A. Electric Well Logging
B. Gamma Ray Logging
C. Caliper Logging
D. Overview of Well Logging Applications
E. Electric Logging Equipment
F. Logging and Groundwater Investigations
G. Logging of Limestone and Dolomite Formations
H. Identifying Lignite Coal Beds
I. Locating Minerals
In electrical well logging, two electrical properties are measured in the borehole: potential and resistivity. It has been observed that in a borehole, the electrical potential varies according to the nature of the beds traversed. For example, salt water sands and brackish water sands are usually more negative than the associated shale or clay. On the other hand, fresh water sands may either be more negative or more positive than the associated formation.
Borehole potentials are caused by electrochemical reactions taking place between the formations and the mud column. Potential measurements are made by recording, in terms of depth, the potential changes between an electrode in the hole and another electrode at the surface, usually in the mud pit. An idealized potential (S.P.) curve is represented on the left side of Figure 1. From a potential curve it is possible to pick up the boundaries of many formations and to obtain information on the nature of some of these formations. Potential and resistivity are simultaneously recorded.
The electrical conductivity of a bed is controlled by the nature, quantity and distribution of the water contained in the bed. Because these factors vary appreciably from one bed to another, conductivity measurements made in a borehole can be used to pick up formation changes and to obtain information on the nature of the formation traversed.
In practice it is not the conductivity but its reciprocal, the resistivity, which is measured. The resistivity curve is obtained by recording either the resistance changes of an electrode placed in the hole (Single Electrode method), or the apparent resistivity given by a multiple-electrode arrangement. The measurements are plotted in terms of depth and the resulting record is called a resistivity curve. An idealized resistivity curve is shown on the right side of Figure 1. It can be seen that fresh water sands and dense formations have a much greater resistivity than salt water sands, clays and shales.
The equipment required to make single-electrode measurements is much simpler and less expensive than that needed for multiple-electrode measurements, but single-electrode measurements have less lateral penetration than multiple-electrode measurements have, therefore they do not always permit distinguishing oil sands from water sands invaded by mud. On the other hand, a single-electrode curve is as good, if not better, than a multiple-electrode resistivity curve for all other problems, especially for obtaining correlation between wells and for determining the depth and thickness of each bed.
Alternating current of low frequency is used for this measurement. As the logging electrode travels in the hole, changes in formation resistivity cause changes in the electrode resistance, which in turn cause voltage changes in the logging circuit. These changes are rectified and recorded as the resistivity curve.
The lateral penetration of a single-electrode measurement is about ten times the electrode diameter, i.e., 18" with the electrodes having a diameter of 1-1/2" and a height of 8". These dimensions were selected to give approximately the same resistivity values that would be obtained with a short normal resistivity curve.
The combination of a potential curve and of one or several resistivity curves placed side by side constitutes an electric log. Such logs are extremely valuable for geologic studies (correlation between wells, subsurface mapping, research on sedimentation), for seismic problems (determination of the best shooting point in a shot hole), for the location of fresh water bearing beds, for determining the exact thickness and position of sand, clay or shale beds, etc.
Requirements to Obtain Good Logs
It is not possible, with conventional logging instruments, to obtain a good electric log in the section of the hole that does not contain water or water base mud. It is therefore necessary that the hole be filled with water base drilling mud or water in the section where the electric log is needed. If this is not possible, a gamma ray probe may be used to obtain a good log.
Unusual Logging Conditions
If the hole is losing mud to the formation and the mud level has dropped appreciably at the time the electric log is to be made, the hole should be filled before the measurements are started. This is usually done by dumping water in the hole. If the composition of this water is different from that of the mud used for drilling, the potential and resistivity curves will probably exhibit a shift at the interface, and the amplitude of the kicks in the section having the saltiest water may be less than in the other section. These differences are usually small unless one water is much saltier than the other.
If some of the water or mud enters permeable beds or fractures during logging, the potential curve will be unstable and it will probably not repeat well in the part of the hole above the lowest point taking water. It may also exhibit a considerable drift. Such potential drift and instability are observed no matter the type of logging equipment used, and cannot be suppressed. Drift is generally encountered only in shallow formations, i.e., where the potential curve is generally flat and not very useful, even when the water level does not drop. Because the resistivity curve is generally not affected by this movement of water the usefulness of the log is not impaired. The same instabilities are observed also in artesian wells and they cannot be suppressed either, unless the flow of water is stopped.
