US3382933A

US3382933A - Process for drilling geopressured formations without encountering a kick

Info

Publication number: US3382933A
Application number: US522215A
Authority: US
Inventors: Clarence E Hottman
Original assignee: Shell Oil Co
Current assignee: Shell USA Inc
Priority date: 1966-01-21
Filing date: 1966-01-21
Publication date: 1968-05-14
Anticipated expiration: 1985-05-14

Description

DEPTH IN FEET "SSARDH ROOM May 14, 196s CQ E. HOTTMAN 3,382,933

PROCESS FOR DRILLING GEOPRESSURED FORMATIONS WITHOUT ENCOUNTERING A KICK Filed Jan. 2l. 1966 2 Sheets-Sheet l FIG. l C. E. HOTTMAN HUM aff/0M HIS ATTORNEY May 14, 1968 c. E. HOTTMAN 3,382,933

PROCESS FOR DRILLING GEOPRESSURED FORMATIONS Filed Jan. 2l, 1966 WITHOUT ENCOUNTERING A KICK 2 Sheets-Sheet 2 10100 .003 10P 0500110500050 11,000 moo 300.114 ,7 11,500 1.000 l 12,100 1.420 1 12,000 1.500 CHINE 12,100 1.009 l WWW-10,100 1.429 l' 13,500 -f 1.290 l 13,000 1.111 l 14,100 1.000 l 14,450 .952 l REs1s1|v|rY I l l l l I lNvENToR:

c. 1:. HoTTMAN 011s ATTORNEY United States Patent Oliice 3,382,933 Patented May 14, 1968 3,382,933 PROCESS FOR DRILLING GEOPRESSURED FOR- MATlONS WITHOUT ENCOUNTERING A KICK Clarence E. Hottman, Houston, Tex., assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware Continuation-impart of application Ser. No. 144,685,

Oct. 12, 1961. This application Jan. 21, 1966, Ser.

2 Claims. {CL 175-50) ABSTRACT OF THE DISCLOSURE A proces-s for drilling a borehole in a region that is apt to contain geopressured formations. 'Ihe section of the borehole penetrating the hydropressured formations lis drilled using a rnud weight equivalent to the hydropressure plus a swabbing factor. At frequent intervals the borehole is logged to measure a porosity property of the shale sections with the logged dat-a being used Ito determine the trend with depth of the property. The drilling is continued until the property diverges from the predicted trend. When the property diverges, the drilling is stopped and the borehole cased at least to the depth at which the property diverged.

This invention is a continuation-impart of copending patent applications Ser. No. 144,685, tiled Oct. 12, 1961, now Patent No. 3,235,026; Ser. No. 226,937, :filled Sept. 28, 1962, now Patent N-o. 3,237,094, and Ser. No. 293,491, tiled July S, 1963.

This invention pertains to a process for drilling an oil well or the like and, more particularly, to a process for drilling a borehole that is near or penetrates a geopressure formation.

The term geopressure is used to describe a subterranean earth formation in wh-ch the fluid pressure of the pores exceeds the hydrostatic pressure for the height of the column of earth formation l-iquids above the formation. Similarly, the term hydropressure is used to `describe a formation in which the uid pressure of the pores is substantially equal to the hydrostatic pressure for the depth -of the formation. Hydropressures are generated primarily by the weight of fluids contained in the formations overlaying the formation of interest while geopressures are generated primarily Vby ya portion of the weight of the solids overlaying the formation of interest being added to the weight of the uids.

The predominant fluid present in subterranean earth formations is water containing various amounts of dissolved s'alt, principally sodium chloride, rang-ing from fresh wa-ter to saturated salt water. The dens-ities of formation waters generally vary between 0.433 p.s.i. per foot for fresh water to 0.443 p.s.i. per foot for average sea water and 0.52 p.s.i. per foot for saturated salt water. In the drilling art these terms iare normally expressed in pounds per gallon or abbreviated p.'p.g. When these terms are converted to p.p.g. such densities of the formation waters vary between labout 8.33 p.p.g. for fresh water and 10.0 p.p.g. fo-r saturated salt Water.

