Coal seam water injection range monitoring and optimization method based on acoustic emission array monitoring technology

Abstract

A method for monitoring and optimizing the range of coal seam water injection based on acoustic emission array monitoring technology, comprising the following steps: setting an acoustic emission array on a coal rock sample, establishing a monitoring path network, and then collecting data based on the monitoring path network; confirming the characteristic signal data of water transmission to the monitoring path; enveloping the water transmission contour line based on the characteristic signal data; sequentially using a water transmission correction control point algorithm and a fracture structure correction control point algorithm to correct and adjust the control points; performing envelope processing again based on the corrected control points to generate a three-dimensional envelope image of the water transmission position of the injected coal sample; and verifying the accuracy of the above algorithm by comparing it with the pore and fracture structure reconstructed by computer tomography. The present invention corrects the collected control points in view of the complex fracture structure of the coal rock itself; improves the accuracy of the envelope line results; the fracture correction algorithm is more accurate and more realistically close to the actual measurement results, thereby improving the accuracy of the envelope results.

Description

Technical Field

[0001] The present invention relates to the technical field of coal rock wetting range monitoring and analysis, and in particular to a coal seam water injection range monitoring and optimization method based on acoustic emission array monitoring technology. Background Art

[0002] Laboratory research on coal seam hydraulic fracturing mainly relies on true triaxial seepage equipment, but traditional monitoring methods are unable to monitor the transmission range of coal seam hydraulic fracturing under true triaxial stress conditions. Therefore, how to monitor this range in true triaxial equipment should be solved as a top priority. Acoustic emission monitoring technology can respond sensitively to changes in the density of coal and rock, and the acoustic emission sensor can be embedded in the pressure head of the true triaxial experimental equipment. Therefore, a method for continuously monitoring the transmission range of coal seam hydraulic fracturing should be studied based on acoustic emission technology. The main deficiencies of current related research are as follows:

[0003] First, the acoustic emission technique was used to study the crack propagation location and determine the relationship between water content and wave parameters. Most scholars studied the changes in water content in rocks by studying the changes in waves actively excited by the acoustic emission instrument, while a few scholars determined the changes in water transmission locations by studying the changes in wave parameters.

[0004] Secondly, when using acoustic emission technology combined with CT scanning and reconstruction technology for analysis, CT reconstruction images are mainly used for qualitative analysis. In addition, crack parameters obtained by CT scanning are rarely introduced into the data analysis process to correct the data results.

[0005] Therefore, in order to solve the above problems, a new experimental method for monitoring the transmission range of coalbed water injection was designed based on the acoustic emission array monitoring technology. Summary of the invention

[0006] The embodiment of the present application provides a method for monitoring and optimizing the coal seam water injection range based on acoustic emission array monitoring technology, and utilizes it to solve the above-mentioned technical problems.

[0007] To achieve the above technical objectives, the technical solution adopted by the present invention is: a coal seam water injection range monitoring and optimization method based on acoustic emission array monitoring technology, the specific steps are as follows:

[0008] S1: Set up an acoustic emission array on the coal rock sample, establish a monitoring path network, and then collect data based on the monitoring path network;

[0009] S2: When the water injection port captures the P wave signal in the monitoring path, the characteristic signal data of water transmission to the monitoring path is confirmed; based on the characteristic signal data, the water transmission contour is enveloped;

[0010] S3: using the water transport correction control point algorithm and the fracture structure correction control point algorithm to correct and adjust the control points in sequence;

[0011] S4: performing envelope processing again based on the corrected control points to generate a three-dimensional envelope image of the water transmission position of the injected coal sample;

[0012] S5: The accuracy of the above algorithm is verified by comparing it with the pore and fracture structures reconstructed by computed tomography.

[0013] As a preferred technical solution: in step S3,

[0014] When the monitoring path does not consider large-scale cracks, the water transport correction control point algorithm is used to complete the correction of the control points: the vertical line from the water injection point to the monitoring path is the shortest path for water transmission, and the probability of water transmission to the vertical foot position is the highest; the calculated water transmission distance on the monitoring path is symmetrically distributed on both sides of the vertical foot, and two new water transmission contour control points are obtained;

[0015] When the effective monitoring path passes through a large area of ​​cracks, the crack correction method is used to correct the water transmission contour control points: the monitoring path with the intersection of the lines as the center and the surface symmetrically distributed on the straight line is the distance of the water transmission task on the monitoring path.

