DETECTION OF WATER FLOW OUTSIDE A WELL CASING

Abstract: A well tool (14) is positioned in a borehole at a depth to be investigated. The tool houses a neutron source (36) which irradiates the borehole environs with neutrons of sufficient energy to cause the nuclear reaction O¹⁶ (n,p) N¹⁶ . Three gamma ray detectors (34,35,39) are spaced longitudinally from the source and from one another and detect gamma rays resulting from the decay of the unstable isotope N ¹⁶. A pulse height analyser (85) makes individual counts of the three sets of detected gamma rays, and the count signals are utilized in predetermined relation­ ships to provide a signal representative of the linear flow rate of an undesired water flow outside said casing in a direction from said source towards the detectors. The provision of three detectors enables the linear flow rate signal to be obtained without the necessity of obtaining counts of background gamma radiation.

Description: The present invention relates to well logging methods in general and, more particularly, to radio active well logging methods. A method for providing a signal representative of undesired water flow outside a well casing in a producing well includes locating a well tool sized and adapted for passage through a well bore having a source of high energy neutrons at least some of which have sufficient energy to cause a nuclear reaction O¹⁶ (n,p) N¹⁶ and at least three (3) gamma ray detectors spatially arranged in a predetermined manner with said source and each other in a well bore at a depth to be investigated. The environs of the borehole is irradiated with high energy neutrons from said source. The gamma ray detectors detect gamma rays resulting from the decay of the unstable isotope N¹⁶ comprising a portion of any undesired outside casing water flow present. Count signals representative of the detection are generated. The count signals are combined according to a first predetermined relationship to derive at least two net count signals for two of the three detectors. The net count signals are combined according to a second predetermined relationship to derive the signal representative of the linear flow of the undesired water flow behind the well casing. The object and advantages of this invention will appear more fully from a consideration of the detailed description which follows taken together with the accompanying drawings wherein one embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustration purposes only and are not to be construed as defining the limits of the invention. Figure 1 is a schematic representation of a water flow detection system in a cased well bore according to the present invention. Figure 2 is a timing diagram of the neutron burst and voltage wave forms occurring in a well logging sonde shown in figure 1. A conventional Behind Casing Water Flow (BCWF) monitor, such as of the type described and disclosed in U.S. Patents 4,032,778; 4,032,780 and 4,035,640, determines the linear flow velocity of water flowing within and/or behind casing from the ratio of the net count rates of two detectors spaced at distances S o1 and S o2 from the target of a neutron generator. The velocity v of the flowing water is then determined from the following equation (1) v= λ (S₀₂ - S₀₁)/[1n(C₁ N /C₂ N ) + 1n K] where C₁ N and C₂ N are the net count rates of detector 1 and detector 2, respectively, λ=9.7215 x 10⁻² sec⁻¹ which is the decay constant of N¹⁶ and K is a constant. The net count rates C₁ N and C₂ N are determined by subtracting the detector background count rates C₁ B and C₂ B from the measured count rates C₁ and C₂ of detectors 1 and 2, respectively, as follows. (2) C₁ N = C₁ - C₁ B (3) C₂ N = C₂ - C₂ B To obtain the net count rates, the background count rates must be either known or measured. In many instances the background count rates can be measured by using the BCWF tool in such a position that the water flows in the direction from the detector towards the neutron generator or by measuring the background count rates in a section of the well where there is no water flow. Both techniques are severely affected by instabilities of neutron generator output since background and flow monitoring are made at considerably different times and/or at different locations (formations, etc.) in the well. The former method cannot be used when water