NO145743B - PROCEDURE AND APPARATUS FOR MEASURING UNWANTED WATER INTRODUCTION IN A DRILL. - Google Patents

Description

The present invention relates to a method

and an apparatus for determining the volume flow rate of

an unwanted flow of water behind the well casing in a production wellbore.

Unwanted liquid communication along the lined parts

in a borehole between production zones has long been a problem in the oil industry. Communication of fresh water or salt water from a nearby water-bearing sand formation to an oil production sand formation can contaminate the produced oil to such an extent that oil production from the borehole can become commercially unstable as a result of the "water influx". In wells close to the surface for use as a source of fresh water for water supplies to a city or the like, the contamination of the drinking water supply by migration of salt water from nearby stretches of sand could likewise contaminate the drinking water supply to such an extent that the water becomes unsuitable as drinking water for humans without extensive purification

Eng.

In both cases mentioned, the experience of the years has

showed that the contamination of drinking water or oil production sand formations can often occur as a result of unwanted communication with water from nearby stretches of sand along the ring between the steel pipe that is used to repel the walls of the borehole and the borehole wall itself. Steel pipes used for this purpose are usually cast in place. If a good primary cementation is achieved when developing the borehole, problems with fluid communication between production zones do not arise.

However, in some areas with very loosely consolidated, highly permeable sand formations which are typical oil production areas, the sand formations may later collapse near the borehole, even if satisfactory primary cementing has been carried out. This can enable the migration of water along the outside of the concrete skin from nearby water-bearing sand areas to the production zone. The problem of unwanted fluid communication also occurs when the primary cementation breaks down as a result of fluid flows in the vicinity. In a similar way, an otherwise good cementation may contain longitudinal channels or voids, which allow unwanted fluid communication between nearby hydrous sand areas and the production zone.

Another problem that can cause unwanted water communication along the borehole between oil production zones and nearby water-bearing sand areas is the so-called "micro-ring" between the pipe and the cement. This phenomenon occurs because the pipe, when the cement is pushed from the bottom of the pipe string up into the annular space between the pipe and the formations (or through pipe perforations), is usually exposed to a high hydrostatic pressure differential in order for the cement to be pressed into the annulus.

The high pressure differential can cause pipe expansion. When this pressure is later weakened by production from the borehole, the previously expanded pipe can contract from the cement mantle that has formed around it in the annulus between the pipe and the formations. This contraction can leave a void between the pipe and the cement mantle, which can be called a microring. In some cases, if there has been a strong pipe expansion during the 'primary cementing (e.g. in a deep well, where a high hydrostatic pressure is needed) the pipe can contract from the cement mantle, leaving behind a microring which is wide enough for fluid to communicate from nearby water-bearing sand formations along the microring and into the production perforations, creating an unwanted water influx.

So far, a number of attempts have been made to calculate and locate the occurrence of cement channels. A number of attempts have also been made to locate and confirm the occurrence of so-called microring-fluid communications. It is perhaps primarily about the use of acoustic cement adhesion measurement. With such a measurement, the amplitude of an acoustic wave energy that is propagated along the pipe from an acoustic transmitter to one or more acoustic receivers is examined. If the pipe is firmly bound to the cement and the formations, the acoustic energy that is propagated along the pipe will in principle radiate outwards from the pipe into the cement and the surrounding formations and thereby reduce the amplitude of the signal. But if a pipe is poorly bonded to the cement or this is poorly connected to the formations, there will be a cavity and the acoustic energy should remain in the pipe and be received by the acoustic receivers with far greater amplitude than when there is a good connection between the pipe, the cement and the formations.

However, acoustic cement adhesion measurement cannot always reliably record the presence of a microring which in some cases enables unwanted fluid communication between water-bearing sand formations and nearby production zones. If the microring is small enough and fluid-filled, the acoustic energy propagating along the tube can be coupled across it. It has nevertheless been shown that even such a small microring can enable unwanted liquid communication between production zones. Similarly, poor cementation may remain undetected using the acoustic measurement method, if the cement mantle is penetrated by various channels or voids asymmetrically arranged on the surface of the mantle. Such channels or cavities can enable unwanted liquid flow, although the main concrete body is well bound to the pipe and the formations and thus allows the acoustic energy to pass satisfactorily from the pipe, out through the cement and into the formations. Such methods capable of acoustic cement <1>bond measurement have thus proven not to be fully satisfactory for recording potential, unwanted fluid communication paths in a developed borehole.

Another known method for locating cavities or channels in the cement mantle is to inject radioactive tracer substances such as iodine 131 or the like, through production perforations to the production formations and into any cavities in the ring surrounding the drill pipe. The theory behind this method is that if the tracer material can be forced back along the flow path of the unwanted liquids, its radioactive properties can then be recorded outside the pipe by radiation detectors. However, this type of measurement in boreholes has usually proved unsatisfactory, especially in loosely consolidated sand formations where unwanted fluid communication is likely to occur.

In particularly permeable formations, such as loosely consolidated sands, the production formation itself can absorb most of the radioactive tracer material that is forced through the perforations. Very little, if any, tracer material can be forced back along the unwanted fluid flow path, especially if this means that the tracer flow must be forced either against the formation fluid pressure or upward against gravity. Therefore, such tracer measurement techniques for recording cement channels or voids outside the drill pipe have usually been found to be ineffective.

The previous attempts can generally be characterized as attempts to investigate the cement mantle. The present invention relates to a method and an apparatus for recording the unwanted water flow itself in cement channels or cavities outside the pipe in a production well. The characteristic features of the invention appear from the claims.

The core engineering measurement technique used according to

the invention comprises activation with high-energy neutrons of oxygen element nuclei which comprise a portion of the unwanted water flow itself. A high-energy neutron source is placed in the borehole in the area to be examined for cement channels or unwanted fluid communication along the mantle. A source for approximately 14 MeV monoenergetic neutrons is used to irradiate the area with such neutrons. An oxygen 16 nucleus becomes after capturing an approx. 10 MeV neutron converted into radioactive nitrogen 16. The radioactive nitrogen 16 disintegrates with a half-life of approx. 7.1 seconds by emission of a beta particle and high-energy gamma radiation with an energy of approx. MeV or more. At a sufficiently high flux of 10 MeV neutrons that irradiate the unwanted water flow in a concrete cavity or a micro-ring channel, sufficient radioactive nitrogen 16 is formed in the unwanted water flow itself to be detectable by means of a pair of detectors which are arranged at a distance from each other in the longitudinal direction. This measurement can be used directly as an indication of the speed of the water flow in the cement channels. New techniques have also been developed for determining the flow volume velocity of water in such cement channels, microrings or cavities by degradation of the high-energy gamma ray spectrum by Compton scattering of gamma rays formed by the disintegration of

the radioactive nitrogen 16. The approximate distance from a single gamma ray detector to the center of the water flow path can thereby be determined.

A further feature of the invention is that by using a pulsating, instead of continuous, neutron source for the mentioned measurements, a more accurate flow registration can be achieved in that the background gamma radiation, which is caused by relatively prompt thermal or epithermal mutual neutron influence in the vicinity of the borehole, is reduced.

