Coal and rock mass hydraulic fracturing effect evaluation method based on micro-seismic monitoring

Abstract

The invention relates to the technical field of hydraulic fracturing, in particular to a coal and rock mass hydraulic fracturing effect evaluation method based on micro-seismic monitoring, which comprises the following steps of engineering geology investigation and survey, and fracturing data collection, screening and processing. Calculating the theoretical fracturing volume Vr of the coal and rock mass; calculating an actual fracturing volume Vf based on an octree; calculating a hydraulic fracturing fracture volume ratio K, and evaluating the integral fracturing effect of hydraulic fracturing; evaluating a fracturing crack expansion range based on the micro-seismic frequency nuclear density; the fracturing crack propagation strength is evaluated based on the micro-seismic energy nuclear density; and local reinforcement pressure relief is carried out on the area with insufficient fracturing. According to the method, the octree is used for calculating the fracturing volume, errors caused by non-response areas of micro-seismic events are effectively removed, and the requirement for large-scale data gridding volume calculation generated by fracturing can be met.

Description

Technical Field

[0001] The invention relates to the technical field of hydraulic fracturing, in particular to a method for evaluating the effect of hydraulic fracturing of coal and rock mass based on microseismic monitoring. Background Art

[0002] With the depletion of shallow coal resources, coal mining has gradually extended to deeper areas. The deep coal and rock masses are in a more complex geomechanical environment. The intensity and probability of dynamic disasters such as rock bursts, coal and gas outbursts are greater. Traditional coal and rock permeability and pressure relief measures can no longer meet the needs of safe and efficient mining in coal mines. Long-distance directional drilling hydraulic fracturing measures can achieve permeability and pressure relief of coal and rock masses on a large scale, weaken and transform coal and rock masses, cut coal and rock masses with high integrity into multiple blocks, release the elastic energy accumulated in coal and rock in advance, and reduce the risk of rock bursts and gas outbursts in coal and rock. For high-gas mines, hydraulic fracturing forms an intricate fracture grid system, increases the permeability of coal and rock, avoids high gas accumulation, and is conducive to gas extraction. It is an advanced technical means that can achieve large-scale coal and rock weakening under existing deep mining.

[0003] There are many factors that affect the effect of hydraulic fracturing. Therefore, the fracturing effects in different areas are different under the same fracturing parameters. Therefore, it is necessary to evaluate the overall fracturing effect and identify the areas where fracturing is insufficient, so as to provide a basis for further precise reinforcement and pressure relief measures. At present, the evaluation of hydraulic fracturing effect is mainly based on water injection pressure curve, post-fracturing overburden drilling detection, in-hole transient electromagnetic detection, and microseismic monitoring. Among them, microseismic monitoring is a commonly used method for monitoring coal and rock fractures in mining due to its high monitoring accuracy and good monitoring effect.

[0004] Hydraulic fracturing creates cracks in coal and rock within a certain spatial range artificially, forming a complex fracture grid in the coal and rock, thereby achieving the purpose of pressure relief and permeability enhancement. Microseismicity can be used to monitor the signals of coal and rock fractures of different scales during fracturing, and calculate their spatial position and energy. Each microseismic event represents the location of coal and rock fractures, and its energy represents the size of the coal and rock fracture cracks. At present, the evaluation of hydraulic fracturing effects based on microseismic data is limited to the statistical analysis of microseismic source location, frequency, and energy, and there is a lack of effective methods that can directly evaluate the fracture volume, fracture extension range, and extension intensity.

[0005] In view of this, the present invention proposes a method for evaluating the hydraulic fracturing effect of coal rock mass based on microseismic monitoring, which solves the above technical problems. Summary of the invention

[0006] The purpose of this section is to summarize some aspects of embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the specification abstract and the invention title of this application to avoid blurring the purpose of this section, the specification abstract and the invention title, and such simplifications or omissions cannot be used to limit the scope of the present invention.

[0007] A method for evaluating the effect of hydraulic fracturing of coal rock mass based on microseismic monitoring comprises the following steps:

[0008] Engineering geological survey, fracturing data collection, screening and processing;

[0009] Calculate the theoretical fracturing volume Vr of coal rock mass;

[0010] Calculate the actual fracturing volume Vf based on the octree;

[0011] Calculate the hydraulic fracturing fracture volume ratio K and evaluate the overall hydraulic fracturing effect;

[0012] Evaluate the extension range of hydraulic fractures based on microseismic frequency kernel density;

[0013] Evaluation of fracture propagation strength based on microseismic energy kernel density;

[0014] Local reinforcement and pressure relief are adopted for areas with insufficient fracturing.

