US3105900A

US3105900A - Method of injectivity profile logging comprising injecting radioactive tracer near interface of fluids

Info

Publication number: US3105900A
Application number: US780059A
Authority: US
Inventors: Richard H Widmyer
Original assignee: Texaco Inc
Current assignee: Texaco Inc
Priority date: 1958-12-12
Filing date: 1958-12-12
Publication date: 1963-10-01
Anticipated expiration: 1980-10-01

Description

Oct. 1, 1963 R; H. WIDMYER 3,105,

METHOD OF INJECTIVITY PROFILE LOGGING COMPRISING INJECTING RADIOACTIVE TRACER NEAR INTERFACE OF FLUIDS 2 Sheets-Sheet 1 Filed Dec. 12, 1958 fawn United States Patent 3,195,9(20 METHOD OF DHECTIVTIY PROFELE L6G- GING CGMPRISING INJECTWG RADIU- ACTEVE TRACER NEAR INTERFACE 6F FLUES Richard H. Widmyer, Houston, Tex., assignor to Texaco End, a corporation of Delaware Filed Dec. 12, 1958, Ser. No. 789,959 6 Claims. (Ci. fish-43.5)

This invention relates to a method and apparatus for studying subsurface earth formations and more particularly to a method and apparatus for measuring or determining the permeability of the earths formations traversed by a well or borehole. The term permeability is used herein in the broad sense as meaning the relative receptibility of the permeable formations to the ingress of fluid as influenced by the applied pressure of the fluid and the varying back pressures in various strata, and is not used in the technical sense of the measured permeability of the removed rock in millidarcys as defined by Darcys law.

Knowledge of the permeability of the subsurface formation in petroleum operations is of considerable importance in secondary recovery operations for the production of oil from subsurface formations. In secondary recovery operations, a fluid, for example, water, oil, or gas, is pumped into an injection well located at a point remote from an oil producing well and directed into a given formation or vertical interval of the injection well to urge oil which may be contained therein toward the producing Well. In order to determine whether the fluid pumped into the injection well is being efliciently injected into the given formation, the amount of fluid passing into various vertical increments of the wall of the injection well must be *known. A log which indicates the vertical increments of the injection Well into which fluid is entering and the amount of fluid flowing into each increment is sometimes referred to as an injectivity profile or permeability log of the well.

A method of obtaining an injectivity profile or permeability log of a well or a particular formation traversed by a borehole has been described in US. Patent 2,7 00,734, granted to Edmond F. Egan and Gerhard Herzog on January 25, 1955. In the patented method two streams of fluid are introduced into a well, one stream passing through a string of tubing extending downwardly to a point below the formation of interest and the other stream passing downwardly through the annular space between the tubing and the casing or the wall of the well. The streams are introduced or pumped into the well simultaneously and each stream is carefully metered at the surface. The fluid pumped down through the tubing will, after filling the exposed portion of the well below the tubing, flow upwardly around the tubing until it meets the fluid flowing downwardly through the annular space, thus forming an interface between the two streams or bodies of fluid. In order to locate the interface between the two streams a small amount of tracer material, such as a radioactive substance, is added to one of the streams before it enters the well so that all the fluid in this stream will be radioactive, while the other stream will be non-radioactive. The depth in the Well at which the interface lies may be readily located by lowering a detector, for example a radiation detector, into the well and simultaneously and continuously recording the depth of the detector and the output signal therefrom. The response of the detector will change abruptly when the detector passes from the radioactive fluid into the non-radioactive fluid or vice versa.

ice

In order to determine the amount of fluid that is entering into a vertical increment of the formation of 'interest, the rates of injection of each of the two streams are varied but the sum of the rates are maintained con stant. By changing the ratio of the amount of the radioactive fluid to the amount of non-radioactive fluid injected into the well, the interface will be forced to move to another depth in the well. The difference in the amount of either of the fluids injected into the well is the amount of fluid that is entering the vertical increment of the formation between two interfaces. It can be seen that by making appropriate changes in the ratio of the amount of radioactive fluid to the amount of nonradioactive fluid pumped into the well the interface can be moved in a number of steps through the well past the formation of interest to provide an accurate log of the permeability of the formation. The length of each of the vertical increments between the successive'interfaces depends upon the amount of change of the rates of the two streams and the permeability of the increment. After each adjustment or change in the rates of the two sreams and after the well fluids have been stabilized, the rate of flow of the two streams is noted and the radiation detector is passed through the Well to determine the depth of the interface. Accordingly, it can be seen that in this manner an injectivity profile log is made of a formation which clearly shows the permeability of the various vertical increments of the formation.

