CA1045203A - Method of determining hydrocarbon saturation in shally formations - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

In a subsurface formation, where shaliness affects the results of typical electrical well logging measurements, partial water and partial hydrocarbon saturations are determined from measurements of dielectric constant. A relation defining dielectric constant change as a function of partial water saturation is used to determine the partial water saturation and partial hydrocarbon saturation. The relation between dielectric constant and conductivity due to shaliness in a partially water saturated formation is also used to determine partial water saturation and partial hydrocarbon saturation of said formation. The value of "shaliness exponent"
for the formation is determined from measurements of dielectric constant as well.

Description

~04S203 ~ This application is a divisional of application S.N.
237,297 filed 8 Oceober 1975 and although it includes all o~ the principal disclosure of S.N. 237,297 it is directed to certain aspects of the inven-tion only, the parent application being directed to other aspects and another divisional application S.N. ~ o ~ o O ~ filed ~u4ne ~, ~7 being directed to still further aspects.

BACKGROUND OF THE INVENTION
A technique typically employed to determine the presence of hydrocarbons in earthformations is to electrically log boreholes drilled in these formations. Normally, formations saturated mostly with hydro-carbons will exhibit a high electrical reslstivity, while formations satur-ated mostly with water or brine will exhibit a low electrical resistivity.
However, it has been determined that the presence of shale in a formation substantially decreases the resistivity of the formation to the extent that commercially producing reservoirs in shaly formations display a resistivity that would otherwise indicate nonproductivity.
U.S. Patent 3,895,289 issued 15 July 1975, discloses a relation ~ ~ ~
between dielectric constant of a shaly sand formation, 100% water saturated, ~ -and a conductivity parameter related to shaliness, and provides a method ~ ~
.
of determining if there is enough shale in a formation that it must be taken into account when evaluating electrical logging results; but does not disclose a method for quantitatively determining the partial hydrocarbon saturation and partial water (brine) saturation of the formation.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a schematic representation of logging equipment arranged in a borehole and at the earth's surface to conduct e~rthmeasure-ments in accordance with the present invention.

1. g~

.. . ~ . .

. , 104~Z~3 FIGURES 2A and 2B illustrate side and front views of a bow sprlng suitable for use with the apparatus of FIGURE 1.
FIGURE 3 is another schematic representation of logging equipment arran8ed in a borehole and at the earth's surface to conduct earth measurements in accordance with the present invention.
FIGURES 4A and 4B illustrate side and front views of a bow spring for use with the apparatus of FIGURE 3.
FIGURE 5A is an electrical diagram, partially in schematic and partially in block form, of electrical equipment suitable for use in connection with the equipment of FIGURES 1, 2A, 2B, 3, 4A and 4B.
FIGURE 5B is an electrical diagram illustrating the equivalent ~ -electrical circuit of an earth formation during an electrical resistivity measurement in a borehole useful in understanding the operation of the apparatus of FIGURES 1, 2A, 2B, 3, 4A, and 4B.
FIGURE 6 is a waveform diagram useful in understanding the circuit of FIGURE 5A.
FIGURE 7 is the graphical illustration of core conductivity (C ) as a function of equilibrating solution conductivity (C ).
FIGURE 8 is a graphical representation of the measured change in dielectric constant with partlal water saturation for four shaly sand ;
formation samples.
SUMMARY OF THE INVENTION
The degree of water (brine~ saturation of a sub`surface for-mation saturated partially with an aqueous saturant and partially with electrically inert matter is determined from measurements of dielectric constant.
It has been discovered that the dielectric constant of a shaly sand formation changes as a function of water (brine) saturation, snd ~hae thi~ variation is defined by the relation K/Ko = S P, where:

. .

