RU2579820C1 - Acoustic logging method - Google Patents

Abstract

FIELD: mining.

SUBSTANCE: invention is intended for determining of coordinates of fractured zones crossing a measuring well drilled in the roof of mine working. Method is based on experimentally established law of influence of a fractured zone on correlation characteristics of the noise acoustic signal emitted into the massif. Method involves excitation in the well of an acoustic signal and its reception after passing through the investigated area of the near-well massif in two points located symmetrically above and below the point of radiation, measuring and combined processing of parameters of received signals. Herewith, there is excited a signal in the form of stationary random noise with the average equal to zero. Signal is received in points located at a distance from radiation points not exceeding 0.3 of correlation radius of the emitted signal in non-destructed mine rock. There is measured a cross-correlation coefficient of signals at reception points and intervals of these signals autocorrelation. At that, the cross-correlation coefficient is used to determine presence and degree of fracturing of the near-well massif between the reception points, and the ratio of measured correlation intervals is used to determine location of the fractured zone relative to the point of reception.

EFFECT: high accuracy of obtained data.

1 cl, 6 dwg

Description

The invention relates to geophysical methods for studying the near-wellbore space of a rock mass, mainly to acoustic methods for detecting fractured zones intersected by a well in rock formations of mining roofs.

There is a known method of acoustic logging, which consists in placing a tube of brittle material rigidly connected to the walls of the borehole using cement mortar, continuously moving the borehole probe inside the tube, emitting and receiving pulsed elastic vibrations with it, which are judged by changing characteristics along the length of the tube on the presence and location of bundles in the near-wellbore region of the massif [1] (USSR Author's Certificate No. 996972, class G01V 1/40, published in BI No. 6, 02/15/1983).

The disadvantage of this method is its low sensitivity with respect to detectable bundles, the disclosure of which does not change or changes little in time. This is due to the fact that such delaminations do not lead to cracks in the brittle material of the tube placed in the well and, as a result, to significant changes in the acoustic characteristics of the received acoustic signal, which propagates mainly along the walls of the tube.

A known method of acoustic logging, which consists in the excitation of an acoustic signal in the well and receiving it after passing through the investigated section of the near-wellbore array at two points located symmetrically above and below the radiation point, measuring and jointly processing the parameters of the received signals [2] (USSR Author's Certificate No. 437036, class G01V 1/40, published in BI No. 27, 07.25.1974).

In this method, the registration by two receiving transducers of the signals reflected from the heterogeneities of the array is carried out, the subtraction of these signals and the registration of the difference signal in the form of the dependence of the arrival time on the well depth.

The disadvantage of this method is the inability to use it to detect the presence and location of fracture zones of an array crossed by a well, and the steppe of rock fracturing in these zones.

The technical result of the proposed method is the ability to detect the presence and location of fracture zones of an array intersected by a well and the degree of fracture of rocks in these zones.

To achieve the specified technical result in the method of acoustic logging, which consists in exciting an acoustic signal in the well and receiving it after passing through the studied section of the near-wellbore array at two points located symmetrically above and below the radiation point, measuring and jointly processing the parameters of the received signals, the signal is excited in the form stationary random noise with an average equal to zero, they are received at points lying from the radiation point at a distance not exceeding 0.3 of the radius of the core the ratio of the emitted signal in the undisturbed rock, measure the cross-correlation coefficient of the signals at the points of reception and the autocorrelation intervals of these signals, while the cross-correlation coefficient is used to judge the presence and degree of fracture of the near-borehole array between the points of reception, and the location of the fractured is judged with respect to the measured autocorrelation intervals zone relative to the point of radiation.

When using a noise acoustic signal, it is necessary to take into account the following features: the random nature of such a signal makes it possible to largely avoid the resonant and interference distortions inherent in periodic signals; the average value of the noise signal, equal to zero, provides the value of the correlation coefficient R = 0 when the signals under study are not correlated; the stationarity of the process eliminates the stringent requirements for the integration time in the correlator of the receiving device.

The proposed method is based on experimentally established and based on computer modeling, the patterns of changes in the correlation characteristics of a stationary noise acoustic signal with an average of zero when it propagates between the radiation point and two symmetric reception points in the geomedium containing and not containing zones of increased fracturing. The essence of these patterns is as follows.