In the method of producing an injectivity profile log, as described in the above mentioned patent, the radioactive substance is introduced into one of the two streams at the surface of the earth and under ordinary surveying conditions the method has proven to be very satisfactory.

However, it has been observed that injectivity profile surveys conducted at very low injection rates, such as, at total rates of four barrels per hour or less, required a long time, for example, from 12 to 36 hours, to complete a survey, which includes tracer displacement time as well as the establishing of various rates of flow in the two streams. After displacement of the tracer to the top of the completed zone or zones of interest the major time consuming factor is the movement and stabilization time between stream percentage changes.

In order to better understand the amount of time consumed during injectivity profile surveys which are conducted at very low injection rates, assume a total injection rate into the borehole of tWo barrels per hour in a well having a 28# 7 inch outside diameter casing and a 2 /2 inch nominal tubing and a typical rate change increment of 10% of the total flow, that is, 0.2 barrel per hour. Since the annulus between the casing and tubing holds 0.0295 barrel per foot, then the interface travel time for a full 10% change would be approximately 9 minutes per foot. To move an interface just 10 feet at that rate alone would take 1 /2 hours. .As is often the case only a portion of the rate change causes the interface movement, for example, assume that a stable interface had been obtained at a 30% casing rate and the casing rate was then increased to 40%. If, however, the actual entry percentage at the last interface was 36% then the net change to move the interface would be only 4% and it would require over 3 /2 hours to move the interface 10 feet. In actual practice, several sets of perforations are often encountered, located an appreciable distance apart, as well as a possible open hole section which may have an undetermined increased or varying hole diameter which would further increase or make uncertain the travel time required. An additional amount of time is required to ascertain interface stabilization as well as to sharpen the contrast of radioactive-non-radioactive fluids as the interface diffuses somewhat when the interface is moved. With high injection rates these objections are not significant, however, with low injection rates they are paramount. i

To date, three methods have been used to decrease the time required to complete a survey. The first is to increase the amount of rate change and hope that the interface movement will not move along the wall of the borehole too great a distance, and if it does appear to be moving beyond the desired range of control, then to decrease the amount of rate change. While this procedure may prove effective where injection is confined to a short interval, in the alternative case it may consume as much or more time as a systematic small rate change. The second of the three methods is to first run a high injection rate survey and then be guided by this in running the low rate survey. While theoretically this should be satisfactory for a well with a low injection fluid level, it has proven that wells requiring pressure for injection can have very different injection profiles for high and low rates. The last of the three methods involves decreasing the annulus volume between the casing and the tubing by increasing the tubing size. Although this may decrease the interface movement time by as much as 50%, the total time involved is still appreciable. This third method has the distinct disadvantage of making mandatory the use of the casing annulus for the tracer fluid since the large volume tubing would have too high aradioactive background should the stream therein be tagged. Thus, the initial displacement time through the casing annulus would be increased as compared to tagging the tubing. Although each of the three methods reduce the survey time under certain condiitons, none of the above three methods are always applicable nor do they reduce the time required to that reasonably desired to complete a survey, that is, within one day and preferably within a few hours.

From the above statements, it is apparent that the major time consuming elements of the injectivity profile logging technique as practiced heretofore and as described in the above-mentioned U.S. patent are: (l) the tracer displacement time and (2) the interface movement time. A further analysis of this technique shows that once a given annulus-tubing rate has been set, a stable interface between the two fluids is formed regardless of the location of the tracer. When both the annulus and the tubing are full of fluid the interface forms immediately. When an injection fluid level exists in the well there may be a short interval of time for the fluid levels in the annulus and tubing to adjust after the rates have been changed.