Logging Shallow Formations
Even when the hole is well conditioned for electric logging measurements and there is no loss of mud into the formation, it frequently happens that the potential curve drifts appreciably to the left in the upper part of the hole. This is a natural phenomenon and it cannot be corrected.
When logging fresh water sands, it sometimes happens that the potential curve "reverses" i.e., the potential in sands is more positive (i.e., it kicks more to the right) than that in clay. This reversal may happen, in particular, when the drilling mud is saltier than usual. The usefulness of a reversed potential curve is not impaired when the logging operator is aware of this possibility. Nothing can be done about this condition, except replacement of the used mud by fresh mud.
Radioactivity is the emission of rays caused by the spontaneous change of one element into another. Although several types of rays are emitted, only gamma rays have enough penetration to be of practical use in logging the natural radioactivity of rocks.
Radioactivity of Rocks
All natural rocks contain some radioactive material. However, compared to that of uranium or radium ore, even of low grade, the radioactivity of most rocks is very small. The radioactivity of a rock is usually expressed in terms of equivalent amount of radium per gram of rock required to produce the same gamma ray intensity. Although there is no fixed rule regarding the amount of radioactivity a given rock may have, shales, clays and marls are generally several times more radioactive than clean sands, sandstones, limestones and dolomites.
Because shales, clays and marls have radioactivities that are of the same order, the term "shale" will generally be used here to denote any of these three formations. Similarly, for the sake of simplification, the term "sand" will be used to denote either sands, sandstones, limestones, and dolomites, since these four rocks have radioactivities of the same order.
The radioactivity of clean sands, i.e., sands free of shaly materials (shale, clay, marl) is generally very low. Sands that contain some shaly material have a somewhat higher radioactivity, and the increase is proportional to the amount of shale contained. Therefore, shaly sands and sandy shales generally have a radioactivity that is between that of clean sands and that of shale. In a given area, the radioactivity of shales does not generally vary too much, so that a gamma ray log is an approximate measurement of the quantity of shale contained in a formation.
of shale varies from area to area. In the tertiary and more recent formations,
i.e., those usually found in the
Some organic marine shales have a much higher radioactivity than the ordinary shales in the same area. However, they are generally relatively thin and are not found too frequently. When present, marine shales make excellent geologic markers.
Interpretation of Gamma Ray Logs
The interpretation of gamma ray logs can be summarized as follows:
1. In a given area, only the relative radioactivity of the various rocks is of significance.
2. Rocks of low radioactivity include primarily clean sands, sandstones, limestones, and dolomites. Anhydrite, salt, lignite and coal have also a low radioactivity. Their radioactivity increases when they are shaly.
3. Ordinary shales have a much higher radioactivity than the rocks listed above. The radioactivity of sandy shales is less than that of shales. Shales are sufficiently high in radioactivity and can generally be easily distinguished from the other rocks on a gamma ray log.
Most of the gamma rays emitted by the formation can penetrate casing, so that a gamma ray curve can be obtained in cased holes, although the amplitudes of the curve are somewhat reduced. For example, a 5/16 inch thickness of steel reduces the gamma ray intensity about one fourth.
Effect of Mud
The mud has two effects on the gamma ray curve:
1. It absorbs a small percent of the radiation and therefore reduces the log amplitude; unless the hole diameter is very large (more than 24") this effect is very small and can be ignored.
2. The shale or clay contained in the mud increases the radioactivity background, so that even clean sands show a slight radioactivity on the log. If the mud is uniform, this small increase is constant from top to bottom. However, if the shale has settled at the bottom of the hole there will be an increase in the radioactivity measured in this interval that has to be considered when interpreting the log. The effect of the mud on the gamma ray log is the same whether the mud is fresh or salty. Because this effect is usually very small, a gamma ray log is very useful in wells containing salty mud since, in this case, the electric log is generally poor.
Effect of Hole Size
The larger the hole, the smaller the gamma ray intensity reaching the probe. However, this effect is small and can generally be neglected.
Application of Gamma Ray Logs
Gamma ray logs are used in the following instances.
