Lithology Logs
Spontaneous Potential (SP) Log
Spontaneous potential (SP) was one of the first logging measurements ever made. It was discovered by accident, appearing as a DC potential in the borehole that caused perturbations to the old electric logging systems. Its usefulness was soon realized and it is one of the few well log measurements to have been in continuous use for over fifty years.
The SP has a number of useful functions, which include correlation; lithology, porosity and permeability indications; and a measurement of Rw (hence formation water salinity).
Figure 1 shows a typical SP log. It is represented (in track 1) on the left as a solid curve and shows departures to the left from a base-line or shale-line reading on the right, to a sand line on the left in the "cleanest" nonshale zones. The scale of the log is in millivolts, abbreviated mV. Notice that there is no absolute scale in mV, only a relative scale of so many mV per division.
The SP can be recorded very simply by suspending a single electrode in the borehole and measuring the voltage difference between the electrode and a "ground" electrode that usually takes the form of a "fish," making electrical contact with the earth at the surface. A generalized illustration of the SP recording system is shown in Figure 2 . Such SP electrodes are built into many logging tools. For example, the SP can be recorded together with an induction log, a laterolog, a sonic log, and a sidewall core gun, once there is a conductive mud in the hole.
When mud filtrate salinities are lower than connate water salinities (i.e., Rmf is > Rw), the SP deflects to the left (the SP potential is negative). This is called a normal SP. When the salinities are reversed (i.e., salty mud and fresh formation water, Rmf < Rw), the SP deflects to the right. This is called a reverse SP. Other things being equal, there is no SP at all when Rmf = Rw.
It is quite common to find fresh water in shallow sands and increasingly saline water as depth increases. Such a progression is shown in Figure 3 , where the SP appears to be deflecting to the left deep in the well but is reversed nearer the surface.
In sand A, Rw is less than Rmf; i.e., formation water is saltier than the mud filtrate. In sand B, the SP deflection is less than in sand A and thus a fresher formation water is indicated. In sand C, the SP is reversed, indicating that formation water is fresher than the mud filtrate and thus Rw is greater than Rmf. Somewhere in the region of 7000 ft it may be guessed that Rmf and Rw are equal.
Quite apart from water salinity variations, SP deflections also respond to depositional changes. Characteristic SP shapes are produced in channels, bars, and other depositional sequences where sorting, grain size, or cementation changes with depth. These shapes are also called "bells" or "funnels." Figure 4 illustrates some of these patterns.
Introduction
The spontaneous potential (SP) curve is a recording of naturally occurring physical phenomena in in-situ rocks. The SP curve records the electrical potential (voltage) produced by the interaction of formation connate water, conductive drilling fluid, and shale. Although relatively simple in concept, the SP curve is quite useful and informative. Among its uses are that it
- differentiates porous and permeable rocks from clays and shales
gives a qualitative indication of bed shaliness
aids in lithology identification
determines Rw (formation water resistivity)
In the log in Figure 5 , the SP is recorded in track one. opposite shales, the SP curve usually defines a more-or-less straight line on the log, called the shale baseline. opposite permeable formations, the curve shows excursion from the shale baseline; in thick beds, the excursions tend to reach an essentially constant deflection called the sand line. The SP log is measured in millivolts (mV).
Recording the SP
The SP can be recorded very simply by suspending a single electrode in the borehole and measuring the voltage difference between the electrode and a ground electrode (usually taking the name and the form of a "fish"), making electrical contact with the earth at the surface. A generalized illustration of the SP recording system is shown in Figure 6 . Such SP electrodes are built into many logging tools. The SP cannot be recorded in oil-base muds, which allow no conductive path.
The Source of the SP
The SP is an indicator of formation water salinity. To understand how the SP can be used to find Rw, let us discuss its origin.