[0016] As a preferred technical solution: the crack correction method is a method of correcting control points using crack structures, and the specific process is as follows:

[0017] The linear parametric equation through points A and B is:

[0018]

[0019] The plane equation of the crack is:

[0020]

[0021] According to the above formula, the intersection point M (x12, y12, z12) can be solved;

[0022] Since there are multiple monitoring paths passing through the triangular area of ​​the crack, when the crack surface approaches the monitoring path vertically, the wetting area is often symmetrically distributed on both sides of the intersection M on the straight line AB; determine the angle between the straight line AB and the crack surface IJK;

[0023] Selecting a line segment of the monitoring path having an intersection point in the triangular region of the crack and determining an equation for the angle relationship between the line segment and the surface;

[0024]

[0025] Select the corrected control point at the maximum angle.

[0026] As a preferred technical solution: during low-pressure water injection, large fracture surfaces are the main structures that dominate water transmission. These surfaces are extracted on the digital plane in the form of triangular fractures. The spatial fracture network composed of triangular fractures dominates the transmission of water on the surface. The obtained digital fracture information is introduced into the fracture correction method to correct the contour of the control point.

[0027] As a preferred technical solution: in step S1, the coal rock sample is a cube structure; the acoustic emission array is an embedded acoustic emission transducer array, and the acoustic emission transducers are embedded on the six surfaces of the coal rock sample, and the acoustic emission transducers on each surface are arranged in a nine-point structure.

[0028] As a preferred technical solution: in step S3,

[0029] When the distribution directions of the cracks and water bodies are consistent and one end of the cracks is close to the water source, the distribution of water bodies in the coal matrix will be symmetrical with the cracks as the symmetry axis;

[0030] When the crack direction is approximately perpendicular to the water distribution direction, the water distribution is not symmetrical with the crack as the symmetry axis;

[0031] The fracture correction algorithm is selected to optimize the coal seam water injection range. When selecting the correction algorithm, the fracture correction algorithm can be selected only when the distance from the injection point to the monitoring path is at least 1.29 times the distance from the injection point to the fracture.

[0032] As a preferred technical solution: the distance from the injection point to the monitoring path is at least a multiple of the distance from the injection point to the fracture. The specific algorithm is as follows:

[0033] Assume that the straight line L (x, y, z) is the intersection of the plane composed of the three points (x0, y0, z0), (x1, y1, z1), and (x2, y2, z2) and the plane where the triangular fracture surface is located, and the distance from the injection point (x0, y0, z0) to the straight line L (x, y, z) is Lb, and the distance from the injection point (x0, y0, z0) to the straight line where the monitoring path is located is La;

[0034] When La> nLb and n≥ 1.29, the distribution directions of the cracks and water bodies are closer to each other. The water source will first be transmitted into the cracks, and then propagate from the cracks to the matrix along the radial method, and then propagate to the vertical foot position. At this time, the water body distribution is symmetrical with the cracks as the symmetry axis, and the envelope result is corrected by the intersection of the crack surface and the monitoring path straight line.

[0035] The present invention is an improvement on the traditional coal rock wetting range monitoring and analysis experimental method. In view of the complex fracture structure of coal rock itself, the water transport correction control point algorithm and the fracture structure correction control point algorithm are gradually adopted to correct the collected control points; the calculated transmission range and large fractures reconstructed by CT scanning are used to correct the control points of the water transmission contour to improve the accuracy of the envelope line results; the fracture correction algorithm is more accurate and more realistically close to the actual measurement results, thereby improving the accuracy of the envelope results. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a schematic diagram of the propagation of ultrasonic waves in pores and coal matrix;

[0037] Figure 2 is the transducer layout diagram;

[0038] Figure 3 is the envelope control point distribution map;

[0039] Figure 4 is the data envelopment diagram;

[0040] Figure 5 It is a schematic diagram of correction based on the location of the fissure;

[0041] Figure 6 yes Figure 5 A magnified image of

[0042] Figure 7 It is a schematic diagram of the correction method of the crack structure influence of the present invention;

[0043] Figure 8 is a schematic diagram of digital fracture surface reconstruction in the present invention;

[0044] Figure a is the surface map of the connected cracks; Figure b is the digital space information map of the cracks;

[0045] Fig. 9 Secondary correction of control point distribution map for fracture structure;

[0046] Fig.10 It is the secondary corrected envelope diagram of the crack structure;

[0047] Fig.11 This is a schematic diagram of the distribution of cutting water delivery distance;

[0048] Fig.12 It is a schematic diagram of the structure of pores and cracks in different parts reconstructed by CT scanning of the present invention;

[0049] Fig.13 Comparison diagram of envelope results and experimental results.

