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The geophysical techniques inside a drilled hole include: 1. Electric Logging 2. Radioactive Logging 3. Induction Logging 4. Sonic Logging 5. Fluid Logging 6. Down-Hole Photography.
Technique # 1. Electric Logging:
A four electrode arrangement is commonly employed in measuring resistivity from bore holes similar to the four electrodes used in surface resistivity method. A current (I) is passed between the electrodes A and B while voltage is measured between electrodes M and N. One current electrode is always on the ground potential and its effect can be taken as negligible. Conventionally there are two systems of electrode arrangements called the ‘normal’ and ‘lateral’, Fig. 8.10.
In the normal arrangement, the distance MN is large compared to the distance AM. If AM is small, say 40 cm, it is called a ‘short normal’ and if it is longer, say 160 cm, it is called a ‘long normal’. In the lateral arrangement MN is very small compared to the distance AM. If O is the midpoint of MN and AO is 1.8 m, then it is called 1.8 m lateral. Usually in electrical logging both normal and lateral devices are used to obtain maximum information. The lateral measures the resistivity of the formation beyond the zone of invasion, Fig. 8.11.
Invasion:
During the drilling operations, the mud in the borehole is usually conditioned so that the hydrostatic pressure inside is greater than the pressure of the formations. The mud filtrate pushes the interstitial water of the formation up to a certain distance called ‘flushed zone’ though its influence is still further up to what is called the ‘invaded zone’, beyond which the formation is uncontaminated. The solid particles of the infiltrating fluid are deposited on the wall of the borehole forming a mud cake, which considerably reduces the infiltration. The different zones and their resistivities are shown in Fig. 8.11.
In actual logging, the logging tool with three electrodes built up in it (one being on the ground surface) called the ‘sonde’ is connected to a multiconductor cable passing over a sheave and via winch and electronic recorder mounted on a truck to a power source. The sonde is lowered into the hole, Fig. 8.12, and the recorder with moving pen traces the electrical resistivity on graph paper continuously with depth, as the sonde is withdrawn.
This resistivity is the apparent resistivity Ra. Different sondes are used for electrical resistivity, S.P. (spontaneous potential) and radioactive logs. Oklahoma Logmaster, USA was used in UNDP investigations in South India and it recorded separate graphs for resistivity—short and long normal, lateral, SP and gamma logs. An idealised electric log is shown in Fig. 8.13 which will help to read the electric logs correctly and locate the different zones encountered down the drilled hole.
Formation Factor:
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The formation resistivity factor F depends on the lithology of the aquifer-
F = R0/Rw …(8.7)
Where R0 = resistivity of the rock saturated with conducting fluid (assumed equal to true formation resistivity Rt) and Rw = resistivity of the saturating fluid.
Also F = a/фm …(8.8)
Where a, m = constants depending on rock property and mineralogical composition, and m is called the cementation factor and ф = effective porosity.
Archie’s formula F = 1/фm …(8.9)
For sands F= 0.81/ф2 …(8.9)
In compacted formations (limestone and dolomite)
F = 1/ф2
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For granular rock (Humble’s formula)
F = 0.62/ф2.15
F is usually consistent for a given sedimentary unit within a depositional basin and may be determined by laboratory analysis of samples or from the Eq. (8.7), when both resistivity logs and formation water samples are available. Effective porosity may also be determined from the electric logs.
Example 1:
The resistivity of a sample of formation water reduced to a standard temperature of 27°C is 15.2 ohm-m. If the formation resistivity read from the electric log is 131 ohm-m, determine the effective porosity of the formation, assuming a cementation factor of 2 in the Archie’s formula.
Solution:
F = R0/Rw = 1/фm
Assuming R0 = Rt
131/15.2 = 1/ф2
Effective porosity ф = 34.1%
Single point resistivity. In this method, only two electrodes are employed, one in the tool and one on the ground. A current is passed between the two electrodes. The amount of current that will flow will be a function of the resistivity of the material close to the electrode in the borehole.
Thus the measurement of current flow under a constant applied voltage will enable resistivity measurements to be made. The simplicity and economy of the equipment is an advantage of this method.
The limitations of this method are:
(i) It is not usually possible to determine the true resistivity accurately enough for quantitative interpretation in terms of porosity or lithology.