~ - -: . :
: ,:: : . - .. .: . , ~ , '. : ~
, . , ~ ' ' :

~04~
K = dielectric constant K = dielectric constant at 100% water saturation Sw = water saturation p = shaliness exponent It has also been discovered that this relation is not significantly affected by a change in salinity of the saturating aqueous solution.
When a borehole is drilled into a formation, normally the formation fluids in the vicinity of the borehole wall will be displaced by drilling fluids. In a formation saturated with fluids saturants, ;
10 such as water and fluid hydrocarbons, partial water saturation of the formation can be determined from "shallow" measurements of dielectric constant near the borehole wall, and from deeper measurements of dielectric constants in portions of the formation not penetrated by drilling fluids, if the value of "p" is known for the formation.
In another embodiment of the invention, partial water saturation is determined utilizing only "deep" dielectric constant measure-ments. In this embodiment, the conductivity relation for a partially water saturated shaly sand formation.

Ct = F* (C~ S~ + BQv S~ P) is employed, where:
Ct = specific conductivity of the shaly sand formation, mho cm 1, F* = formation resistivity factor for shaly sands C~ = specific conductivity of the aqueous saturant, mho cm 1 S~ = partial water saturation n = desaturation exponent B = equivalent conductance of clay exchange cations as a function of C~ at 25C, mho cm2meq 1 Qv = cation exchange capacity per unit pore volume, meq ml 1 p = shaliness exponent - - . ~ . . - :

.. ~ , , :. . : :

~045ZiO3 The value of Ct, F*, C~ are obtained from routine well logging methods.

Although the value of "n" varies somewhat for different types of shaly sand, it has been determined that a value of 2 can be used for "n"
without introducing excessive error. The quantity BQv S~ P, a conductivity parameter related to shaliness, has been found to be related to dielectric constant, and the value thereof is directly determined by correlating the measured dielectric constant with the relation between dielectric constant and BQv S~ P. From the information thus obtained, S~ , the aqueous portion of formation saturants, can be calculated.

A primary advantage of this invention is that it permits a determination of the probable productivity of a hydrocarbon reservoir located in a shaly sand formation from well logging information.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The conductivity equation for shaly sands, 100% saturated with an aqueous solution, is as follows:

CO = F* (C~ + BQV ) The paper entitled "Electrical Conductivities in Oil Bearing Shaly Sands"
by M.H. Waxman and L.J.M. Smits, Society of Petroleum Engineers Journal, June 1968, page 107 describes the theoretical basis for this equation.
In the equation, the terms have the following meanings:
CO = specific conductivity of shaly sand, 100% saturated with aqueous solution, mho cm 1, F* = formation resistivity factor for shaly sand, which has been found to be related to porosity, ~ by what is known as Archie's first empirical equation F* = ~-m (See Archie, G.E., "The Electrical Resistivity Log as an Aid in Determining Some Reservoir Characteristics", Trans., AIME (1942), Vo. 146, p. 54-67. The constant, 3~ m, has a value of approximately 2, but varies somewhat ~ depending on the saDd characteristics.) :

C ~ = speclfic conductivity of the saturating aqueous solution, mho cm 1 Q v = cation exchange capacity per unit pore volume, meq ml 1 B = equivalent conductance of clay exchange cations as a function of C~ at 25C, mho cm2 meq 1 It is known that a relationship exists between dielectric constant, measured at frequencies less than about 50 KHz, of a 100% brine saturated shaly sand formation and the term BQ v , the conductivity parameter related to shaliness.
lQ By determining the BQ vvalue of a number of shaly sand earth samples at 100% water saturation and the corresponding value of dielectric constant for each sample, a correlation can be prepared that permits a determination of the BQv value of a portion of an earth formation from a measurement of dielectric constant.
We have determined experimentally that the dielectric constant of a shaly sand formation also varies as a function of water saturation.
The dielectric constant of shaly sand formation samples measured at water saturation less than 100% has been found to be related to dielectric constant measured at 100% water saturation, by the relation:

K = KoS~ P
where:
K = dielectric constant Ko = dielectric constant at 100% water saturation S ~ = partial water saturation p = shaliness exponent -FIGURE 8 is a graphical representation of the change in dielectric constant with a change in water saturation for a particular set of shaly sands. This graph indicates a value for "p" of about 0.8, but the value of "p" may be different for other formations having different textures cIay d~tribution, or other characteristics. From information available at this time it-appears that the value of "p" can vary from 0.7 to 1.3.