In accordance with the present invention, the injectivity technique heretofore employed has been improved so as to eliminate or at least to greatly decrease the displacement and tracer interface travel times while still maintaining the inherent advantages of the known injectivity techniques. In the practice of oneaspect of the present invention the tracer material is introduced into one of the injected streams in the borehole in the vicinity of the borehole interval which is to be logged. In carrying out the invention an exploring unit including means for emitting, continuously, intermittently, or at will, a tracer material into a borehole stream and means for detecting the presence of the tracer material in the stream is suspended in the borehole between the wall of the borehole and a tubing disposed therein. In the case of a radioactive tracer material, radioactivity detectors are located preferably above and below the tracer emitting means so that when the tracer material is emitted into the annulus stream the direction of movement of that stream, that is, upwardly or downwardly, is ascertained by noting which detector indicates an increase in radioactivity.

For a better understanding of the invention, reference may be had to the accompanying drawing in which:

FIG. 1 is a vertical sectional view through a portion of a borehole traversing a permeable formation showing the V arrangement therein of apparatus including an exploring unit used in accordance with this invention.

FIG. 2 illustrates a slightly modified exploring unit which may be substituted for the exploring unit illustrated in FIG. 1.

FIG. 3 illustrates another modified exploring unit which may be substituted for the exploring unit shown in FIG. 1, and

FIG. 4 is a vertical sectional view through a portion of a borehole traversing a permeable formation showing a modification of the apparatus illustrated in FIG. 1.

Referring to the drawing and particularly to FIG. 1, there is shown a borehole 10 traversing a permeable formation 12 the permeability profile of which is to be determined. The upper portion of the borehole 10 is lined with a casing 14 having aclosed casing head 16 at the surface. A string of tubing 18 extends downwardly into the borehole to a depth below the formation 12. 7

At the surface a first inlet pipe 20 is connected through ing 18 and the casing 14. In the pipe 20 is a pump 22 and a flow meter 24. Likewise an inlet pipe 26. is-connected to the interior of the tubing 18. In this pipe 26 is a pump 28 and a flow meter 30. The pumps .28 and 22 control, respectively, the flow rates of a firstfluid 32 which flows downwardly through the annulus between the tubing 18 and the wall of the borehole 10 and a second fluid 34 which flows downwardly through the tubing 18. Under these conditions there is a depth or location in the annular space at which the first and

second fluids

32 and 34 form an interface 36. This interface 36, as is well known, can be moved up or down along the wall of the formation 12 by changing the flow rates of the two

fluids

32 and 34 while maintaining constant the sum of the flow rates thereof. By locating the interface 36 for various ratios of flow rates of the two fluids and by noting the change in the flow rates of at least one of the two fluids, the injectivity profile of a given formation can be readily determined, as disclosed in the above mentioned patent.

Two pumps 22 and .28 have been illustrated in FIG. l

of the drawing but it should be understood that one common pump or pressure source for both annulus and tubing fluids may be used when the annulus and tubing fluids are like fluids. divided into two separately metered streams by the use of a two-way proportioning valve, thus to more easily maintain a constant total injection rate.

In accordance with an important aspect of this invensupported by a cable 40 and preferably further includes a first detector 44 located a short distance above the tracer emitter 42 and a second detector 46 located a short distance below the tracer emitter. A suitable shield 48 is disposed between each detector and the tracer emitter 42. The exploring unit 38 may alsoinclude an electronic section 50 wherein the output signals from

thedetectors

44 and 46 may be amplified before they are transmitted through cable 40 to the recorder 52 located at the surface. The location of the exploring unit 38 may be continuously recorded by a cable measuring device 54 emitted therefrom will be carried downwardly with the annulus fluid and will be detected by the lower radiation The pumped fluid can be subsequently.

detector 46. As the source is lowered the lower detector 46 will indicate a fairly sharp cut oil in radioactivity in the annular space surrounding this detector after the tracer emitter 42 is lowered to a point below the interface 36, since the radioactive substance from the emitter 42 in the borehole fluid no longer moves toward and past the lower detector 46 but only away from the lower detector 46. The decrease in radioactivity in the lower detector 46 will correspond with an increase in radioactivity in the upper detector 44 since the tracer material is now moving toward the upper detector 44. Accordingly, it can be seen that an indication of a change in radioactivity in either or both of the

radiation detectors

44 and 46 is an indication of the location of the interface in the annulus. Since the radioactive substance is introduced into one of the two streams at a point relatively near the detectors, interfaces for a large number of rate percentages can be established in a relatively short time since the tracer need move only a very short distance for detection.