The pressure existing -in any formation at any particular depth is generated substantially by and is equal to hydrostatic pressures of the waters contained in the formations overlaying the formation of interest. Thus, the format-ion pressure will be equal to the weighted average of the water contained in the formation overlaying the formation of interest. Of course, there are cases where formation pressures would be the result of the Weighted average of waters contained in porous formations that outcrop a considerable distance from the formation of interest. In such cases, the formation fluid pressures correspond to the height to which the outcropping formation extends above the formation of interest, and these pressures are also referred to as hydropressures. The average pressure gradient of hydropressures in the coastal areas of the Gulf of Mexico reflecting the Weighted average of the formation Waters is approximately 0.465 p.s.i. per foot or 8.94 p.p.g.

Geopressure formations occur in various areas of the earth, and the Gulf Coast area of the United States consists predominantly of formations that are apt to be geopressure for-mations. More particularly, in formations located both onshore and offshore in the states of Louisiana and Texas, a pressure gradients in the subterranean earth formations are apt to exceed a hydrostatic pressure gradient of 0.465 and be within a range of geopressure pressure gradients of from about 0.8 to 1.0 per foot.

When a well is drilled it is customary to weight the drilling mud from 0.5 to 2.0 p.p.g. greater than the pressure gradient of the hytdropressure formations. This additional weight or overbalance is used to control the swabbing effect of the dr-ill string. The term swabbing effect `is used to describe the phenomenon that occurs when the drill string is withdrawn from the well and acts as :a piston being with-drawn from a `closed end cylinder. Thi-s reduces the pressure at the bottom of the well and formation fluids can be drawn vinto the well. To prevent a swabbinginduced intiow of fluids the mud weight is increased by an `amount commensurate with the swabbing effect.

Most hydropressure formations will withstand the 0.5 to 2.0 overbalance Without fnacturing. The exception would be formations that contain extremely weak areas or thief zones. Such formations are usually located within few thou-sand feet below the surface of the earth.

In the past, it has been the practice to increase the weight of the drilling mud -as one approached the expected locations of geopressure formations. This involves a risk since the increase in the mud Weight increases the possibility that the hydropressure formation will be hydraulically fractured. While some hydropressure formations will hold mud Weights of 14.5 ppg., i.e., an overbalance of 4.5 ppg., many will not.

`In the past, great difculty has been encountered in drilling Wells that are located near or penetrate geopressure formations. Even in cases where the particular Well does not penetrate a geopressure formation, difficulties are encountered as a result of the measures taken in anticipation of the well penetrating a geopressure formation.

Some of the problems that larise in the prior methods of drilling in areas that are apt to contain geopressure form-ations are as follows:

(1) The drilling -mud that is circulated is weighted to contain the anticipated geopressures. In some eases, the mud is weighted to an extent that overlaying hydropressure formations lare hydraulically fractured by the heavy drilling mud. A well in a hydropressure formation can generally be drilled with a mud providing :a pressure gradient which is only 0.5 to 2.0 ppg. greater than the pressure gradient for the hydropressure section. While some hydropressure formations can withstand greater pressures than others, many hydropressure formations contain Weak spots and fare apt to be fractured by a pressure gradient of from about 2 to 4 p.p.g. more than the hydrostatic pressure gradient in the formations.

(2) The Well is cased at much more frequent inter vals in order to avoid the possibility of the heavy mud weights .accidentally fracturing the formations. Thus,

since the wells are cased more often there is the possibility that the later installed portions of casing will become so small that it will be impossible to reach the target depth. It is readily appreciated that a certain minimum casing size is required to permit the rotary drilling equipment to be inserted within the cased borehole.

(3) The differential of the high mud weight pressure over pore-fluid pressure increases the occurrence of the drill string sticking in the borehole. While it is possible to retrieve some stuck drill strings, many case-s occur where the drill string cannot be freed and it must be left in a hole. This makes it necessary for the well to be abandoned or the borehole to be sidetracked around the stuck drill string.