5.

. - ~ . , . . ~ ... .

It has also been determined experimentally that this relatlon is not substantially affected by a change in salinity of the water. This discovery is of particular significance because it permits partial water saturation to be determined from measurements of dielectric constant.
The measured dielectric constant does, however, change as a function of the frequency of the electrical signal used for measuring dielectric constant. Dielectric constant has been observed to decrease with increasing frequency. At frequencies above about 50 KHz, dielectric constant resulting from shaliness may not be great enough to be useful in practicing the invention.
During normal well drilling operations, native formation fluids adjacent the wellbore will be displaced by drilling fluids. If water based drilling fluids are used a portion of the formation adjacent the wellbore will become nearly 100% water saturated. Values for K
can then be determined by measuring dielectric constant in said portion of a formation adjacent the borehole. By measuring dielectric constant in substantially the same portion of the formation, but at a depth from the borehole surface not penetrated by drilling fluids, the partial water (brine) saturation of the formation can be determined for any sand formation 2a having a known value of "p".
This embodiment of the invention, therefore, permits the partial water saturation of a shaly sand formation saturated partially by water and partially by an electrically inert fluid to be dete~rmined. The electrically inert fluid portion of such formation saturants, which may be fluid hydrocarbons, is therefore equal to l-S~ . Partial hydrocarbon saturation is hereafter referred to as S H.
It is understood that if the electrically inert saturant is comprised of solid matter that will not be displaced by the aqueous drilling fluid, the "shallow" dielectric constant measurement as described hereinabove may not determine an accurate value for K . However, values for K can also be determined for a portion of a formation by measuring dielectric 6, . ~ . . . .

1~)45203 , constant in an ad~acent zone in sald formatLon that is water saturated, if such a water leg exists.
In a second embodiment of the invention, only the deep dielectric constant measurement is employed. Although additional well-logging information must be known in order to practice this second embodiment, it can also be employed for determining partial water saturation in formation zones partially saturated by a solid electrically inert satur-ant, such as kerogen when no water leg is present in said formation. The following discussion should be of assistance in understanding the second embodiment of the invention.
In 1942, Archie (see Archie, G.E., supra) proposed that the resistivity index of clean (nonshaly) sandstone partially saturated with water follows the relation:

Rt = 1 (1) R S t-where:
Rt = resistivity of a partially water saturated formation ohm -m Ro = resistivity of a 100% water saturated formation ohm -m I = resistivity index S~ = partial water saturation r~ = desaturation exponent (equal to about 2 for nonshaly sandstone formations) Rewriting this equation in terms of conductivity:

Ct = COS ~ (2) Another parameter which has been found to be useful is the formation factor F. Formation factor for nonshaly sands is ~
defined as: -, .

.