(ii) The measured resistance is seriously affected by variations in the diameter of the well and the mud resistivity.
Spontaneous Potential:
The spontaneous or self-potential (SP) in a drill hole is due to electrochemical and electrokinetic or streaming potentials. Electrochemical potentials are due to differences in concentrations of activities of the formation water and the mud filtrate, called the liquid junction potential, and membrane potential due to the presence of shale layers. The streaming potential is due to electro-filteration of the mud through the mud cake.
If the permeable formation is not shaly, the electrochemical potential (static SP or SSP)
Ec = -K log aw/amf …(8.10)
Where aw, amf = chemical activities of the interstitial water and mud filtrate, respectively and K= coefficient proportional to the absolute temperature of formation (K = 71 at 25 °C).
The chemical activity of a solution is related to the salt content and hence to the resistivity. The SP due to electrochemical activity may be written as-
SP = -K log Rmf/Rw …(8.10)
Where Rmf = resistivity of the mud fluid and Rw = resistivity of the formation water.
The SP curve generally provides the best logging approach to the determination of water quality.
The SP log is obtained by recording the potential differences, against depth, between a fixed surface electrode and a movable electrode in the borehole. Since the potentials associated with shales and clays are normally the least negative, the SP curve is a straight line called the ‘shale baseline’. Opposite the permeable formations the SP curve shifts either to the left (negative) or to the right (positive) depending on the relative salinities of the formation water and the mud filtrate.
The shale baseline is drawn through as many deflection minima as possible. A sand line may then be drawn through negative deflection maxima and if fluid salinity is constant these lines will be parallel to each other and the zero baseline. The boundaries of the permeable formations are located at points of maximum SP slope rather than half amplitude as on many other logs.
The SP curve may be used to calculate formation water resistivity, locate bed boundaries, distinguish between shales and sandstone or limestone in combination with other logs, and for stratigraphic correlation. The SP log is affected by hole diameter, bed thickness, water or mud resistivity, density and chemical composition, and cake thickness, mud filtrate invasion and well temperature.
Static SP:
The SP currents flow through the borehole, the invaded and the non-invaded part of the permeable formation and the surrounding shales. In each medium the potential along a line of current flow drops proportionately due to the resistance encountered by the SP current. If the SP currents could be prevented from flowing by insulation plugs, the potential difference in the mud equals the total e.m.f. The SP curve which would be recorded in such idealised conditions is called the static SP or SSP opposite clean formations.
Example 2:
Electric and lithologic logs of Borehole no. 1008 at Pattukotai, Tamil Nadu are given in Fig. 8.14.
(i) Locate the main aquifer zones at their boundaries based on electric logs between 15 and 150 m depth.
(ii) Locate the clay zones, if any, between the aquifers.
(iii) Determine the conductivity in micromhos/cm, TDS in ppm of the formation water and porosity of the formation material in the aquifers located.
A graph showing Rm/Rw against SSP is shown in Fig. 8.15. Assume a formation temperature 38 °C and a cementation factor of 2 in the Archie’s formula. The SP deflection may be taken as equal to SSP. Mud resistivity (Rm) = 8.1 ohm-m.
Solution:
From a study of resistivity and SP logs between 15 and 150 m the following conclusion can be drawn:
(i) The main aquifer zones are located at half amplitudes on normal resistivity curves as first aquifer between depths 28 and 52.5 m, maximum resistivity Rt1 = 77.5 ohm-m; second aquifer between depths 82.5 and 142.5 m, maximum resistivity Rt2 = 82.5 ohm-m
(ii) Clay zones are located at
First clay layer between 52.5 and 69 m
Second clay layer between 72 and 78 m
(iii) Midpoint of the first aquifer zone is at 82.5 + 142.5/2 = 40.25 m. Maximum SP deflection at this point is +8 mv. Assuming SP ≈ SSP (static SP), from Fig. 8.15 Rmf/Rw = 0.76. Resistivity of the formation water in the first aquifer,
8.1/Rw1 = 0.76, Rm ≈ Rmf
Therefore, Rwt = 10.7 ohm-m
Midpoint of the second aquifer zone is at (82.5 + 142.5)/2 = 112.5 m.
