A coal rock mass hydraulic fracturing effect evaluation method based on microseismic monitoring

Abstract

The present application relates to the technical field of hydraulic fracturing, and in particular to a coal rock mass hydraulic fracturing effect evaluation method based on microseismic monitoring, comprising the following steps: engineering geological investigation, fracturing data collection, screening and processing; calculating the theoretical fracturing volume Vr of the coal rock mass; calculating the actual fracturing volume Vf based on octree; calculating the hydraulic fracturing fracture volume ratio K, evaluating the overall fracturing effect of hydraulic fracturing; evaluating the fracturing crack propagation range based on the microseismic frequency kernel density; evaluating the fracturing crack propagation intensity based on the microseismic energy kernel density; and taking local reinforcement and pressure relief in the insufficient fracturing area. The present application uses octree to calculate the fracturing volume, effectively removes the error caused by the non-response area of microseismic events, and can meet the requirement of large-scale data grid calculation volume of fracturing.

Description

Technical Field

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

[0002] With the depletion of shallow coal resources, coal mining has gradually extended to deeper areas. Deep coal and rock masses are exposed to a more complex geomechanical environment, resulting in greater intensity and higher probability of dynamic hazards such as rock bursts and coal and gas outbursts. Traditional coal and rock permeability enhancement and pressure relief measures can no longer meet the needs of safe and efficient coal mining. Long-distance directional drilling hydraulic fracturing measures can achieve large-scale permeability enhancement and pressure relief of coal and rock masses, weaken and transform coal and rock masses, cut high-integrity coal and rock masses into multiple blocks, and release the elastic energy accumulated in the coal and rock in advance, reducing the risk of rock bursts and gas outbursts. For high-gas mines, hydraulic fracturing forms an intricate network of fractures, increasing coal and rock permeability, preventing high gas accumulation, and facilitating gas extraction. It is an advanced technical means for achieving large-scale coal and rock weakening under existing deep mining conditions.

[0003] Numerous factors influence the effectiveness of hydraulic fracturing. Therefore, different areas can experience varying results under the same fracturing parameters. Therefore, it's necessary to evaluate the overall fracturing effect and identify areas of inadequate fracturing to provide a basis for implementing precise reinforcement and pressure relief measures. Currently, hydraulic fracturing effectiveness evaluation is primarily based on injection pressure curves, post-fracturing borehole inspection of overburden, in-hole transient electromagnetic detection, and microseismic monitoring. Microseismic monitoring, due to its high accuracy and effectiveness, is a commonly used method for monitoring coal and rock fractures in mining operations.

[0004] Hydraulic fracturing artificially creates fractures within a defined spatial range in coal rock, forming a complex network of fractures within the rock, thereby achieving pressure relief and increased permeability. Microseismic data can be used to monitor signals of fractures at various scales in the coal rock during fracturing and calculate their spatial location and energy. Each microseismic event represents the location of a fracture in the coal rock, and its energy represents the size of the fracture opening. Currently, the effectiveness of hydraulic fracturing is evaluated solely through statistical analysis of microseismic source location, frequency, and energy. Effective methods for directly evaluating fracture volume, fracture extension range, and fracture intensity are lacking.

[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 the embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the abstract and title of this application to avoid obscuring the purpose of this section, the abstract and the title of the invention, and such simplifications or omissions should not be used to limit the scope of the present invention.

[0007] A method for evaluating the effect of hydraulic fracturing of coal and 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 crack 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 and 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 investigated, including the buried depth and inclination of coal seams, and the hydraulic fracturing construction plan is collected, including the position, thickness, and hardness of the fractured 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 cross-sectional base map are collected;

[0016] The collected microseismic data were 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 were eliminated.

[0017] 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 total volume Vr of the theoretically fractured coal and 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 coal rock layer thickness is less than the fracturing radius, the fracturing height is the coal rock layer thickness, that is, h = m; if the coal rock layer thickness is greater than the fracturing radius, the fracturing height is the fracturing radius, that is, h = r).

[0020] As a preferred embodiment of the method for evaluating the hydraulic fracturing effect 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. The generation of fractures during the fracturing process generates microseismic events of different energy levels. Therefore, the spatial distribution, energy, and frequency characteristics of microseismic events can reflect the development of fractures in the coal rock. Based on this, 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 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 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 child node; when the microseismic event distribution density is greater than the threshold, stop dividing and mark the node.