By first placing a pair of high-energy gamma radiation detectors spaced apart in the longitudinal direction above and then below the neutron source, one can further distinguish between liquid flow inside and outside the pipe using only a few relatively durable assumptions. In a further feature according to the invention, registration of unwanted liquid flow in a production zone under production conditions is made possible by using a water flow detector device with a small diameter, which is dimensioned so that it can be passed through production pipelines and which is operated according to the same flow measurement principles as mentioned above . According to the invention, a technique has also been developed to record unwanted fluid flow behind the pipeline in the same direction as the desired fluid flow in a nearby production pipeline that passes through a production zone that is being investigated for channels in a complete multibore pipe. In this case, both the water flow in the production pipeline that passes through the examined zone and the unwanted water flow in concrete channels or cavities outside the pipe can go in the same direction and can be equally recorded separately. In other techniques which make use of the new features according to the invention, operations are provided for separating water flows in and out of the pipe on the basis of the direction of flow during measurement.

The invention will now be described in more detail with reference to the drawing, where

fig. 1 schematically illustrates the geometry of a single detector water flow probe,

fig. 2 represents the schematic geometry of a dual detector water flow probe,

Fig. 3 is a graphical representation of the response in a water flow measurement system with a continuous neutron source during flow and without flow, Fig. 4 is a graphical representation of the response in a water flow measurement system with a pulsating neutron source during flow and without flow, Fig. 5 is a graphical representation showing gamma radiation spectral degradation, when the gamma radiation source is moved to a different distance from the detector, Fig. 6 is a graphical representation showing the pulse-number ratio at two detectors at a mutual distance as a function of the distance, Fig. 7 is a schematic cross section of a double, concentric cylinder gamma radiation detector, Fig. 8 is a graphical representation of the response of the detector according to fig. 7 as a function of the detector's distance from the gamma radiation source, Fig. 9 is a schematic representation of the lower part of a module water flow detection probe according to the invention, Fig. 10 is a schematic representation of a water flow detection system in a drill pipe according to the invention, Fig. 11 is a schematic diagram illustrating the time management and data transfer of the water flow measurement system according to the invention,

Fig. 12 is a schematic diagram illustrating

a technique for water flow recording in a production borehole and

Fig. 13 is a schematic diagram illustrating a water flow detection technique in a multi-production pipe.

Before giving a detailed description of the component systems used to measure the water flow rate outside the pipeline according to the present invention, it will be useful to look at the theoretical basis for the measurement according to the principles of the invention.

The techniques according to the present invention are based on the formation of the unstable radioactive isotope nitrogen 16 in the water stream that flows outside the pipeline and must be recorded. This is achieved by bombarding the flowing water with high-energy neutrons with energy in excess of approx. 10 MeV. This bombardment can cause the formation of mutual nuclear interaction between the unstable nitrogen isotope 16 and the oxygen nuclei forming the water molecules in the flow, whereby the nuclear reaction is 0"*" (njpJN1^.

In fig. 1 we imagine a liquid-tight borehole probe 14 containing a 14 MeV neutron generator 11 and a gamma radiation detector 12. The center of the gamma radiation detector 12 is located S mm from the center of the neutron source 11. We further imagine a channel of water 13 which flows parallel to the axis of the probe 14 and whose center is located R mm from the center of the probe 14, and which flows from the neutron source 11 towards the detector 12. It can be shown that C, the counting rate as a result of the disintegration of the induced radioactive nitrogen 16 activity recorded by the detector 12 is given by equation 1.

where V = water flow volume (mm^/sec.)

\ = 0.0936 sec = the decay constant for N^"

a = the effective irradiation length of the water flow, when

this passes the source (mm)

b = the effective detection length of the water flow, when

this passes the detector (mm)

v = the linear speed of the water flow (mm/sec.)

øn = the neutron output from the source (neutrons/cm/sec.)

G = a geometric and power factor of the detector

K(R) = a function depending on the distance R (mm) from the center of the probe to the center of the water flow

S = distance between source and detector (mm)

£Q = (a constant) = NQP 6 a/MXb, where NQ is Avogadro's number, M = the molecular weight of water, P is the density of water and 6 is the microscopic cross section of oxygen for neutrons - Equation 1 can be rewritten as follows:

where f = E ø G.

Wed

The quantities S, a and b are characteristic of the water flow probe 14 and are measurable or calibratable quantities. ZQ is characteristic of the physical properties of water, the water flow probe, and the 0<16>(n,p)N<16> reaction and can also be measured. If the source and detector geometry are fixed and the neutron output is fixed, equation 2 will indicate that C/V for a given value of R is a function of v, the linear flow velocity of the water, and not a function of the geometry of the water flow (ie, the ring size, the cement channel etc.).

With reference to fig. 2, a second double detector measuring probe 24 is assumed which contains a 14 MeV neutron generator 21 and two gamma radiation detectors 22 and 25, arranged Sl and S2 mm from the center of the neutron source 21. Between the source and the detector, a shielding material 26 is arranged here. With reference to equation 2, the counting rate ratio recorded in the detectors 22 and 25 can be expressed as follows:

When equation 3 is solved for v, the linear flow velocity, it appears that:

In Equations 3 and 4, X = 0.0936 SEC 1 , S2-S^ is a known physical dimension of the probe 24 and C-^ and C2 are the measured count rate quantities. Equation 4 then states that the linear flow rate can be determined without knowledge of the geometry of the flow or the distance R, measured from the center of the probe to the center of the water flow 23.

When measuring the water flow (in or) behind the pipe, it is the volumetric flow rate V rather than the linear flow rate v that is the quantity of primary interest. If the flow volume rate V can be accurately determined, it can be decided whether a cement injection (or improvement of the casting work) must be carried out so that fluid communication between fresh water sand formations and oil-producing formations is prevented. It is therefore necessary to set v, the quantity that can be determined on the basis of equation 4, in relation to V using a parameter that can either be measured or estimated with an acceptable degree of accuracy. The parameter R, the distance between the center of the probe and the center of flow, can be used for this purpose.

It is assumed that the neutron flux, which irradiates a given, growing volume of water, decreases in intensity as a function of l/R 2 when the increase in volume is moved a distance R from the source. In a similar way, it is assumed that the radiation registered by the detector decreases as a function of l/R 2 when the distance R is increased from the detector.

Based on the two assumptions mentioned above, the term K(R) in equation 2 can be expressed as follows:

where P is a calibration constant. Equation 6, which appears in this way, is only an approximate equation, based on the above assumptions. Laboratory experiments have, however, confirmed that equation 6 for approximation is representative of the K(R) behavior of the function.

Using equations 6 and 2, we can write down the flow volume velocity V as follows:

where Q = p.f1 and i = 1 or 2 (representing the dual detectors). Equation 7 states that if v is obtained from equation 4 and R is known or can be estimated, the flow volume V can be obtained from the count rate recorded in one of the detectors 22 or 25 (1 or 2 in equation 7) using the corresponding value for Si. Two separate techniques for determining R will be discussed in more detail.

The above discussion has illustrated that by using a borehole measuring probe containing a 14 MEV neutron source and two gamma radiation detectors, the linear flow velocity v can be obtained independently of the flow geometry and the position of the moving water, if the water flow runs parallel to the axis of the measuring probe. Similarly, theory has suggested that the flow volume V can be obtained if the distance from the center of the probe to the center of the water flow can be measured or estimated with acceptable accuracy. In the case of a water flow in a concrete channel or ring, an assessment of R that lies in

the concrete jacket surrounding the pipeline, be reasonable.