[0015] As a preferred solution of the method for evaluating the effect of hydraulic fracturing of coal rock mass based on microseismic monitoring provided by the present invention, engineering geological survey and basic data collection are carried out. The basic data for hydraulic fracturing in mines are surveyed, including the buried depth of coal seams and the inclination of coal seams, and the hydraulic fracturing construction plan is collected, including the position, thickness, hardness, etc. of the fracturing coal and rock layers. The microseismic data generated during the coal seam fracturing process are collected. The microseismic data format must include time, x, y, z coordinates, microseismic energy, etc. The microseismic event positioning plane and profile base map are collected;

[0016] The collected microseismic data are preliminarily used to locate the earthquake source in the cross-section and plan views; in order to improve the accuracy of the fracturing effect evaluation, the microseismic data outside the fracturing range are eliminated.

[0017] As a preferred solution of the method for evaluating the effect of hydraulic fracturing of coal rock mass based on microseismic monitoring provided by the present invention, the total volume Vr of the theoretically fractured coal rock layer is calculated according to the coal mine hydraulic fracturing design and construction plan:

[0018] V r =l×[2r+(n-1)d]×h

[0019] Where r is the fracturing radius, d is the fracturing borehole spacing, n is the number of fracturing boreholes, l is the length of the horizontal section of the fracturing borehole, and h is the thickness of the fracturing coal rock layer (if the thickness of the coal rock layer is less than the fracturing radius, the fracturing height is the thickness of the coal rock layer, that is, h = m; if the thickness of the coal rock layer is greater than the fracturing radius, the fracturing height is the fracturing radius, that is, h = r).

[0020] As a preferred solution of the method for evaluating the effect of hydraulic fracturing of coal rock mass based on microseismic monitoring provided by the present invention, hydraulic fracturing forms an artificial large-scale fracture network system in the coal rock mass, and the generation of fractures during the fracturing process forms microseismic events of different energy levels. Therefore, the spatial distribution and energy and frequency characteristics of microseismic events can reflect the development of fractures in coal rock, and accordingly, an octree is introduced to evaluate the fracture volume generated by fracturing;

[0021] The specific process of calculating the fracturing volume based on the octree is as follows:

[0022] (1) Construct the octree root node. Define the theoretical fracturing volume as the octree root node (i.e., the 0th-level node). The center position and side length of the root node determine its position in three-dimensional space.

[0023] (2) Recursively divide the octree child nodes. Divide the root node equally into 8 first-level sub-regions (first-level sub-nodes) along the three dimensions of x, y, and z. These first-level sub-regions are the first-level nodes of the octree. Eliminate the empty nodes (excluding microseismic events) and select the non-empty nodes. Divide the non-empty nodes in the first-level nodes equally into 8 second-level sub-regions (second-level sub-nodes) along the three dimensions of x, y, and z. These second-level sub-regions are the second-level nodes of the octree. Eliminate the empty nodes (excluding microseismic events) and select the non-empty nodes.

[0024] (3) Set the target density threshold and make conditional judgments. Set the target threshold for the spatial distribution density of microseismic events at non-empty nodes, and make judgments on the nodes at each level. When the microseismic event distribution density is less than the threshold, continue to divide the next subnode; when the microseismic event distribution density is greater than the threshold, stop dividing and mark the node;

[0025] (4) Performing octree partitioning on all microseismic events during the fracturing period according to (2) and (3) until the number of non-empty nodes of all microseismic events is greater than the target threshold, stopping the cyclic iteration, and obtaining the octree grid of all non-empty nodes of microseismic events;

[0026] (5) Calculate the edge length of the non-empty nodes at the i-th level that meet the microseismic event density threshold. Since the root node is at the 0th level, the edge lengths of the root node along x, y, and z are Each time the nodes of the next level are divided, they are divided into eight equal parts, that is, they are divided into two equal parts along each edge length. Then the edge length of the non-empty node of the i-th layer is:

[0027]