When two sodium chloride solutions of differing concentration are brought into contact, ions from the solution with a higher concentration tend to migrate toward the solution of lower concentration until equilibrium occurs ( Figure 7 ). However, since C1- ions move faster than Na+ ions, a conventional current flows from the less concentrated solution to the more concentrated solution. The electrical current resulting from the combined sodium and chlorine ion movement is known as the liquid junction effect.
In terms of the solutions present in a formation, mud filtrate can be substituted for the less concentrated solution and formation water for the more concentrated solution. The potential is referred to as the liquid junction potential (Elj). The greater the contrast in salinity between mud filtrate and formation water, the larger this potential ( Figure 8 ).
Another "battery" that exists in the formation arises from the molecular construction of shale beds. Shales are permeable to Na+ ions, but not so permeable to C1- ions. A shale thus acts as an ionic sieve. This phenomenon occurs because of the crystalline structure of clay minerals. Their exterior surfaces exchange sites where cations may cling temporarily. This same surface conductance effect manifests itself in the electrical behavior of shaly sands.
Since Na+ ions effectively manage to penetrate through the shale from the saline formation water to the less saline mud column, a potential is set up known as the membrane potential (Em). Figure 9 indicates the process.
The total SP ( Figure 10 ) can now be appreciated as the sum of the two components:
Etotal = Elj + Em
The total potential, measurable in the borehole by an electrode, is also referred to as the electrochemical component of the SP.
When mud filtrate salinities are lower than connate water salinities (i.e., Rmf > Rw) the SP deflects to the left (the SP potential is negative). This is called a normal SP. When the salinities are reversed (i.e., salty mud and fresh formation water, Rmf < Rw) the SP deflects to the right. This is called a reverse SP. Other things being equal, there will be no SP at all when Rmf = Rw
It is quite common to find fresh water in shallow sands and increasingly saline water as depth increases. Such a progression is shown in Figure 11 , where the SP appears deflecting to the left deep in the well but is reversed nearer the surface.
In sand A, Rw is less than Rmf; i.e., formation water is saltier than the mud filtrate. In sand B, the SP deflection is less than in sand A, indicating a fresher formation water. In sand C, the SP is reversed, indicating formation water that is fresher than the mud filtrate (Rw > Rmf). We may guess that, at about 7000 ft, Rmf
and Rw are equal.
Spontaneous Potential: Rw Determination
Rw from the SP
In order to perform quantitative analysis of the SP, the relationship between the SP and the resistivities of the mud filtrate and the formation water must be determined. It can be shown that
SP = -K log (Rmf/Rw)
where SP is measured in millivolts and K is a constant that depends on temperature. By inspection, Rw can be found if SP, K, and Rmf are known.
The SP should be read in a water-bearing sand, provided it is clean (no shale is present) and sufficiently thick to allow for full development of the potential. K can be estimated from the temperature of the formation. A good approximation is
where T is the formation temperature in F.
Rmf can be estimated from direct measurement on a sample of mud filtrate prepared by placing a circulated mud sample in a mud press. This data is usually entered on the log heading. Care should be taken when using these values, however, since logging engineers have been known to take shortcuts and quote Rmf as some fraction of Rm usually 0.75 Rm This may be a fair estimate, but is not necessarily correct.
Likewise, circulated mud samples are not always collected by rig personnel in the correct manner. Even when properly collected, samples are not always representative of the mud in the hole at the time a particular formation was drilled.
Experiments of the sort reported by Williams and Dunlap (1984), where Rm and Rmf were measured on a daily basis as a well was drilled, tend to support the contention that Rmf is the least well-defined parameter in SP log analysis. A comparison between the values of Rm and Rmf
, as reported on log headings with the actual values measured on a daily basis, shows some alarmingly large differences. In Figure 1 , we see that both the Rm and Rmf reported on the log heading for this well were low by a substantial factor.
