US3068355A - Method of locating and studying permeable subsurface formations - Google Patents

Description

Dec. 11, 1962 EGAN METHOD OF LOCATING AND STUDYING PERMEABLE SUBSURFACE FORMATIONS Original Filed April 20, 1953 IN VEN TOR.

EDMOND FEGA 1v TTQENEY United States Patent Ofifice 3,068,355 Patented Dec. 11, 1962 3,0d8,355 METHUD OF LG'CATHNG AND STUDYING PERMEABLE SUBSURFACE FQRMATHGNS Edmond F. Egan, Dailas, Tex., assignor to Texaco Inc., New York, N .Y., a corporation of Delaware Continuation of abandoned application Ser. No. 349,816, Apr. 20, 1953. This application May 29, 1959, Scr. No.

This application is a continuation of application Serial No. 349,816, filed April 20, 1953.

The importance of secondary recovery or repressuring in oil fields has increased greatly during the last few years, and there is consequently a real need for a better understanding of the engineering aspects of these projects. Detailed studies of reservoir performance of water flooding conditions can be undertaken only when certain basic data are available and among the data which is necessary is information concerning the vertical distribution of the injected water in a water injection well.

In US. Letters Patent No. 2,700,734, granted January 25, 1955 to Gerhard Herzog and Edmond F. Egan, based on an original application Serial No. 349,784, filed April 20, 1953, and now abandoned, a method is disclosed for making an injection profile of a permeable formation traversed by a bore hole. In the method disclosed in that application, two streams of similar fluids such as Water are pumped down in the well, one of these streams being pumped down through a string of tubing to a point below the formation to be examined, while the other si'eam is pumped down through the annulus or annular space between the tubing and the casing or the walls of the hole. One of these fluids, preferably that which is pumped down through the annular space between the tubing and the walls of the hole, is made radioactive by the addition of a small amount of a radioactive substance, while the other fluid which is pumped down through the tubing is nonradioactive. These fluids will meet in the annulus somewhere above the lower end of the tubing and an interface will exist between the fluids at that point. The depth of this interface in the bore hole will depend upon the rates of injection of the two fluids and can be caused to move up or down through the annular space around the tubing by varying inversely these rates of injection. The position or depth of the interface is located by passing a radiation detector downwardly through the tubing and noting the change in response of the detector as it passes from the radioactive fluid into the nonradioactive fluid. The sum of the rates of injection of the two fluids is maintained constant and, as stated above, by increasing the rate of injection of the radioactive fluid while decreasing the rate of injection of the non-radioactive fluid, the interface can be made to move downwardly through the hole past the exposed walls of the formation to be examined. By adjusting the rate of injection by increments and noting the depth of the interface after each change, one can determine the amount of the radioactive fluid which is passing into the section of the formation between adjustments of the injection rates.

The present invention is somewhat related to that described in the preceding paragraph but involves a different method of locating the fluid interface. The method to be described can be used to accomplish several different results.

During the drilling of a Well, gas, oil, or water will sometimes flow into the well if the reservoir pressure exceeds the hydrostatic pressure of the drilling mud column. To stop the flow, it is sometimes necessary to cement off the zone from which the fluid is entering the hole. If several permeable zones have been penetrated, it becomes necessary to locate the zone that is flowing fluid into the hole. This zone can be located by following the method which will be described hereinafter.

Lost circulation is a common problem in the drilling of a well. Permeable zones with a reservoir pressure less than the hydrostatic pressure of the drilling mud column will take fluid and make it impossible to maintain proper circulation. When the zone where circulation is being lost has been accurately located, it can be cemented off or material may be pumped into the zone to lessen its permeability and permit a filter cake to build build up on the walls of the formation. As will be described, by forcing a liquid having a density less than the drilling mud downwardly in the hole through the annulus around a tubing string and measuring the depth of the interface between the less dense liquid and the drilling mud while increasing the rate of injection of this liquid, a point or depth will be found at which the interface will cease falling, showing that the less dense liquid is flowing into the formation or lost circulation zone. Remedial steps can then be taken.