Firstly, when these signals propagate in each particular rock type that does not contain zones of increased fracturing, there is a characteristic correlation radius r. This is the distance l within which the recorded acoustic signals from one source are correlated with each other (i.e., their cross-correlation coefficient R> 0.1), and for l> r these signals are independent of each other and for them R <0, one. Given the natural heterogeneity of the properties of the same rocks even in the absence of zones of increased fracture in them, the two signals can be independent of each other at a distance of 0.6 r. Thus, cross-correlation measurements of signals at two reception points symmetrical with respect to the radiation point make sense only provided that they lie at a distance l≤0.3 r from the radiation point.

Secondly, the presence of the zone and the degree of fracture of the rocks between the points of reception affect the coefficient R of the mutual correlation between the signals recorded at these points. If, in the absence of fracturing, R → 1, then with its increase, fracturing decreases the relationship of signals recorded at the receiving points. Physically, this effect is understandable, given that fracturing leads to both a decrease in r and a different change in the amplitude, frequency, and phase characteristics of the signals recorded at two points of reception.

Thirdly, the intervals of signal autocorrelation at the points of reception are the smaller, the more the fractured portion of the array between the radiation point and the corresponding receiving point is violated, which means that the ratio between the indicated intervals carries information about which of the receiving points is the fractured zone closest to.

The acoustic logging method is illustrated in FIG. 1 - FIG. 6, where: in FIG. 1 shows an arrangement of an ultrasonic probe at a depth of well H _{1 of a} homogeneous array that does not contain a fracture zone intersected by the well; in FIG. 2 is an arrangement diagram of an ultrasonic probe at a depth of H _{2 of an} array containing a fractured zone between a radiation point and an upper receiving point of a noise acoustic signal; in FIG. 3 is a location diagram of an ultrasonic probe at a depth of H _{3 of an} array containing a fractured zone located at the same distance from the points of reception of a noise acoustic signal; in FIG. 4 is an arrangement diagram of an ultrasonic probe at a depth of H _{4 of an} array containing a fractured zone located between a radiation point and a lower receiving point of a noise acoustic signal; in FIG. 5 shows the results of measuring the mutual correlation coefficient R of acoustic noise signals at the receiving points, and FIG. 6 - relations of the autocorrelation intervals of these signals when the ultrasonic probe is located at depths of H ₁ , H ₂ , H ₃ and H _4, respectively.

The circuits shown in FIG. 1-4, include a well 1, in which an ultrasonic probe 2 is placed, which is discretely moved to the depth of the well 1. An ultrasonic probe 2 comprises a radiating acoustic transducer 3, an upper receiving acoustic transducer 4, and a lower receiving acoustic transducer 5. Radiating acoustic transducer 3 connected to the output of the generator 6 noise stationary signal with an average of zero. The upper receiving acoustic transducer 4 and the lower receiving acoustic transducer 5 are connected to the inputs of the correlation analyzer 7. The radiating acoustic transducer 3 contacts the borehole wall 1 at point 8 of the radiation, the upper receiving acoustic transducer 4 contacts the receiving point 9 and the lower receiving acoustic transducer 5 point 10 of reception. The reception points 9, 10 are symmetrical with respect to the radiation point 8 and are located at a distance not exceeding 0.3 of the correlation radius r of the emitted acoustic noise signal.

The results of measuring the cross-correlation coefficients R received by the acoustic noise signals in FIG. 5 are presented: a value 12 of this coefficient obtained at a depth H _{1 of the} location of the ultrasonic probe 2 in well 1; a value of 13 obtained at a depth H _{2 of the} location of the probe 2 in the well 1; a value of 14 obtained at a depth of H _{3 the} location of the probe 2 in the well 1; a value of 15 obtained at a depth of H _{4 the} location of the probe 2 in the well 1.