In the absence of any reported value for Rmf, a value can be estimated from Figure 2 , which also serves for estimation of Rmc. A fit of this empirical chart gives
Rmf = (Rm)1.065 x 10((9 - W)/13)
Rmc = (Rm)0.88 x 10((W - 10.4)/7.6)
where W is the mud weight in lb/gallon.
Mother statistical approximation for predominantly NaCl muds is
Rmf = 0.75 Rm
Rmc = 1.5 Rm
In all cases, direct measurement on a sample of mud filtrate is preferred. Even after determining values for SP, K, and Rmf, there are still minor problems to be solved. The equation
SP = -K log (Rmf/Rw)
does not explain adequately the true electrochemical behavior of salt solutions. The actual SP development is controlled by the relative activity of the formation water and mud filtrate solutions. Thus the SP equation should read
SP = -K Log (Aw/Amf)
where Aw and Amf are the activities of the connate water and the mud filtrate, respectively.
The resistivity of a solution is roughly proportional to the reciprocal of its activity at low salt concentrations, but at high concentrations there is a marked departure from this rule. A way to compensate for this departure is to define "effective" or "equivalent" resistivities for salt solutions that are, by definition, inversely proportional to the activities (Rwe = 0.075/Aw at 77 F). A conversion chart is then used to go from an equivalent resistivity (Rwe) to an actual resistivity (Rw). The SP equation can then be rewritten to the strictly accurate formula
SP = -K log (Rmfe/Rwe)
Spontaneous Potential: Influencing Factors
Factors Affecting the SP
SP readings are usually accurately and easily measured. However, there are some circumstances where SP readings need careful handling.
Oil-base muds completely lack an electrical path through the mud column, hence no SP can be generated.
Shaly formations reduce the measured SP. This phenomenon permits the formation shaliness to be determined if a clean sand with the same water salinity is available for a legitimate comparison.
Hydrocarbon saturation reduces SP measurements. Thus, only water-bearing sands should be selected for Rw determination from the SP.
Unbalanced mud columns, with differential pressure into the formation, can cause "streaming" potentials that augment the SP. This effect, known as electrokinetic SP, is noticeable in depleted reservoirs, and is impossible to handle quantitatively.
Resistivities may be very high in hard formations, except in the permeable zones and in the shales. These high resistivities affect the distribution of the SP currents, hence the shape of the SP curve.As illustrated in Figure 1 , the SP currents flowing from shale bed Shl toward permeable bed P2 are largely confined to the borehole between Shl and P2 because of the very high resistivity of the formation in this interval. Accordingly, the intensity of the SP current in the borehole in this interval remains constant. Assuming the hole diameter is constant, the potential drop per foot is constant and the SPcurve is a straight line.
In these formations, SP current can leave or enter the borehole only opposite permeable beds or shales, and the SP curve shows a succession of straight portions with a change of slope opposite every permeable interval (with the concave side of the SP curve toward the shale line) and opposite every shale bed (with the convex side of the SP curve toward the shale line). The boundaries of the permeable beds cannot be located with accuracy by use of the SP in such highly resistive formations.
Bed thickness can affect the SP measurement quite dramatically. In thin beds, the SP does not fully develop. Figure 2 illustrates the factors involved in SP reduction.
In the terminology used here SP refers to the observed SP deflection on the log and SSP (static SP) to the value it would have had all disturbing influences been removed. Among the disturbing factors may be bed thickness, diameter of invasion, Rxo/Rm ratio, neighboring shale resistivity (Rsh), hole diameter (dh), and mud resistivity (Rm). In general, the SP reduction is greatest in thin beds, where Rxo/Rm is high and where invasion is deep.
Many SP correction charts are available in the literature, some more complex than others. It is virtually impossible to include on one chart all the variables involved in making necessary corrections. Figure 3 shows a practicable chart, with most of the required variables (di, Rxo/Rm , and h) known or estimated.