When old wells are converted to water injection wells, it becomes necessary to determine if all of the injection water is going into the sand or formation being water flooded, and, as has been stated above, it is frequently desirable to have an injection rate profile of the formation. In accordance with one embodiment of this invention, this is accomplished by pumping fluids of different density in the tubing and through the space between the tubing and the walls of the hole with separate pumps, the rates of injection of the two fluids being carefully metered at the surface and the sum of the rates of injection of the two fluids being maintained constant. By measuring or locating the depth of the interface between the two fluids while varying the ratio of the rates of injection of the two fluids by increments, one can determine the amount of the fluid being injected into each section of the formation.

Well completions in limestone reservoirs are frequently diflicult if a gas-oil contact or oil-water contact is exposed in the well bore and acidization is required. Selective acidization is a procedure of pumping acid only into the oil zone and not in the gas or water zone. This selective acidization is accomplished by controlling the acid level in the well bore with a pump on the tubing and a separate pump on the annulus, i.e., the annular space between the tubing and the walls of the hole or the casing. The acid-water interface can be located with the method to be described, and, by properly varying the rates of injection of the acid and the other liquid in the annulus,

3 the interface can be held at a depth such that the acid will be forced into, say, the oil zone rather than into a gas zone overlying the oil zone or, in another instance, the acid can be forced into an oil zone overlying a water One of the principal objects of this invention is the location of an interface between two different fluids in an oil well where it is necessary to determine the depth of this interface or to maintain the interface at a predetermined depth in the carrying out of any of the procedures which have been mentioned.

In accordance with the invention, an instrument containing a source of gamma rays and a gamma ray detector separated from the source by a suitable shield is passed through the well, preferably through the tubing if such is present. When the instrument is entirely surrounded by a liquid of a certain density, the detector will provide a certain output or response, and this is recorded at the surface by any suitable means. When the instrument passes into the other liquid having a different density, there will be a sudden change in the response or output of the detector, and this will be indicated clearly on the record. By noting the depth of the instrument as it is passing through the bore hole, this point or change in the detector output will indicate that the instrument is passing from the liquid of one density into the liquid of the other density, and the interface between the liquids will, of course, be opposite that point.

For a better understanding of the invention, reference may be had to the accompanying drawing in which FIGURE 1 is a vertical sectional elevation through a section of a bore hole traversing a formation from which a liquid or a gas is flowing into the well;

FIGURE 2 is a vertical sectional elevation through a well traversing a permeable formation into which it is desired to force an injection fluid, such as for the purpose of repressuring an oil field, and

FIGURE 3 is a sectional elevation through a portion of a well traversing a zone or formation containing both oil and gas and in which it is desired to force an acid into the oilcontaining portion only.

Referring to the drawing, a well or bore hole is shown as traversing several subsurface formations, including formation 12, from which it may be assumed that liquid such as water is flowing into the well as indicated by the arrows. The upper portion of the well is provided with a casing 14 having a closed casing head 16. A suitable pump 18 is connected to the casing head 16 through a meter 20 and is adapted to pump a fluid such as water into the upper portion of the well. The lower portion of the well contains the fluid 22, which may be drilling mud which has been left in the hole after the drill pipe was removed. In case the well is not under high pressure, the upper level 24 of the fluid 22 will probably be somewhere above the formation 12. On the other hand, if the well is flowing, the fluid 22 may actually be flowing out of the top of the well. If there is any appreciable amount of water flowing into the well from formation 12, this will dilute the fluid 22, and the density of the fluid above the lower boundary of the formation 12 will be less than the density of the drilling mud 22 below this boundary. This, of course, is true since the water from the formation 12 will tend to dilute or lighten the fluid above the flowing formation. An interface 24 will exist, therefore, between the diluted or less dense liquid above the formation 12 and the undiluted or denser liquid below that formation.