автокорреляции сигнала, принятого в верхней точке 9 приема, и интервала $${\tau }_{ик}^{н}$$

автокорреляции сигнала, принятого в нижней точке 10 приема, представлены на фиг. 6 значениями этого отношения: 16, которое соответствует глубине H₁ расположения ультразвукового зонда 2 в скважине 1; 17, которое соответствует глубине Н₂ расположения ультразвукового зонда 2 в скважине 1; 18, которое соответствует глубине Н₃ расположения ультразвукового зонда 2 в скважине 1; 19, которое соответствует глубине Н₄ расположения ультразвукового зонда 2 в скважине 1.Measurement Results Relative to Interval $${\tau }_{\mathrm{and}\mathrm{to}}^{\mathrm{at}}$$

autocorrelation of the signal received at the upper point 9 of the reception, and the interval $${\tau }_{\mathrm{and}\mathrm{to}}^n$$

the autocorrelation of the signal received at the lower receiving point 10 is shown in FIG. 6 by the values of this ratio: 16, which corresponds to the depth H _{1 of the} location of the ultrasonic probe 2 in the well 1; 17, which corresponds to a depth H _{2 of the} location of the ultrasonic probe 2 in the well 1; 18, which corresponds to a depth H _{3 of the} location of the ultrasonic probe 2 in the well 1; 19, which corresponds to a depth H _{4 of the} location of the ultrasonic probe 2 in the well 1.

The method of acoustic well logging is as follows.

A measuring well 1 is drilled in the roof of the mine working, in which an ultrasonic probe 2 is placed containing an emitting acoustic transducer 3 and a symmetric upper receiving acoustic transducer 4 and a lower receiving acoustic transducer 5. The distance l between the radiating acoustic transducer 3 and each of the receiving acoustic transducers 4 and 5 are changed so that the condition l≤0.3 r is satisfied, where r is the correlation radius of the emitted acoustic signal in the undisturbed rock. The value of g is obtained on the basis of preliminary measurements on samples of the corresponding rocks that do not contain disturbances in the form of cracks.

The emitting acoustic transducer 3 is connected to the output of the generator 6 of a stationary electric noise signal with an average equal to zero, and the receiving acoustic transducers are connected to the corresponding inputs of the correlation analyzer 7.

The ultrasonic probe 2 is discretely moved deep into the well 1 with a step ΔН and at each step provide reliable contact conditions for the emitting acoustic transducer 3 at the radiation point 8, the upper receiving acoustic transducer 4 at the receiving point 9 and the lower receiving acoustic transducer 5 at the receiving point 10.

и $${\tau }_{ик}^{н}$$

указанных сигналов в этих точках. Затем определяют отношение $${\tau }_{ик}^{в}$$

/ $${\tau }_{ик}^{н}$$

интервалов корреляции шумовых акустических сигналов, измеренных в верхней точке 9 приема и в нижней точке 10 приема.At each discrete depth H _{i of the} measuring well 1, at which the acoustic transducers 3, 4, 5 are in contact with its wall, the cross-correlation coefficient R of the acoustic noise signals at the receiving points 9, 10 and the autocorrelation intervals are measured using a correlation analyzer 7 $${\tau }_{\mathrm{and}\mathrm{to}}^{\mathrm{at}}$$

and $${\tau }_{\mathrm{and}\mathrm{to}}^n$$

indicated signals at these points. Then determine the ratio $${\tau }_{\mathrm{and}\mathrm{to}}^{\mathrm{at}}$$

/ $${\tau }_{\mathrm{and}\mathrm{to}}^n$$

correlation intervals of acoustic noise signals measured at the upper receiving point 9 and at the lower receiving point 10.

и $${\tau }_{ик}^{н}$$

автокорреляции принятых акустических сигналов, а значит отношение интервалов автокорреляции этих сигналов ( $${\tau }_{ик}^{в}$$

/ $${\tau }_{ик}^{н}$$

) имеет значение 16 на фиг. 6, т.е. стремится к 1.If on the basis of 2l between the upper receiving point 9 and the lower receiving point 10, the rock mass does not contain a fractured zone 11 crossing the measuring well 1 (see Fig. 1), the changes in the signal characteristics at the receiving points 9 and 10 will be approximately the same and insignificant. As a result, the cross-correlation coefficient R of these signals will tend to 1 (R → 1), which is reflected by a value of 12 in FIG. 5. For the same reasons, the intervals at the points 9.10 of reception will also be close $${\tau }_{\mathrm{and}\mathrm{to}}^{\mathrm{at}}$$

and $${\tau }_{\mathrm{and}\mathrm{to}}^n$$

autocorrelation of received acoustic signals, which means the ratio of the intervals of autocorrelation of these signals ( $${\tau }_{\mathrm{and}\mathrm{to}}^{\mathrm{at}}$$

/ $${\tau }_{\mathrm{and}\mathrm{to}}^n$$

) has a value of 16 in FIG. 6, i.e. tends to 1.