[0050] Fig.14 Schematic diagram of the correction algorithm for determining crack morphology.

[0051] Fig.15 This is the spatial distribution diagram of the envelope control points of 10 data combinations.

[0052] Fig.16 The wetting range contour diagram is a combination of 10 data sets.

[0053] Fig.17 Comparison chart of envelope results of 10 data combinations and experimental results.

[0054] Fig.18 The bar graph is a quantitative comparison analysis of the envelope results of 10 data combinations and the experimental results. DETAILED DESCRIPTION

[0055] In order to illustrate the technical solution described in this application, a specific embodiment is provided below for illustration.

[0056] An acoustic emission array is arranged on the coal and rock samples to construct a monitoring path network, and then data is collected based on this network. On a specific monitoring path, once the water injection port captures the P-wave signal, it is regarded as the landmark data that water has reached the path. Based on these characteristic signals, the water transmission contour line is enveloped; the water transport correction control point algorithm and the fracture structure correction control point algorithm are used in turn to accurately adjust the control point position. Based on the adjusted control points, the envelope processing is performed again to generate a three-dimensional envelope image of the water transmission position in the water-injected coal sample. By comparing the pore and fracture structure reconstructed by computer tomography and the water transmission contour line data generated by various algorithms, we have determined a more accurate algorithm to verify the accuracy of the above algorithm.

[0057] The present invention verifies the feasibility of the designed experimental method and the accuracy of the envelope of the water transmission contour of coal seam water injection. In order to improve the experimental efficiency and eliminate the interference of unnecessary conditions, a water transmission range monitoring experiment without triaxial stress conditions was designed. In order to simulate the ultra-low pressure water injection environment, the water injection pressure was set to 500Pa and the water injection time was set to 0,48 hours. Complete wetting requires 168 hours. The development of cracks during the water transmission process after coal seam water injection was not considered. The above-mentioned groups of acoustic emission monitoring data were collected. After the experiment, a plane perpendicular to the borehole was cut at the top of the borehole. The water transmission contour was then drawn and presented in a two-dimensional coordinate system. In addition, the depicted water transmission contour was compared with the envelope of the water transmission range to verify the envelope result.

[0058] The specific steps are as follows:

[0059] S1: Set up an acoustic emission array on the coal rock sample, establish a monitoring path network, and then collect data based on the monitoring path network;

[0060] In this embodiment, the experimental coal sample was collected from the coal rock of the Gaojiabao Coal Mine in the No. 4 coal seam of Zhongmei, Shaanxi Province, China. The coal rock sample was selected as a cubic structure of 100 mm × 100 mm × 100 mm; the sample was dried at 80°C for 24 hours; a cylindrical hole with a diameter of 10 mm and a depth of 50 mm was drilled in the center of the water injection surface. The acoustic emission array is an embedded acoustic emission transducer array, and the acoustic emission transducers are embedded on the six faces of the coal rock sample. The acoustic emission transducers on each face are arranged in a nine-point structure. The number of test channels of the acoustic emission meter is 16, but the total number of transducers arranged on all sides of the cubic test piece is 53. Therefore, the data of each transducer is collected by line switching. Ultrasonic signals can be amplified by a high-power amplifier to improve the penetration of the wave. The transducer and the signal generator are of the same model, the size of the piezoelectric ceramic is 18.8 mm (diameter) × 1 mm (thickness), the piezoelectric ceramic is PZT-5 series, and the center frequency is 150 kHz, see Figure 2 .