[0028] Where: is the length of the edge of the non-empty node that meets the density threshold at the i-th level. i is the total level of the octree division (i=1, 2, 3, ...n), j is an edge of the node (j=1, 2, 3);

[0029] (6) Calculate the volume of non-empty nodes at the i-th level that meet the microseismic event density threshold. After obtaining the side length of the i-th level non-empty nodes, the volume of each non-empty node can be obtained as:

[0030]

[0031] Where: V i 0 is the volume of a single non-empty node at the i-th level that meets the density threshold;

[0032] (7) Calculate the total volume of all non-empty nodes that meet the microseismic event density threshold at the i-th level. Traverse all non-empty nodes that meet the threshold at the i-th level and add up the total volume of non-empty nodes that meet the density threshold at the i-th level:

[0033]

[0034] Where: V i is the total volume of all non-empty nodes that meet the density threshold at the i-th level. h is the number of non-empty nodes that meet the density threshold at the i-th level (h = 1, 2, 3, ...s);

[0035] (8) Calculate the total volume of non-empty nodes that meet the density threshold at all levels, which is the fracturing volume:

[0036]

[0037] As a preferred solution of the method for evaluating the effect of hydraulic fracturing of coal and rock mass based on microseismic monitoring provided by the present invention,

[0038] (1) Based on the theoretical fracturing volume Vr and the fracturing crack expansion volume Vf calculated above, the hydraulic fracturing crack volume ratio K can be obtained as follows:

[0039]

[0040] (2) The degree of fracture development during the fracturing process is evaluated based on the hydraulic fracturing fracture volume ratio, thereby measuring the overall fracturing effect of hydraulic fracturing. When the fracture volume ratio is: K < 0.05, the fracturing effect is poor; when the fracture volume ratio is 0.05 < K < 0.2, the fracturing effect is good; when the fracture volume ratio is: K > 0.2, the fracturing effect is good.

[0041]

[0042] As a preferred solution of the method for evaluating the effect of hydraulic fracturing of coal rock mass based on microseismic monitoring provided by the present invention, the above evaluation of the fracturing effect based on the calculation of the fracturing volume and the crack volume ratio based on the octree can obtain the overall fracturing effect, but it is impossible to determine whether the extension range of the fracturing cracks covers the entire theoretical fracturing range. Therefore, the microseismic event frequency kernel density value is used to evaluate the extension range of the fracturing cracks;

[0043] (1) Calculate the frequency kernel density of microseismic events in coal and rock mass and draw spatial cloud maps;

[0044] Based on the collected microseismic events during fracturing, the energy kernel density is calculated as:

[0045]

[0046] Where: N is the kernel density value of microseismic event frequency, (∑N i ) j is the total number of microseismic events in unit space j, V j is the volume of unit space j, i is the number of microseismic events, and j is the number of unit spaces;

[0047] (2) Assess the extent of hydraulic fracturing fracture extension;

[0048] The frequency kernel density value of microseismic events during fracturing is calculated and a spatial cloud map is drawn. The map can be used to define the scope of crack expansion and identify the expansion of fracturing cracks. Areas with smaller frequency kernel density values ​​are sparse crack expansion areas, and areas with larger frequency kernel density values ​​are dense crack expansion areas.

[0049] As a preferred solution of the method for evaluating the effect of hydraulic fracturing of coal and rock mass based on microseismic monitoring provided by the present invention,

[0050] The kernel density value of microseismic event frequency can reflect the extension range of hydraulic fracturing cracks, but cannot reflect the intensity of crack extension, that is, the crack opening degree. Therefore, the kernel density value of microseismic event energy is used to evaluate the intensity of crack extension in different areas within the hydraulic fracturing range, judge the degree of hydraulic fracturing sufficiency in different areas, and identify areas with insufficient hydraulic fracturing.

[0051] (1) Calculate the energy density of coal-rock microseismic energy and draw spatial cloud maps;

[0052] Based on the collected microseismic events during fracturing, the energy kernel density is calculated as:

[0053]

[0054] Where: E is the energy density of microseismic events, (∑E i) j V is the total energy of all microseismic events in unit space j; j is the volume of unit space j, i is the number of microseismic events, and j is the number of unit spaces;

[0055] (2) Evaluate the hydraulic fracturing fracture propagation strength;

[0056] The energy kernel density value of microseismic events during fracturing is calculated and a spatial cloud map is drawn; the map can be used to identify the intensity of fracturing crack expansion. Areas with smaller energy kernel density values ​​are areas with lower crack expansion intensity, indicating insufficient fracturing; areas with larger energy kernel density values ​​are areas with larger crack expansion intensity, indicating sufficient fracturing.