A copending application by C. A. Stuart entitled Process For Drilling Geopressured Formations, Ser. No. 357,485, filed Apr. 6, 1964 now abandoned and assigned to the same assignee as this application, discloses a method for drilling geopressure formations that brieiiy comprises the steps of: drilling the surface portion of the well and then installing a surface casing to approximately 2,000 to 4,000 feet, depending on geological factors and depth of the first salt water formations. The drilling is continued using a lightweight mud that is approximately 0.5 to 1.5 p.p.g. heavier than the pressure gradient of the hydropressure formations until a kick is observed. The drilling is halted, and the kick is controlled. After the kick is controlled and after an adequate penetration of the geopressure formation has been attained, the well is cased and the drilling is continued.

While the method disclosed in the above copending application has been successfully used, it has the disadvantage of drilling until a kick, `as defined therein, is observed. In some geological provinces, it is possible to drill a considerable distance into the geopressure formation before the kick is observed. This problem occurs primarily where the mutation or transition zone between the hydropressure and geopressure formations contains a long shale section having a low permeability. In these instances, by the time that a permeable formation containing geopressured fiuid is encountered the pressure gradient may be so high that it will be impossible to control the well by using a heavier mud weight.

In such a situation it is apt to be impossible to withdraw the drill string and the well is likely to be lost. When the mud weight is increased to an extent necessary to control the kick, the overlying hydropressure formations are apt to fracture and allow the mud circulation to be lost. The problem of losing a well in this manner will occur where -a considerable depth of a geopressure formation is penetrated before a kick occurs.

The term kick is lused herein to describe the fluid entry of heaving shale that occurs when the pressure of the formation pore fiuid exceeds the pressure generated by the column of drilling mud filling the well by an amount causing materials in the earth formations to enter the borehole. When this occurs a response, such as a pressure impulse or kick, is observable at the surface. In addition to kicks that are caused by the formation pressure exceeding the pressure generated by the column of mud, kicks can occur as a result of heaving shale. The term heaving shale is used to `describe the cases where the pressure of the shale formation causes the shale section adjacent the 4borehole Wall to crumble or heave in and may in extreme cases cause the drill pipe to stick. Similarly, swab kicks can occur when the drill string is withdrawn from the well as a result of the swabbing action of the drill string. The swabbing action temporarily lowers the pressure exerted by the column of drilling mud and thus permits the formation pressure to generate an incipient kick that was not observable until the drill string was withdrawn. All of the various types of kicks are described in the above-referenced copending application.

The present invention is directed to drilling a borehole near, into or through geopressure formations, while preventing any materials in the earth formations from entering the borehole in response to an underbalance in a fiuid pressure gradient within the borehole, i.e., an underbalance, in respect to the iiuid pressure gradient within the earth formations around the borehole. The invention utilizes a combination of drilling and logging techniques. In this invention a technique of drilling with a lightweight mud, eg., a technique of the type described in the abovementioned copending patent application Ser. No. 357,485 is combined with the technique of determining the trend with depth of a shale formation property that is responsive to the density or porosity of a subterranean shale formation., e.g., a technique of the type described in copending patent applications Ser. No. 144,685, filed Oct. 12, 1961; Ser. No. 226,937, filed Sept. 28, 1962; and Ser. No. 293,491, filed July 8, 1963.

The present invention solves the above problems of drilling a borehole near or into geopressured formations vby drilling the hydropressured section of the -borehole using the low mud weight technique of the abovereferenced copending application. During the drilling of the hydropressured section at frequent intervals the borehole is logged to measure a porosity property of the shale formations penetrated by the borehole. The logged data is then plotted to determine the trend with depth of the porosity property of the shale sections. Drilling of the borehole is continued using a mud weight that is substantially equivalent to the hydrostatic pressure plus a swabbing factor. The hydrostatic pressure is determined from the measured porosity property. The drilling is continued using the above methods until the measured porosity property diverges from the predicted trend of the property, at which point the drilling is halted. The borehole is then cased at least to the depth at which the porosity property diverged from the predicted trend before the drilling is resumed.