[0061] The six faces of the cubic coal seam sample are labeled S1, S2, S3, S4, S5, and S6, and an acoustic emission transducer array is arranged on each face, as shown in FIG. Figure 2 As shown. The transducer collects data from the monitoring path between any transducers on two different surfaces, thereby forming a three-dimensional monitoring path network. When a monitoring path exhibits a phenomenon that P waves cannot be monitored before water injection but become monitorable after water injection, the spatial position information of the monitoring path is picked up, that is, data is collected based on the monitoring path network. Subsequently, these picked effective monitoring paths are used to determine the water transmission profile. The perpendicular point from the incident point to the effective monitoring path is the shortest distance point. In the probability analysis, the perpendicular point is the position with the maximum probability that the water transmission surface will first reach the monitoring path.

[0062] S2: When the water injection port monitors or captures the P wave signal in the same monitoring path, the characteristic signal data of water transmission to the monitoring path, i.e., the P wave, is confirmed. Based on these characteristic signal data, the water transmission contour is enveloped. That is, when the monitoring path shows that the P wave cannot be monitored before water injection but can be monitored after water injection, the path is a valid monitoring path, and each group of valid monitoring paths has corresponding ultrasonic monitoring data. The spatial position information of the monitoring path is picked up, the water transmission contour is determined using the spatial position information, and the control points of the water transmission contour are determined by the inscribed circle method.

[0063] Under different water injection times, the wave parameters of the excited ultrasonic waves are different after propagation on the monitoring path. The variation characteristics are used to analyze the ultrasonic propagation parameters before and after water injection in the fractured coal; Figure 1 The following is a schematic diagram of the propagation of ultrasonic waves in pores and coal matrix. There are a large number of small holes and a small number of large cracks distributed inside the coal body, which constitute different propagation paths. The processing methods are as follows:

[0064] (1) When there is only a small hole in the ultrasonic propagation path, the transmission and non-transmission areas of the water body conform to the Wyllie model, such as Figure 1 As shown in the figure, in this model, the internal pore filling medium affects the internal propagation velocity of ultrasonic P waves. The Wyllie model approximates the porous coal seam as consisting of a porous coal seam skeleton (pm) and the medium filling the pores (pl). The relationship between the ultrasonic propagation velocity is

[0065] (1)

[0066] Where Vp is the propagation velocity of the P wave in the entire coal body, m / s. Vpl is the wave velocity when the P wave propagates in the pores, m / s, and Vpm is the propagation velocity of the P wave in the coal skeleton, m / s. When the pores and cracks are filled with air, the propagation velocity of the P wave in the entire coal body is Vpa, m / s. When the pores and cracks are filled with water, the propagation velocity of the P wave in the entire coal body is Vpb, m / s.

[0067] (2) When a larger crack path appears during the ultrasonic propagation process, the propagation consumption of the longitudinal wave in the crack at the crease increases, and the propagation form changes, which requires further analysis. The scale of the crack is much larger than the pore. Therefore, the propagation position of the ultrasonic wave in the crack is an interface propagation problem. At this time, the acoustic impedance of the fracture medium and the coal matrix can be determined by analyzing the formation of ultrasonic propagation. The main principles are as follows:

[0068] (2)

[0069] In the formula, is the acoustic impedance, kg / (m2·s). When it is close to 0, the acoustic impedance matches. At this time, the main ultrasonic wave is refracted from medium 2 to medium 1. When it is close to 1, the acoustic impedance is not matched. The main ultrasonic wave is reflected at the interface between medium 1 and 2, and the energy of the refracted wave is almost 0.

[0070] The effective monitoring path is determined by its characteristic mark. Each set of effective monitoring paths has corresponding ultrasonic monitoring data. Statistical analysis of these data can obtain the monitoring path when water is transported.

[0071] The three-dimensional spatial scatter plot of the control points and the water delivery contours based on the envelope of the control points are shown in Figure 2. Figure 3-4 shown. Figure 3 shows the distribution of control points, and Figure 4The data envelope is shown. The analysis shows that the small number of control points results in limited accuracy of the water delivery contour envelope. The surface of the envelope shows that the contour of the water delivery range presents a relatively complete shape, and the overall appearance is similar to an ellipsoid. The elements on the surface of the envelope are large, the shape of the water delivery contour is poorly expressed, and the water delivery contour can only be roughly displayed.

[0072] S3: Correct the control points using the water transport correction control point algorithm and the fracture structure correction control point algorithm in sequence.