1. To log cased holes (no electric log can be obtained in cased holes).
2. To log dry holes (no electric log can be obtained in holes that do not contain water or mud).
3. To log holes containing salt water or salty mud (the electric logs obtained in such holes are generally poor).
4. To supplement the information given by the electric log (identification of formations, estimating the amount of clay in sands, etc.)
5. To locate radioactive ores, uranium in particular.
6. To help locate lignite and coal beds.
7. To help locate clay and fresh water sands.
The caliper logging system provides a means for a continuous recording of borehole diameter versus depth. These logs are generally run in uncased wells, but for many applications have proven very valuable when used within casing. The caliper tool has three arms that may be motored open or closed by control from the surface. Caliper arms are ordinarily supplied in two lengths to provide a maximum extension of either 15 inches or 30 inches. Caliper logs are used to determine hole and casing diameter, locate caved zones, casing, and the absence of casing; and permit the recognition of mud cake. (i.e., permeable zones). An example of a caliper log is shown in Figure 2.
Geophysical logs and, more particularly, electric logs give a detailed and continuous picture of the formation penetrated in the course of drilling and are one of the most useful tools currently available for subsurface investigation. They are used in:
1. Core Holes
2. Shot Holes
3. Water Wells
4. Oil & Gas Wells (conventional and progress logging)
5. Mineral Exploration
6. Soil Mechanics
An electric log gives a detailed picture of all the beds penetrated by the drill. By correlating the logs obtained in the wells of a given area, accurate geological maps are obtained showing structures, faults and changes in lithology and sedimentation.
Seismograph Shot Holes
Electric or gamma ray logs made in shot holes provide the seismologist with invaluable information. The logs permit selecting the best shooting point and they supply qualitative indications of changes in surface velocity. Further, by correlating the logs made in the area of interest, an accurate geologic map can be obtained.
An electric log run in a water well permits determining the exact depth and thickness of each aquifer and estimating the quality of the water.
Oil & Gas Wells (Conventional Logging)
Once an oil or gas field is discovered and the basic reservoir data are obtained in several key wells, all the information that is generally needed in the subsequent wells is an electric log that gives, besides correlations, the exact depth and thickness of the producing zones. Electric loggers are perfectly adapted to this type of logging. Either single-electrode or multiple-electrode equipment can be used. The latter are also useful to decide the depth of the oil-water contact. The logs are also invaluable in secondary recovery work (water flooding) and for underground storage investigations.
Oil & Gas Wells (Progress Logging)
In certain areas several electric logs are run in each well as the drilling progresses. This is done for the sole purpose of ascertaining, by means of correlation with the logs of other wells in the vicinity, the stratigraphic position of the well as the drill goes deeper. For this purpose, a single-electrode electric log or a simplified multiple-electrode log is sufficient and can be obtained to depths of approximately 8000 feet with portable loggers. Skid units and truck mounted equipment are also available for deeper wells. This procedure permits making appreciable savings, not only on the logging costs but also on rig time since the equipment is always available when needed.
Electric logs can be used for locating coal, lignite and certain ores.
Electric logs are used frequently for investigating the foundations of dams, bridges, highways, etc.
There are several types of electric logging equipment and they are generally classified in two groups: Single-electrode equipment and multiple-electrode equipment. Each group can be further divided depending on whether the equipment is portable.
This is the simplest and least expensive. Further, it can generally be operated by any member of the drilling crew. The single-electrode equipment gives a Potential curve (S.P.) and a single-electrode resistivity curve. The single-electrode resistivity curve is equivalent to a very short normal curve and, therefore, it gives good detail under normal conditions, even in very thin beds. Single-electrode equipment is recommended for correlation work (core holes, shot holes, progress logging), water wells, mining applications and soil mechanics studies. It is also used with success in oil and gas wells when the main purpose of the logs, besides obtaining correlations, is to determine the depth and thickness of the beds penetrated by the drill. For the estimation of petroleum saturation and/or formation porosity, and for logging wells that contain salt water or salty mud, single-electrode equipment is not generally recommended; multiple-electrode equipment is preferable. However, in holes of the same size, drilled with the same type of mud through the same producing horizons, changes in fluid contents and porosity are often reflected on the resistivity curve obtained with single-electrode equipment. Because the latter is relatively inexpensive, many operators have been prompt to take advantage of this empirical method for making quantitative determinations.