The use of non-NaCl muds (such as KCl) affects the derivation of Rw from the SP. Cox and Raymer (1976) cover the subject matter in detail. A quick solution in the case of a KCl mud problem is simply to take the observed SP deflection, subtract 25 mV, and then treat it as a NaC1 mud case. The Rmf to Rmfe relationship is slightly different for KCl filtrates than for NaCl filtrates. Again a quick rule of thumb is to add 30% to the measured Rmf and treat it as an NaCl filtrate.
The SP as a Shale Indicator
The presence of shale in art otherwise "clean" sand tends to reduce the SP. This effect can be used in estimating the shale content of a formation. If SPsand is the value observed in a clean, water-bearing sand and SPshale is the value observed in a shale, then any intermediate value of the SP may be converted into a value for the shale volume (Vsh) by the relationship
Gamma Ray Measurement
Natural and Spectral Gamma Ray (GR)
Logs Gamma ray logs are used for four main purposes:
correlation and bed boundary determination
evaluation of the shale content of a formation
mineral analysis
perforating depth control and the tracing of radioactive fluid movement
GR logs measure the natural gamma ray emissions from subsurface formations. Because gamma rays can pass through steel casing, measurements can be made in both open and cased holes. In applications not pertinent here, such as pulsed neutron logging, induced gamma rays are measured.
Figure 1 shows a typical GR log. It is always presented in track 1 on a linear grid and is scaled in API units. Gamma ray activity increases from left to right. Gamma ray tools consist of a detector and the associated electronics for passing the gamma ray count rate to the surface. These tools are in the form of double-ended subs that can be sandwiched into practically any logging tool string; thus, the GR log can be run with practically any tool available.
Gamma rays originate from three sources in nature: the radioactive elements in the uranium and thorium groups, and potassium. Uranium 235, uranium 238, and thorium 232 all decay, via a long chain of daughter products, to stable lead isotopes. An isotope of potassium, K40, decays to argon.
An "average" shale contains 6 ppm uranium, 12 ppm thorium, and 2% potassium. Since the various gamma ray sources are not all equally effective, it is more informative to consider potassium equivalents (i.e., the amount of potassium that would produce the same number of gamma rays per unit of time). Reduced to a common denominator, the average shale contains uranium equivalent to 4.3% potassium, thorium equivalent to 3.5% potassium, and 2% potassium.
But an average shale is hard to find. Since a shale is a mixture of clay minerals, sand, silts, and other extraneous materials, there can be no standard gamma ray activity for shale. Indeed, the main clay minerals vary enormously in their natural radioactivity. Kaolinite and chlorite have no potassium, whereas illite contains between 4% and 8% potassium. Montmorillonite contains less than 1% potassium. Occasionally, natural radioactivity may be due to the presence of dissolved potassium or other salts in the water contained in the pores of the shale.
Each radioactive decay produces a gamma ray that is unique. These various gamma rays have characteristic energy levels and occur in characteristic abundance, as expressed in counts per time period. Counting how many gamma rays a formation produces can be carried a step further to counting how many from each gamma ray energy group it produces. If the number of occurrences is plotted against the energy group, a spectrum will be produced that is characteristic of the formation logged. The relationship between gamma ray energy and frequency of occurrence, shown in Figure 2 , is used as the standard for measurement in the natural gamma spectroscopy tools.
Figure 3 shows such a spectrum, where energies from 0 to approximately 3 Mev have been split into 256 specific energy "bins." The number of gamma rays in each bin is plotted on the Y-axis. This spectrum can be thought of as a mixture of the three individual spectra belonging to uranium, thorium, and potassium. Some unique mixture of these three radioactive "families" would have the same spectrum as the observed one. The trick is to find a quick and easy method of discovering that unique mixture. Fortunately, on-board computers in logging trucks are capable of quickly finding a "best fit" and producing continuous curves showing the concentration of U, Th, and K.