[0073] In order to further increase the amount of control point data and improve the accuracy of the water delivery contour envelope, the control point selection method was optimized and corrected, thereby improving the accuracy of control point selection, such as Figure 5-6 , and gradually use the water transport correction control point algorithm and the fracture structure correction control point algorithm to correct the control points.

[0074] In the same monitoring path, the P-wave velocity monitored before water injection is v0, and the P-wave velocity monitored after water injection for 48 hours is v1. When fully wetted, the velocity of the P-wave is v2. The water penetration distance is L2, and the distance of the water non-penetration area is L0. The P-wave velocity in the L0 area is assumed to be v0, and the P-wave velocity in the L2 area is assumed to be v2. The length of the monitoring path is L1, and the calculation formula for L2 is

[0075] (3)

[0076] Some monitoring paths do not receive P waves; therefore, the P wave velocity must be calculated. There is a certain relationship between the propagation speeds of P waves and S waves in porous media, and they often present a certain ratio, called the aspect ratio. At this time, the unmonitored P wave velocity can be determined by the average direction of the entire coal sample.

[0077] (4)

[0078] By substituting the obtained P-wave velocity into formula (3), the water transmission distance L2 can be solved. Subsequently, the calculated water transmission distance on the monitoring path can be introduced into the correction process of the control point.

[0079] The correction process of the control points is as follows:

[0080] First, when large-scale cracks are not considered, the vertical line from the injection point to the monitoring path is considered to be the shortest path for water transmission, and the probability of water transmission to the vertical foot position is the highest. Therefore, the calculated water transmission distance on the monitoring path is symmetrically distributed on both sides of the vertical foot. In this way, two new water transmission contour control points are obtained, and the first correction of the control points is completed, that is, the algorithm for correcting the control points using the water transmission distance. According to the measured wave velocity, the distance of L2 can be calculated using dance. Here, let the length section of L2 be s, and assume that the water transmission distance is symmetrically distributed on the straight line of the monitoring path, with the vertical point as the center. Since point E is on straight line AB, it satisfies

[0081] (5)

[0082] So there is

[0083] (6)

[0084] According to formulas (5) and (6), E(x10, y10, z10) and D(x9, y9, z9) can be solved.

[0085] Then, in actual situations, coal rock has a complex fracture structure. Drilling excavation leads to cracks around the boreholes, and these large fracture surfaces are the main channels for water propagation in the surrounding rock. Therefore, when the monitoring path is effective, after a large area of ​​fracture area, the distance of the water delivery task on the monitoring path is centered on the intersection of the line and the surface is symmetrically distributed on the straight line monitoring path. At this time, the correction of the control point of the water transmission profile no longer follows the first correction result, but is replaced by the fracture correction method.

[0086] When many monitoring paths pass through the same crack area, this correction method is suitable for situations where the angle between the straight line and the plane is large. The algorithm for correcting control points using crack structure is as follows:

[0087] The linear parametric equation through points A and B is:

[0088] (7)

[0089] The plane equation of the crack is:

[0090] (8)

[0091] According to formulas (7) and (8), the intersection point M (x12, y12, z12) can be solved. According to the "algorithm for correcting control points using water delivery distance", the coordinates of correction points F and G can be calculated. Since the crack triangle may have multiple monitoring paths passing through it, when the crack surface approaches the monitoring path vertically, the wetting area is often symmetrically distributed on both sides of the intersection point M on the straight line AB. Therefore, it is necessary to determine the angle between the straight line AB and the crack surface IJK, such as Figure 7 shown.

[0092] A line segment of the monitoring path having an intersection point in the triangular area is selected, and an angle relationship equation between the line segment and the surface is determined.

[0093] (9)

[0094] Then select the corrected control point at the maximum angle.

[0095] During the simulation, it was found that when the cracks and water bodies are distributed in the same direction and one end of the crack is close to the water source, the distribution of water bodies in the coal matrix will be symmetrical with the cracks as the symmetry axis, while when the crack direction is approximately perpendicular to the water body distribution direction, the distribution of water bodies will not be symmetrical with the cracks as the symmetry axis. Therefore, in order to determine how to more reasonably select crack data to correct the envelope results, the crack data correction selection conditions are analyzed, such as Fig.14 shown.

[0096] The main calculation principle for correction is as follows.