When assessing the applicability and limitations of water flow registration behind a pipe, it is necessary to examine the accuracy with which v can be measured. When we remember that equation 4 is used to calculate v and that equation 4 contains C-^/ C2, which is the recorded counting rate in the nearest and farthest detector of a flow detection system as illustrated in fig. 2, it must be noted that the ratio C^/C2 is associated with an inherent statistical error, the nuclear disintegration process of the nitrogen 16 isotope being of a statistical nature. This statistical error in C1/C2 is an inverse function of the magnitude of C1 and C^. The error in the ratio C3/C,, is therefore affected by any parameter that affects the magnitude of C-^ and C2- Such parameters as the source-detector distances S^ and S2, the distance R from probe center to flow center, the cross-sectional area F of the flow, the efficiency of the gamma radiation detectors G, the counting time interval T, the neutron flux output øn and the bakgruhns gamma radiation counts recorded in the absence of currents can all affect the measurement. It should be noted that although most of these parameters do not appear directly in equation 4 and therefore do not affect the magnitude of v, they do affect the accuracy and precision with which v can be measured.

In fig. 3 shows a typical set of gamma radiation energy spectra recorded with and without water flow. The intensity of the recorded gamma rays at a single detector arranged at a distance from the radiation source is plotted as a function of energy in fig. 3. 7.12 and 6.13 MeV gamma-ray photopeaks characteristic of the N-y disintegration and their corresponding avoidance peak pairs are well defined under flow conditions. In conditions without flow, some pointed structure is also seen. This is due to the activation of oxygen 16 in the formation and borehole near the source and is registered by the detector even at a distance of 863.6 mm, which is used for data in fig. 3 and 4. This background spectrum also contains radiation from thermal neutron capture gamma rays from the formation, pipe and probe. It will be seen that this source of background radiation can be eliminated by pulsing the neutron source in the manner described below.

Most of the gamma radiation due to the prompt neutrons will occur within a millisecond after a neutron pulse<1> ceases. If the neutron source e.g. is turned on for one millisecond and the gamma radiation detection is delayed for 3 milliseconds. after the neutron burst before the detectors are activated, gamma radiation caused by prompt neutrons will disintegrate to a negligible level. By then counting the gamma radiation induced by oxygen activation that remains for approx. 6 milliseconds, the relatively high level of background radiation, as illustrated in fig. 3, be significantly reduced. This entire pulse-delay count cycle is then repeated approx. 100 times per second. It may of course be desirable for other reasons to operate the neutron source continuously and this is possible, as shown in fig. 3, but a greater background counting rate will then be induced.

Although the power cycle of the neutron source during pulsed operation is only 10% in this mode of operation, the neutron output, when the source is on, is approximately a factor of 10 times greater than the continuous neutron output, if the source is operated continuously. The integrated neutron output is thus approximately the same in pulsating or continuous operation. When pulsing, the detectors' power cycle is approximately 60% (i.e. 6 to 10 milliseconds). If the count-acceptance energy window illustrated in fig. 3 (about 4.45 'MeV to about 7.20 MeV) used for continuous operation were used for pulsed operation, the net count rate of the disintegration of the unstable isotope N"^ would be reduced to about 60% of what is the case for continuous operation. But during pulsed operation virtually none of the prompt neutron-gamma radiation is recorded. As there is no major component of element activation radiation except that from the unstable N 16 isotope above 2.0MeV, it is possible to o extend counting -the acceptance energy window from about 2.0 to about 7.20 M^y, in pulsed operation. This change of the counting energy window thus includes additional counts from the energy-degraded Compton effect- 6.13 and 7.12 MEV gamma radiation which as a result of the oxygen activation and will therefore increase the counting rate for displacement of the losses as a result of the detectors' approximately 60% power cycle during this pulsating operation. Fig. 4 dramatically illustrates the reduced background effect when using pulsating operation. In fig. 4, the same distance between source and detector (8 63.6 mm) is used as in fig. 3, and the extended counting energy window on

the detector is used as mentioned earlier.

During pulsating operation of the neutron generator, the magnitude of the signal from the oxygen activation reaction will thus remain approximately the same, while the background radiation is significantly reduced by eliminating the registration of prompt N-Y radiation. This increase in the noise ratio of the desired count signal reduces the statistical error of the magnitude C-^/C^.

Equation 2 shows that the counting speed at a detector C varies with e s/w. This implies that the distance to the detector S should be as small as possible to maximize the counting rate C and thus reduce the statistical error in the measurement of v to a minimum. But with regard to the two detector-flow measuring probes in fig. 2, equation 4 implies that if the distance between the two detectors (S2-S^) becomes too small, v will become insensitive to the ratio of counting rates C-^/C^• It is therefore necessary to find a practical compromise when choosing detector distances S -^ and S2> so that the statistical and non-statistical errors in v are reduced to a minimum. Suitable experimental techniques have been derived for determining optimal distances and S2- These distances for typical pulsed neutron sources used in the system according to the present invention are discussed in more detail in the description of the equipment below. Although the operating theory regarding the instrumentation is still valid at other distances, those skilled in the art will understand that the distances mentioned in the description below are not self-evident without an experimental basis.

Referring to equation 7, it should be noted that it is possible to measure the volumetric rate of water flow behind the pipe, provided that a technique for determining R, the radial distance from the center of the detector to the center of the water flow, can be determined or estimated. For reasons that will become apparent from the following description, it is sometimes impossible to estimate R with the necessary accuracy in order to be able to predict the flow volume velocity V. It is, however, possible to measure R with certain techniques that will be described in more detail in the following .

The first technique for determining R can be considered a gamma radiation spectral degradation technique. In fig. 5 schematically shows two gamma radiation spectra which result from the disintegration of radioactive nitrogen 16, produced by oxygen activation with a water flow measuring probe of the type that can be used according to the present invention - The spectra seen in fig. 5 is taken on the same detector in the flow measuring probe and illustrates the counting rate in the detector, which is due to a water flow whose center is R., and R2mm from the center of the detector. The dashed curve in fig. 5 illustrates a gamma radiation spectrum resulting from the disintegration of radioactive nitrogen 16 and a water flow whose center is located at a distance R approximately corresponding to 7.5 mm from the center of the water flow probe detector. In fig. 5, R2 is thus greater than R^. The double arrows in fig. 5 also indicates two energy range counting windows A and B. Window A comprises 7.12 and 6.13 MeV photo and avoidance peaks from the radioactive nitrogen 16 which is primarily direct radiation reaching the detector without Compton scattering. Window B is a radiation energy window for the detection of gamma radiation that has primarily been degraded in energy by Compton scattering.

If CA(R) is defined as the count rate recorded

in window B for arbitrary R, it appears that:

The ratios C^/Cg in equation 8, which appear in this way, are due to the fact that a larger fraction of the primary 6.13 and 7.12 MeV gamma radiation is degraded by scattering or inelastic interaction with the intervening material when the distance R between the activated water flow and the detector is increased. By calibrating a system for water flow detection on the basis of the spectral degradation as a function of the radial distance R, a tool is provided for determining the unknown radial distance R to the center of the flow. This distance R can then in turn be used in connection with equation 7 for quantitative determination of the water flow volumetric velocity.