In a gamma ray spectral log note that in track 1 both total gamma ray activity (SGR) and a "uranium-free" (CGR) version of the total activity are displayed. Units are API. In tracks 2 and 3, the concentration of U, Th, and K are displayed. Depending on the logging service company the units may be in counts/sec, ppm, or %.
Introduction
Gamma ray logs are used for three Main purposes:
correlation
evaluation of the shale content of a formation
mineral analysis
Gamma ray logging tools measure the natural gamma ray emissions from subsurface formations. Since gamma rays can pass through steel casing, measurements can be made in both open and cased holes. Other applications, measuring induced gamma rays (e.g., in pulsed neutron logging), are not discussed here.
Figure 4 shows a typical gamma ray log. It is normally presented in Track I on a linear grid and is scaled in API units, defined below. Gamma ray activity increases from left to right. Gamma ray tools consist of a gamma ray detector and the associated electronics for passing the gamma ray count rate to the surface. Shaped like double-ended subs that can be sandwiched into practically any logging tool string, they can be run with practically any tool available.
Origin of Natural Gamma Rays
Gamma rays originate from three Main sources in nature: the radioactive elements in the uranium group, the thorium group, and potassium. Uranium 235, uranium 238, and thorium 232 all decay, via a long chain of daughter products, to stable lead isotopes.
An isotope of potassium, K40, decays to argon, giving off a gamma ray. It should be noted that each type of decay is characterized by a gamma ray of a specific energy (wavelength) and that the frequency of occurrence for each decay energy is different. Figure 5 shows this relationship between gamma ray energy and frequency of occurrence. This is an important concept, since it is used as the basis for measurement in the natural gamma spectroscopy tools.
Naturally Occurring Radioactive Minerals
An "average" shale contains 6 ppm uranium, 12 ppm thorium, and 2% potassium. Since the various gamma ray sources are not all equally effective, it is more informative to consider this mix of radioactive Materials on a common basis, e.g., by reference to potassium equivalents (i.e., the amount of potassium that produces the same number of gamma rays per unit of time). Reduced to a common denominator, the average shale contains uranium equivalent to 4.3% potassium, thorium equivalent to 3.5% potassium, and 2% potassium. An average shale is hard to find. Shale, being a mixture of clay minerals, sand, silts, and other extraneous Materials, exhibits no "standard" gamma ray activity. Indeed, the main clay minerals vary enormously in their natural radioactivity: kaolinite has no potassium, illite between 4% and 8%, montmorillonite less than 1%. occasionally, natural radioactivity may be due to the presence of dissolved potassium or other salts in shale pore water.
Operating Principle of Gamma Ray Tools
Traditionally, two types of gamma ray detectors have been used in the logging industry: Geiger-Mueller and scintillation detectors. Today, practically all gamma ray tools use scintillation detectors containing a sodium iodide crystal ( Figure 6 ). when a gamma ray strikes the crystal, a single photon of light is emitted. This tiny flash of light then strikes a photo cathode made from cesium-antimony or silver-Magnesium.
Each photon, upon hitting the photocathode, releases a spray of electrons. These, in turn, are accelerated in an electric field to strike another electrode, producing an even bigger "shower" of electrons. This process is repeated through a number of stages until a final electrode conducts a small current through a measure resistor to give a voltage pulse signaling that a gamma ray has struck the sodium iodide crystal. The system has a very short "dead time" and can register Many counts per second without becoming swamped by numerous signals.
Calibration of Gamma Ray Detectors and Logs
One problem of gamma ray logging is choosing a standard calibration system, since all logging companies use a variety of counters encased in different steel housings of various sizes and shapes. On very old logs, the scale might be quoted in micrograms of radium equivalents/ton of formation. For Many reasons this method was found to be unsatisfactory to calibrate for gamma ray logs, so an API standard was devised. A test pit (installed at the university of Houston) contains an "artificial shale" ( Figure 7 ). A cylinder 24 ft long and 4 ft in diameter contains a central 8-ft section consisting of cement mixed with 13 ppm uranium, 24 ppm thorium, and 4% potassium. On either side, completing the sandwich, are 8-ft sections of neat Portland cement cased with 5-1/2 in. J55 casing. The API standard defines the difference in radioactivity between the neat cement and the radioactively doped cement as 200 API units. Any logging service company May place its tool in this pit to make a calibration.