Maximum SP deflection at this point is -17.2 mv and from Fig. 8.15, Rmf/Rw = 1.8. Resistivity of the formation water in the second aquifer-
8.1/Rw2 = 1.8, Rm ≈ Rmf
Therefore, Rw2 = 4.5 ohm-m
The electrical conductivity (EC) of the formation water in micromhos/cm is given by-
EC = 10,000/Rw
For the first aquifer, EC = 10,000/10.7 = 935 µohms/cm
For the second aquifer, EC = 10, 000/4.5 = 2222 µohms/cm
The total dissolved salts (TDS) in ppm of the formation water is given by-
TDS = 0.64 × EC in microohms/cm
For the first aquifer,
TDS = 0.64 × 935 = 600 ppm
For the second aquifer,
TDS = 0.64 × 2222 = 1422 ppm
The formation factor
F = R0/Rw = 1/фm , R0 ≈ Rt, m = 2
For the first aquifer,
77.5/10.7 = 1/ф2
Porosity of the aquifer material, ф = 0.372, or 37.2%
For the second aquifer,
82.5/4.5 = 1/ф2
Porosity of the aquifer material, ф = 0.233, or 23.3%
Technique # 2. Radioactive Logging:
Radioactive logs are of two general types—those which measure the natural radioactivity of formations (gamma ray log) and those which detect radiation reflected from or induced in the formations from an artificial source (neutron logs). Radioactive logs can be used in cased holes where most other types of logging will not work.
Gamma Ray Logs:
The minerals in shales and clay emit more gamma rays than the minerals in gravels and sands. Thus gamma logs can be used to differentiate between sands, shales and clay. The probe is essentially a geiger counter or scintillometer and can be run in open or cased holes, Fig. 8.16 (a).
Gamma-Gamma:
The gamma rays from a source in the probe are scattered and diffused through the formation. Part of the scattered gamma rays re-enter the hole and are measured by an appropriate detector. The higher the bulk density of the formation, the smaller the number of gamma-gamma rays that reach the detector. The count rate plotted on a gamma- gamma log is an exponential function of bulk density. By knowing the bulk density, the porosity of the formation (n) can be determined (uncased holes only) from the equation-
n = ρg – ρb/ρg – ρf …(8.12)
Where ρg = grain density; ρb = bulk density and ρf = fluid density.
In a logging equipment used by Geological Survey of Canada 10 to 35 millicuries of cobalt60 is used as a gamma source attached below a sodium iodide detector.
Neutron Logging:
Neutron rays are useful in determining the porosity of formations. A ‘fast neutron’ source is used to bombard the rock. When any individual neutron collides with a hydrogen ion (of a water molecule), some of the neutron’s energy is lost and it slows down. A large number of slow neutrons, as recorded by a slow neutron counter, indicates a large amount of fluid, i.e., high porosity.
Each radiation produces a pulse in the circuit. The number of pulses per unit time is recorded. This can be done in cased or uncased holes, Fig. 8.16 (b). The gamma ray log does not indicate casing or presence of fluid while the neutron log is sensitive to both casing and fluid in the hole as well as in the formation, Fig. 8.16 (c).
Technique # 3. Induction Logging:
Induction logging measures the conductivity (reciprocal of resistivity) of formations by means of induced alternating currents. Insulated coils (for induction), rather than electrodes, are used to energise the formations, and the bore hole may contain any fluid or be empty but the hole must be uncased. It is specially used to investigate thin beds because of its focusing abilities and its greater radius of investigation. It is a superior method for surveying empty holes and holes drilled with oil based mud.
Technique # 4. Sonic Logging:
The sonic log records the time required for a sound wave to travel through a specific length of formation. Such travel times are recorded continuously against depth as the sonde is pulled up the bore hole. The sonic log is recorded as transit time (∆t) in microseconds per metre, with zero on the right. The speed of sound in subsurface formations depends on the elastic properties of the rock, the porosity of the formation (n) and their fluid content and pressure. The sonic log enables the accurate determination of porosity of the formation.
n = (1/V – 1/Vm) / (1/Vf – 1/Vm) …(8.13)
Where Vm = velocity matrix; Vf = velocity fluid and V = velocity formation.