-104S203 , ~,, R (3) where:
R = resistivity of a 100% water saturated formatlon, ohm -m R~ = resistivity of the saturating water, ohm -m Rewriting equation (3) in terms of conductivity:
C = 1 C (4) and substituting the value of CO from equation (2) into equation (4):
Ct = F C ~S ~n (5) which is the equation of conductivity of a partially brine saturated nonshaly sandstone formation.
By using the relationship that has been found to exist between dielectric constant at partial water saturation, and dielectric constant at 100% water saturation, shaliness effects on the conductivity of partially .
water saturated formations can be taken into account. As stated above, dielectric constant in a 100% water saturated formation, Ko~ is proportional to the shaliness conductivity parameter, BQ v , from the relation: -C = _ (C ~ + BQ v) (6) As stated hereinabove, dlelectric constant, K, also changes with decreasing water saturation according to the relation:
. K = KoS~ P (7) It follows, therefore, that since dielectric constant changes with water saturation by the factor, S~ P, the conductivity due to shaliness, also changes with water saturation by this same factor, Therefore, the complete equation for the conductivity of shaly sand, Ct, partially saturated with brine and partially saturated with an electrically inert saturant can be written in the form:
t -F* (C~ S~ + B~v S~ ) (8) . . .
: . ' ~ :, "'','', ' . " ~ ~ . ' 1~, , In order to use this equation, certain inEormation routinely obtained from well logs is necessary. lhe value of C ls obtained from resistivity logs. The value of F* is determined from porosity which is routinely determined for each reservoir. The value of C ~ , the water conductivity, is determined from actual measurements of the brine that is present in the reservoir, or from the s.p. (self potential) log. The value of " ~" has beén experimentally determined to be approximately equal to 2 in shaly sandstone formations.
As stated above, BQv S~ P is proportional to dielectric constant.
As also disclosed in U.S. Patent 3,895,289 dielectric constant at 100%
water saturation, K , is related to the conductivity parameter, BQ v ;
that is Ko BQ v . Since it has been established that K = KoS~ P, it follows that K ~ BQ v S ~ P, and that K is related to BQ v Sw P in the same manner that Ko is related to BQ . Therefore, the graphical relation between K and BQ v which can be determined as described hereinafter also de-fines the relation between K and BQv S ~P, and a value for BQ v S ~ P is determinable by correlating the measured value of dielectric constant with said graphical relation.
After the value of the term BQ v S~ P is determined from a dielectric constant measurement, the value of all the terms in the equation:
C = 1 (C ~." S ~ + BQVS,~, ) (9) : ~
t F
are now known except for S ~ , the partial water saturation, which can now be calculated. The fractional portion of the saturants composed of electrically inert matter is equal to l-S ~ . For a formation saturated with water and hydrocarbon, the foregoing method provides a method of quanti-tatively determining the amount of hydrocarbons present in the formation.
In order to practice this invention, it is necessary to measure the dielectric constant of a formation. One embodiment of the invention requires the making of a "deep" and "shallow" dielectric constant measure-ment. The second e~bodiment of this invention requires only the "deep"measurement, but the relation between dielectric constant and the conductivity parameter, BQ vS ~P, must have been determined, and other routine well logging information must be known.
FIGURE 1 illustrates apparatus Por making shallow dielectric constant measurements. The apparatus is comprised of an elongated housing 1 to which is affixed in the usual manner a number of bow springs 5 each of which carries a plurality of electrode contacts 7A and 7B (see FIGURES 2A
and 2B) positioned on the bow springs so as to be urged into contact with the walls of a borehole 3. The housing 1 is suspended from a logging cable 11 which is wound on a reel drum 15 which extends over a sheave 13 in the usual manner so as to be suitably positioned in the borehole.
- Electrical leads within the logging cable 11, that conduct electrical signals from the downhole equipment contained in housing 1, are connected to electrical leads within an electrical conduit 17, which in turn, connect to the surface electrical equipment package 18. Electrical equipment package 18 may include, among other things, suitable recording and display apparatus.
The electrodes 7A and 7B on the bow springs 5 are supported by electrical insulators and are spaced apart a sufficient distance so that electrical currents and flux lines flowing therebetween will penetrate the desired distance into the earth formation. This spacing may preferably be about 5 inches. Electrodes 7A and 7B are connected to electrical leads 9 which are electrically insulated from any borehole fluids. Leads 9 connect, into the downhole electronic equipment contained in housing 1. ~
FIGURE 3 illustrates apparatus for making "deep" dielectric ~ -constant measurements, in a portion of the formation not penetrated by drilling fluids. This apparatus is identical to the apparatus in FIGURE 1 except that the electrode contacts are spaced further apart. Each of the Bow springs (see FIGURES 4A and 4B) contains only one of the electrodes of an electrode pair. Electrical currents and flux lines will flow between electrade ~A located on an upper bow spring and electrode 8B positioned on a lower bow spring. The spacing between the electrodes of an electrode pair may preferably be about 5 feet.
lQ.