Field calibration is performed using a portable jig that contains a radioactive "pill." Placed over the center of the gamma ray detector, the jig produces an increase over the back-ground count rate equivalent to a known number of API units, depending on the tool type and size and the counter it encloses.
Time Constants
All radioactive processes are subject to statistical variations. For example, if a source of gamma rays emits an average of 100 gamma rays each second over a period of hours, the source will emit 360,000 gamma rays per hour (100/sec. x 60 seconds x 60 minutes). If the count is measured for 1 second, however, the actual count might be more or less than 100, thus forcing a choice. A relatively quick gamma ray count gives a poor estimate of the real count rate, while a long count yields a more accurate estimate of the count rate at the expense of much time. The logger must choose between various time constants, according to the radioactivity level measured. The lower the count rate, the longer the time constant required for adequate averaging of the variations.
In the logging environment, gamma rays can be counted for a short period of time (e.g., one second) with the recognition that during that time period, the detector will have moved past the formation whose activity is being measured. Thus, the logging speed and the time interval used to average count rates are interrelated. The following rules of thumb are generally recognized.
Logging Speed | Time Constant |
3600 ft/hr | 1 sec |
1800 ft/hr | 2 sec |
1200 ft/hr | 3 sec |
900 ft/hr | 4 sec |
In the future, when the efficiency of gamma ray detectors and their associated electronics improves by one or two orders of Magnitude, the use of a time constant will be obsolete except in the cases of extremely inactive formations with intrinsically low gamma ray count rates.
Perturbing Affects on Gamma Ray Logs
Gamma ray logs are subject to a number of perturbing effects, including
sonde position in the hole (centered/eccentered)
hole size
mud weight
casing size and weight
cement thickness
Since there are innumerable combinations of these effects, an arbitrary standard set of conditions is defined as a 3-5/8 in. OD tool eccentered in an 8-in. hole filled with 10-lb mud. A series of charts exists for Making the appropriate corrections. Note that if a gamma ray log is run in combination with a neutron density tool, it is run eccentrically. If run with a laterolog or an induction log, it is usually centered.
Gamma Ray Spectroscopy
Each radioactive decay produces a gamma ray that is unique in terms of energy level and abundance, expressed in counts per time period. The simple method of counting how Many gamma rays a formation produces can be carried a step further to count how Many from each gamma ray energy group it produces. If the number of occurrences is plotted against the energy group, a spectrum is produced that is characteristic of the formation logged.
Figure 8 shows such a spectrum, where energies from 0 to approximately 3 MeV have been split into 256 specific energy "bins." The number of gamma rays in each bin is plotted on the Y-axis. This spectrum can be thought of as a mixture of the three individual spectra belonging to uranium, thorium, and potassium. A unique mixture of these three radioactive "families" has the same spectrum as the observed one. The trick is to find a quick and easy method of discovering that unique mixture. Fortunately, on-board computers in logging trucks are capable of quickly finding a "best fit" and producing continuous curves showing the concentration of U, Th, and K.
Figure 9 illustrates a gamma ray spectral log. Note that in Track I both total gamma ray activity (SGR) and a "uranium free" version of the total activity are displayed (units are API). In Tracks II and III the concentrations of U, Th, and K are displayed. Depending on the logging service company, the units may be in counts/sec, ppm, or percentage.
Lithodensity Tool
Photoelectric Factor (Pe) Log
This Peor lithodensity log, run with the lithodensity tool (LDT), is another version of the standard formation density log. In addition to the bulk density (b), the tool also measures the photoelectric absorption index (Pe) of the formation. This new parameter enables a lithological interpretation to be made without prior knowledge of porosity.