[0097] During the distribution of water in the injected coal sample, what kind of proportional relationship (n is the proportional coefficient) should be present between the distance from the injection point to the monitoring path and the distance from the injection point to the fracture, so that the water body will be symmetrically distributed on the monitoring path with the intersection of the fracture surface and the monitoring path as the center. This problem is the core issue of the optimization algorithm.

[0098] This embodiment counts all the data groups in this experiment that meet the monitoring path passing through the crack surface, and arranges all the data according to the size of the proportional coefficient n, reducing the crack data one by one, and gradually expanding the range of n (each group of data corresponds to a value of n), and compares and analyzes each group of corrected results with the experimental results until the optimal result is obtained, thereby obtaining the range of n. The specific method is as follows.

[0099] like Fig.14As shown, it is assumed that the straight line L (x, y, z) is the intersection of the plane composed of three points (x0, y0, z0), (x1, y1, z1), and (x2, y2, z2) and the plane where the triangular fracture surface is located. And the distance from the injection point (x0, y0, z0) to the straight line L (x, y, z) is Lb, and the distance from the injection point (x0, y0, z0) to the straight line where the monitoring path is located is La. When La> nLb, it is believed that the cracks and the water body distribution directions are closer to each other, and the water source will first be transmitted into the cracks, and then propagate from the cracks to the matrix along the radial method, and then propagate to the vertical foot position. At this time, the water body distribution is symmetrical with the cracks as the symmetry axis, and the intersection of the crack surface and the monitoring path line is used to correct the envelope result.

[0100] In this embodiment, the range of n is determined according to the change of the screening data. When all cracks are used for correction, there are 10 data combinations, that is, there are 10 intersections for crack correction. The number of combinations can be changed by changing the n value. When n=1.29, the number of remaining groups is 9, when n=1.3, the number of remaining groups is 8, when n=1.3505, the number of remaining groups is 7, when n=1.351, the number of remaining groups is 6, when n=1.36, the number of remaining groups is 5, when n=1.39, the number of remaining groups is 4, when n=2, the number of remaining groups is 3, when n=2.3, the number of remaining groups is 2, when n=3, the number of remaining groups is 1, and when n=4.7, the number of remaining groups is 0. Therefore, 10 groups, 9 groups, 8 groups, 7 groups, 6 groups, 5 groups, 4 groups, 3 groups, 2 groups, and 1 group of data are used for correction respectively, and the difference between the envelope results and the experimental slicing results is compared to determine the optimal range of n, such as Figure 15-18 shown.

[0101] Through the solution, it is found that with the increase of the number of groups using crack correction, the wetting range contour structure of the envelope becomes more and more complex, and the local structure display becomes more and more delicate. When 1 set of correction data is used, it is found that two positions are quite different from the experimental results. When 2 sets of correction data are used, it is found that three positions are quite different from the experimental results. With the increase of the number of groups, the wetting range contour gradually changes. When 5 sets of correction data are used, an inclined surface is added in the lower right corner. This surface is the area of ​​the crack surface around the borehole. The shape of the wetting range at this position is similar to the contour of the crack, which is an enlarged display of the crack contour. Then when the number of correction data groups increases to 9 groups, the errors of the two positions where there were obvious errors before are significantly reduced. Observing the three-dimensional contour, there is a significant change in the upper part. At this time, the contour of the envelope is closer to the true wetting range contour. When 10 sets of correction data are used, the envelope result has obvious errors, and the envelope control point exceeds the entity area. The accuracy of the envelope data at this time begins to decline. It can be seen that at the beginning, as the number of crack correction data sets increases, the resolution of the envelope's wetting range area contour gradually increases. However, when it increases to a certain extent, the crack correction data used will produce deviations. At this time, the crack may contain less water, and the water in the matrix has not been transported to the crack area. The crack is close to the boundary and far from the water injection point. Using this crack for data correction will result in a large error. Fig.18 From the quantitative comparative analysis of the bar graph, it can be found that the difference between the envelope result and the actual result is getting smaller and smaller. Fig.18 The correlation coefficients between the envelope results and the experimental results in the 10 groups of data are 0.94272, 0.92244, 0.95261, 0.95261, 0.95261, 0.95261, 0.95261, 0.95261, 0.95261, 0.95851, and 0.95851. The data show that the accuracy is relatively high when 9 and 10 groups of data are used for correction.