[0057] As a preferred solution of the method for evaluating the effect of hydraulic fracturing of coal and rock mass based on microseismic monitoring provided by the present invention,

[0058] According to the frequency kernel density and energy kernel density cloud map of microseismic events, the insufficient fracturing area is identified, that is, the area with small frequency kernel density and energy kernel density has little fracturing crack expansion and small crack opening. This area has not achieved the expected fracturing effect and needs to implement local pressure relief measures, such as blasting pressure relief, local hydraulic cutting, etc.

[0059] As a preferred solution of the method for evaluating the effect of hydraulic fracturing of coal and rock mass based on microseismic monitoring provided by the present invention, the critical breaking length lm of the rock burst induced by the low and high key layers is calculated according to the above formula to determine the length range of the key layer for active structural regulation: l max <l m ; Determine structural control measures and implement active pressure-relieving control.

[0060] Beneficial effects of the present invention:

[0061] 1. This method uses octree to calculate the fracturing volume, effectively removing the error caused by the unresponsive area of ​​the microseismic event, and can meet the requirements of large-scale data gridding calculation volume generated by fracturing.

[0062] 2. This method starts from the perspective that fracturing is the artificial creation of cracks in coal and rock, thereby forming microseismic events. It makes full use of the spatial distribution characteristics, frequency, and energy characteristics of microseisms. First, the fracturing effect is evaluated as a whole, and then the fracturing effect in a specific area is further determined. The evaluation results are more reliable and accurate than the previous conventional single method of microseismic source positioning.

[0063] 3. This method determines the insufficient fracturing area based on the evaluation of the fracturing effect, and then implements precise reinforcement and pressure relief measures. Compared with the previous method that the fracturing effect of the specific area is unclear after fracturing and the local pressure relief measures cannot be accurately implemented, it can make the fracturing effect transparent and the reinforcement and pressure relief measures precise.

[0064] 4. This method is easy to implement in field engineering practice, and can analyze the fracturing effect within a short time after fracturing, and then strengthen the pressure relief, realize active structural regulation, and ensure safe and efficient mining in the mine. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative labor.

[0066] in:

[0067] Figure 1 This is the spatial distribution profile of the microseismic sources of hydraulic fracturing;

[0068] Figure 2 This is the spatial distribution plan of the microseismic sources of hydraulic fracturing;

[0069] Figure 3 is the theoretical hydraulic fracturing volume;

[0070] Figure 4 This is the distribution pattern of coal rock fractures after hydraulic fracturing;

[0071] Figure 5 This is a schematic diagram of the octree hierarchical division structure;

[0072] Figure 6 This is the kernel density distribution diagram of the frequency of hydraulic fracturing microseismic events;

[0073] Figure 7 This is the energy kernel density distribution diagram of hydraulic fracturing microseismic events;

[0074] Figure 8 Strengthen pressure relief measures for areas where fracturing is insufficient;

[0075] Fig. 9 is an octree two-dimensional volume grid;

[0076] Fig.10 is an octree 3D volume grid. DETAILED DESCRIPTION

[0077] In order to make the purpose, features and advantages of the present invention more obvious and easy to understand, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described below are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0078] Example

[0079] 1. Engineering geological survey, fracturing data collection, screening and processing

[0080] (1) Engineering geological survey and collection of basic data. Investigate the basic data for hydraulic fracturing in mines, including coal seam burial depth and coal seam inclination, and collect hydraulic fracturing construction plans, including the position, thickness, and hardness of the fracturing coal and rock layers. Collect microseismic data generated during coal seam fracturing. The microseismic data format must include time, x, y, z coordinates, microseismic energy, etc. Collect the plane and profile base map of microseismic event location.

[0081] (2) Screening microseismic data within the theoretical fracturing range.

[0082] The collected microseismic data are initially used to locate the earthquake source in the cross-section and plan views, such as Figure 1 , 2 In order to improve the accuracy of fracturing effect evaluation, the microseismic data outside the fracturing range are removed.