In fig. 6 are the results of an experimental calibration of the ratio of count rates CA/Cg/ measured under unknown flow conditions during the test as a function of R plotted with associated signs of normal deviation errors.

In fig. 6, results from a Monte Carlo computer calculation for a 6.13 MeV point gamma radiation source at different distances R from a gamma radiation detector are also plotted. The Monte Carlo calculations are based on probability theory and are used to predict the undegraded or undegraded gamma radiation flux as a function of the radial distance from the source to the detector using known laws of physics regarding the Compton effect. It should be noted that there is excellent agreement between the experimental curve and the Monte-Carlo calculations, as indicated by the data points in Fig. 6.

In the two water flow detector probes which will be discussed in more detail below, the ratio of the counting rates at the two selected energy windows CA and Cg from the nearest detector can be measured. The distance R from the center of the water flow to the center of this detector can then be determined by comparing the background-corrected count rates at these two energy windows with the relationship illustrated in fig. 6, so that R, i.e. the distance from the center of the detector to the center of the water flow, can be determined. The counting rate ratio in the closest detector is used for this purpose because this will have a greater counting rate and will thus provide greater statistical accuracy. However, it should be noted that this ratio will also apply to the A detector and, if desired, the A detector's counting speed can be used alternatively or additionally to the nearest detector's counting speed ratio for this purpose. The counting rates at the two different detectors can be used to calculate v, the linear flow rate, and by then using the relationship in equation 7, the flow volume rate V can be derived, when R has been determined in this way.

In fig. 7 and 8 an alternative technique for measuring R is schematically illustrated. In fig. 7, the cross section of a double crystal gamma radiation detector is schematically illustrated. This detector comprises an internal crystal 71 which has an approximately cylindrical shape and comprises a sodium or cesium iodide-activated detector crystal with radius r^ and length L, and which is arranged in the center of a cylindrical mantle crystal 72. The detector crystal 72 also comprises a sodium - or cesium iodide-thallium-activated crystal of the type known to those skilled in the art for the detection of high-energy gamma radiation and which has an inner radius r2 and an outer radius r^. Two separate photomultiplier tubes can be independently connected optically to the detector crystals 71 and 72 and used for independent detection of scintillation or flashes of light resulting from the mutual influence of the high-energy gamma radiation and the crystalline structure, so that two separate counting rates CQ and of the two cylindrical detectors can be recorded 7 2 and 71 respectively.

If one considers the activated water flow radioactive nitrogen 16 as a point gamma radiation source located R mm from the center detector 71, it can be shown that the ratio of counts CQ recorded in the outer crystal and C-^, counts recorded in the inner crystal, is given by equation 9:

In equation 9, K is a constant which includes a shielding effect of the outer crystal against the inner crystal for gamma ray flux. If equation 9 is numerically integrated as a function of R using the dimensions given in fig. 8, a curve is obtained like the fully drawn curve in fig. 8.

Fig. 8 illustrates a graphical rendering of the ratio Cq/Cj as a function of R, the fully drawn curve using the dimensions shown in the figure. It appears from fig. 8 that R can be obtained from the ratio Cq/C^ if this ratio can be measured with sufficient accuracy. The two dashed curves in fig. 8 includes the envelope curve for + 2% accuracy in determining the ratio C^/C^ and illustrates the fact that R can be determined with an accuracy of 12.7 mm, if R is less than or equal to 127 mm, when measuring the ratio Cq/C ^. with + 2% accuracy. If it is desired to maintain an accuracy greater than +12.7 mm when measuring R, a longer counting interval is required to achieve the ratio Cq/C-j. with greater accuracy than + 2%.

To summarize this technique for measuring the flow volume rate V, recording the flow volume rate V can be obtained from the relationship given in equation 7, provided that the water flow can either be estimated or measured by one of the techniques discussed above. The linear flow rate v is obtained as discussed above. Under certain water flow conditions, R can be accurately measured by one of the above mentioned techniques and then used to calculate V, the flow volume rate.

In some cases it may be necessary to estimate R. This

can be done by assuming that the water flow is in a channel or a cavity in the concrete ring that surrounds the pipeline outside the borehole. In such a case, R can be estimated to be 12.7 to 25.4 mm larger than the known outside pipe diameter. In this case, the water flow volume can then be similarly obtained using equation 7. In both cases, techniques for determining the linear flow rate v and a quantitative measurement of the flow volume rate V of the water in a concrete channel or an annular cavity behind the pipe in a borehole are described above. In the following sections, water flow detection systems and operational measurement techniques will be treated in more detail for use under different borehole and production conditions for detecting and measuring the water flow outside the pipeline in a borehole.

The equipment used to carry out the water flow measurements mentioned above is based on the activation of oxygen 16 nuclei by capturing neutrons with an energy equal to or greater than 10 MeV. This necessitates the use of a neutron generator that can generate sufficient intensity of neutrons with an energy of 10 MeV or more, so that the measurement can be carried out. The most commonly available neutron generators are those that rely on the deuterium-tritium reaction to generate this flux of high-energy neutrons with sufficient intensity for measurements of this type. The deuterium-tritium reaction neutron generators are usually called accelerator-type neutron sources.

Neutron generators of the accelerator type usually comprise an evacuated casing with a target material at one end which is impregnated with a high percentage content of tritium. This target material is kept at a high negative potential (approx. 128

kV) with respect to the deuterium nuclear source to be accelerated on it. At the opposite end of the evacuated container there is an ion source and a deuterium nuclear source, usually called

"replenish". During operation, such accelerator sources will generate a concentration of deuterium ions from the ion source, which are focused into a beam by electrostatic lenses and accelerated by the high negative potential towards the target material which is impregnated with tritium cores. As a result of the high acceleration voltage, the electrostatic coulomb repulsion between deuterium nuclei and tritium nuclei is overcome and the thermonuclear fusion reaction takes place during the generation of a relatively high intensity of neutrons with an energy of approx. 14 MeV.

In the construction of the equipment for carrying out the water flow measurements as mentioned above, because it is necessary to use a neutron source of the accelerator type, a problem will arise in the physical construction of the borehole part of the system. This problem is due to the fact that a high voltage supply is required to generate the approx. 125 kV potential required by the neutron source to accelerate the deuterium ions. The most efficient high voltage source of this type is probably a multi-stage Cockroft-Walton Voltage Multiplier Circuit.

A high voltage generation device such as that required by the accelerator tube, when placed in a downhole meter, requires considerable longitudinal extension to stack the voltage multiplier stages longitudinally along the length of the downhole meter while providing sufficient insulation around these voltage multiplier stages to to prevent voltage destruction of the insulators.

In fig. 9A, 9B and 9C are schematically illustrated lower probe for water flow detection measurement. The probe consists of several component sections that can be physically interchanged to perform steps in the detection of water flow behind the pipe according to the principles discussed above. The upper end of this probe is provided with a head 91 which is approx. 254 mm long. A control and detector electronics section 92 is attached to the head and is approx. 1905 mm long. The detector section 93 contains two gamma radiation detectors which may comprise thallium activated sodium iodide crystal detectors

(approx. 50.8 mm by 101.6 mm cylinders) and an iron shield which is placed at the opposite end of the neutron generator. Under the detector section in fig. 9A, the neutron generator and power supply section is arranged and comprises the neutron generator 94 and power source 95 for 125 kV high voltage. The distances which

preferred between the neutron source and the detectors in the combined device is approx. 584.2 and 1066.8 mm, respectively, as shown in fig. 9. The neutron source and power supply section is approx. 2387.6 mm long. Finally, a protective "bull plus" unit 96 is arranged at the lower end of the probe which serves to protect the lower end of the probe should it come into contact with the bottom of the borehole or an obstacle in the borehole.