-104SZ~3 FIGURE 5A illustrates an electrical schematic diagram, partially in block form, of apparatus suitable for use with the apparatus previously described for measuring capacitance in a borehole, from which dielectric constant is determined. The appara~us shown in FIGURE 5A is divided into two sections, the surface equipment and the downhole equipment. The surface equipment is connected to cable 17 (shown in FIGURES 1 and 3) and interconnected with the downhole equipment through cables 17 and 11, and electrical leads inside housing 1. The surface equipment includes power supply 21 which preferably is a DC supply, recording oscillograph 23 and recorder 25 for making a time recording of a voltage derived from the downhole equipment. Power supply 21 is electrically connected through the cables 17 and 11 to rectangular wave generator 27 within the housing 1. Rectangular wave generator 27 may be a multivibrator. The output of the generator is connected by lead 37 to contact member 7A (assuming that the configuration of FIGURE 1 is used), and through resistor 39 of low resistance to contact member 7B. Contact electrodes 7A and 7B are designed ~ ~
to have a very large contact area with the earth formation so that the -contact impedance will be quite low. The contact electrodes may be platinized electrodes having sufficient diameter to minimize contact im- ;
pedance by making the contact resistance and the contact impedance as - low as possible. (The combination of the contacts and the borehole wall is the equivalent of a resistor and a capacitor in parallel.) The voltage appearing across resistor 39 is applied to the input of gated amplifier 29. A gating signal is derived from lead 37 of generator 27 so that, in effect, the output voltage of generator 27 switches ;-the amplifier 29 on and off. The output signals of the gated amplifier are applied through leads 31 to the recording oscillograph 23 in the surface equipment. The output signals from the gated amplifier are the gated signal as ~hown in FIGURE SD and a signal equal to the generator 27 output as shown in FIGUR~ 6A. The voltage appearing across resistor 39 is also trans~itted 11 .

~04s~ao3 "
to the earth's surface to be recorded by recorder 25. If desired, and if the recording oscillograph is provided with sufficient input circuits, the signal appearing across resistor 39 may be simultaneously recorded by recording oscillograph 23.
Because of the capacitive reactance between the contacts 7A
and 7B resulting from the effective capacitor produced by the contacts and the formation in contact therewith, the current passing through resistor 39 will lead the voltage produced by generator 27. The contact impedance of contacts 7A and 7B is very low and will not introduce significant errors into the measurements. The capacitive reactance will be produced almost entirely by the effective capacity, Cf, of the earth. The dotted lines in FIGURE 6B illustrate the wave form that would be produced were there no capacitive reactance in the circuit. The solid rule line represents the current wave form that will typically be produced.
The electrical characteristics of the formation sample can be represented by a capacitive component Cf and a resistive component Rf in parallel as illustrated in FIGURE 5B. The current from generator 27 flows through the formation sample and through resistor 39. Designating the pulsed voltage amplitude across resistor 39 as V and the resistance value of resistor 39 as R, it is apparent that I = Vr . If the pulsed voltage amplitude produced by generator 27 is designated as E , then the formation sample resistance Rf is given by:
E _ V (E V )R
Rf = o I = o - r ~ (10) Vr From the above it can be seen that the effective resistance of the formation can be determined from the recordations of the voltage generated by generator 27, from the voltage produced across resistor 39, and the resistance of resistor 39.

Manifestly, the time integral "T" of each voltage pulse appearing at the output of generator 29 and illustrated by the wave form of FIGURE 4D

is given by the formula:

12.

.
;'. ~ - '. ' ~04s:ao3 T = IR(ReffCf) (ll) where R = R R
eff f (12) Rf+R
so that:
Cf = T = TE
IRReff V R(E - Vr~ (13) From the above it is apparent that the capacitance of an earth sample can be measured using the apparatus described above.
The instrument may be calibrated to measure dielectric constant directly. Initially a number of earth samples having a wide range of dielectric constants are obtained. The samples may have been previously obtained from coring operations in the earth, or they may be specially ~ ~
obtained for thç purpose of calibrating the instrument. The dielectric ;~ ~;
constant of each of the samples is then obtained by techniques well known to the art such as described in the texts: "Solids State Magnetic and Dielectric Devices", Library of Congress Catalog Card Number 59-6769, John Wiley & Sons, New York, 1959; and "Theory of Dielectrics" by a. Frohlich, University Press, Oxford, 1958. For example, an earth sample may be placed between conductive plates of known dimensions and the capacitive reactance of the capacitor resulting therefrom can then be measured. The `
dielectric constant of the earth sample can be calculated from the area of the plate and the spacing between the plates. Such techniques have been well known to the art for many years and will not be further discussed ;
herein.
After the dielectric constant of the various earth samples have ~ -been obtained, these samples or earth samples obtained from the same forma-tions, having the same dielectric constant, are placed in contact with the contacts 7A and 7B. The thickness of each formation sample should be great enough so that the electric lines of force between the pairs of contacts . .