By modifying the exploring unit illustrated in FIG. 1, the amount of tracer injected into the stream may be very small. Such a modified exploring unit is illustrated in FIG. 2. This modified unit 38a may be supported by the cable 49a and may include a tracer emitter 42a, upper and

lower detectors

44a and 46a, shields 48a and an electronic section 50a. Additionally, the modified exploring unit is provided with a borehole fluid conduit 56 having upper and lower fluid ports 58 which preferably are constructed in the form of scoops or funnels so as to encourage fluid movement through the conduit 56. The borehole fluid conduit 56 is arranged and adapted to permit a portion of the fluid in the annulus to move. past the tracer emitter 42a and the upper and

lower detectors

44a and 46a. Since only a portion of the annular stream passes through the conduit 56, a smaller amount of tracer material need be emitted into the fluid passing through the conduit since the dispersion of the tracer material will be relatively small. As shown in FIG. 2, the modified exploring unit 38a may also include a signal transmission conduit 60 in order to conveniently transmit the signals from the lower detector 46a to the electronics section 50a.

Another exploring unit which is a modifiication of the exploring unit illustrated in FIG. 1 f the drawing, is the exploring unit illustrated in FIG. 3. This exploring unit 3812 may be supported in a borehole by a cable 4% and may include a single detector 44b located between an upper tracer emitter 42b and a lower tracer emitter 420. A shield 48b is located between each tracer emitter and the detector 44b. An electronics section may be disposed at the upper end of the exploring unit 3815. When using the exploring unit 38 illustrated in FIG. 3 the direction of the stream flow is established by determining which tracer emitter causes an increase in radioactivity at the detector 42b. The source of the tracer material in th fluid at the detector 44b may be identified by, for example, controlling at the earths surface the rate of injection of the tracer material into the borehole fluid from the emitters. Consequently, for a first interval of time tracer material may be emitted from only one of the two emitters and then for a second interval of time from the other emitter. The location of the interface is determined by moving the exploring unit through the annulus until it is noted that the source of the material at the detector 44b has changed. If desired a borehole fluid conduit similar to the conduit 56 illustrated in FIG. 2 of the drawing may be included in the exploring unit 38b of FIG. 3.

Regardless of which of the three exploring units described hereinabove is used the injectivity profile of a given formation or Zone is determined in the usual manner of interface depth versus rate percentage or volume of flowing fluid as described hereinabove.

The tracer emitter may include a radioactive substance consisting of a solution of radioactive iodide 131 enclosed by a semi-permeable membrance so as to continuously release the radioactive substance into the borehole fluid. Alternatively, the tracer emitter may be in the form of an electrode which is connected to a suitable source of potential producing a flow of current through the electrode in order to release a radioactive tracer material from the electrode by electrolytic dissociation thereof into the borehole fluid, as described more fully in US. Patents 2,658,- 725 and 2,659,046. When using the electrode type of tracer emitter the amount of tracer can be readily c I1- trolled from the earths surface by controlling the flow of current through the electrode. A suitable electrode may be made out of copper or lead in which radioactive phosphorus has been dissolved Alternatively, a radioactive isotope may be produced in situ by exposing a non-radioactive element to thermal neutrons. By producing the radioactive isotope in the borehole near the zone or formation of interest a tracer having a short half-life of the order of minutes may be utilized. More specifically, a non-radioactive selenium or magnesium source may be placed in the borehole so as to capture thermal neutrons from a neutron generator to produce radioactive selenium Se having a 17 minute half-life or radioactive magnesium Mg having a 10.2 minute halflirfe, respectively. The source of thermal neutrons may be a known neutron generator, for example, a radium-beryllium or pol'oni-um-beryllium neutron generator surrounded by a suitable moderator employing, for example, hydrogenous material.