Shown suspended within the well from a conductor cable 26 is an instrument indicated by a sealed housing 28 and containing near its lower end a source 30 of radioactivity such as a small amount of radium. Above the source 30 is a gamma ray detector 32 of any suitable type, the output of which passes to a preamplifier 34 which is connected to the lower end of the cable 26. The detector 32 is separated from the source 30 by means of a shield 36 of lead or any other substance capable of absorbing those gamma rays from the source 30 which otherwise would tend to pass directly to the detector 32. The cable 36 passes over a suitable cable-measuring device 38 and from there to an amplifier 40, the output of which is connected to a recorder or recording device 42 preferably adapted to record the output or response of the detector 32 on a moving tape or strip as the detector passes through the bore hole.

In order to locate the depth of the formation or zone 12 from which water is passing into the hole, it is necessary to locate the interface 24 between the heavier or denser liquid 22 and the lighter or less dense liquid 44. To this end the instrument 28 is passed through the bore hole, either downwardly or upwardly. Gamma rays from source 30 pass outwardly of the instrument into the surrounding liquid, and some of these gamma rays are scattered within the liquid and return to be intercepted by the detector 32, and the amount of the scattering and thus the intensity of the gamma rays striking the detector 32 will depend upon the density of the liquid in which the scattering is taking place. Thus, assuming that the instrument 28 is being lowered through the hole and is above the formation 12, the response of the detector 32 will be at a certain level, and this will be recorded by the instrument 42, preferably in correlation to the depth of the instrument 28 in the hole as measured by the device 38. As soon as the instrument 28 passes downwardly into the liquid 22 of higher density, however, the response of the detector 32 will change and this will be recorded by the recording instrument 42. The point at which this change in the output or response of the detector 32 occurs will, of course, be opposite the interface 24 between the two liquids of different densities, and, by noting the depth in the hole in which this change in detector response occurs, one will know the depth of the interface and thus the lower boundary of the formation 12 from which the water is entering the hole. Any suitable remedial steps can then be taken to seal off the exposed surface of the formation 12. For example, cement can be used to seal in the formation so that no additional water will enter the hole.

While still referring to FIG. 1, let us assume that the permeable zone or formation 12 is not flowing into the well but that drilling fluid 22 has been passing from the hole outwardly into the formation. Such a formation is sometimes referred to as a zone of lost circulation. It is, of course, desirable to locate this zone so that steps can be taken to seal 01f the exposed surfaces thereof to prevent the loss of additional drilling fluid. We will assume that the upper level 24 of the drilling fluid 22 is somewhere in the hole above the formation 12. A liquid 44 having a lesser density than the drilling fluid 22 will be pumped into the casing by means of the pump 18, and this liquid will force the interface 24 downwardly in the hole until it reaches the depth of the lower boundary of the zone of lost circulation 12. The lighter liquid 44 will then pass outwardly into the zone 12, and the interface 24 will remain substantially fixed as in the position shown in FIG. 1. By passing the instrument 28 through the hole, as has been described hereinabove, the depth of the interface 24 can be accurately determined, and subsequently steps can be taken to seal off the exposed surface or wall of the formation 12 as by cementing or by pumping a material into the zone to lessen the permeability and permit the mud filter cake to build up on the walls of the zone so that drilling operations can continue.