When it is essential to obtain true resistivity data, Multiple-Electrode equipment should be used. This equipment gives the S.P. and two, or more, resistivity curves. Multiple-Electrode equipment is also recommended for the logging of wells that contain salty mud or salt water.
Gamma Ray Logs
Gamma ray logs are used in the following instances:
1. In air drilled holes and in cable tool holes.
2. In cased holes (instead of electric logs that are meaningless).
3. In holes that contain salty mud where even multiple-electrode logs may not give a usable record.
4. When data on the clay or shale content of certain formations are needed, for example for a better interpretation of electric logs.
5. For locating coal or lignite beds that, sometimes, are not clearly seen on electric logs.
6. For uranium, potash and phosphate exploration (these minerals are generally more radioactive than the other types of rocks).
Gamma ray logs can be obtained whether the well is cased or not. Gamma ray logs can always be obtained no matter the nature of the fluid in the hole (air, water, mud, oil).
Caliper logs, either alone or with other types of logs, are useful for solving certain problems. Their main applications are:
1. To compute the amount of cement needed for a cementing operation.
2. To compute the amount of gravel needed for gravel packing.
3. To help
identify certain formations subject to caving. For example, shales
are generally more subject to caving than sandstones and limestones,
therefore they can readily be seen on a caliper log. A somewhat similar
condition exists where potash is associated with common salt. The salt
saturated brine used for drilling dissolves more potash than salt,
consequently, the hole enlargements shown on the caliper log in the salt
section generally correspond to the potash layers.
Advantages of Single-Electrode Resistivity Curve.
An important advantage of single-electrode resistivity curves is that they give considerable detail: under usual conditions beds having a thickness of one foot or more can be located and their boundaries can be accurately picked.
Another advantage of this curve is that its response is dependable: for every increase (or decrease) in formation resistivity there is an increase (or decrease) on the resistivity curve. Conversely, every increase shown by the curve corresponds to an increase in the formation resistivity. On the other hand, the response of multiple-electrode logging systems is not always consistent, for example, increases in formation resistivity are sometimes recorded as decreases. Also there are sometime spurious deflections on multiple-electrode logs several feet from bed boundaries.
The two foregoing advantages of the single-electrode resistivity log result in curves that sharply delineate lithology changes. With this log supplemented by the S.P. curve, it is generally possible to identify the type of formation traversed by the well and, in the case of water bearing beds, to estimate changes in the groundwater salinity. The qualitative interpretation of the data is easy and does not require charts.
Single-electrode equipment gives curves from the bottom of the hole up to the casing shoe or to the mud level, whichever is deeper.
Single-electrode equipment is small (and generally highly portable), rugged, simple, relatively inexpensive, and can be operated by a member of the drilling crew after a few hours' instruction.
Shortcomings of Single-Electrode Resistivity Curve.
There are two cases when single-electrode resistivity measurements may not be as efficient as multiple-electrode ones. One is when the hole diameter is larger and the mud is salty. In this case, the curves lose some of their detail. Thin beds cannot be seen and the boundaries of thick beds cannot accurately be picked.
The other case is when true resistivities are needed, for example, when it is necessary to estimate the oil saturation in petroleum reservoirs. The single-electrode resistivity curve is not well adapted to this problem for the following reasons:
1. As pointed out above, a single-electrode resistivity measurement is basically identical to that obtained with a very short normal device. A glance at conventional departure curves for beds of finite thickness shows that the resolving power of a very short normal device for determining true resistivities is small, more particularly for beds whose resistivities are much greater than the adjacent formation resistivity, and/or those which are invaded by mud. In the case of invaded beds, not only is the resolving power poor, but true resistivity determinations cannot be made, even in theory, unless the extent of invasion and the resistivity of the invaded zone are known.
2. Apparent resistivities obtained from single-electrode measurements are greatly affected by changes in hole diameter. Therefore, it is illusory to try to determine the true resistivity of a formation unless a caliper log is available or unless it is known that the formations of interest do not appreciably cave.
These shortcomings are found in any type of single-electrode equipment, but they can be remedied if a special logging procedure is used and if special features are embodied in the logger.