[0083] 2. Calculation of theoretical fracturing volume Vr of coal rock mass

[0084] like Figure 3 As shown in the figure, according to the coal mine hydraulic fracturing design and construction plan, the total volume Vr of the theoretically fractured coal rock layer is calculated as:

[0085] V r =l×[2r+(n-1)d]×h

[0086] Where r is the fracturing radius, d is the fracturing borehole spacing, n is the number of fracturing boreholes, l is the length of the horizontal section of the fracturing borehole, and h is the thickness of the fracturing coal rock layer (if the thickness of the coal rock layer is less than the fracturing radius, the fracturing height is the thickness of the coal rock layer, that is, h = m; if the thickness of the coal rock layer is greater than the fracturing radius, the fracturing height is the fracturing radius, that is, h = r).

[0087] 3. Calculation of actual fracturing volume Vf based on octree

[0088] Hydraulic fracturing forms a large-scale artificial fracture network system in coal rock mass, such as Figure 4 The generation of cracks during fracturing generates microseismic events of different energy levels. Therefore, the spatial distribution, energy and frequency characteristics of microseismic events can reflect the development of cracks in coal and rock. Based on this, the octree is introduced to evaluate the volume of cracks generated by fracturing.

[0089] The specific process of calculating the fracturing volume based on the octree is as follows:

[0090] (1) Constructing the octree root node. The theoretical fracturing volume is defined as the octree root node (i.e., the 0th-level node). The center position and side length of the root node determine its position in three-dimensional space.

[0091] (2) Recursively divide the octree child nodes. Divide the root node equally into 8 first-level sub-regions (first-level sub-nodes) along the three dimensions of x, y, and z. These first-level sub-regions are the first-level nodes of the octree. Eliminate the empty nodes (excluding microseismic events) and select the non-empty nodes. Divide the non-empty nodes in the first-level nodes equally into 8 second-level sub-regions (second-level sub-nodes) along the three dimensions of x, y, and z. These second-level sub-regions are the second-level nodes of the octree. Eliminate the empty nodes (excluding microseismic events) and select the non-empty nodes. Figure 5 shown.

[0092] (3) Set the target density threshold and make conditional judgments. Set the target threshold of the spatial distribution density of microseismic events in non-empty nodes, and make judgments on the nodes divided at each level. When the microseismic event distribution density is less than the threshold, continue to divide the next subnode; when the microseismic event distribution density is greater than the threshold, stop dividing and mark the node.

[0093] (4) Loop iterative division: divide all microseismic events during fracturing into octrees according to (2) and (3) until all non-empty nodes of microseismic events are greater than the target threshold, and then stop loop iteration. Fig.10 Octree grid with non-empty nodes for all microseismic events shown.

[0094] (5) Calculate the edge length of the non-empty nodes at the i-th level that meet the microseismic event density threshold. Since the root node is at the 0th level, the edge lengths of the root node along x, y, and z are Each time the nodes of the next level are divided, they are divided into eight equal parts, that is, they are divided into two equal parts along each edge length. Then the edge length of the non-empty node of the i-th layer is:

[0095]

[0096] Where: is the length of the non-empty node edge that meets the density threshold at the i-th level. i is the total level of the octree partition (i=1, 2, 3, ...n), and j is a node edge (j=1, 2, 3).

[0097] (6) Calculate the volume of non-empty nodes at the i-th level that meet the microseismic event density threshold. After obtaining the side length of the i-th level non-empty nodes, the volume of each non-empty node can be obtained as:

[0098]

[0099] Where: V i0 is the volume of a single non-empty node at level i that meets the density threshold.

[0100] (7) Calculate the total volume of all non-empty nodes that meet the microseismic event density threshold at the i-th level. Traverse all non-empty nodes that meet the threshold at the i-th level and add up the total volume of non-empty nodes that meet the density threshold at the i-th level:

[0101]

[0102] Where: V i is the total volume of all non-empty nodes that meet the density threshold at the i-th level. h is the number of non-empty nodes that meet the density threshold at the i-th level (h = 1, 2, 3, ... s).