As stated hereinbefore, when old wells are converted to water injection wells, it becomes necessary to determine if all of the injection water is going into the sand being water flooded, and sometimes it is desirable to have an injection rate profile of the sand. This can be accomplished with the method described by pumping fluids of different density into a string of tubing extending below the zone to be flooded and into the annulus or annular space around the tubing. In FIG. 2 of the drawing, a section of a bore hole or well is shown as traversing a formation or zone 12a into which it is desired to inject a liquid such as water for the purpose of repressuring an oil field. The upper portion of the well is shown as being cased at 14a, and a closed casing head 16a is attached to the upper end of the casing. A string of tubing 48a is shown as extending downwardly to a point below the formation 12a. A pump 18a is connected to the casing head 16a through a meter 20a, and a second pump 50 is connected to the upper end of the tubing 48a through a meter 52. By pumping a fluid 44 such as a water containing a large amount of dissolved salts which increase the density of the fluid downwardly through the tubing 48a while at the same time pumping another fluid 44a such as water containing small amounts of dissolved salts downwardly through the annular space around the tubing 48a, an interface 24a will be formed between the two fluids. By maintaining constant the sum of the two rates of injection of the fluids pumped at 18a and 50 and by varying inversely the rates of injection by the two pumps, the interface 24a can be made to move up or down through the hole through the annular space around the tubing 48a. As is described in the aforementioned Patent No. 2,700,734 of Gerhard Herzog and Edmond F. Egan, by stopping the pump 18a while continuing to pump the denser fluid by means of the pump 50, the interface 24a will remain at a depth opposite the upper boundary of the formation 12a. Then, by increasing the pumping rate at 18a while correspondingly decreasing the pumping rate at 50 by a predetermined amount, the interface 24a will move downwardly past the exposed surface of the formation 12a to a new depth. The density-responsive instrument 28 can then be passed through the tubing 48a to locate accurately the depth of the new position of the interface and by knowing the rate of injection of the pump 18a, the amount of the liquid 44a entering the upper section of the formation 12a can be accurately determined. By further increasing the rate of injection at 18a in additional increments, an injectivity profile of the formation 12a can be obtained showing the amounts of liquid and thus the permeabilities of the different vertical sections of the formation.

FIG. 3 shows a section of a well bore transversing a formation or zone 12b, the upper portion of which is a gas zone and the lower portion of which is an oil zone, the two zones being separated by the contact plane 54. The tubing 48a extends downwardly below the formation 12b, and an acid 56 is pumped downwardly through the tubing while a liquid 58 having a lower density than the acid is pumped downwardly through the annular space around the tubing. By adjusting the rates of injection of the two liquids and by positioning the detecting instrument 28 opposite the gas-oil contact 54, the interface 24b can be fixed at this depth. The operator will then be sure that the acid 56 passing downwardly through the tubing 48a is all passing into the oil zone of the formation 12b rather than into the gas zone. In case an oil zone overlies a water zone in a formation such as indicated at 12b, substantially the same method can be used for injecting the acid into the overlying oil zone. In this case, however, the acid will be pumped downwardly through the annular space around the tubing 48a while the liquid pumped downwardly through the tubing will be a liquid having a higher density than the acid.

It will be seen that a method has been provided for locating an interface between two liquids having different densities, and this can be applied to several uses, such as the locating of a zone from which water or other liquid is flowing into a well bore; a zone of lost circulation; for the making of an injectivity profile of the zone to be flooded with water; for the selective acidization of an oil zone or section of a formation, etc.

Obviously, many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof, but only such limitations should be imposed as are indicated in the appended claims.

I claim:

1. In the drilling of a bore hole through the earth involving circulation in the bore of drilling mud having a density greater than the density of water, the method of locating a zone of water invasion from a formation along the bore hole comprising the steps of stopping circulation of the drilling mud, allowing time for water from an invasion zone to enter the well in order to establish an interface in the bore hole opposite the invasion zone between the drilling mud in the hole below said zone and the liquid of lesser density opposite said zone due to the water entry, passing a source of gamma rays through said liquids so that gamma rays from said source will be scattered in the surrounding liquids, measuring in the vicinity of said source the intensity of said scattered gamma rays, said intensity being different when measured in the different liquids, and noting the depth at which a sudden change occurs in the measured intensity, said depth corresponding to the depth of said interface.

2. In the drilling of a bore hole through the earth involving circulation in the bore of drilling mud the method of determining the depth of a zone of lost circulation in the bore hole comprising the steps of interrupting circulation of the drilling mud, forcing a liquid of lesser density than said drilling mud into the hole from the surface for sufiicient time to establish an interface opposite the zone of lost circulation between the drilling mud below the zone and the liquid of lesser density which is forced into the hole, passing a source of gamma rays through the hole so that gamma rays from the source will be scattered in the liquid surrounding the source, measuring variations in the intensity of said scattered gamma rays and noting the depth at which a sudden change in the density occurs, said change being indicative of the interface between the two liquids and thus the depth of the lower boundary of the zone of lost circulation.