The physical principle upon which the Pe log is based is that gamma rays interact with matter in various ways, depending upon their energy. However, only two reactions are of interest when dealing with relatively low energy gamma rays originating from the chemical sources currently used in logging tools. These reactions are the Compton scattering of gamma rays by electrons, and the photoelectric absorption of gamma rays by electrons.
The photoelectric effect occurs when a gamma ray collides with an electron and is absorbed in the process, so that all of its energy is transferred to the electron. The probability of this reaction taking place depends upon the energy of the incident gamma rays and the type of atom. The photoelectric absorption index of an atom increases as its atomic number, Z, increases.
Pe = (0.1 Zeff)3.6
The Compton effect occurs over a wide energy range, whereas the photoelectric effect only occurs when lower energy gamma rays are involved.
The lithodensity tool is similar to a conventional density logging device, and uses a skid containing a gamma ray source and two gamma ray detectors held against the borehole wall by a spring-actuated arm ( Figure 1 ). Gamma rays are emitted from the tool and are scattered by the formation, losing energy until they are absorbed through the photoelectric effect.
At a finite distance from the source, a gamma ray energy spectrum exists such as that shown for a rock of density p in Figure 2 . This figure also shows that an increase in the formation density (2 > 1) results in a decrease in the number of gamma rays detected over the whole spectrum.
For formations of constant density but different photoelectric absorption coefficients, the gamma ray spectrum is only altered at lower energies, as indicated in Figure 3 . Observing the gamma ray spectrum, we notice that region H only supplies information relating to the density of the formation, whereas region L provides data relating to both the electron density and the Pe value. By comparing the counts in the energy windows H and L, the Pe can be measured. The gamma ray spectrum at the short spacing detector is only analyzed for a density measurement, which is used to correct the formation density determined from the long spacing spectrum for effects of mud-cake and rugosity.
The lithodensity tool skid and detector system produces greater counting rates than those obtained with the conventional density tools, resulting in lower statistical variations and better repeatability of the measurements. The geometry of the skid is such that the density reading has a sharper vertical resolution than that of the conventional density measurement. The Pe measurement also has this high resolution, which has applications in identifying fractures and laminar formations.
The photoelectric absorption coefficient is virtually independent of porosity, there being only a slight decrease in the coefficient as the porosity increases. Similarly, the fluid content of the formation has little effect. Simple lithologies, such as pure sandstone and anhydrite, can be read directly from logs using Pe curves. Look for the following readings in the most commonly occurring reservoir rocks and evaporites.
Material | Pe |
Sand | 1.81 |
Shale | 3-4 |
Limestone | 5.08 |
Dolomite | 3.14 |
Salt | 4.65 |
Anhydrite | 5.05 |
Although there is a degree of variation in the log readings due to impurities, we can identify four main lithologies in this example: sandstone up to 265 m, anhydrite from 255 to 210 m, dolomite from 210 to 185 m, and halite above 185 m. In some ambiguous cases, we must also refer to the density or neutron porosity readings.
Lithologic Density
The lithodensity logs are improved and expanded versions of the standard formation density log. In addition to the bulk density ( b), these tools measure the photoelectric absorption index (Pe) of the formation. This parameter enables a lithological interpretation to be made without prior knowledge of porosity.
Physical Principle
Gamma rays interact with matter in various ways, depending upon their energy. However, only two interactions are of interest when dealing with relatively low-energy gamma rays originating from the chemical sources currently used in logging tools. These interactions are the Compton scattering of gamma rays by electrons, and the photoelectric absorption of gamma rays by electrons. Compton scattering has already been discussed in the context of the conventional density tool measurements.
The photoelectric effect occurs when a gamma ray collides with an electron and is absorbed in the process, so that all of its energy is transferred to the electron. The probability of this reaction taking place depends upon the energy of the incident gamma rays and the type of atom. The photoelectric absorption index of an atom increases with increasing atomic number, Z.