[0102] Based on the above analysis, when 9 sets of correction data are used in this embodiment, the envelope profile accuracy reaches the maximum value, and at this time, the minimum value of n is 1.29. That is, when selecting the correction algorithm, the crack correction algorithm can be selected only when the distance from the injection point to the monitoring path is at least 1.29 times the distance from the injection point to the crack (n ≥ 1.29).

[0103] S4: Envelope processing is performed again based on the corrected control points to generate a three-dimensional envelope image of the water transmission position of the injected coal sample.

[0104] During low-pressure water injection, these large fracture surfaces are the main structures that dominate water transport. Therefore, these surfaces are extracted into the digital plane in the form of triangular fractures, and the spatial fracture network composed of triangular fractures dominates the water transport on the surface, such as Figure 8As shown. The digital information of the water transmission control points obtained can be introduced into the above algorithm to correct the contour of the control points. The control points obtained by the above two algorithms are subjected to envelope processing, as shown Figure 9-10 As shown, these are the control points after secondary correction of the fracture structure.

[0105] S5: The accuracy of the above algorithm is verified by comparing with the pore and fracture structures reconstructed by computer tomography.

[0106] After combining the analysis envelope of the three-dimensional surface, the water conveyance distance is introduced for the correction, the number of contour control points increases, and the accuracy of the envelope result is improved. After further correction, the fracture surface reconstructed by CT is used, and the large-scale fracture surface structure of coal is reconstructed by CT scanning. The contour position control points are determined more accurately, and the shape distribution is uniform. The transmission contour reflects the influence of the structure. In the lower part, the influence of the coal body structure on the water conveyance task contour is shown in detail. Therefore, with the increase of the crease correction number, the envelope of the water conveyance contour becomes more detailed.

[0107] The change of water transmission range is mainly affected by the injection pressure and the internal structure of coal. As the distance from the tangent surface to the injection surface increases, the water pressure in the borehole caused by gravity gradually increases. In order to analyze the influence of internal structure and water pressure on the water transmission range, the pore and fracture structures reconstructed by CT scanning at the corresponding slice position were analyzed, such as Fig.11 shown.

[0108] Fig.12It shows that in the area where water injection boreholes exist, the fracture structure is mostly connected to the water injection boreholes. In the cut planes at 30, 40, and 50 mm, the number of fractures connected to the boreholes is the largest in the 30 mm slice plane. This result shows that even if the water pressure is the highest at 50 mm, the water transmission distance at this location is smaller than that at 30 mm. This also shows that when the contact area between water and the well wall is the same, the fractures dominate the water transmission. However, after the water injection boreholes disappear, the distribution of fractures in the range of 60–90 mm is similar; only in the range of 60–70 mm does the water transmission range slowly decrease. In the range of 70–90 mm, the water transmission distance decays rapidly with the decay of water pressure, indicating that water pressure plays a vital role in water transmission at the location of the cut plane (not in direct contact with the water injection surface). The permeability is judged based on the distribution of fractures. At 10 mm, there is little difference in the development of horizontal and vertical fractures, but most of the fractures in the vertical direction are connecting fractures, and there are few virtual connection areas. Therefore, the transmission range of water in the vertical direction should be greater than that in the horizontal direction. At 20mm, the vertical distribution of cracks is greater than the horizontal distribution of cracks. Therefore, the range of vertical water transmission should be greater than that of horizontal water transmission. At 30mm, the distribution of vertical cracks is greater than the horizontal distribution of cracks; however, more virtual joint cracks are found in the vertical direction. Therefore, the range of water transmission in both directions should be similar. At 40mm, the horizontal cracks are larger than the vertical cracks. Therefore, the range of horizontal water transmission should be larger than that of the vertical transmission. At 50mm, the distribution of cracks in both directions does not change much, but as the diameter of the water injection borehole decreases, the dominant role of cracks near the borehole increases. Several vertically connected cracks are found in the area close to the borehole; therefore, the distance of vertical water transmission becomes longer. Fig.13 In the figure, the upper figure is the cross-sectional view of the water transmission CT slice; the lower figure is the envelope diagram after correction by the crack correction method.

[0109] The new method of monitoring coal injection water delivery contour by embedded acoustic emission transducer array proposed by the present invention is proposed in true triaxial equipment. In the same monitoring path, before water injection, the longitudinal wave cannot be monitored. After water injection, it can be used as a characteristic mark to determine whether water has reached the monitoring path. The selection and correction algorithm of water delivery control points are studied and the envelope of water delivery contour is optimized.