13.
, ~, .

10~SZ03 will pass only through the formation sample. The equipment illustrated in FIGURE 3A ls then actuated so that a substantially rectangular wave pulse train with a frequency spectrum predominantly less than 50 KHz, as illustrated in FIG~RE 4A, is generated by generator 27.
A number of earth samples of known dielectric constant are successively placed in contact with the electrodes and the area of the lntegrated signal (which is the time integral "T") recorded by oscillograph 23 is measured for each sample. Thus there is obtained a relationship between the integral of the gated signal and the dielectric constant of the earth samples placed between the electrodes. The dielectric constant of any unknown earth sample can be obtained by measuring the parameters described above and correlating with the calibration curve. As stated hereinabove, the dielectric constant of shaly sand formation samples has been observed to vary with frequency; therefore, waveforms having substan~
tially the same frequency spectrum must be used for all dielectric con-stant measurements made in calibrating instruments, preparing calibra-tion curves, and measuring formation dielectric constant.
As stated earlier, it has been determined that the dielectric constant of a brine saturated formation, measured at frequencies less than about 50 KHz, is proportional to cation exchange capacity. `~
For the purpose of establishing the relationship between dielectric constant and cation exchange capacity, ~v , earth samples whose dielectric constant has been determined are subjected to laboratory analysis for the purpose of determining the cation exchange capacity per unit pore volume of these samples. The particular laboratory analysis to which the earth samples are subjected is not part of the invention and may be any standard known prior art type of analysis such as has been described and shown to he useful in the paper entitled "Electrical Conductivities in Oil Bea~i~g Shaly Sands" by M.H. Waxman and L.J.M. Smits, Soclety of Pe-troleum , 14.

.

~45Z~3 Engineers Journal, June, 1968, page 107. One particular method commonly used in the prior art comprises repeated equilibration of crushed rock samples with concentrated barium chloride solutions, washing to remove excess barium ions, followed by conductometric titration with standard MgS04 solution. The latter procedure is also described in the article "Conductometric Titration of Soils for Cation Exchange Capacity" by M.M. Mortland and J.L. Mellor, Proc. Soil Science Society of America (1954), Column 18, page 363. Another technique that may be used involves chromatographic measurements using ammonium acetate solutions as described in the artlcle "Effect of Clay and Water Salinity on Electro-Chemical Behavior of Reservoir Rocks" by H.J. Hill and J.D. Millburn, appearing in Transactions of the AIME, (1956), Volume 207, pages 65-72.
As stated earlier, the conductivity equation for 100% brine sat-urated shaly sands is:

Co F* (C", ~ BQ V ) FIGURE 7 graphicslly illustrates this relation, showing the change in the conductivity of a brine saturated shaly earth sample with increasing conductivity of the saturating solution. It is evident from FIGURE 6 that except for very low salinity levels the relationship between CO, the sat-urated core conductance, and C~ , the saturating solution conductance, is linear, and the value of B in this range is a constant. Subsequently the ~ .
value of B will be treated as a constant sinCe this introduces very little ~ -~
error.
Since dielectric constant, measured at frequencies less than about 50 KHz, is proportional to Qv , the cation exchange capacity per unit pore volume, with B being treated as a constant, dielectric constant is also proportional to BQv .
The conductivity of a number of 100% water saturated shaly sample8~ having known values for Qv , is determined at different levels of salinity. The relationship between the conductivity of the saturating 104S21~3 solution alone and the conductivity of a water saturated shaly sample will appear similar to FIGURE 6. The pro~ection of the straight portion of the line on the hori~ontal axis represents the value of BQv . The values of BQ vvs Qvfor each sample are plotted and from this graph, the value of BQv for the earth formations of ~nterest can be obtained as a function of Q v , the cation exchange capacity, thereby permitting the value of BQ v as a function of dielectric constant to be determined.
As an alternative to the preceding method of determining a value of BQv as a function of dielectric constant, the initial step of determining the value of Qv , alone, can be skipped, and the process for determining the value of BQv can be employed on each of the samples after the value of dielectric constant is measured. Thus there is obtained a relationship between dielectric constant at 100% brine saturation, K , and the conductivity parameter, BQv . It will be noted that this method requires only one laboratory analysis rather than two for determining a value of BQv ;
As explained earlier, the conductivity parameter related to .
shaliness in partially water saturated shaly sands, BQVS~P, is related to dielectric constant K, the same way that the conductivity parameter related to shaliness in 100% water saturated shaly sands, BQv is related to the dielectric constant of 100% water saturated shaly sands, K ;
therefore, the graphical relationship that permits BQv to be determined from a measurement of Ko, also permits BQVSWP to be determined from a measurement of K.
The foregoing description of a preferred embodiment of the in~
vention discloses a method of determining the aqueous portion of formation saturants. The remainder of the formation saturants, comprised of electrically inert matter, is therefore equal to l-S~ . The electrically ~ -inert formation can be hydrocarbons or other matter such as sulfur.

16.

-',' '`' ' 1~4S2~
Coring operations or other forma~ion testing will indicate the nature of the electrically inert matter. If tests indlcate the presence of hydrocarbons, the foregoing disclosure is a very useful method of ob-taining quantitative evaluation of the aqueous and hydrocarbon phases of for-S mation saturants.
It is understood that if oil based drilling fluids are employed, the foregoing methods for obtaining partial water saturation of a formation may not be practical. Under such circumstances, it would be necessary to determine the value of K by measuring dielectric constant on cores in the laboratory.

: ~.

-;:. , ~ . : -,, . : . , : : ::, . , .

:':: :-. . .. , ,.. ,, . ~

Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a method for determining the value of "p" for a type of shaly sand subsurface formation wherein a core sample is extracted from said type of formation, the steps of:
measuring dielectric constant at a frequency lower than about 50 KHz of said core sample at selected levels of partial water saturation, thereby determining values for dielectric constant corresponding to various levels of partial saturation;
utilizing said corresponding values of dielectric constant and partial water saturation to graphically determine the value of "p" for said type of formation, from the relation K = Ko S.omega. P
wherein:
K = dielectric constant Ko = dielectric constant at 100% water saturation S.omega. = partial water saturation and p = shaliness exponent

2. A method of electrical well-logging a portion of shaly sand subsurface formation saturated with native formation saturants comprised partially of water and partially of an electrically inert fluid, comprising the steps of:
extracting a core sample from said formation;
measuring dielectric constant of said core sample at a selected frequency lower than about 50 KHz at selected levels of partial water saturation thereby determining values for dielectric con-stant corresponding to said selected levels of partial water saturation;

utilizing said corresponding values of dielectric constant and partial water saturation to graphically determine the value of "p" for said portion of said formation, from the relation, K = KoS.omega. P where:
K = dielectric constant Ko = dielectric constant at 100% water saturation S.omega. = partial water saturation p = shaliness exponent making a first measurement of dielectric constant at said selected frequency at a first location in said formation wherein said native formation saturants have been displaced by an aqueous saturant where-by a value for "Ko" is determined;
making a second measurement of dielectric constant at said selected frequency in a second location in said formation adjacent said first location, saturated with native formation fluids whereby a value for "K" is determined, and determining the partial water saturation, S.omega. , of said portion of said formation, from the relation K = KoS.omega. P.

3. The method of claim 2 wherein said first measurement of dielectric constant is made in a portion of said formation adjacent a borehole drilled in said formation using an aqueous drilling fluid and said aqueous saturant is comprised of said aqueous drilling fluid.