PEG. 4 illustrates an other arrangement of the apparatus which may be used in accordance with the method of this invention. Parts in FIG. 4 which are similar to parts in FIG. 1 have the same reference numeral. In the arrangement of FIG. 4, a fluid radioactive substance is preferably stored in a tank 62 at the surface of the earth and introduced into a stream in the borehole in the vicinity of the formation or zone to be logged by means of a pump 64 and a vertically moveable string of small tubing 66. A radiation detector 440 is mounted on the small tubing 66 a short distance above the outlet thereof, however, a detector may also be disposed below the outlet and preferably suspended from the small tubing 66. The radiation detector 44c is preferably shielded from the radioactive substance passing through the small tubing 66 and "from the radioactive substance at the lower end of the tubing. The location of the detector 440 is determined by measuring the length of the nroveable string of small tubing 66 in the annular space in the borehole. With the lower end of the small tubing 66 and the detector 44c disposed below the interface 36, as shown in FIG. 4, the radioactive substance will flow upwardly with the fluid and surround the radiation detector 440 causing a strong output signal to be produced in the detector 440. Wh n the small tubing 66 is raised so that the lower end thereof is above the interface 36, the radioactive substance will no longer pass upwardly with the fluid but rather will pass downwardly into the permeable formation and thus away from the radiation detector 44c. Consequently, when the lower end of the small tubing 66 is raised above the inter- :face 36 the response from the radiation detector 440 will be substantially less than when the lower end of the small tubing 66 is below the interface 36. By raising and lowering the small tubing 66 and by observing the response of the radiation detector 44c the exact location of the interface can be determined. It can be seen that in this embodiment of the invention the rate of injection of the radioactive substance into the borehole fluids can also be easily controlled, and that the output from the radiation detector 440 can also be transmitted to a surface recorder 52a through a suitable cable 40c which passes over a cable measuring device 54a.

Although it has been mentioned hereinabove that the invention is of particular importance in injectivity pr file logging when low rates of injection into a permeable formation are encountered it should be understood that the invention may also be practiced when high injection rate surveys are made.

, The invention as thus far described relates to the location of a single interface in the borehole. However, it should be understood that the invention is not limited to the single interface method of determining subsurface formation permeabilities. Another method contemplated within the scope of this invention is the two interface method of obtaining a permeability log, as described in a copending application of A. S. McKay, filed December 23, 1957, and having Serial Number 704,814. In the two interface method three streams of fluid are introduced into the borehole forming two interfaces therein, the rate of flow of the stream introducing the fluid into the subsurface formations between the twointerfaces is generally held constant throughout the survey while the ratios of the flow rates of the other two streams are varied, and the sum of the flow rates of the three streams is' always held constant. Each of the two interfaces formed in the prac: tice of this method may then be located in accordance with the teachings of this invention for each ratio of flow rates of the two variable streams.

Furthermore, the method and apparatus of this invention may be employed when it is desired to selectively treat a specific zone or subsurface formation by utilizing the three fluids or two interface technique described in the copending application of Alexander S. McKay and Edmond F. Egan having Serial No. 456,002, filed September 14, 1954, now US. Patent No. 2,869,642.

It should be also understood that this invention is not limited to the use of the emission of a radioactive substance from the tracer emitters. Any substance which may be detected in a stream may be utilized. A nonradioactive tracer, for example, a concentrated brine which would alter the resistivity of the fluid passing through the conduit 56 could be used and this tracer could be detected by a suitable resistivity indicating device. Furthermore, the exploring unit may comprise a fluid movement detector which may be electrically or mechanically sensitive to the fluid movement and disposed in the annular space between the wall of the borehole 10 and the tubing 18 to locate the interface between the two fluid streams. These fluid movement detectors maybe in the form of torsion wires, impellers or fluid activated bridges having electrical outputs which may be transmitted to the surface recorder.

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

I claim:

'1. A method of making a log of a. permeable formation traversed by a borehole which comprises in combination the steps of simultaneously introducing into said borehole two separate fluids forming an interface therebetween at said permeable formation, introducing into one of said fluids at a first point in the vicinity of said permeable formation a tracer material, deter-mining at a point in the vicinity of said permeable formation spaced a short distance from said first point the presence of said tracer material in said one fluid, then repeatedly introducing the tracer material into one of the two fluids at different points spaced vertically from said first point and determin-v ing in the vicinity of said permeable formation for each of the different points the presence of the tracer material in said one fluid until the absence of the tracer material is noted, and repeating these operations for a dilferent ratio of flow rates of the two fluids while maintaining the total flow rate constant.