The present combination of drilling and logging techniques provides the advantages of both preventing the fracturing of the weaker formations and avoiding the need for using too many strings of casings, as well as providing a significant portion of the fast drilling ladvantages that are inherent in the technique of Idrilling with a lightweight mud. This combination makes it possible to obtain those advantages without the necessity of ever encountering and controlling an observable kick.

Since the invention is directed primarily to drilling into geopressure formations and since these formations occur primarily in the Gulf Coast area of the United States the specific examples employed in the description of the process will be limited to wells in this area.

In the present process the surface portion of the well is drilled and cased. Conventional techniques are suitable, and the use of lightweight drilling mud is a preferred procedure for obtaining a rapid rate of penetration. The surface portion of the well normally refers to the first 2,000 to 4,000 feet of a well that is to be drilled to total depth of about 10,000 feet. Normally, wells in the Gulf Coast area of the United States are drilled to total depths of'between 7,000 and 20,000 feet. In a well having a total depth of 7,000 to 20,000 feet the surface portion of the well would normally occupy the first 2,000 to 4,000 feet, depending on the geological structure and depth of the first salt water formation and the like weak zones in the near surface subterranean earth formations. This is the depth that is conventionally drilled before the setting of the first casing string, and it is a zone in which no geopressures are apt to be encountered. Of course, prior to drilling this depth, it is customary to install a surface casing consisting of a relatively short section, generally a few hundred feet, of a relatively large diameter pipe.

In a wildcat well, after the surface portion of the borehole is drilled, the drill string is preferably removed and the :borehole is logged before this portion is cased. On the other hand, in a well being drilled in an area where data are available from other wells that encountered similar subterranean earth formation, or where the trend with depth of a porosity-responsive shale property is to be determined by techniques, such as neutron logging techniques, that can be used in cased Wellls, it is normally not necessary to log the surface portion of the borehole prior to setting `the casing.

In general, in accordance with the present invention, after the surface portion of the borehole has been drilled and cased to a depth sufficient to seal o the relatively weaker near-surface earth formations, drilling is continued below the casing while circulating a drilling mud which is maintained at a weight substantially equivalent to the hydropressures plus a swabbing factor. This swabbing factor preferably amounts to a mud weight which is from about 0.5 to 1.5 p.p.g. greater than the Weight of the formation iiuids.

During the drilling operation, measurements are made continuously or intermittently of porosity, or density, responsive properties of the shale formations that are encountered as the borehole is deepened. Several logging techniques that respond to such properties of shale formations are commercially available and other available logging techniques distinguish shale formations from formations of other types. For example, acoustic velocity well logging, resistivity logging, and neutron logging all respond to changes in the porosity of formations while self-potential logs distinguish between shales and other types of formations. From the measurements of the density, or porosity, responsive properties of the shales, determinations are made of the trend with depth that is exhibited by the measured properties of the shales that are encountered at increasingly deeper depths. The trend that is first established, by measurements Within or near the surface portion of the borehole and/or by measurements within similar zones of other wells that encountered a similar sequence of earth formations, is the trend in the hyd-ropressured shale formations. The drilling, measuring, and trend-determining operations are continued as the borehole is deepened, and, when a geopressure formation is encountered, the measurements made near the bottom of the deepened borehole diverge from an extrapolation of the trend exhibited at shallower depths.

Particularly suitable procedures for conducting such porosity-, or density, responsive measurements and trend-` determining operations are described in greater detail in the copending patent applications, Ser. No. 144,685, directed to acoustic logging techniques, Ser. No. 293,491, directed to resistivity logging techniques, and Ser. No. 226,937, directed to utilizing such measurements and trend determinations of substantially any densityresponsive physical propertyY of the shales in order to determine the depth of the top of a geopressure formation.

