US3237094A

US3237094A - Method utilizing formation resistivity measurements for determining formation fluid pressures

Info

Publication number: US3237094A
Application number: US226937A
Authority: US
Inventors: Charles L Blackburn; Clarence E Hottman; Robert K Johnson
Original assignee: Shell Oil Co
Current assignee: Shell USA Inc
Priority date: 1962-09-28
Filing date: 1962-09-28
Publication date: 1966-02-22
Anticipated expiration: 1983-02-22

Description

1w 4 1 s wl! 8 [am E mn am 3 m m f 3M n 10.0. m M h Tn B E S N Kms T ws E. G C N A SEZ [7.0 H MTW A O O R Em M BHJ E qu G H www. mLEK. T EN? OM EQC.R .J ID 0 V V.. Num :5m RMP OD URN m B 0 Kwn F MF N E CI O O O O MR 0. Mmm O m 2 3 4 5 m w w m www i2 Bmw. T152 m12 om; am. n 1 .Frs-v AP E nl LGI T .NN E CII M ZH l0- IR Il. LE l M um l w O., Ungarl .nv .l nu .D 9 m m Lw s 7 6T Ru 3 6M 8 l E 9 2 1.. l D; i .mw n n a. 2 if@ .w 0 0 O H Lw n o M X f. E.. F e m m.. M m if Ew zma v u i ,l n Q( n D. I u .V viiIlv (CIS (Rc AT 77 F) Ii"J Fell 22, 1966 c. L. BLACKBURN ETAL 3,237,094

METHOD UTILIZING FORMATION RESISTIVITY MESUREHENTS DETERMINING FORMATION FLUID PRESSURBS -Fned sept. 28, 1962 2 sheets-snee: a

SEA WATER soLuTloN TEMPERATURE, F

-b U1 O O DEPTH, FEET I 2 V4 e `|o R OHM-METERS FIG. 5

loo i l l l l lNvENToRs:

0.4 o5 0.6' 0.7 0.a os C' BLACKMN c. E. Ho'rTMAN FLUID PRESSURE GRADIENT. puff' FOR SANDS R K JOHNSON FIG. s BYSWK @2M THEIR ATTORNEY l IIIIIIII- borehole.

' conductor-13 to a generator 14 located at. thesur'face simulate the communication between the clay particles while the plates themselves simulate the clay particles. Upon the application of pressure to the uppermost plate the height of the 4springs between the plates remains unchanged as long as no water escapes from the system. Thus, in the initial stage the applied pressure is supported entirely by the equal and opposite pressure of the water. As the water escapes from the system through the perforations in the plate the. uppermost plate will move downward slightly and the springs will carry part of the applied load. :Asmorewater escapesthe springs will carry additional load until `finally the complete axial load will be borne by the springs` and the system will reach a state of equilibrium.

mally high pressures in the fluids contained in permeable rocks which are enclosed within such shales.

Referring now to FIGURE l, there is shown a simplified diagramk of an electrical logging tool. The tool consists of two electrodes 10 and 11 that are lowered into a The electrode l0 .is coupled by means of a with tl-e generator 14 being coupled to a ground 15. Thus the generator 14 will induce a circulating electrical current in the formation surrounding the borehole 12. TheV electrode 11 is coupled by means of a conductor 16 to a recorder 17 located at the surface.

The recorder 17 measures the potential existing between the electrode 11 and ground and this potential will be related to the resistivity of the formation adjacent the electrode 11. The recorder 17 can record the infomation in various forms with informationl being normally recorded in ohm meters. The recorded information will be recorded in relation to the depth of the electrodes in the wellbore 12. Of course, in actual practice the electrodes are disposed on an instrument that is lowered into the wellbore at the end of a logging cable and the information transmitted to the surface where it is recorded.

A normal resistivity log that is provided by various logging companies is referred to as a short normal electrical resistivity log. This log is particularly adapted for use with this invention since it is not normally necessary to apply any borehole correction for the conditions usually encountered in abnormally pressured sections. While this type of resistivity log is desirable other types of electrical logs may be used, as for example induction, resistivity or normal resistivity. The shale resistivity (Rsh') as'measured on a short normal electrical log is dependent on four rock variables: porosity (qs), cementation factor (m), temperature (T) and resistivity of the fluid saturating the shale formation (-Rw). The functional relations between these various factors can be expressed symbolically by the following formula:

. Rsh=f("1,Rw,,T (I) In the normally pressurized tertiary sections of the gulf coast of the United States, three of the variablesporosity, cementation and temperaturecan be expressed as functions of depth, as follows:

or remains constant as a function of increase indepth is dependent upon the rate of change of the dependent variable in Equation l above.

Referring now to FIGURE 2, there is shown a plot of the resistivity with respect to depth for a well drilled that as the well enters the top of the abnormal pressure in the gulf coast area. From lthis plot it is easily seen formation the resistivity decreases rapidly. The data, plotted in FIGURE 2 is uncorrected for the above variables and from this data it is seen that the variables do not vary linearly with depth but as some oth-erfunction of depth. v

The well illustrated in FIGUR-E 2 was drilled using normal techniquesand obtaining resistivity logging data at the various intervals. These data were then plotted as shown in 'FIGURE 2 and a determination made of the expected rate of increase in resistivity with depth. As the well was drilled deeper the rate of increase in the resistivity was noted and the point at which thisrate of increase changed or, as shown in FIGURE 2, actually reversed and became a decrease in resistivity was used as the top of the abnormally pressured section. As shown `in the data in FIGURE 2, this occurred at approximately 11,000 feet- At this point, the procedures required for drilling in abnormally pressured sections were then instituted and the well deepened to its desired depth.