Since the transit time ∆t = 1/V,
n = ∆tlog – ∆tmatrix/∆tfluid – ∆tmatrix …(8.14)
This log will also give an indication of rock type and fracturing.
Technique # 5. Fluid Logging:
Fluid logging includes the use of sondes to measure the temperature, quality and movement of fluids in a drill hole. (These characteristics of the fluid column may or may not truly reflect conditions in the aquifer system).
Temperature Logging:
The rate of increase of temperature with depth (geothermal gradient) depends on the locality and heat conductivity of the formations. Temperatures encountered in drill holes are dependent not only on the natural geothermal gradient but also on the circulation of the mud. Temperature logs may be used to identify aquifers or perforated sections, contributing water or gas to a well, to provide data on the source of water, as an aid in identifying rock types and for calculating fluid viscosity and specific conductance from fluid- resistivity logs.
Temperature log can be used to distinguish moving and stagnant water in a well and identify the source of recharge or injected waste water. Temperature logging can also be used to verify that the cement on the outside of the casing has formed a proper bond because cement generates a great amount of heat as it sets, Fig. 8.17. Higher temperatures are usually recorded in caved sections where greater volumes of cement are deposited, permitting correlation with electric logs.
Fluid Resistivity Logging:
Fluid resistivity logging is the measurement of resistivity of the fluid (water quality) between two closely spaced electrodes in the hole. The fluid resistivity log may be used to locate points of influx or egress of waters of different quality, to locate the interface between salt and fresh water, to correct head measurements for fluid density differences, to locate waste waters and to follow the movement of saline tracers. The resistivity of the fluid column is also important in interpreting SP, resistivity and neutron logs which may be affected by salinity changes.
Flow Meter and Tracer Logging:
The devices used to measure vertical flow in water wells include the impeller flow meter, the radioactive tracer ejector-detector and the brine ejector-detector. Fluid movement from one aquifer to another can be measured by an impeller flow meter which records the number of impeller revolutions against time. Flow meters are useful for relatively high velocities. A magnetic-type flow meter for down-hole current measurements is shown in Fig. 8.18.
Speed and direction of ground water flow can be detected by the use of dyes, soluble salts, radioactive tracers, electrical methods, heat dissipation and other means.
The common dyes are fluorescein, uranine and eosin. A common dye is sodium fluorescein, which can be detected in very low concentrations. Powdered fluorescein has a reddish-brown colour when dry; when dissolved in water it appears, by reflected light, a brilliant green. 1 part in 40 million can be detected by the naked eye.
The tracing dye is placed in a central well at equal distances from which test wells have been sunk. The direction of flow is the direction from the central well to the well in which the reagent is first detected. The tracer is injected into the well at some point and a detector records the time it takes for the tracer to reach a second point. Very low velocity movements (of the order of metres per day) can be recorded with such a tracer set up.
Other in-hole tracers have been used to measure permeability. This includes insoluble radioactive tracers which are concentrated in the most permeable beds, and a single-well pulse technique relating to the recovery time under pumping conditions of a slug of tracer placed in the well.
Single well dilution technique was adopted in the Atomic Energy Establishment, Trombay, to study the amount and velocity of subsurface water flow. A radio-isotope solution was injected into a confined section of a well and measurement of the exponential dilution of the isotope solution with time as unlabelled water slowly moves into the well, was made. The velocity of flow (V) was calculated by the following equation-
V = (2.3Q/YAt) log (C/C0) …(8.15)
Where Q = volume of water in the well; A = cross-section of the well; t = time lapse after injection; C0, C = initial and final (after time t) concentrations of tracer; Y = coefficient to take account of deformation of the hydro-dynamic field due to the presence of the well; (Y = q/qw); q = flow rate of water passing through the well and qw = flow rate of water in the aquifer (formation) across the same cross-section of the well.
Radioactive tracers are very suitable for tracing the movement of ground water as against organic dyes since their mass concentrations as low as 10-6 to 10-18 can easily be measured by geiger counters and related circuits. Precautions should be taken against radiation effects. Radioactive tracers commonly used are Bromine82, Calcium45, Cobalt60, Tritium (H3), Iodine131, Phosphorous32, Rubidium86, and Iridium192.