[0110] CT reconstruction of pore and fracture structures The scan results are used to verify the accuracy of the crack correction method, which is as follows:

[0111] (1) After water injection into fractured coal, the penetration depth of longitudinal waves is enhanced; some monitoring paths cannot monitor longitudinal waves before water injection; however, longitudinal waves can be monitored. This phenomenon is used as a sign of characteristic water transport into the monitoring path; it is found that the ultrasonic energy saved due to the reduction of the acoustic impedance mismatch phenomenon is greater than the ultrasonic energy wasted due to the change of the medium in the fracture.

[0112] (2) The water delivery rate after distance correction and fracture position correction is higher than the control point initially determined. The fracture correction algorithm has high accuracy. Water transmission is affected by water pressure and fractures. The fractures directly connected to the injection well are the main structures that dominate water transmission. The transmission contour around the water borehole is ellipsoidal in shape, and the shape of the fracture can be reflected locally, which is based on the comparison with the CT scan slice.

Claims

1. A method for monitoring and optimizing coal seam water injection range based on acoustic emission array monitoring technology, characterized in that: The specific steps are as follows: S1: Set up an acoustic emission array on the coal and rock samples, establish a monitoring path network, and then collect data based on the monitoring path network; S2: When the water injection port captures the P wave signal in the monitoring path, the characteristic signal data of water transmission to the monitoring path is confirmed; based on the characteristic signal data, the water transmission contour is enveloped; S3: using the water transport correction control point algorithm and the fracture structure correction control point algorithm to correct and adjust the control points in sequence; When selecting the correction algorithm, the fracture structure correction control point algorithm can be selected only when the distance from the injection point to the monitoring path is at least 1.29 times the distance from the injection point to the fracture; The distance from the injection point to the monitoring path is at least a multiple of the distance from the injection point to the fracture. The specific algorithm is as follows: Assume that the straight line L (x, y, z) is the intersection of the plane composed of the three points (x0, y0, z0), (x1, y1, z1), and (x2, y2, z2) and the plane where the triangular fracture surface is located, and the distance from the water injection point (x0, y0, z0) to the straight line L (x, y, z) is Lb, and the distance from the water injection point (x0, y0, z0) to the straight line where the monitoring path is located is La; (x1, y1, z1) and (x2, y2, z2) are the transducer position points; When La> nLb and n≥ 1.29, the distribution directions of the cracks and water bodies are closer to each other, and the water source will first be transferred into the cracks, and then propagate from the cracks to the matrix along the radial method, and then propagate to the vertical foot position. At this time, the water body distribution is symmetrical with the cracks as the symmetry axis, and the intersection of the crack surface and the monitoring path straight line is used to correct the envelope result; S4: performing envelope processing again based on the corrected control points to generate a three-dimensional envelope image of the water transmission position of the injected coal sample; S5: The accuracy of the above algorithm is verified by comparing it with the pore and fracture structures reconstructed by computed tomography.

2. The method for monitoring and optimizing coal seam water injection range based on acoustic emission array monitoring technology according to claim 1 is characterized in that: During low-pressure water injection, large fracture surfaces are the main structures that dominate water transmission. These surfaces are extracted on the digital plane in the form of triangular fractures. The spatial fracture network composed of triangular fractures dominates the water transmission on the surface.

3. The method for monitoring and optimizing coal seam water injection range based on acoustic emission array monitoring technology according to claim 1 is characterized in that: In step S1, the coal rock sample is a cubic structure; the acoustic emission array is an embedded acoustic emission transducer array, and the acoustic emission transducers are embedded on six surfaces of the coal rock sample, and the acoustic emission transducers on each surface are arranged in a nine-point structure.

4. The method for monitoring and optimizing coal seam water injection range based on acoustic emission array monitoring technology according to claim 1 is characterized in that: In step S3, When the distribution directions of the cracks and water bodies are consistent and one end of the cracks is close to the water source, the distribution of water bodies in the coal matrix will be symmetrical with the cracks as the symmetry axis; When the direction of the crack is approximately perpendicular to the direction of water distribution, the distribution of the water is not symmetrical with the crack as the axis of symmetry.

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