Today's water well drillers operate over larger areas and they sink deeper wells than their predecessors of some forty years ago. They use rotary drilling equipment instead of cable tools. For these reasons yesterday's approach to the drilling of water wells is now quite often inadequate.
In order to take full advantage of present day's opportunities, the progressive water well driller is borrowing techniques that are now standard in oil well drilling. Among those, electric logging is probably the simplest and most effective. This technique is no longer too expensive for water well work because portable loggers have brought its cost to a level permitting its regular use in ground water development work.
In continuously cored holes the examination of cuttings permits obtaining logs that are reasonably accurate. This is not so when rotary drilling is used, as the samples taken contain cuttings from several feet of hole and debris resulting from the erosion of the exposed formation by the mud stream. Electric logging, because of its accuracy, reliability and simplicity, is the usual answer.
Conventional electric logs consist of a potential curve and of one or more resistivity curves recorded side by side as a graph. Electric logs can only be recorded in open holes.
The potential curve is the continuous recording in terms of depth of the natural electric potential in millivolts. It is sometimes called the S.P. curve (Spontaneous Potential). The potential in clay (or shale) is generally used as reference, for convenience. Potential readings in aquifers vary in direction and amplitude according to the respective salinities of the drilling mud and the formation water.
A resistivity curve is obtained by lowering one or several
electrodes and recording - also in terms of depth -appropriate electrical
measurements. As the logging tool travels in the hole the recorded variations
of formation resistivity constitute a resistivity curve. When only one electrode is in the hole
the graph obtained is called "single electrode resistivity
curve". The graph obtained when the downhole
tool carries several electrodes is called "
Basically, an electric logger consists of a downhole tool - sometimes called logging electrode or "sonde" - connected to a reel mounted logging cable, a depth measuring device, a control panel and an automatic recorder. The other end of the logging cable is connected to the surface control panel and the recorder by a means of a collector. The measuring device is mechanically or digitally connected to the chart drive mechanism so that the chart moves in synchronism with the sonde. Recording pens moving back and forth, according to the amplitude of the signals (potential and resistivity) received from the sonde, draw the corresponding curves.
"Two Curve" loggers are the simplest and least expensive. They permit recording a potential and a single electrode resistivity curve and are satisfactory for usual water well drilling applications. "Multiple Electrode" loggers allow recording a potential and one or more resistivity curves. The latter loggers are somewhat more complicated and expensive but they permit obtaining quantitative information regarding the fluid content and other characteristics of the formation penetrated by the drill. Loggers of both types are available.
The more electric logs are made in an area and correlated with pumping tests and production performances, the more efficiently the subsequent wells can be completed. For example, because the composition of the water of a given formation is relatively uniform over a large area, it is generally possible to calibrate, for the area, the readings of the electric log in terms of water salinity. If water samples and an electric log are available in a test hole, it is a simple matter to determine limiting resistivity and potential values beyond which the water can be used for domestic or industrial purposes. This information can then be applied for all the wells drilled in the general area.
Although electric logs cannot determine the yield of an aquifer, they are extremely useful in helping solve this problem since they make it possible to "count" the net sand (or gravel) thickness of the aquifer. This information, combined with the pumping and other tests made in the area, generally permits fairly accurate prediction of the yield of most aquifers.
After several electric logs are available in a given area, cross sections and maps can be prepared, from which it is possible to predict the net sand (or gravel) thickness at any location. From this information, supplemented by the pumping tests and production performances of the wells, the depth at which other wells must be drilled in order to produce at a given rate can be predicted. This is important since the drilling costs can thus be estimated accurately in terms of the quantity of water wanted.
As seen from the foregoing discussion, there are many advantages to making electric logs in all wells drilled for water. These advantages are enhanced if proper consideration is given to the few limitations of this modern technique. If these limitations are kept in mind, the electric log, in conjunction with the driller's log, can be used to great advantage in eliminating guesswork in the development of ground water supplies. It can thus be appreciated how the routine use of electric loggers in water wells will supply a wealth of information rapidly, simply and at very reasonable cost.
Gamma Ray Logs
Gamma ray logging equipment is used to record the variation of natural radioactivity of the formation in open and cased holes. Gamma ray logs can be obtained whether the well is filled with water or any type of drilling fluid.