The problem that arises is due to the length (2387.6 mm) of the high voltage source. Those skilled in the art will appreciate that for detection of a water flow that is directed upward, the flow must first pass the neutron source and then the detectors during its movement. This indicates the shape as shown in fig. 9B where the detector section 93 of the measuring apparatus is arranged above the high voltage supply and neutron generator sections 94 and 95. But for detection of a downward water flow, the form shown in fig. 9C where the downward water flow must first pass the neutron source and then the gamma radiation detectors, so that the flow can be measured as discussed above. In this form, the neutron source power supply section 94, 95 must be arranged above the detector section 93 in the borehole apparatus.

As the gamma detectors must be arranged within a reasonable distance from the neutron generator's target material, the tritium-impregnated target material in the neutron source 94 must be arranged as close to the shield part of the detector section 93 as possible. This requires the design of a neutron source 94 and power supply 95 section which is reversible (ie can be connected for operation at both ends) when changing from the form shown in FIG. 9B to that shown in FIG. 9C, so that both upward and downward water flows can be registered. In a similar manner, all component parts of the borehole apparatus in fig. 9 constructed in modular form. These modules can be connected by liquid-tight screwing together and sealed against penetration of borehole fluid by seals in each joint.

Lower probe which is schematically illustrated in fig. 9A,

B and C are also provided with centralizing means 97 which may comprise cylindrical rubber arms or the like which extend out into contact with the inner wall of the drill pipe when the probe is lowered for measurement. These centralizing arms 97 keep the probe body central in the tube to help maintain cylindrical symmetry of the measurements. If the probe was able to rest against the pipe side, it could fail to detect water flows on the opposite side of the pipe due to a lack of sensitivity due to the increased distance between the neutron source and detectors and the flowing water.

The electronics section 92 for the borehole probe functions as described in more detail below, for controlling the operation of the neutron source 94 and supplying high voltage for the operation of the detectors located in the probe's detector section 93. The electronics section 92 also provides synchronization pulses at the beginning of each neutron blast. The electronics section 92 further contains circuits for sending electrical pulse signals from the detectors and synchronization pulse signals to the measuring cable up to the surface.

In fig. 10 is a measurement system for boreholes according to the present invention schematically shown in a borehole together with the surface equipment. A borehole probe 104 which in practice is constructed according to the module system as shown in fig. 9A, B and C, is suspended in a borehole 100 by means of an armored measuring cable 111 and centralized by means of centralizing means 105, as previously mentioned, in relation to the drill pipe 102. The borehole, which is provided with a pipe, is filled with well fluid 101. The borehole probe in fig. 10 is provided with dual gamma radiation detectors 124 and 125, which are shown mounted in fig. 9C for recording downward water flow outside the pipe 102. The borehole probe is also provided with a 125 kV power source and a neutron generator 126 of the previously mentioned type. The electronics section 127 of the probe 104 corresponds to the electronics section 92 in fig. 9A, B and C.

Earth formations 123, 107, 108 and 109 are penetrated by the wellbore 100. A cement channel 110 on one side of the drill pipe cement casing 103 is illustrated and allows unwanted fluid flow in a downward direction from a hydrous sand formation 107 contaminating a production sand formation that is separate from the water sand formation 107 by a slate layer 108. When the measuring instrument 104 is arranged as shown and the detector source is designed as illustrated in fig. 10, the instrument 104 is capable of detecting unwanted water flow from the hydrous sand formation 107 through the concrete channel 110 to the production sand formation 109. Perforations 106 in the pipe 102 allow fluid from the production sand formation to enter the wellbore 100 and also allow unwanted water flow down along the concrete channel 110 and into the borehole 100. In the design shown in fig. 10, high-energy neutrons from the neutron source 126 penetrate through the steel tube 102 and activate the oxygen element in the water flow from the hydrous sand formations 107 through the concrete channel 110. The water flowing in the channel 110 then continues past the detectors 124 and 125 and gamma radiation from the disintegration of the radioactive nitrogen 16 is recorded by the detectors 124 and 125 in the manner previously discussed. Electrical pulses of a height proportional to the energy of the impending gamma rays detected by the detectors 124 and 125 are sent to the electronics section 127 of the downhole instrument and are connected from there to the conductors of the cable 111 and sent to the surface in a form that will be further discussed.

In fig. 11 shows a timetable for the use of the various instruments according to fig. 10, together with the pulse waveforms that appear in the measuring cable 111. The electrical pulse signals representing the energy of the gamma radiation on the detectors 124 and 125 are illustrated in the upper part of fig. 11, while the lower part of fig. 11 is a schematic representation of the time division in connection with the operation of the system as shown in fig. 10. It should be noted that a neutron burst with a duration of 1 millisecond is initiated at time T = 0 and lasts for the time T = 0 plus 1 millisecond. Simultaneously with the initiation of the neutron blast in the borehole instrument, a synchronization pulse with large amplitude and negative polarity is generated by the electronic section 127 in the borehole probe and is connected to the conductors in the measuring cable 111. The amplitude of the synchronization pulse is made larger than any possible data pulse amplitude from the detectors. Electrical pulse signals representing randomly occurring gamma rays affecting the detectors D1 and D2 in the probe 104 are continuously connected to the conductors in the cable 111 for transmission to the surface via the electronics section 127. The pulses from the detector D1 are applied to the conductor as voltage pulses with negative polarity, while pulses representing gamma rays registered at the detector D2, the cable's conductor is applied as voltage pulses with positive polarity. At the surface, a pulse separator 115 is used to separate detector D1 pulses from detector D2 pulses on the basis of their electrical polarity. The pulses with negative polarity are sent as an input to a synchronization pulse detector 118 and as an input to a time gate 116. The positive pulses from the detector D2 are sent as an input to a time gate 117.

The sync pulse detector 118 detects the large amplitude negative sync pulses on the basis of the amplitude and applied conditioning pulses to the time gates 116 and 117 beginning at time 4 milliseconds. after the initiation of the neutron blast. It is thus a 3 millisec. time interval between the end of the neutron blast and the conditioning of the time gates 116, 117 by means of the synchronization detector and timing pulse generator circuit 118.

The output from the two detectors D1 and D2 is continuously fed to the measuring cable 111, but is prevented from reaching subsequent circuits, since the gates 116 and 117 only allow the randomly occurring data pulses to reach the processing circuits during the 5.85 millisecond interval that begins at 4 milliseconds. after T = 0 and lasts until 9.85 millisec. after T = 0, as illustrated in the time diagram in fig. 11.