outside a casing flows simultaneously and in the opposite direction from that flowing within the casing. Therefore it appears to be of great advantage to eliminate the knowledge and/or measurement of background count rates altogether from the determination of water flow velocities. The present invention uses at least three detectors to avoid using the aforementioned techniques and hence their problems. The measured count rates C₁, C₂, C₃ for three detectors spaced S₀₁, S₀₂, S₀₃ from the neutron generator target are given by (4) C₁ =C₁ N + C₁ B (5) C₂ = C₂ N + C₂ B (6) C₃ = C₃ N + C₃ B It was determined that the ratio of C₁ B and C₂ B was constant for a variety of multiple casing configurations even though the individual count rates C₁ B and C₂ B varied widely. Therefore it follows that the ensuing relationship is valid for all detector background rates, (7) C j B = K ij B · C i B where j and i designate the various detectors, and the K i are constants which are predetermined. Substituting equation 7 with i = 1 and j = 2 and 3 into equations 4, 5 and 6 the following equations 8, 9 and 10, are obtained (8) C₁ = C₁ N + C₁ B (9) C₂ = C₂ N + K₁₂ B C₁ B (10) C₃ = C₃ N + K₁₃ B C₁ B Equation ] can be rearranged and used to express C₂ N as function of C₁ N and velocity v as follows (11) C₂ N = C₁ N . K₁₂ . e ⁻ S 12 (λ/v) where S₁₂ = S₀₂ - S₀₁ which is the differential spacing of detectors 1 and 2, and K₁₂ is a constant. An equation similar to equation 11 can be written for C₃ N and C₂ N as follows (12) C₃ N = C₂ N · K₂₃ . e ⁻ S 23 (λ/v) where S₂₃ = S₀₃ - S₀₂, which is the differential spacing of detectors 2 and 3, and K₂₃ is a constant. Substituting equation 11 into equation 9 and equation 12 into equation 10 we obtain two pairs of equations (13) C₁ = C₁ N + C₁ B (14) C₂ = C₁ N · K₁₂ . e⁻ S 12 (λ/v) + K₁₂ B .C₁ B and (15) C₂ = C₂ N + K₁₂ B C₁ B (16) C₃ = C₂ N K₂₃e⁻ S 23 (λ/v) + K₁₃ B C₁ B Equations 13 and 14 can now be solved for C₁ N and equations 15 and 16 for C₂ N (17) C₁ N = [K₁₂ B C₁ - C₂]/[K₁₂ B - K₁₂e⁻ S 12 (λ/v) ] (18) C₂ N = [K₁₃ B C₂ - K₁₂ B C₃]/[K₁₃ B - K₁₂ B ·K₂₃e⁻ S 23 (λ/v) ] Equations 17 and 18 contain now only the known and predetermined constants K ij B and K ij , the measured count rates C j , the known differential spacing, S ij and unknown water flow velocity v. The background count rates have been eliminated. Combining equations 11, 17 and 18 and rearranging the terms, we obtain the implicit function F(x) = 0 with x = 1/v. Equation 19 can be iteratively solved for x = by the method of Newton of successive interpolations. The algorithm to obtain successive approximations X r to the true value X is as follows This technique requires that a first assumption for X=X o be supplied. When F(X r ) suffices the inequality O ≦ F(X r ) ≦ δ where δ is a very small number, the value X r is assumed to be a good approximation to the true value of X=. Referring now to Figure 1, a well logging system in accordance with the concepts of the present invention is shown in a borehole environment with the surface equipment portion thereof and is illustrated schematically. A downhole sonde 14 is suspended in a well borehole 10 by an armored well logging cable 21 and is centralized by centralizers 15 with respect to the interior of the well casing 12. The cased borehole is filled with a well fluid 11. The downhole sonde 14 is provided with three gamma ray detectors 34, 35 and 39 which are shown mounted for detecting water flow in a downward direction behind the casing 12. The arrangement for detecting water in an upward direction behind casing 12 would have detectors above the neutron source as shown in our US Pat. 4032778 for such water flow. The downhole sonde 14 is also provided with a 125 KV power supply and neutron generator 36 of the type providing neutrons having an energy in excess of approximately 10 MeV. Bombardment of the stream of water flowing behind the casing with high energy neutrons causes nuclear interactions to produce the unstable nitrogen isotope 16 from the oxygen nuclei comprising the water molecules in the flowing stream, the nuclear reaction being O¹⁶ (n,p) N¹⁶. Earth formations 33, 17, 18 and 19 are penetrated by the borehole 10. A cement channel 20 on one side of the cement sheath 13 of the cased well bore is illustrated allowing undesired fluid flow in a downward direction from a