Pe = (0.1 x Zeff)3.6
While the Compton effect occurs over a wide energy range, the photoelectric effect results only with lower-energy gamma rays. Figure 4 plots the mass absorption coefficient, ยต, against the gamma ray energy and also shows that the photoelectric effect, unlike Compton scattering, is dependent on the formation type.
Measurement Theory
The lithodensity tool, similar to a conventional compensated density device, uses a skid containing a gamma ray source and two gamma ray detectors held against the borehole wall by a spring-actuated arm ( Figure 5 ). Gamma rays, emitted from the tool with an energy of 662 keV, are scattered by the formation, losing energy until they are absorbed through the photoelectric effect.
At a finite distance from the source, a gamma ray energy spectrum exists ( Figure 6 ). This figure also shows that an increase in the formation density results in a decrease in the number of gamma rays detected over the whole spectrum. Fore formations of constant density but different photoelectric absorption coefficients, the gamma ray spectrum is altered only at lower energies. In the gamma ray spectrum in Figure 7 , region H only supplies information relating to the density of the formation, whereas region L provides data relating to both the electron density and the photoelectric absorption index.
By comparing the counts in the energy windows H and L, one may measure the photoelectric absorption index. The gamma ray spectrum at the short-spacing detector is analyzed only fore a density measurement, which is used to correct the formation density determined from the long-spacing spectrum for effects of mudcake and rugosity.
The lithodensity tool skid and detector system produces greater counting rates than are obtained with the conventional density tools, resulting in lower statistical variations and better repeatability of the measurements. The geometry of the skid is such that the density reading has a sharper vertical resolution than that of the conventional density measurement. The Pe measurement, also with high resolution, has applications in identifying fractures and laminar formations.
Interpretation of the Pe Curve
The photoelectric absorption coefficient is virtually independent of porosity and fluid content, decreasing only slightly as porosity increases. Simple lithologies, such as puree sandstone and anhydrite, can be read directly from logs using the Pe curve (PEF, or photoelectric factor) alone. Look for the following readings in the most commonly occurring reservoir rocks and evaporites.
Material | Pe |
Sand | 1.81 |
Limestone | 5.08 |
Dolomite | 3.14 |
Salt | 4.65 |
Anhydrite | 5.05 |
Figure 8 illustrates an example of these logs . Although there is a degree of variation in the log readings, caused by impurities, four main lithologies can be identified in this example: sandstone up to 265 m, anhydrite from 255 to 210 m, dolomite from 210 to 185 m, and halite above 185 m. In some ambiguous cases, the density or neutron porosity readings must also be referenced.
Exercise 1.
On the log shown in Figure 1 , read the maximum SP deflection from the shale line to the sand line.
Sokution 1:
The scale in Figure 1 is 200 millivolts (mV) across the track or, for a track divided into 10 divisions, 20 millivolts per division.
Since the deflection is 7 divisions, the deflection is equal to 20 x 7 = 140 millivolts.
The solution is 140 mV.
Exercise 2.
In the example shown in Figure 1 , determine which element is responsible for the high activity seen on the total gamma ray intensity curve at the point marked A.
Solution 2:
Through Section A, the only element that increases notably is uranium, hence uranium is responsible for the increased GR activity in that shale.
Exercise 3.
The SP deflects 25 mV from the base line in a 4-ft-thick sand. Ri/Rm = 50 and di = 30 in.
a. Find by what percentage the SP has been reduced.
b. Compute the corrected value for the SP.
Solution 3:
In Figure 1 (SP correction for bed thickness) we see that the SP has been reduced 60%, or by a factor of 1.67. The theoretical value of SSP (static SP) should be
(25 mV) (1.67) = 41.67 42 mV
The solutions are
a. The SP is reduced by 60%
b. SSP = 25/0.6 = 41.7 mV
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