When the time gates 116 and 117 are opened by the conditioning pulse from the synchronization pulse detector 118, the data pulses from the borehole detector pair 124, 125 are connected as inputs to the pulse height analyzers 119 and 120 respectively. These pulse height analyzers perform the spectral energy separation of gamma rays recorded by the borehole instrument 104 at each detector 124 and 125 according to the energy windows as mentioned before. The aforementioned spectral degradation technique can thus be used to derive the distance R from the detector center to the center of the flowing water in the concrete channel 110 by the method discussed in connection with the calibration chart in fig. 6. For this purpose, the energy discriminated pulse height information from the pulse height analyzers 119, 120 is fed to a small computer 121, which may comprise a universal digital computer of the PDP-11 type, manufactured by the Digital Equipment Corporation of Cambridge, Massachusetts. The computer 121 may then, when it receives the energy discriminating information, use the counting ratio technique as discussed above, with respect to the ratio in fig. 6, to determine R, the distance of the center of water flow from one or both

detectors.

It will be obvious to those skilled in the art that such a universal digital computer for determining R can be programmed, e.g. in a commonly used programming compiler language such as FORTRAN or the like, for carrying out the necessary calculations for deriving the water flow rate v and R. Output signals which are representative of this desired information are led from the computer 121 to a recording device 122. As indicated by the dashed line 113, the recording device 122 can be electrically or mechanically connected to a disk wheel 112 for displaying the quantities of interest as a function of the measuring instrument's depth in the borehole. In a similar way, the count information processed in the multi-channel pulse height analyzer 119, 120 can be directed to the data recording device 122 and recorded or displayed as a function of the depth of the measuring instrument 104 in the borehole.

The above description has dealt with the theory and equipment that can be used for recording unwanted water flow in concrete channels or cavities behind the pipe in a borehole. The remaining sections will deal with operating methods under different types of borehole conditions for applying the mentioned methods and devices. The first condition for operation of a flow recording system using the principles according to the invention concerns the operation in a borehole which is provided with pipes and produces liquid under formation pressure through perforations directly into the borehole. This situation corresponds to the borehole that is schematically described in connection with fig. 10.

In fig. 10 shows borehole equipment as previously described in a borehole with perforated pipe. Unwanted water flow from a water-bearing sand formation 107 communicates along a concrete channel 110 through a shale crack 108 to a producing sand formation 109, where it enters the borehole through pipe perforations 106. Although fig. 10 illustrates a case where unwanted water production is emitted by downward flowing water from hydrous sand 107 to the production formation 109, it is obvious that it is equally likely that unwanted water communication along a similar concrete channel (not shown) occurs from a hydrous sand formation that lies below the production layer 109 In practice, it will rarely be known exactly from which direction the unwanted water flows. It is actually the purpose of the equipment and the method according to the invention to enable a registration of such channels or unwanted liquid flow from both directions.

It will be recalled that it is necessary to activate the oxygen element nuclei in the water flow to enable the production of the radioactive nitrogen 16 whose radioactive disintegration is recorded by the longitudinally spaced detectors 124, 125 in the borehole instrument 104. As the recording of fluid flow cannot be predicted with accuracy , it is necessary to use module instrumentation as discussed in connection with fig. 9A, B and C, so that the modules can be mounted for recording water flow in an upward or downward direction behind the pipe.

In experimental use of such modular equipment, it has been shown that the tool is highly discriminating when it comes to detecting water flow directions. In practice, it has been found that if the apparatus is mounted for the detection of upward water flows, its response to a downward water flow in the investigated borehole stretch will be approximately as illustrated in fig. 4 for "no water flow" using a pulsed neutron source or as illustrated in fig. 3 for "no water flow" using a continuous neutron source. The apparatus thus turns out to indicate the exact direction of the water flow past the neutron source 126, depending on whether the detectors 124, 125 are located above or below the neutron source 125. To detect upward-flowing water, the detectors are placed above the neutron source and to detect downward-flowing water, they are placed below the neutron source.

With this directional indication for the eye and with reference to fig. 10, the following sequence of operations will be necessary to accurately locate the unwanted water flow or channel formation, which is shown in FIG. 10.

First, the apparatus must be mounted with the detectors 125, 124 arranged at a distance from each other in the longitudinal direction, below the neutron source 126 for detection of downward water flow, as shown in fig. 10. Next, the apparatus must be lowered to a depth slightly above the perforated space 106 and measurements of the disintegration of the radioactive nitrogen 16 in the downward flowing water in the concrete channel 110 is carried out over a suitable period of time, e.g. for about. 5 min. While the downhole tool 104 is arranged somewhat above the perforated space 106, it will remain insensitive to any fluid flow in the pipe 102 in an upward direction, such flow will pass the detectors 124, 125 first, but not the neutron source 126 before the detectors. Thus, only the downward flowing water in the concrete channel 110 will be activated and registered by the borehole equipment with this design.

The modular device is then removed from the borehole and the source-detector device is replaced, so that the detector is mounted above the neutron source in the probe as shown in fig. 9C. The apparatus is then lowered to a point slightly below the perforation 106 in the borehole and the oxygen activation measurement cycle is repeated for a suitable period of time. This enables the detection of any upward water flow along concrete channels near the pipe. With this design, the borehole apparatus will be insensitive to produced liquid in the pipe 102 which is moved down past the detectors 124 and 125.

In this way, the detectors' response to an unwanted liquid flow along concrete channels or cavities can be used in connection with equation 4 to determine the linear flow velocity v of the unwanted water flow in the concrete channel. The direction of such a flow will of course also be defined at the same time.

In a similar way, the volume velocity V of any registered, unwanted fluid movement can be obtained by estimating or measuring the distance R to the center of flow from the centers of the detectors using one of the above mentioned techniques. If it is not desirable to follow such a measurement technique, the approximate flow volume velocity V can be estimated by assuming that the distance R is from 12.7 mm to 25.4 mm greater than the outside diameter of the pipe. Using equation 7, the volume velocity V can then be derived quantitatively.

The mentioned techniques are described as stationary measurements. This is probably the most accurate form of flow detection according to the invention. It has also been established experimentally that the water flow measurement system according to the invention can be operated with the borehole measuring device in motion. In this case, if the apparatus is moved at a low speed, which is precisely known, e.g. about. 1.5 m per min. etc., the device in the example according to fig. 10 is first placed in the borehole with the detectors below the neutron source and starting just above the pipe perforations in the area to be inspected. The instrument is then slowly, continuously lowered past the perforations 106 over a fixed, short distance below the perforations. In a similar manner, the borehole apparatus can then be removed from the borehole, detectors and source are exchanged and the apparatus is lowered to a fixed position below the perforations 106 and moved slowly upwards past the perforations 106. This movement is continued over a fixed distance above the perforations. In this mode of operation, when the detectors 124, 125 are arranged below the source, the apparatus will remain relatively insensitive to its downward movement. With the detectors arranged above the source, the device will remain relatively insensitive to its upward movement. Thereby it is possible to register at least qualitatively, by continuous measurement, whether there is any unwanted liquid communication along the concrete mantle and to register any such communication as a function of the borehole depth in the manner previously discussed in connection with fig. 10. •If it is desired to move the borehole instrument upward with the detectors 124, 125 arranged below the neutron source 126 or if it is desired to move the instrument downward with the detectors 124 and 125 arranged above the neutron source, the movement of the apparatus will only add a constant known, linear velocity to the response to water flow in the direction corresponding to the device's sensitive direction. As the speed of movement is known, this constant value can be compensated for by subtracting when the linear flow rate v and flow volume rate V are calculated in the computer system 121 in fig. 10. Unless the rate of the unwanted water flow is exactly the same as the rate at which the apparatus is moved through the borehole, so that there will be no relative motion, the unwanted flow would still be recorded under these conditions.