water sand 17 which contaminates a producing sand 19 separated from the water sand 17 by a shale layer 18. With the well logging instrument 14 placed in the position shown and with the detector source configuration illustrated in Figure 1, the instrument 14 is capable of detecting undesired water flow from the water sand 17 through the cement channel 12 into the producing sand 19. Perfora­ tions 16 in the casing 12 allow fluid from the producing sand to enter to well borehole 10 as well as allowing the undesired water flow down the cement channel 20 to enter the borehole 10. In the configuration shown in Figure 1, high energy neutrons from the neutron source 36 penetrate the steel casing 12 and activate the elemental oxygen, as noted before, in the water flow from water sand 17 through cement channel 20. The water flowing channel 20 then continues past detectors 34, 35 and 39 sometime later and gamma rays resulting from the decay of the radioactive nitrogen 16 are detected in the manner previously described by the detectors 34, 35 and 39. Electrical pulses whose height is proportional to the energy of the impending gamma rays detected by the detectors 34, 35 and 39 are transmitted to the electronic section 37 of the downhole instrument which includes multiplexing means for multiplexing the signals from the detectors, and from there are coupled to the well logging cable 21 conductors and transmitted to the surface in a form which will be described in more detail subsequently. Referring now to Figure 2, a timing chart for the neutron burst, the sync pulse and downhole detectors 34, 35 and 39 is shown. It will be observed that a neutron burst, generally for a duration of one millisecond, is initiated . Simultaneously with the initiation of the neutron burst in the downhole instrument, a large amplitude negative polarity synchronization (or sync) pulse is generated by the electronic section 37 of the downhole instrument and coupled to the conductors of well logging cable 21. The amplitude of the sync pulse is made greater than any possible data pulse amplitude from the detectors. As can be seen in Figure 2, the signals provided by detector 34 multiplex signal, causes the data pulses provided by detector 34 to be applied to cable 21. Similarly the multiplexing signals for detectors 35 and 39 when at a high logic level causes the pulses provided by detectors 35 and 39 to be provided to logging cable 21. The signals are conducted uphole where a sync separator 60 separates the negative sync pulse and provides it to a conventional type logic module 63. Logic module 63 has internal logic elements and clocks providing the appro­ priate signals and timing pulses necessary in the operation of the surface equipment. The data pulses from detectors 34, 35 and 37 are applied to amplifiers 64, 65 and 69. Since the gain requirement differs according to the detector, amplifier 64 has a gain associated with detector 34; amplifier 65 has a gain associated with detector 35 and amplifier 69 has a gain associated with detector 39. The gains of amplifiers 64, 65 and 69 are adjustable. The amplified signals from amplifiers 64, 65 and 69 are provided to linear gates 74, 75 and 79, respectively. Logic module 63 in accordance with the sync pulse from sync separator 60 provides multiplexing signals to control gates 74, 75 and 79 so that they pass data pulses associated with detectors 34, 35 and 39, respectively to mixer router means 82. Mixer router means 82 provides the spectra information and the routing information to pulse height analyzer 85 so that all of the data pulses associated with a particular detector are entered into a different memory section of pulse height analyzer 85. Pulse height analyzer 85 provides signals representative of the count rates to logic module 63 and to computer means 88 which may be a general purpose computer of the type manufactured by Hewlett-Packard as their part 9826. Logic module 63 also provides an output to computer means 88. Computer means 88 will then solve equation 19 in an iterative manner to determine the velocity of water flow. Computer means 88 provides analog signals to strip chart recorder 90 which is also receiving depth information from sheave wheel 22. Computer means 88 also receives and provides digital signals to disc memories 92 and digital signals to print/plot means 93.