An ideal tracer:
(i) Must be susceptible to quantitative determination in very low concentrations.
(ii) Should not be present in the natural water.
(iii) Must not react with the natural water to form a precipitate.
(iv) Must not be absorbed by the porous media.
(v) Must be cheap and readily available.
No tracer completely meets all the above requirements and a reasonably satisfactory tracer can be selected to fit the needs of a particular situation.
Dating of Ground Water:
Tritium is produced in the atmosphere by cosmic radiation (and thermonuclear explosions) and is in abundance in rain. After the rain water infiltrates into the ground there is in no further addition of tritium and the tritium concentration diminishes exponentially. Thus, from ground water samples obtained, particularly from confined aquifers recharged from a single recharge area, the age of the ground water can be estimated.
If several water samples are obtained from wells scattered over a basin, the direction and rate of ground water movement might be determined. The equipment required for measuring the radiation of extremely low levels of naturally occurring tritium is very expensive and time consuming which limit its application.
Focussed Resistivity Logs:
The conventional sondes are 40, 160, 180 cm long and so tend to give average resistivity values unless the formations are fairly thick. To get over this focussed resistivity devices such as the laterolog, induction log, microlog and the microlaterolog have been developed.
A microlog or contact log is a resistivity log, measured with the electrodes spaced very closely (2.5 to 5 cm), in an insulating pad which is pressed against the walls of the drill hole, Fig. 8.19. For a 2.5 cm spacing, a depth of only 2.5 cm is investigated by the micro-resistivity study. The thickness of the mud cake has a significant major effect on the value of micro-resistivties. As the tool is lowered into the hole, the springs are clamped into the tool; upon reaching the bottom the spring arms are released. The average hole diameter and the micro-resistivities of each stratum exposed in the hole are recorded as the tool is drawn up the hole.
Microcaliper Log:
The microcaliper log records the average hole diameter and is run in conjunction with the microlog or contact log. The hole diameter will be equal to the size of the drilling bit, when a hard sandstone or limestone is traversed. Under normal conditions the well bore becomes enlarged in shale beds because the shales become wet with the mud fluid, slough off and cave into the hole. The microcaliper will indicate an enlarged hole up to the maximum spread of the caliper arms. Such information is useful for determining areas of formation caving, casing lengths, packers and perforations.
Cement Bond Logging:
A cement bond log indicates whether or not cement is tight against the outside casing wall. This log is based on the principle that the signal strength of an acoustic signal travelling along the casing is greatly reduced where the cement is well bonded to the pipe, compared to no bonding or poor bonding.
Technique # 6. Downhole Photography:
Drift Indicator or Photoclinometer:
By means of a camera, pendulum and compass all housed in a probe, the inclination and direction of drill hole deviation (drift) can be determined.
The US Army Engineers developed the NX borehole camera at a cost of $ 80,000 to fit a 7.5 cm borehole and give a 360° scan of the borehole wall and used 8 mm colour movie film.
More recently, television cameras have been developed which can provide immediate and continuous visual inspection of a borehole wall—live and in colour. It can also be recorded on a video tape to be replayed later. They are usually less than 7.5 cm diameter and use 1,000 watts lighting apparatus. The camera or television transmitter is lowered down the hole, usually at constant rate. Depth is calibrated using a cable marked at intervals.
Photographs or television can be used to identify geologic formations in open holes, as part of well completion survey, to check damaged walls, to aid in removing foreign matter from a well, and to assist in development or well cleaning.
Geophysical logs are often used in conjunction with drill-time logs (bit penetration rate) and bore logs of the sample cuttings obtained from different depths during drilling to aid in identifying formation characteristics, Fig. 8.13. The record of bit penetration rate can be quantitative (cm/min) or even qualitative, termed as fast, slow or very slow.
Currently some sophisticated techniques that are being developed are the neutron lifetime logging, several types of spectral logging, acoustic amplitude logging, nuclear magnetic logging and computer interpretation and collation of geophysical logs. Special sondes like the limestone sonde have been developed for specific purposes.
Geophysics is a specialised field by itself and information given here should serve only as an introduction for further study and interpretation of the results in ground water studies.