In various locations, particularly where the transition zone between-the hydropressure and geopressure formations is apt to occur in a long shale section containing substantially no permeable streaks from which fluid is apt to be produced into the borehole, it is particularly advantageous to determine the earth formation fluid pressure gradients in conjunction with the above measuring and trend-determining operations. In such situations it is desirable to determine both the depth at which measurements made near the bottom of the borehole diverge from an extension of the trend exhibited at shallower depths and the fluid pressure gradient that corresponds to the magnitude of that divergence. Thisk information is then utilized as a basis for increasing the mud weight as required to maintain a weight commensurate with the pressure plus a swabbing factor of the fluid in the earth formations near the bottom of the borehole. The necessary mud weight increases are made although the shale property measurement information may be the only indication that the formation fluid pressure has increased above a hydropressure. This combination of operations makes it possible to penetrate to a selected depth below the top of a geopressure with confidence that, if and when the borehole is extended into a highly permeable geopressure reservoir, the weight of the mud in the borehole will be substantially adequate to contain the pressure at which fluid tends to be produced into the borehole. Particularly suitable procedures for making such fluid pressure gradient determinations are described in greater detail in the copending patent applications (Ser. No. 144,685 and Ser. No. 293,491).

In general, the drilling, measuring, and trend-determining operations specified above are continued While increasing the mud weight as required to maintain a weight substantially commensurate with the pressure plus a swabbing factor of the fluid in the earth formations near the bottom of the borehole until the borehole has penetrated a selected distance below a depth at which measurements of a density-responsive shale property near the bottom of the borehole diverge by a significant amount from an extrapolation of the trend of that property at shallower depths. In respect to the shale property trends that are exhibited by the hydropressured shales, it is possible to determine such a trend for a given property from data obtained by measurements of an equivalent but different property in the upper portions of either the same borehole or another borehole that encounters similar earth formations. However, it is generally preferable to utilize measurements of the same shale properties in each of the measuring and trend-determining operations.

The frequency at which the shale property measurements and trend determinations are made may, of course, be varied with variations in the likelihood that the borehole is approaching a geopressure. When the borehole nears a depth at which geopressures have been encountered in comparable wells, the measurements should be made at least within relatively frequent intervals; and, where long transition zones in an extensive shale formation are apt to be encountered, it may be desirable to make both such measurements and the formation fluid pressure gradient determinations within short intervals, even though such a frequency of measurements may necessitate more drill string round trips than would otherwise be required. The selected depths to which the borehole is extended, below the depth at which measurements near the bottom diverge from the extrapolated trend, are affected by numerous factors. If the mud weight required to confine the earth formation fluid pressure corresponds to a pressure gradient approaching one likely to fracture shallower formations, this distance should be relatively short. On the other hand, if the transition to high geopressures is relatively slow and a deep well is planned, this distance should be as long as feasible. In any case, the selected distance should be sufficient to obtain measurement data or other information, such as mud and/or drill string responses, indicative of the fact that the borehole has been extended to a depth below the top of a geopressure.

When the borehole has been extended for the selected distance beyond the depth at which measurements made at the bottom of the hole diverge from the extrapolated trend, the drilling is halted, or interrupted, and casing is run and set in order to case the borehole from the surface to the selected depth below the top of a geopressure. The Well is then completed, plugged, or drilled deeper. Where the well is drilled deeper, the above-described drilling meas-uring, and trend-determining operations are employed in the manner described above. In such welldeepening operations, the use of the determinations of the earth formation fluid pressure gradients is particularly advantageous. In numerous locations geopressure formations are apt to overlay hydropressure formations that might easily be fractured by a mud weight sufficient to contain a high degree of geopressurization. In such situations, when the formation fluid pressure gradient becomes relatively low, additional casing may be required to allow the yuse of a lighter mud.

From the above description it can be seen that the process of the present invention for drilling geopressured formations comprises the following steps: First, the surface portiorx of the borehole is drilled and cased, using substantially any drilling and casing techniques. After the surface portion is drilled it may be desirable to log the borehole, by making measurements that are responsive to the porosity of the formations penetrated by the borehole. Normally the surface portion of the borehole will not be logged, the exception being a Wildcat well.