Claims: 1. A method of providing a signal representative of the linear flow rate of an undesired water flow outside a casing of a cased well borehole, comprising: positioning a well tool in the borehole at a depth to be investigated, said well tool having a source of neutrons of sufficient energy to cause the nuclear reaction O¹⁶ (n,p) N¹⁶ and gamma ray detector means spaced longitudinally from said source; irradiating the borehole environs with neutrons from said source; detecting at said detector means gamma rays resulting from the decay of the unstable isotope N¹⁶ and generating count signals representative of the detected gamma rays; and utilizing said count signals in predetermined relationships to provide said signal representative of the linear flow rate of an undesired water flow outside casing in a direction from said source towards said detector means; characterised by: providing said well tool with first, second and third said gamma ray detectors spaced longitudinally from said source and from one another in said well tool; said detecting step comprises detecting the gamma rays at each of said first, second and third detectors and generating respective first, second and third said count signals; and said utilizing step comprises utilizing said first, second and third count signals in predetermined relationships to provide said linear flow rate signal without the necessity of obtaining counts of background gamma radiation. 2. A method according to claim 1 characterised by: removing the well tool from the borehole after said irradiating and detecting steps at said position; reversing the longitudinal juxtaposition of said source and of said detectors; repositioning the reversed juxtaposition tool in the borehole; and repeating the irradiating, detecting and utilizing steps so as to provide said linear flow rate signal of an undesired water flow outside said casing in a direction opposite to the first-mentioned flow direction. 3. A method according to claim 2 characterised in that said positioning step comprises positioning the tool above or below perforations in said casing and said repositioning step comprises repositioning the tool respectively below or above said perforations. 4. A method according to any one of claims 1 to 3 characterised by performing said irradiating and detecting steps while moving said well tool at a predetermined velocity past the depth to be investigated. 5. A method according to any one of claims 1 to 4 characterised in that said source of neutrons is a pulsed source, said irradiating step comprises repetitively irradiating the environs with bursts of fast neutrons from said source. 6. A method according to claim 5 characterised in that said detecting step comprises detecting the gamma rays at each of said first, second and third detectors substantially in time intervals between successive said bursts of neutrons. 7. A method according to claim 6 characterised in that said detecting step commences approximately 3milliseconds after the cessation of each neutron burst. 8. A method according to any one of claims 5 to 7 characterised in that said neutron bursts are of approximately a 1 millisecond duration and are repeated approximately 100 times per second. 9. A method according to any one of claims 1 to 8 characterised in that said detecting step is effected in the gamma ray energy range of from approximately 3.25 MeV to approximately 7.20 MeV. 10. Apparatus for providing a signal representative of the linear flow rate of an undesired water flow outside a casing of a cased well borehole, comprising: a well tool (14) for positioning in the borehole at a depth to be investigated; a source (36), in said well tool, of neutrons of sufficient energy to cause the nuclear reaction O¹⁶ (n,p) N¹⁶ in any such water in the borehole environs irradiated by neutrons from said source; gamma ray detector means (34,35,39), in said well tool, for detecting gamma rays resulting from the decay of the unstable isotope N¹⁶; means (84) for generating count signals representative of the detected gamma rays; and means (88) for utilizing said count signals in predetermined relationships to provide said signal representative of the linear flow rate of an undesired water flow outside said casing in a direction from said source towards said detector means; characterised in that: said well tool (14) is provided with first, second and third said gamma ray detectors (34,35,39); said detectors (34,35,39) arranged to detect the gamma rays at each said detector; said generating means (85) generates respective first, second and third said count signals; and said utilizing means (88) utilizes said first, second and third count signals in predetermined relation­ ships to provide said linear flow rate signal without the necessity of obtaining count of background radiation.

Inventors: PAAP, HANS JUERGEN, DR.
Applicants: TEXACO DEVELOPMENT CORPORATION
Application year: 1986
Main classification: E21B47
IPC codes: G01V5, E21B47