In some cases, it may be desirable to try to determine the location and quantity of an unwanted fluid flow along concrete channels or cavities in a production well that is completed and in production by gas lift. Such techniques are quite common in certain geographical areas where there are relatively large quantities of natural gas that can contribute to production. In such cases it is always desirable to try to measure the unwanted liquid flow under operating conditions. If production in the suspicious zone is stopped to carry out measurements of unwanted fluid flow, pressure differentials that prevail during production from the production zone and which contributed to the unwanted fluid flow will be lost when the zone is taken out of production.

In gas lift, a production zone is usually driven through a relatively small diameter production pipeline (76.2 mm) which is passed through a packing which is anchored in the pipe, usually approx. 15-18 m above the production perforations. Gas lift valves are provided in the production pipeline above the packing and above the perforations that allow production through the pipeline when the liquid level is below the valve. These valves also allow the introduction of natural gas under pressure into the annular space between the production pipe and the drill pipe. This pressure is used to force the well fluid up into the production line. The gas that is allowed to enter the production line forms a kind of bubble emulsion with the well fluid produced from the perforations and lifts this to the surface in the production line as a result of the gas pressure. . In the case of gas lifting, it is therefore obvious that the production pipeline must not be removed from the borehole, if the production conditions are to be maintained, otherwise the effect of the gas lifting equipment will be limited. This would of course stop the flow of production fluid and probably change unwanted fluid flow, so that it becomes difficult if not impossible to detect.

In order to carry out detection of unwanted liquid flow at the same time as gas rise of production liquid takes place, a measuring instrument is required that is dimensioned and adapted for passage through the production line. Such an apparatus can be constructed with the same shape as that discussed in connection with fig. 9 and 10. This means that a neutron generator tube and scintillation detectors of suitable size are arranged in an apparatus housing that has a total outer diameter that does not exceed approx. 25.4 - 26.9 mm. This device is then passed through the production line to the desired location for measurement of unwanted flow.

In fig. 12, the method for measuring unwanted liquid flow during gas lift is schematically indicated. A drill pipe 201 is cast in place and a production zone producing through perforations 202 is isolated from the rest of the borehole by a gasket 203, through which a production line 204 passes to bring the produced fluid to the surface. A gas lift valve 205 is provided for suitable introduction of natural gas under pressure to the production line.

For detection of unwanted downward liquid flow, a continuous tubular apparatus 206 according to the invention is carried with a source detector device as shown in fig. 12A, through the production line to the area just above the production perforations 202. With the apparatus as shown in fig. 12A, a downward fluid flow can be detected according to previously discussed techniques. If the borehole apparatus is designed as shown in fig. 12B, with the detectors arranged above the neutron source, and lowered through the production line to the production zone and below the perforations 202, an upward unwanted liquid flow along the pipe will be recorded as previously discussed in connection with the larger devices.

When carrying out these measurements, the borehole apparatus 206 can either be arranged stationary, first above and then below the perforations with the detectors first below and then above the neutron source, or the apparatus 206 can be slowly lowered past the perforations or slowly pulled up past the perforations. In both cases, the operating conditions for determining the location, the linear flow rate and the flow volume rate of the unwanted flow in concrete channels or cavities outside the pipe will remain the same as previously discussed.

In multi-zone wells, two or more production zones are produced which lie at different depths and are isolated from each other by gaskets in the pipeline, through multi-pipelines. In such a case, the flow from a deeper production zone must of course pass through the shallower production zone(s) in its production pipeline. As this production from the deeper production zone may contain some water, the detection of unwanted liquid flow behind the pipeline in the upper production zone is made somewhat difficult by this factor. The problem that arises is therefore how to distinguish between the aqueous fluid flow in a nearby production line that passes through a shallower production zone that is isolated by gaskets across the perforations in such a shallower zone.

This situation is schematically illustrated in fig. 13.

In the diagrams in fig. 13A and 13B is a shallow production zone 303 isolated by seals 304 and 305 from the rest of the wellbore and a lower production zone 306. The lower production zone 306 produces by gas lift through a production line 307 which passes completely through the shallower seal isolated production zone 303. Upper production zone 303 produces through one set of perforations 308, while lower production zone 306 produces through another set of perforations 309.

To make the problem in connection with the detection of unwanted liquid flow behind the pipe 310 in the upper zone less complicated, the lower zone can be taken out of production during measurement.

If the two production zones are sufficiently close to each other, and the unwanted water flow in the upper zone comes from an aquifer between the two zones, this shutdown of the lower zone can affect the water flow conditions in the upper production zone and thus make it impossible to measure the unwanted fluid flow. According to the present invention, however, unwanted liquid in the upper zone operated with gas lift can be registered despite the presence of water components in the production line 307 passing through this zone. In this case, however, the measurement technique will require further theoretical explanation.

Referring to the previously discussed theory of spectral degradation of gamma radiation due to differing thickness of the scattering material between the source and the detectors for water flow occurring at different distances from the detectors, the rate C recorded in an energy range or window in (i = A, B) for the detector j (j = 1,2) after background correction is expressed as follows

T

where C .. is the counting speed from water flowing in product-1 1 D

tion pipeline that passes through the upper zone and C F.. is the count velocity from the water behind the pipeline in the upper production zone in fig. 13. It can be shown that the ratio between the two detector count rates as a result of the conduction flow in the energy window A is as follows:

where K = AAS

AS is the detector distance

vTer the linear velocity of the fluid flow i

the pipeline (mm/sec.), and

X = 0.0936 sec"<1>

Correspondingly, the ratio between the counting rates at the two detectors as a result of water flow outside the pipeline in the energy window A can be reproduced as follows:

where v is the linear flow rate of the unwanted water flow behind the pipeline and K is as previously defined.

The counting rate C A , x, can therefore be expressed as follows: but the counting rate ^ ^an is also expressed as follows:

By solving equation 14 for C T_ and putting mn in (13), the counting rate C , can be expressed as follows:

Similarly, an equation can be developed for the counting rate in the energy window CD , which can be expressed as follows:

ice, l

but here too the counting rate C Fn_ can be expressed as follows: If one now puts equation 17 into equation 16, equation 18 appears for the total counting rate CD n but also the counting rate C pA 2 is expressed by equation 19: where the expression L(R^) is a function of R^,, the distance between the center of the probe and the center of flow behind the pipe. It should be noted that this function is illustrated for a particular experimental geometry in the graphical representation in fig. 6. An approximate analytical expression for the function L(R) for a particular probe geometry can then be developed and expressed in equation 20.

If equation 19 is inserted into equation 15, equation 21 is obtained.

If equation 18 is inserted into equation 21, equation 22 is obtained:

Equation 22 can be solved for the unknown function L(R^) which is reproduced in equation 22-a.