After the surface section of the borehole is cased, the borehole is deepened by drilling below the cased portion while circulating mud maintained at a weight substantially equivalent to the hydropressures plus aswabbing factor. Measurements are made, continuously or intermittently, in respect to a porosity-responsive property of the shale formations that are encountered as the borehole is deepened and determinations are made of the trend with depth that is unique to that property in the shales that are encountered at the increasingly deeper depths. These drilling, measuring, and trend-determining operations are continued, while increasing the weight of the drilling mud as required to maintain a weight substantially commensurate with the pressure plus a swabbing factor of the fluid in the earth formations near the bottom of the borehole, until the borehole has penetrated for a selected distance below a depth at which measurements of the shale property near the bottom of the borehole diverge by a significant amount from an extrapolation of the trend exhibited at shallower depths. After the selected distance of penetration has been obtained, the drilling is halted and the borehole is cased to at least the depth at which the measurements at the bottom of the borehole began to diverge from an extrapolation of the trend exhibited at shallower depths. In addition, in a preferred embodiment, determinations are also made of the uid pressure gradients that correspond to the amounts by which shale property measurements made near the bottom of the borehole diverge from the extrapolated trend and the mud weight is increased as required to maintain a weight commensurate with that fluid pressure gradient plus a swabbing factor.

In some situations, as, for example, an exploratory well, it may be desirable to drill deeper without casing the well. In this situation the mud weight can be tailored to the fluid-pore pressure of the formation using the log data from the shale sections. While it is possible to drill a portion of the geopressure formation by this method, a point is reached where a further increase in the mud weight will result in fracturing of the hydropressure forr mation and lost circulation. At this point, the well must be cased or further drilling abandoned. Most hydropressure sections of a well will hold a mud weight of up to about 14 ppg. without fracturing or causing other lost circulation problems. Y

From the above description, it is seen that the method of the copending application Ser. No. 357,485 has been modified by providing for logging the well and continually maintaining a mud weight commensurate with the fiuidpore pressures being encountered as geopressure formations are approached. Normally the general depth of the geopressure formations will be known and the well can be drilled to this general depth before the frequent logs and frequent determinations of the earth formation fluid pressure gradients are necessary in order to maintain such a weight in the drilling mud. In those wells where no information is available regarding the general depth of the geopressure formations, the measurements and pressure determinations must be made at more frequent intervals.

The following logging procedure has proven to be generally effective in the Texas Gulf Coast area. lln a given region where the borehole is approaching a depth at which geopressures are anticipated, in view of similar encounters within a similar region, the well is preferably logged about every 500 feet until the first indication of geopressure formations, and the well is then logged about ever 200 feet to 300 feet. In addition to logging the well, determinations are preferably made of the pressure gradient within the earth formations being drilled as the geopressure formations are approached. When the pressure gradient reaches a range such as about 0.6 to 0.75 p.s.i./ft. the well is preferably cased or abandoned.

Example I This invention was employed in drilling a well with the results indicated in FIGURE 1. The upper portion of the borehole was drilled and logged and then cased to a depth of 3,000 feet with a 13%-inch Icasing string. The logging operations included measurements of differential acoustic transit times, At, and self-potentials of the surrounding earth formations. The depths at which shale formations were encountered were determined from the selfpotential measurements. The trend of the acoustic transit times of the shales was plotted, in microseconds, against the depths of the shales, as shown in FIGURE 1. The mud used was a conventional water base mud having a weight proportional to the amount of weighting material that was added. The various weights, in p.p.g., at which the mud was maintained at depths below about 8,000 feet are indicated in the column Actual Mud Wt. Carried While Drlg. At a depth of about 8,600 feet, a shale acoustic transit time measurement near the bottom of the borehole was found to diverge by 5 units from the Value indicated by an extrapolation of the trend with depth at shallower depths, as is indicated under the column At SH ABN-t SH N. This amount of divergence is equivalent to a formation uid pressure gradient that corresponds to a mud weight of 11.1 p.p.g., as indicated under the column Equivalent Mud Wt. As indicated by the data tabulated in the three columns mentioned above, the actual mud weight was maintained substantially cornmensurate with the pressure plus a swabbing factor of the fluid in the earth formations near the bottom of the borehole while drilling the borehole from a depth of about 8,000 feet to about 13,100 feet.