3. The method of controlling the injection of water into a porous sand in a water-injection well provided with a string of tubing which comprises pumping a liquid into the well through said tubing, simultaneously pumping the water to be injected downwardly through the annular space around the tubing, said first-mentioned liquid having a density greater than said water, simultaneously with said pumping operations passing a source of gamma rays through the well so that gamma rays from the source will be scattered in the liquid surrounding the source, measuring variations in the intensity of said scattered gamma rays and noting the depth at which a suddent change in the density occurs, said change being indicative of the interface betwen the two liquids, and controlling the rates of the liquids being pumped while observing the depth of said interface so as to maintain the interface at the proper depth with respect to the depth of said porous sand.

4. The method of controlling the injection of water into a porous sand in a water-injection well provided with a string of tubing which comprises pumping a liquid into the well through said tubing, simultaneously pumping the water to be injected downwardly through the annular space around the tubing, said first-mentioned liquid having a density greater than said water, measuring the depth in the Well of the interface between said liquid and said water by passing a source of gamma rays through the well so that gamma rays from the source will be scattered in the liquid surrounding the source, measuring variations in the intensity of said scattered gamma rays and noting the depth at which a sudden change in the density occurs, said change being indicative of the interface between the two liquids and varying the rates of the liquids being pumped while maintaining constant the sum of the two liquids and observing the depth of said interface so as to maintain the interface at the proper depth with respect to the depth of said porous sand.

5. The method as described in claim 3 in which the injection water is pumped down through the tubing and a liquid less dense than water is pumped down through said annular space.

6. The method of making an injectivity profile of a subsurface formation traversed by a bore hole which comprises pumping a first fluid into the bore hole above said formation, simultaneously pumping a second fluid into the bore hole below said formation, said second fluid having a greater density than the first fluid, thereby establishing an interface between said fluids, determining the depth in the hole of said interface by passing a source of gamma rays through the hole so that gamma rays from the source will be scattered in the fluid surrounding the source, measuring variations in the intensity of said scattered gamma rays and noting the depth at which a sudden change in the density occurs, said change being indicative of the interface between the two fluids, varying the ratio of the two fluids being pumped into the hole while maintaining constant the sum of the pumping rates of the two fluids so as to cause said interface to move along the walls of said formation to another depth, again determining the depth of the interface, and repeating these operations while noting the ratios of the pumping rates of the two fluids for each measured increment of depth of the interface in the hole.

7. In a bore hole having a tubing string extending therein such that an annular space is provided between the tubing and the Walls of the bore hole, the method wherein a first liquid is pumped into the tubing string and a second liquid is pumped into the annulus in order to establish an interface between the two liquids in the annulus, one of said liquids being an acid and the other of said liquids being of diflerent density than the acid, passing a source of gamma rays through said bore hole so that gamma rays from said source will be scattered in the surrounding liquids, measuring in the vicinity of said source the intensity of said scattered gamma rays, said intensity being different when measured in the different liquids and noting the depth at which a sudden change occurs in the measured intensity, said depth corresponding to the depth of the interface established between said two liquids.

8. The method of claim 7 which further comprises monitoring the position of the interface between the liquids while adjusting the pumping rates of the liquids in order to position the interface between a first zone in the formation to be acidized and a second zone from which the acid is to be excluded and maintaining said interface between said zones while pumping acid into the first zone.

9. The method according to claim 1 in which a material is pumped into the thus located invasion zone to lessen its permeability.

10. The method according to claim 2 in which a material is pumped into the thus located zone of lost circulation to lessen its permeability.

11. In a bore hole having a tubing string extending therein such that an annular space is provided between the tubing and the walls of the bore hole, the method wherein a first liquid is pumped into the tubing string and a second liquid is pumped into the annulus in order to establish an interface between the two liquids in the annulus, one of said liquids comprising a well treating material and the other of said liquids being of different density than the treating material, passing a source of gamma rays through said bore hole so that gamma rays from said source will be scattered in the surrounding liquids, measuring in the vicinity of said source the intensity of said scattered gamma rays, said intensity being different when measured in the different liquids and noting the depth at which a sudden change occurs in the measured intensity, said depth corresponding to the depth of the interface established between said two liquids.