`From the above it can be seen that the first 11,000 feet of the welll could be drilled using normal techniques and only the remaining lfew thousand feet would require .the

(RJV- ILA) l-V] X 100 IIn this formula RGN isthe apparent shale resistivity that is indicated by extrapolation of the resistivity curve for the'. normally pressured section of the well to the depth ZE, and RGA is the apparent resistivity indicated by the log of the abnormally pressured section at depth 2,. From this curve it is possible to determine the pressure gradient for any particular position or depth of well and thus the net weight required during drilling operations to contain the pressures that are likely to be encountered. This permits one to tailor the mud used in drilling to the actual conditions that exist in the well. This ability to tailor the mud to the requirements results in a saving since the use of excessive mud weights can be reduced.

As explained above. the resistivity for the well shown in FIGURES 2 and 3 is not a linear relationship with Y depth and thus it is desirable to correct a measured resistivity to obtain a linear relationship. It has been discovered that if the measured resistivities are corrected for temperature alone one will obtain a substantially linear relationship between the logarithm of the measuredl resistivity and depth. A typical curve for correcting measured resistivities is shown in FIGURE 4. One of the curves shown is for correcting resistivities in shale formations, while the second curve is for correcting resistivities measured in saline solutions. Using this curve to correct the resistivity measurements shown in FIGURE 2 and then replotting the infomation, one would obtain a curve similar to that shown in FIGURE 5. It will be noted from the curve shown in FIGURE 5, that the relationship between the. logarithm of shale resistivity versus depth is substantially a linear relationship. Thus, the. conversion of the resistivity to a linear relationship to depth provides an easy means for projecting the resistivity in the normal pressured zones to obtain the data required for plotting resistivity parameters against liuid pressure gradients. Using the data from FIGURE 5. one can obtain the curve shown in FIGURE 6 which is substantially similar to that shown in FIGURE. 3.

Using the curve plotted in FIGURE 5, one could drill a well in thesame manner as that described above with relation to FIGURE' 2." Accordingly, one would'drill using normal techniques until a depth lwas reached at which the ratelofincrease in resistivity' decreased. This decrease in the rateof increase in resistivity would then indicate the top of the abnormally pressured section. At this point, of course, one would institute the techniques required for drilling abnormally pressured sections.

While this invention has been described with relation to two separate techniques it should be appreciated that it is susceptible tomany modifications and changes within its broad spirit and scope. For example, while it is preferable to use the amplied short normal electrical resistivity logs one could also use other types of devices that yield formation. resistivity. While this is possible, care must be exercised in using any resistivity log in order to eliminate as many sources of error as possible.

We'claim as our invention: 1. A-methodfor detecting the fluid pressure gradient resistivities in normally pressured shale formations penesured shale formation comprising:

of a permeable subsurface earth formation penetrated Y' by aborehole, vsaid method comprising: measuringv the individual resistivities of the shale formations penetrated Y, is drilled; and measuring the difference between said recorded resistivities in abnormally pressured shale formations penetrated by the borehole and a projection to the depth of the abnormally pressured shale formations of a measured resistivity or' the trend with depth of the measuring the depth at which the borehole enters an abnormally pressured shale formation;

I measuring the trend with depth of the individual resistivities exhibited by shale formations encountered above said abnormally pressured shale formation;

measuring the resistivity of a shale formation at a selected depth encountered below said abnormally pressured shale formation;

determining the difference between the resistivities exhibited at the selected depth by, respectively, a projection to the selected depth of the-trend of resistivities exhibited by the shale formations encountered above said abnormally pressured shale formation and Y.

the measured resistivity; measuring the amount of ud pressure gradient from preplotted values of fluid pressure gradient in relation to the difference in resistivities that corresponds to the difference between the resistivities exhibited at the selected depth. 3. The methodrof claim 2 in which the trend with depth of said resistivities is measured from logarithms of resistivities that are corrected for the eEects of tem- WALTER L. CARLSON, Primary Examiner.

Claims

1. A METHOD FOR DETECTING THE FLUID PRESSURE GRADIENT OF A PERMEABLE SUBSURFACE EARTH FORMATION PENETRATED BY A BOREHOLE, SAID METHOD COMPRISING: MEASURING THE INDIVIDUAL RESISTIVITIES OF THE SHALE FORMATIONS PENETRATED BY THE BOREHOLE; RECORDING THE RESISTIVITIES WITH RELATION TO THE DEPTH OF THE POINT OF MEASUREMENT IN THE BOREHOLE; CONTINUING TO MEASURE SAID RESISTIVITIES AS SAID BOREHOLE IS DRILLED; AND MEASURING THE DIFFERENCE BETWEEN SAID RECORDED RESISTIVITIES IN ABNORMALLY PRESSURED SHALE FORMATIONS PENETRATED BY THE BOREHOLE AND A PROJECTION TO THE DEPTH OF THE ABNORMALLY PRESSURED SHALE FORMATIONS OF A MEASURED RESISTIVITY OF THE TREND WITH DEPTH OF THE RESISTIVITIES IN NORMALLY PRESSURED SHALE FORMATIONS PENETRATED BY THE BOREHOLE TO DETERMINE THE FLUID PRESSURE GRADIENT OF AN ADJACENT PERMEABLE SUBSURFACE EARTH FORMATION.