Clay and shale are moderately radioactive while clean sand, gravel, sandstone are much less radioactive. The gamma ray curve is therefore a "formation log" and permits distinguishing clay and shale beds from sands, gravels, sandstones and limestones. Shaly and clayey sands or gravels are generally mediocre aquifers. They can usually be identified by their intermediate radioactivity and comparison between electric and gamma ray logs of the same well.
In many areas, desirable aquifers are separated from unwanted ones by very thin impervious beds that are impossible to locate on the driller's log and difficult to pick up on the electric log. It is often possible to locate such thin beds on the gamma ray log.
Particularly when drilled with rotary equipment, the size of the hole does not remain constant. Caliper logs give the means for determining the volume of the hole for operations such as gravel packing or casing cementing.
Limestone and dolomite have essentially the same physical properties. Therefore, the following discussion of limestone logging applies also to dolomite.
As far as electric log interpretation is concerned, limestones can be classified in three types: 1, dense limestone, 2, limestone having intergranular porosity and 3, fractured limestone.
Dense limestone has a very high resistivity and generally very little S.P.
Limestone with Intergranular Porosity
This type of limestone gives the same electric log as that obtained in sandstone, i.e.,
1. The resistivity is less than that of dense limestone, but generally greater than that of clay or shale. The resistivity decreases when the porosity increases and when the formation water salinity increases.
2. The S.P., with respect to that of associated shale or clay, is small if the limestone contains fresh water (from minus 50 to plus 50 mv). If the formation water is brackish or salty, the S.P. is generally negative and large (more than minus 50 mv).
The resistivity of fractured limestone is less than that of dense limestone, and it decreases when the porosity increases. Generally, the S.P. is very small, regardless of the type of formation water.
Gamma Ray Log
An electric log does not always permit distinguishing a fractured limestone from clay or shale, but the distinction can be made if a gamma ray log is also available. Limestone, whether porous or dense, generally has a very low radioactivity provided it does not contain shaly material. Its response on the gamma ray log is, therefore, the same as that of sand and sandstone. On the other hand, clay and shale have a high radioactivity so that it is possible to distinguish limestone from clay or shale with a gamma ray log.
Note that the gamma ray log is affected neither by the type of formation water nor by the salinity of the drilling mud.
Because coal and lignite generally have a very high resistivity, they can be located with an electric log. The single-electrode resistivity curve is usually preferable because it gives a very detailed log. Although dense limestone and similar rocks have also a high resistivity, the differentiation between them and coal or lignite can be made with the driller's log since coal and lignite are soft while dense rocks are hard to drill.
There are a few
coals (some cannel coals) and lignites (in certain
parts of the
With very few exceptions, coal and lignite have an extremely low radioactivity while all the associated formations have a higher one, especially clay and shale. Therefore, a gamma ray log is a very useful supplement to the electric log for locating coal and lignite beds.
Since coal is often friable, the hole usually exhibits enlargements opposite coal beds. These enlargements are readily detected on a caliper log.
A number of
minerals have a much greater conductivity than the usual rocks and can,
therefore, be located from resistivity measurements
made in wells which have cut them. The most common conductive minerals are:
Graphite, Pyrite, Chalcoprite, Pyrrhotite
If the ore body is not cut by the borehole, it can be located only if:
1. There is continuity of conductivity, so that the ore body acts as a single conductive mass of large size,
2. The volume of the ore body is large enough,
3. The ore body is not too far from the borehole in which the measurements are made.
Native copper, although extremely conductive, cannot be directly located from electrical measurements - unless the concentration is extremely high - because the copper nuggets are generally separated by rock that is much less conductive than the copper.
Many minerals (hematite, limonite, blende, coal, etc.) have a high resistivity, generally much higher than that of the formation in which they are found. and they can be located from resistivity measurements made in wells which have cut them. If the mineral is not cut by the borehole, it can be located only if it is close to it and if it is relatively thick, as explained above for conductive minerals.
Radioactive minerals (most uranium ores, many phosphates) can be located by gamma ray logging, provided they have been cut by the well or are only a few inches from it.
Indirect Location of Minerals.
It is possible sometimes to locate indirectly certain minerals, in particular.
1. If they are associated with a formation that can be identified on the log, or
2. If the mineral has produced, by electrochemical action or by dissolution, a halo around the deposit. This halo may be detected by resistivity and/or S.P. measurement.