12. The method of claim 11 which further comprises monitoring the position of the interface between the liquids while adjusting the pumping rates of the liquids in order to position the interface between a first zone in the formation to be treated and a second zone from which the treating material is to be excluded and maintaining said interface between said zones while pumping treating material into the first zone.

13. The method of locating the zones from which fluids of diflerent densities are entering a well which comprises passing gamma rays from a source of predetermined intensity to a detector uniformly spaced therefrom at a plurality of points throughout the well, said gamma rays passing from the source to the detector through a portion of the well fluid in the vicinity of the source and the detector, measuring the intensity of the detected gamma rays at said points, said intensity being indicative of the density of the fluid being traversed by the gamma rays, and correlating said measurements with the depths of said points in the well.

14. The method described in claim 13 in which said measurements are made continuously throughout at least a portion of the vertical dimension of the well.

15. The method of locating the zones from which fluids of different densities are entering a well which comprises passing a source of gamma rays of constant intensity through at least a portion of said well, measuring the intensity of said gamma rays passing through a portion of the well fluid to a detector located at a fixed distance from said source, the intensity of the detected radiation depending upon the densities of the fluid through which said gamma rays are passing, and correlating said measurements with the depth of the source in the well, a high intensity measurement indicating that the source is passing through a light fluid such as gas, a low intensity measurement indicating that the source is passing through a heavier fluid such as water and an intermediate intensity measurement indicating that the source is passing through a fluid having a density between gas and water such as oil.

16. A method for determining the location of respective areas of entry of different fluids into a well bore, each of said fluids having a significantly different effect upon a beam of penetrative gamma radiation, which comprises emitting a beam of penetrative gamma radiation at different locations along the well bore and logging, at a fixed distance from the source of said beam of penetrative gamma radiation, the variations at each location in said beam of penetrative gamma radiation which has passed through and been significantly affected by the well fluids at said location.

17. In a bore hole having a tubing string extending therein such that an annular space is provided between the tubing and the walls of the bore hole, the method wherein a first fluid is pumped into the tubing string and a second fluid is pumped into the annulus in order to establish an interface between the two fluids in the annulus, one of said fluids comprising a well treating material and the other of said fluids being of significantly different density than the treating material, passing a source of gamma rays through said bore hole so that gamma rays from said source are transmitted through a portion of the well fluid in the annulus, detecting gamma rays which pass through a portion of said Well fluid in the annulus to a detector located at a predetermined distance from said source, providing a signal display indicating the intensity of the detected gamma rays in correlation with their location in the bore hole, whereby the location of said interface may be determined by noting a sudden change in the intensity of the detected radiation due to the difference in densities of said two fluids.

18. The method of claim 17 which further comprises monitoring the position of the interface between said fluids while adjusting the pumping rates of said fluids in order to position the interface between a first zone in the formation to be treated and a second Zone from which the treating material is to be excluded, and maintaining said interface between said zones while pumping the treating material into said first zone.

19. The method of claim 11 further characterized in that the source of gamma rays which is passed through the borehole is passed through the tubing string in said borehole.

20. The method of claim 13 which further comprises preventing the passage of gamma rays in a direct path from the source to the detector, whereby variations in the intensity of the detected gamma rays are due primarily to the influence of the well fluid on gamma rays which are scattered by the well fluid in passing through the well 15 fluid from the source to the detector.

10 21. The method of claim 20 wherein the gamma ray source and detector are passed through the well within a tubing string therein to locate fluids of difierent densities which influence the scattering of said gamma rays in the 5 well outside of said tubing.

References Cited in the file of this patent UNITED STATES PATENTS 10 2,335,409 Hare Nov. 30, 1943 2,450,265 Wolf Sept. 28, 1948 2,480,674 Russell Aug. 30, 1949 2,648,014 Arthur Aug. 4, 1953 2,710,925 McKay June 14, 1955 2,926,259 Dewan Feb. 23, 1960

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