At about 10,200 feet the extent of the divergence of measurements near the bottom of the borehole from the trend that was exhibited at shallower depths began to increase at a significant rate. A string of /s-inch protective casing was run in and set to a depth of about 10,300 feet. It should be noted that this involved a penetration of about 2,000 feet below the depth of about 8,600 feet at which measurements near the bottom of the borehole diverged by a significant amount from an extrapolation of the trend that was exhibited at shallower depths. At about 12,500 feet the Weight of the mud was increased in order to attain the specified type of correspondent with the pressure of the fluid in the formations near the bottom of the borehole. At that time the measurement determined fluid pressure gradients were the only significant indications that such an increase in the mud weight was needed.

As indicated in the column Remarks, it is apparent that this combined drilling and logging procedure controlled the well as the borehole was extended through a hydropressure section and deep into a geopressure section without allowing any significant amount of materials in the earth formations to enter the borehole. The importance of maintaining a mud weight substantially commensurate with the pressure plus a swabbing factor of fluid in the formations near the bottom of the borehole is indicated by the fact that in the interval from about 10,200 feet to 10,300 feet, in which interval an 11.1 p.p.g. mud weight was retained although it was by then becoming less than the specified weight, the materials in the earth formation did demonstrate some tendency to enter the borehole.

Example Il This invention can also be practiced by utilizing the trend with depth of, for example, a resistivity property of shales in respect to which the trend with depth is determined by means of a computer. FIGURE 3 shows a computer plot using a least squares lit of the machine output of a computer program to solve the equation:

C=conductivity mmhos (1G00 divided by the shale resistivity),

A0=intercept at surface (i.e., 0 feet),

A1=slope mmhos per foot,

D=depth in feet.

The machine plot of FIGURE 2 determined the top of geopressures to be at 11,000 feet, and this corresponds with a similar determination made by manually plotting the same measurement data. The computer program followed the procedures that were essentially the same as those used in manually plotting the level with depth of shale resistivities. -It converted the data to conductivities and picked the top of the geopressures by (l) supplying the computer with shale resistivity, depths and identication; (2) computing C for each depth; (3) computing C/D for each depth to determine C2; (4) comparing C22 to C21 where C21 is the shallowest point Iand C22 is the point at the next depth, for a ser-ies of depth points until points are found below C22, i.e., C2D C2n+h C2n+2 C2+5; and (5) recording the depth of C2n as the top of the geopressure.

I claim as my invention:

1. A process for drilling a borehole in a region apt to contain a geopressure formation while keeping materials in the earth formations from entering the borehole, which process comprises:

(l) drilling and casing an upper portion of the borehole that extends through the relatively weaker nearsurface earth formations;

(2) drilling below said cased portion of the borehole while circulating mud maintained at a weight substantially equivalent to the hydropressure plus a swabbing factor;

(3) measuring a porosity-responsive property of shale formations encountered by the borehole and determining the trend with depth that is unique to said property of the shales that are encountered at increasingly deeper depths;

(4) continuing the drilling and said measurin-g and trend-determining operations while increasing the weight of the mud as required to maintain a weight substantially commensurate with the pressure plus a swabbing factor of the fluid in earth formations near the bottom of the borehole until the borehole has penetrated a selected distance below a depth at which measurements of said shale property near the bottom of the borehole diverge by a significant amount from an extrapolation of the trend with depth of said property that was exhibited at shallower depths; and then (5 halting said drilling and casing the borehole to at least the depth of said divergence in the trend of said measured properties.

2. The process of claim 1 wherein:

(1) determinations are made of the fluid pressure gradient which corresponds to the amount by which said shale property measurements near the bottom of the borehole diverge from said trend extrapolation; and

(2) the weight of the mud is increased as required to maintain a weight commensurate with said measurement determined -uid pressure gradient plus a swabbing factor.

Gas Journal, May 16, 1960, pp. 172-176, vol. 58 TN 860.039. 166,/4.

JAMES A. LEPPINK, Primary Examiner.