[0103] (8) Calculate the total volume of non-empty nodes that meet the density threshold at all levels, which is the fracturing volume:

[0104]

[0105] 4. Calculate the hydraulic fracturing fracture volume ratio K and evaluate the overall hydraulic fracturing effect

[0106] (1) Based on the theoretical fracturing volume Vr and the fracturing crack expansion volume Vf calculated above, the hydraulic fracturing crack volume ratio K can be obtained as follows:

[0107]

[0108] (2) The degree of fracture development during the fracturing process is evaluated based on the hydraulic fracturing fracture volume ratio, thereby measuring the overall fracturing effect of hydraulic fracturing. When the fracture volume ratio is: K < 0.05, the fracturing effect is poor; when the fracture volume ratio is 0.05 < K < 0.2, the fracturing effect is good; when the fracture volume ratio is: K > 0.2, the fracturing effect is good.

[0109]

[0110] 5. Evaluation of the extension range of hydraulic fractures based on microseismic frequency and kernel density

[0111] The above evaluation of the fracturing effect by calculating the fracturing volume and the crack volume ratio based on the octree can obtain the overall fracturing effect, but it cannot determine whether the extension range of the fracturing cracks covers the entire theoretical fracturing range. Therefore, the kernel density value of the microseismic event frequency is used to evaluate the extension range of the fracturing cracks.

[0112] (1) Calculate the frequency kernel density of coal-rock microseismic events and draw spatial cloud maps

[0113] Based on the collected microseismic events during fracturing, the energy kernel density is calculated as:

[0114]

[0115] Where: N is the kernel density value of microseismic event frequency, (∑N i ) j is the total number of microseismic events in unit space j, V j is the volume of unit space j, i is the number of microseismic events, and j is the number of unit spaces.

[0116] (2) Assessment of hydraulic fracturing crack extension range

[0117] The kernel density value of the frequency of microseismic events during fracturing is calculated and a spatial cloud map is drawn, such as Figure 7 As shown in the figure, the crack expansion range can be defined and the expansion of hydraulic fractures can be identified. The area with smaller frequency kernel density value is the sparse crack expansion area, and the area with larger frequency kernel density value is the dense crack expansion area.

[0118] VI. Evaluation of Fracturing Crack Extension Intensity Based on Microseismic Energy Core Density

[0119] The microseismic event frequency kernel density value can reflect the extension range of the fracturing cracks, but it cannot reflect the intensity of the crack extension, that is, the crack opening degree. Therefore, the microseismic event energy kernel density value is used to evaluate the intensity of crack extension in different areas within the fracturing range, judge the degree of fracturing adequacy in different areas, and identify areas with insufficient fracturing.

[0120] (1) Calculate the energy density of coal-rock microseismic nuclei and draw spatial cloud maps

[0121] Based on the collected microseismic events during fracturing, the energy kernel density is calculated as:

[0122]

[0123] Where: E is the energy density of microseismic events, (∑E i ) j V is the total energy of all microseismic events in unit space j. j is the volume of unit space j, i is the number of microseismic events, and j is the number of unit spaces.

[0124] (2) Evaluation of hydraulic fracturing crack expansion strength

[0125] The energy density of microseismic events during fracturing is calculated and a spatial cloud diagram is drawn, such as Figure 8 shown.

[0126] The figure can be used to identify the strength of fracturing crack expansion. The area with smaller energy core density value is the area with lower crack expansion strength, and the fracturing is insufficient; the area with larger energy core density value is the area with larger crack expansion strength, and the fracturing is sufficient.

[0127] 7. Local reinforcement and pressure relief for areas where fracturing is insufficient

[0128] According to the frequency kernel density and energy kernel density cloud map of microseismic events, the insufficient fracturing area is identified, that is, the area with small frequency kernel density and energy kernel density has little fracturing crack expansion and small crack opening. This area has not achieved the desired fracturing effect and needs to implement local pressure relief measures, such as blasting pressure relief, local hydraulic cutting, etc. Fig. 9 shown.

[0129] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0130] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for evaluating the effect of hydraulic fracturing of coal and rock mass based on microseismic monitoring, characterized in that: The following steps are involved: Engineering geological survey, fracturing data collection, screening and processing; Calculate the theoretical fracturing volume Vr of coal rock mass; Calculate the actual fracturing volume Vf based on the octree; Calculate the hydraulic fracturing fracture volume ratio K and evaluate the overall hydraulic fracturing effect; Evaluate the extension range of hydraulic fractures based on microseismic frequency kernel density; Evaluation of fracture propagation strength based on microseismic energy kernel density; Local reinforcement and pressure relief are adopted for areas with insufficient fracturing.