/ $${\tau }_{ик}^{н}$$

)<<1. Причем и R и $${\tau }_{ик}^{в}$$

/ $${\tau }_{ик}^{н}$$

будут тем меньше, чем больше трещиноватость в зоне 11.In the presence of a fractured zone 11 between the upper receiving point 9 and the radiation point 8 (see Fig. 2), the decorrelation of the acoustic signal at the receiving point 9 will be significantly greater than the acoustic signal recorded at the receiving point 10. As a result, measured at a depth of H _{2, the} value 13 of the quantity R << 1 and the value 18 of the ratio ( $${\tau }_{\mathrm{and}\mathrm{to}}^{\mathrm{at}}$$

/ $${\tau }_{\mathrm{and}\mathrm{to}}^n$$

) << 1. Moreover, both R and $${\tau }_{\mathrm{and}\mathrm{to}}^{\mathrm{at}}$$

/ $${\tau }_{\mathrm{and}\mathrm{to}}^n$$

will be less, the greater the fracture in zone 11.

/ $${\tau }_{ик}^{н}$$

будут близки к 1 (см. фиг. 5 и фиг. 6). Однако, поскольку абсолютная симметрия трещиноватой зоны 11 относительно точек 9 и 10 приема на практике маловероятна, значения 14 величины R и 18 отношения ( $${\tau }_{ик}^{в}$$

/ $${\tau }_{ик}^{н}$$

) будут все же несколько меньше, чем в случае полного отсутствия трещиноватой зоны (см. фиг. 1).If there is a symmetrical location of the fractured zone 11 relative to the points 9 and 10 of the reception (see FIG. 3), the changes in the characteristics of the acoustic noise signals recorded at these points will be approximately the same and, as a result, the value 14 of R and the value measured at a depth of H ₃ 18 relationship $${\tau }_{\mathrm{and}\mathrm{to}}^{\mathrm{at}}$$

/ $${\tau }_{\mathrm{and}\mathrm{to}}^n$$

will be close to 1 (see FIG. 5 and FIG. 6). However, since the absolute symmetry of the fractured zone 11 relative to points 9 and 10 of reception is unlikely in practice, the values of R are 14 and 18 are ratios ( $${\tau }_{\mathrm{and}\mathrm{to}}^{\mathrm{at}}$$

/ $${\tau }_{\mathrm{and}\mathrm{to}}^n$$

) will still be slightly less than in the case of a complete absence of a fractured zone (see Fig. 1).

/ $${\tau }_{ик}^{н}$$

)>1.For the case of FIG. 4, when the fractured zone 11 is located between the emitting acoustic transducer 8 and the lower receiving acoustic transducer 10, the decorrelation of the acoustic signal at the receiving point 10 will be significantly greater than at the receiving point 9. As a result, measured at depth H _{4, the} value 15 of the quantity R << 1, and the value 19 of the ratio ( $${\tau }_{\mathrm{and}\mathrm{to}}^{\mathrm{at}}$$

/ $${\tau }_{\mathrm{and}\mathrm{to}}^n$$

)> 1.