With reference to fig. 13A, the distance R^, which is measured from the center of the probe 303 to the center of the production line 307, is usually known or can be estimated with reasonable accuracy. Equation 20 can therefore be used to calculate the function L(RT) on the basis of R^. Remaining terms on the right-hand side of equation 23 are known (K) or measured quantities, (CA ^, C- n, Cra , and C0 0) . Equation 23 can therefore be solved for v„, the linear flow rate of the water behind the pipeline. Equation 15 can be rewritten as equation 23 and then as follows:

The term vT, which is the linear flow rate in the production line 307, can be calculated from the amount of water produced (which is usually known) and the cross-section of the production pipe 307. The remaining terms on the right-hand side of equation 24 are either known (K), can be calculated (v ) or have been measured (C , and C _). Equation 23 can

tA , x A , z

therefore solve for C _.

When using vT which can be calculated as mentioned above, and the measured quantities C,. , , CA _, C0 , and CD , can equation 22A

A,X A,ZD/J-ti,Z

is solved for L(Rp). This value rv L(Rp) can then be inserted into equation 20 to obtain Rp, the radial distance between the center of the probe 303 and the center of the flow behind the pipe.

Using v„, obtained by equation 23, R_ obtained

FF F

from equations 22A and 20 and C A ^ obtained from equation 24, equation 7 can be used to calculate V^, the volume flow rate of the water behind the pipe, where

and Q is an empirically determined calibration constant.

The above technique is of course equally applicable both above and below the perforations in the upper production zone in the multiwell, so that liquid flow both upwards and downwards on opposite sides of the perforations can be selectively recorded in this way. The technique just described can be described as selective against detection of the known liquid flow in the production zone through the production line, which passes from the lower zone, on the basis that its distance from the detectors is different from the distance for any unwanted liquid flow outside the pipe.

It should be noted that if the two production zones in fig. 13 are in reality several hundreds of feet apart, so that the unwanted fluid flow in a zone is not likely to be affected by production interruption in the deeper zone, the most appropriate technique would be to interrupt production from the lower zone to eliminate influence as a result of this fluid flow through the production line passing through the shallower production zone. If the two zones mentioned above are so close to each other that shutting down the production would not be desirable in order for the operating parameters to be maintained as well as possible in both zones, the technique just mentioned can be used to separate the liquid flow outside the pipe from the one taking place in the production line that passes in the pipe.

It should also be noted that the same theory will apply whether there are two or more production zones that are exploited at a greater depth than the production zone being investigated. In this case, the above-mentioned method and theory can be extended as mentioned above for individual consideration of each flow from all production lines that pass through the examined zone.

When measuring, a probe with a small diameter (25.4 mm - 26.9 mm) will be lowered through the production line to the production zone to be investigated. Stationary oxygen activation count rate measurements will then be taken in the two energy windows A and B both above and below the perforations in the production zone, with the detector arranged first below and then above the neutron source in the previously mentioned manner. The above-mentioned interpretation of these counting rates is used. The flow volume velocity V and the linear flow velocity v of any unwanted liquid flow behind the pipe could thus be registered.

What has been described above will point out other alternative embodiments of the method and apparatus according to the invention. The following claims cover all such changes and modifications that fall within the scope of the invention.

Claims (10)

1. Method for determining the volume flow rate for an unwanted water flow behind the well casing in a production well borehole, characterized by the following steps: formation of fast neutrons capable of 16 ^ 6 causing the nuclear reaction 0 (n,p)N at a source in the well casing (known per se), irradiation with the fast neutrons of the earth formations including elementary oxygen nuclei in the water flow outside the casing, detection in the well casing by first and second detectors which are spaced longitudinally from each other and from said source (in and of itself known) of gamma rays resulting from the disintegration of the unstable isotope N<1>^ formed by the nuclear reaction, formation of electrical signals (C^, C21 which are representative of the detected gamma rays (known per se), combining (equation 4) said signals to derive a signal representative of the linear flow rate (v) of the water flow behind the well casing, forming a signal representative of the radial distance ( R) from said detectors to the water flow, and combining (equation 7) of said linear flow velocity signal with the radial distance signal for to derive a signal representative of the volume flow rate (V) of the water stream behind the casing at the depth of irradiation. 2. Method according to claim 1, characterized in that said step is performed to derive said volume flow rate signal with respect to water flowing in a first direction behind the casing, reversing the relative longitudinal positions of the neutron source and said first and second detectors, and repeating the step to derive a further volume flow rate signal relative to the water flowing in a second opposite direction behind the casing. 3. Method according to claim 1 or 2, characterized in that the irradiation step includes continuous irradiation of the soil formations around the casing with said fast neutrons and that the detection step includes detection by first and second detectors of gamma rays in an energy window that extends between 4.45 MeV and 7.20 MeV. 4. Method according to claim 1 or 2, characterized in that the irradiation step comprises irradiation of the soil formations around the casing with pulses of said fast neutrons, and that the detection step comprises detection by first and second detectors of gamma rays in an energy window that extends between 2.0 MeV and 7 .20 MeV. 5. Method according to claim 4, characterized in that the detection step comprises the detection of gamma rays over a period of time which begins after an interval after the cessation of each said neutron pulse and which ceases before the next said neutron pulse. 6. Method according to one or more of the claims 1 - 5, characterized in that said radial distance signal is formed by generating a signal which is representative of a somewhat larger distance, e.g. from 1 - 4 cm, than the outer radius of the casing. 7. Method according to one or more of claims 1 - 6, characterized in that said radial distance signal is formed by generating first and second count signals (CA/Cg) which are representative of the respective counts of gamma rays in two spaced energy windows at one of the detectors, that a ratio signal (CA/Cg) is produced which constitutes the ratio between first and second counting signals, and (fig. 6) that the signal representative of the radial distance (R) is produced from the ratio signal. 8. Method according to claim 7, characterized in that said two spaced energy windows cover the energy intervals 3.25 MeV - 4.0 MeV and from 4.9 MeV to 6.5 MeV. 9. Method according to one or more of claims 1 - 6, characterized in that the detection step includes, at one of the detectors, detection of gamma rays by both inner and outer coaxially aligned cylindrical detector crystals, and that said radial distance signal is formed by generating a ratio signal (CQ /C^) which constitutes the ratio of signals representative of counts of gamma rays detected by said inner (C^) and outer (CQ) detector crystals respectively, and (fig. 8) that a signal representative of the radial distance (R) is produced from said ratio signal. 10. Apparatus for determining the volume flow rate of an unwanted water flow behind a well casing in a production well borehole, characterized by: a module measuring probe (104) which includes first, second and third modules (94 + 95, 93, 92), the first module (94 + 95) comprises a fast neutron source (S) for irradiating the soil formations around the casing to cause the nuclear reaction 0 16 (ra,p)N 16 in the water in said formations, that said second module (9 3) includes - first and second gamma ray detectors (D-^, D2) which are spaced longitudinally from each other and from said source (S), a neutron screen (128) between the source and the detectors, and electrical signal generators (127) which are connected to said detector to form electrical signals representative of the detected gamma rays, that the third module (92) includes a control device which is connected to said source and to the detectors and a signal transmitter to transmit signals from said electrical ical signal generation means to the earth's surface, that a surface signal processing means (114 - 120) is connected to a signal transmission means for combining said signals to derive a signal representative of the linear flow rate (v) of said water flow to form a signal representative of the radial distance (R) from said detectors to said water stream and to combine the linear flow rate signal with the radial distance signal to derive a signal representative of the volume flow rate (V) of the water stream behind the casing at the depth of irradiation , and that the modules (94 + 95, 93, 92) are intended to be connected to each other with either a first module above the second or the second module above the first in the wellbore.