2. The method for evaluating the effect of hydraulic fracturing of coal and rock mass based on microseismic monitoring as claimed in claim 1, characterized in that: Engineering geological survey and basic data collection, survey the basic data for hydraulic fracturing in mines, including coal seam burial depth and coal seam inclination, collect hydraulic fracturing construction plans, including the position, thickness, hardness, etc. of the fracturing coal and rock layers, collect microseismic data generated during coal seam fracturing, and the microseismic data format must include time, x, y, z coordinates, microseismic energy, etc., collect microseismic event positioning planes and profile base maps; The collected microseismic data are preliminarily used to locate the earthquake source in the cross-section and plan views; in order to improve the accuracy of the fracturing effect evaluation, the microseismic data outside the fracturing range are eliminated.

3. The method for evaluating the effect of hydraulic fracturing of coal and rock mass based on microseismic monitoring as claimed in claim 2, characterized in that: According to the coal mine hydraulic fracturing design and construction plan, the total volume Vr of the theoretically fractured coal and rock formation is calculated as follows: V r =l×[2r+(n-1)d]×h Where r is the fracturing radius, d is the fracturing borehole spacing, n is the number of fracturing boreholes, l is the length of the horizontal section of the fracturing borehole, and h is the thickness of the fracturing coal rock layer (if the thickness of the coal rock layer is less than the fracturing radius, the fracturing height is the thickness of the coal rock layer, that is, h = m; if the thickness of the coal rock layer is greater than the fracturing radius, the fracturing height is the fracturing radius, that is, h = r).

4. The method for evaluating the effect of hydraulic fracturing of coal and rock mass based on microseismic monitoring as claimed in claim 3, characterized in that: Hydraulic fracturing forms a large-scale artificial fracture network system in coal rock. The generation of fractures during the fracturing process forms microseismic events of different energy levels. Therefore, the spatial distribution, energy and frequency characteristics of microseismic events can reflect the development of fractures in coal rock. Based on this, the octree is introduced to evaluate the fracture volume generated by fracturing. The specific process of calculating the fracturing volume based on the octree is as follows: (1) Construct an octree root node and define the theoretical fracturing volume as the octree root node (i.e., the 0th-level node). The center position and side length of the root node determine its position in three-dimensional space. (2) Recursively divide the octree child nodes, and equally divide the root node into 8 first-level sub-regions (first-level sub-nodes) along the three dimensions of x, y, and z. These first-level sub-regions are the first-level nodes of the octree, and the empty nodes (excluding microseismic events) are removed; select the non-empty nodes, and equally divide the non-empty nodes in the first-level nodes into 8 second-level sub-regions (second-level sub-nodes) along the three dimensions of x, y, and z. These second-level sub-regions are the second-level nodes of the octree, and the empty nodes (excluding microseismic events) are removed again, and the non-empty nodes are selected; (3) Setting a target density threshold and making conditional judgments: Setting a target threshold for the spatial distribution density of microseismic events at non-empty nodes, making judgments on nodes at each level of division; when the microseismic event distribution density is less than the threshold, continuing to divide the next child node; when the microseismic event distribution density is greater than the threshold, stopping the division and marking the node; (4) Performing octree partitioning on all microseismic events during the fracturing period according to (2) and (3) until the number of non-empty nodes of all microseismic events is greater than the target threshold, stopping the cyclic iteration, and obtaining the octree grid of all non-empty nodes of microseismic events; (5) Calculate the edge length of the non-empty nodes at the i-th level that meet the microseismic event density threshold. Since the root node is at the 0th level, the edge lengths of the root node along x, y, and z are Each time the nodes of the next level are divided, they are divided into eight equal parts, that is, they are divided into two equal parts along each edge length. Then the edge length of the non-empty node of the i-th layer is: Where: is the length of the edge of the non-empty node that meets the density threshold at the i-th level; i is the total level of the octree division (i=1, 2, 3, ...n), j is an edge of the node (j=1, 2, 3); (6) Calculate the volume of non-empty nodes at the i-th level that meet the microseismic event density threshold. After obtaining the side length of the i-th level non-empty nodes, the volume of each non-empty node can be obtained as: Where: V i 0 is the volume of a single non-empty node at the i-th level that meets the density threshold; (7) Calculate the total volume of all non-empty nodes that meet the microseismic event density threshold at the i-th level. Traverse all non-empty nodes that meet the threshold at the i-th level and add up the total volume of non-empty nodes that meet the density threshold at the i-th level: Where: V i is the total volume of all non-empty nodes that meet the density threshold at the i-th level; h is the number of non-empty nodes that meet the density threshold at the i-th level (h = 1, 2, 3, ...s); (8) Calculate the total volume of non-empty nodes that meet the density threshold at all levels, which is the fracturing volume:

5. The method for evaluating the effect of hydraulic fracturing of coal and rock mass based on microseismic monitoring as claimed in claim 4, characterized in that: (1) Based on the theoretical fracturing volume Vr and the fracturing crack expansion volume Vf calculated above, the hydraulic fracturing crack volume ratio K can be obtained as follows: (2) The degree of fracture development during the fracturing process is evaluated based on the hydraulic fracturing fracture volume ratio, thereby measuring the overall fracturing effect of hydraulic fracturing; when the fracture volume ratio is: K < 0.05, the fracturing effect is poor; when the fracture volume ratio is 0.05 < K < 0.2, the fracturing effect is good; when the fracture volume ratio is: K > 0.2, the fracturing effect is good; 6. The method for evaluating the effect of hydraulic fracturing of coal and rock mass based on microseismic monitoring as claimed in claim 5, characterized in that: The above evaluation of fracturing effect by calculating fracturing volume and crack volume ratio based on octree can obtain the overall fracturing effect, but it cannot determine whether the extension range of fracturing cracks covers the entire theoretical fracturing range. Therefore, the kernel density value of microseismic event frequency is used to evaluate the extension range of fracturing cracks. (1) Calculate the frequency kernel density of microseismic events in coal and rock mass and draw spatial cloud maps; Based on the collected microseismic events during fracturing, the energy kernel density is calculated as: Where: N is the kernel density value of microseismic event frequency, (∑N i ) j is the total number of microseismic events in unit space j, V j is the volume of unit space j, i is the number of microseismic events, and j is the number of unit spaces; (2) Assess the extent of hydraulic fracturing fracture extension; The frequency kernel density value of microseismic events during fracturing is calculated and a spatial cloud map is drawn. The map can be used to define the scope of crack expansion and identify the expansion of fracturing cracks. Areas with smaller frequency kernel density values ​​are sparse crack expansion areas, and areas with larger frequency kernel density values ​​are dense crack expansion areas.

7. The method for evaluating the effect of hydraulic fracturing of coal and rock mass based on microseismic monitoring as claimed in claim 6, characterized in that: The kernel density value of microseismic event frequency can reflect the extension range of hydraulic fracturing cracks, but cannot reflect the intensity of crack extension, that is, the crack opening degree. Therefore, the kernel density value of microseismic event energy is used to evaluate the intensity of crack extension in different areas within the hydraulic fracturing range, judge the degree of hydraulic fracturing sufficiency in different areas, and identify areas with insufficient hydraulic fracturing. (1) Calculate the energy density of coal-rock microseismic energy and draw spatial cloud maps; Based on the collected microseismic events during fracturing, the energy kernel density is calculated as: Where: E is the energy density of microseismic events, (∑E i ) j V is the total energy of all microseismic events in unit space j; j is the volume of unit space j, i is the number of microseismic events, and j is the number of unit spaces; (2) Evaluate the hydraulic fracturing fracture propagation strength; The energy kernel density value of microseismic events during fracturing is calculated and a spatial cloud map is drawn; the map can be used to identify the intensity of fracturing crack expansion. Areas with smaller energy kernel density values ​​are areas with lower crack expansion intensity, indicating insufficient fracturing; areas with larger energy kernel density values ​​are areas with larger crack expansion intensity, indicating sufficient fracturing.

8. The method for evaluating the effect of hydraulic fracturing of coal and rock mass based on microseismic monitoring as claimed in claim 7, characterized in that: According to the frequency kernel density and energy kernel density cloud map of microseismic events, the insufficient fracturing area is identified, that is, the area with small frequency kernel density and energy kernel density has little fracturing crack expansion and small crack opening. This area has not achieved the expected fracturing effect and needs to implement local pressure relief measures, such as blasting pressure relief, local hydraulic cutting, etc.

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