2. A method of making a log of a permeable formation traversed by a borehole which comprises in combination the steps of simultaneously introducing into said borehole two separate fluids forming an interface therebetween at said permeable formation, introducing a tracer material at a plurality of vertically spaced points in the vicinity of said permeable formation into said fluids, and'detecting the tracer material in said fluids at points spaced a short distance from said vertically spaced points for each of said plurality of points to determine the depth in the hole of said interface, varying the ratios of the rates of the two fluids introduced into the borehole while maintaining- :constant the sum of the rates of-the two fluids so as to cause said interface to move along the wall of the bore. hole to another depth, again determining the depth of the interface and repeating these operations while noting the distance moved by the interface for each change in the ratio of the rates of the two fluids introduced into the,

borehole.

3. A method of makinga permeability log of a permeable formation traversed by a borehole which comprises in combination the steps of simultaneously introducing into said borehole two separate fluids forming an interface T therebetween at said permeable formation, introducing into one of said fluids at a first point in the vicinity of said permeable formation a radioactive substance, determining at a point in the vicinity of said permeable formation spaced a short distance from said first point theamount of radioactivity in said one fluid, then repeatedly introducing the radioactive substance into one of the two fluids at different points spaced vertically from said first point and determining in the vicinity of said permeable formation for each of the different points the amount of radioactivity in said one fluid until a change in'the radio-, activity in said one fluid is noted, and repeating these operations for a different ratio of flow rates of the two fluids while maintaining the total flow rate constant.

4. A method of making a log of a permeable formation traversed by a borehole which comprises in combination the steps of simultaneously introducing into said borehole two separate fluids forming an interface thcrebetween at said permeable formation, introducing into said fluids at a plurality of vertically spaced points in the vicinityvof said permeable formation a radioactive substance, and detecting the radioactivity at points in said fluid spaced a" short distance from each of said plurality of points to' determine the depth in the borehole of said interface, I

varying the ratio of the rates of the two fluids introduced into the borehole While maintaining constant the sum of the rates of the two fluids so as to cause said interfaceto move along the wall of the borehole to another depth, again determining the depth of the interface and repeating these operations while noting the distance moved by theinterface for each change in the ratio of the rates .of the two fluids introduced into the borehole.

5. A method of making a log of a zone of interest in a borehole traversing subsurface formations which com- 7 prises introducing a first stream into the borehole from above said zone, introducing a second like indistinguishable stream into the borehole from below said zone so as to form an interface in said zone between said two streams, altering the character of one of said streams commencing at a point adjacent the zone of interest to make said stream distinguishable and detectable, detecting in the. borehole at a known point in the vicinity of said zone of interest the presence of said altered stream, and simultaneously detecting in the borehole at another known point in the vicinity of said zone of interest the presence of the othe of said two streams.

6-. A method in the making of a permeability log of a jacent said first point, moving the point of introduction 7 9 of said radioactive tracer material and said fixed detect-ion points to a new elevation adjacent said subsurface formation, and [repeating said radioactive tracer introduction and said detections at said new elevation.

References (Iited in the file of this patent UNITED STATES PATENTS 2,453,456 Piety Nov. 9, 1948 10 Carpenter -June 10, 1952 Craggs Nov. 11, 1952 Egan et a1 Jan. 25, 1955 Pi-rson Jan. 3 1, 1956 De Witte Nov. 19, 1957 Nestle J an. 13, 1959 McKay et a1 Ian. 20, 1959

Claims

5. A METHOD OF MAKING A LOG OF A ZONE OF INTEREST IN A BOREHOLE TRAVERSING SUBSURFACE FORMATIONS WHICH COMPRISES INTRODUCING A FIRST STREAM INTO THE BOREHOLE FROM ABOVE SAID ZONE, INTRODUCING A SECOND LIKE INDISTINGUISHABLE STREAM INTO THE BOREHOLE FROM BELOW SAID ZONE SO AS TO FORM AN INTERFACE IN SAID ZONE BETWEEN SAID TWO STREAMS, ALTERING THE CHARACTER OF ONE OF SAID STREAM COMMENCING AT A POINT ADJACENT THE ZONE OF INTEREST TO MAKE SAID STREAM DISTINGUISHABLE AND DETECTABLE, DETECTING IN THE BOREHOLE AT A KNOWN POINT IN THE VICINITY OF SAID ZONE OF INTEREST THE PRESENCE OF SAID ALTERED STREAM, AND SIMULTANEOUSLY DETECTING IN THE BOREHOLE AT ANOTHER KNOWN POINT IN THE VICINITY OF SAID ZONE OF INTEREST THE PRESENCE OF THE OTHER OF SAID TWO STREAMS.