/ $${\tau }_{ик}^{н}$$

. Для вычисления радиуса корреляции r в блоке известняка производилось пошаговое увеличение расстояния l между излучающим и приемными акустическими преобразователями. Экспериментально установлено, что при достижении значением l величины в 180 мм коэффициент корреляции R падает ниже значения 0,1. Таким образом, для дальнейших исследований расстояние l было принято равным 50 мм.The described method was tested in laboratory conditions. A through hole with a diameter of 42 mm was drilled in a cubic block of limestone with a side of 400 mm. With the help of television logging equipment, the walls of the drilled well were examined, which showed that at a depth of 230 mm there is a zone of increased fracturing crossing the investigated well. Then, a logging probe was placed in the well, consisting of one emitting acoustic transducer and two receiving acoustic transducers placed at an equal distance on opposite sides of it. The resonant frequency of all converters was 150 kHz, the quality factor was 10. An electric noise signal was applied to the input of the emitting converter from the GSH-1 noise generator in the frequency band 10-500 kHz with an average of zero. Electrical signals from the receiving acoustic transducers were fed to a two-channel ADC with a sampling frequency of 1 MHz connected to a personal computer, on which the correlation coefficients R and the ratio of the autocorrelation intervals were calculated by software $${\tau }_{\mathrm{and}\mathrm{to}}^{\mathrm{at}}$$

/ $${\tau }_{\mathrm{and}\mathrm{to}}^n$$

. To calculate the correlation radius r in the limestone block, a stepwise increase in the distance l between the emitting and receiving acoustic transducers was performed. It was experimentally established that when the l value reaches 180 mm, the correlation coefficient R falls below 0.1. Thus, for further studies, the distance l was taken equal to 50 mm.

/ $${\tau }_{ик}^{н}$$

).The described logging probe was moved deep into the well so that, in the first measurement, the fractured zone was outside the logging probe (see Fig. 1), in the second, the fractured zone was between the upper receiving transducer and the transmitter (see Fig. 2), and in the third, the fractured the zone turned out to be aligned in coordinate with the emitter (see Fig. 3), in the fourth - the fractured zone was between the emitter and the lower receiving acoustic transducer (see Fig. 4). In each of the cases, the correlation coefficient R and the ratio of the correlation intervals ( $${\tau }_{\mathrm{and}\mathrm{to}}^{\mathrm{at}}$$

/ $${\tau }_{\mathrm{and}\mathrm{to}}^n$$

)

/ $${\tau }_{ик}^{н}$$

=0,90, для второго случая R=0,23 и ( $${\tau }_{ик}^{в}$$

/ $${\tau }_{ик}^{н}$$

)=0,31, для третьего случая R=0,71 и $${\tau }_{ик}^{в}$$

/ $${\tau }_{ик}^{н}$$

=0,84, для последнего случая R=0,21 и $${\tau }_{ик}^{в}$$

/ $${\tau }_{ик}^{н}$$

=1,48.According to the measurement results, it was found that for the case of the first measurement R = 0.86 and $${\tau }_{\mathrm{and}\mathrm{to}}^{\mathrm{at}}$$

/ $${\tau }_{\mathrm{and}\mathrm{to}}^n$$

= 0.90, for the second case, R = 0.23 and ( $${\tau }_{\mathrm{and}\mathrm{to}}^{\mathrm{at}}$$

/ $${\tau }_{\mathrm{and}\mathrm{to}}^n$$

) = 0.31, for the third case, R = 0.71 and $${\tau }_{\mathrm{and}\mathrm{to}}^{\mathrm{at}}$$

/ $${\tau }_{\mathrm{and}\mathrm{to}}^n$$

= 0.84, for the latter case R = 0.21 and $${\tau }_{\mathrm{and}\mathrm{to}}^{\mathrm{at}}$$

/ $${\tau }_{\mathrm{and}\mathrm{to}}^n$$

= 1.48.

Thus, the proposed method provides a technical result, which consists in providing the ability to detect the presence and location of fracture zones of an array crossed by a well, and the degree of fracture of rocks in these zones.

Claims (1)

The method of acoustic logging, which consists in generating an acoustic signal in the well and receiving it after passing through the investigated section of the near-wellbore array at two points located symmetrically above and below the radiation point, measuring and processing the parameters of the received signals, characterized in that they excite the signal in the form of a stationary random noise with an average equal to zero, they are received at points lying from the radiation point at a distance not exceeding 0.3 of the correlation radius of the emitted signal in broken rock, measure the cross-correlation coefficient of the signals at the points of reception and the intervals of autocorrelation of these signals, while the cross-correlation coefficient is used to judge the presence and degree of fracture of the near-wellbore array between the points of reception, and the location of the fractured zone relative to the point of reception is judged by the ratio of the measured correlation intervals .

Images