0.65 |
3.99 |
1.59 |
2.18 |
0.72 |
7700 |
|
94-97 |
78.6 |
2.2 |
1.2 |
2.45 |
1.83 |
0.89 |
0.48 |
0.89 |
0.48 |
2.18 |
1.17 |
1.19 |
0.98 |
6200 |
INTERPRETATION
Steps towards Interpretation:
Resistivity of an unconsolidated aquifer is controlled primarily by porosity, packing, water Resistivity,
degree of saturation ,and temperature .
Specific resistivity values can not be stated for different aquifers.
- Casings and metallic objects will indicate very low resistivities.
- On relative basis shale, clay, and salt water sand give low values, freshwater sand moderate to high values.
- Clayey or shale-bearing sediments posses the highest gamma intensity.
- High value of gamma indicates isotopes of potassium, uranium and thorium.
- Natural gamma give low values for quartz sands and carbonate rocks. As well as sand with fresh water.
- For SP the signs of the potential depends on the ratio of the salinityof the drilling mud to the formation
water.
Normal Resistivity
Short Normal: The value recorded by N16” represents the formation
resistivity greatly modulated by the mud fluid resistivity i.e. an overview of the flushed/invaded zone is obtained.
Long Normal: It gives the resistivity of the un-invaded (in some cases
undisturbed) formation.
Combination of Short and Long Normal
The simultaneous recordings done by two types (N16 &
N64) of resistivity measurement. By keeping zeroes of both the Normals at the same point, the interpretation of both the logs
tells about the extent of invasion at a depth of around 50m.
The iner-seperation of the two normal resistivity logs revels the most wanted
and reliable information about the permeability of the formation at around 45 – 55m depth.
Lateral Log: Due to relatively deep penetration, lateral log measures
the resistivity of the uninvaded zone. In lateral logging, while coming up from the bottom, a plateau has occurred (see fig.)
at around 50m depth, when electrode “A” enters the resistivity formation and continues till the reference point
“O” reaches the bed boundary.
SP Interpretation: The interpretation technique is valid for very saline
formation water. The SP log on encountering the shale deposit shows a fairly constant trends and follows a straight line on
the log, called the shale base line. Thus there is a SP base line shift, which is observed due to the changes in RW-value of different formation at a depth range of 45-55m.
SP deflection
(while entering shale/clay formation, current density is maximum at boundaries of permeable formations and hence slope is
maximum) is reduced when salinity is encountered. Abnormally large SP deflection may occur in a formation of very low permeability,
and thus register a zone at the depth mentioned not suitable for aquifer formation.
At around 50m depth, RM>RW, the SP is negative, indicating that the formation water is less resistive than the borehole mud, and thus confirms
that water below 50m is saline.
Gamma Logging: Normally high concentration of radioactive elements in
the gamma log indicates clay and shale. Within the well between 45-55m depth pure shale/clay are encountered hence shale line
is established.
The interpretation has been done on the qualitative basis in connection with
the interpretation of the drilling sample.
INFERENCE: GROUNDWATER
QUALITY BELOW 50m DEPTH IS HIGHLY SALINE.