[0025] (4) Perform 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, then stop the octree iteration and obtain the octree grid of all non-empty nodes of microseismic events;

[0026] (5) Calculate the side 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 length of the three sides of the root node along x, y, and z is Each time the nodes of the next level are divided into eight equal parts, that is, they are divided into two equal parts along each side length. Then the side length of the non-empty nodes in 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 level i. i is the total level of the octree (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 level i 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 level i. h is the number of non-empty nodes that meet the density threshold at level i (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 fracture volume Vr and fracture expansion volume Vf calculated above, the hydraulic fracture volume ratio K can be obtained as follows:

[0039]

[0040] (2) The degree of fracture development during the hydraulic fracturing process is evaluated based on the hydraulic fracturing fracture volume ratio, which is used to measure the overall hydraulic fracturing effect. 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 hydraulic fracturing effect of coal rock mass based on microseismic monitoring provided by the present invention, the above evaluation of the fracturing effect by calculating the fracturing volume and the fracture 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.

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

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

[0045]

[0046] Where: ρ 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) Evaluate the extent of hydraulic fracturing crack expansion;

[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 crack expansion range and identify the fracturing crack expansion situation. 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 intensive crack expansion area.

[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 fractures, but it cannot reflect the intensity of fracture extension, that is, the fracture opening. Therefore, the kernel density value of microseismic event energy is used to evaluate the intensity of fracture extension in different areas within the hydraulic fracture range, judge the degree of hydraulic fracture sufficiency in different areas, and identify areas with insufficient hydraulic fracture.

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

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

[0053]

[0054] Where: ρ E is the energy density value of the microseismic event, (∑E i) j is the total energy of all 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;

[0055] (2) Evaluate the hydraulic 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 ​​indicate lower crack expansion intensity and insufficient fracturing; areas with larger energy kernel density values ​​indicate greater crack expansion intensity and 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] Based on the frequency kernel density and energy kernel density cloud maps of microseismic events, the areas of insufficient fracturing are identified. That is, the areas with low frequency kernel density and energy kernel density have 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 and local hydraulic cutting.

[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 control: 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 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 from an overall perspective, 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 areas with insufficient fracturing based on the evaluation of the fracturing effect, and then implements precise reinforcement and pressure relief measures. Compared with the previous method that could not accurately implement local pressure relief measures without knowing the fracturing effect of the specific area after fracturing, this method 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. It 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 following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0066] in:

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

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

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

[0070] Figure 4 The distribution pattern of coal rock cracks 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 map 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] Figure 9 is an octree two-dimensional volume grid;

[0076] Figure 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 making 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 basic data collection. Investigate the basic data for hydraulic fracturing in mines, including coal seam depth and coal seam inclination, and collect hydraulic fracturing construction plans, including the location, thickness, and hardness of the fractured 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., and collect microseismic event location planes and cross-sectional base maps.

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

[0082] The collected microseismic data are preliminarily 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 eliminated.

[0083] 2. Calculation of the 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 theoretical 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 coal rock layer thickness is less than the fracturing radius, the fracturing height is the coal rock layer thickness, that is, h = m; if the coal rock layer thickness 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 the fracturing process 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 rocks. 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) Construct the octree root node. Define the theoretical fracturing volume as the octree root node (i.e., the 0th layer 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 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 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 perform conditional judgment. Set the target threshold for the spatial distribution density of microseismic events at non-empty nodes and judge the nodes at each level. When the microseismic event distribution density is less than the threshold, continue to divide the next child node; when the microseismic event distribution density is greater than the threshold, stop dividing and mark the node.

[0093] (4) Loop iterative division is performed on all microseismic events during the fracturing period by octree division according to (2) and (3) until all non-empty nodes of microseismic events are greater than the target threshold, and then the loop iteration is stopped. Figure 10 An octree grid with non-empty nodes for all microseismic events is shown.

[0094] (5) Calculate the side 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 length of the three sides of the root node along x, y, and z is Each time the nodes of the next level are divided into eight equal parts, that is, they are divided into two equal parts along each side length. Then the side length of the non-empty nodes in the i-th layer is:

[0095]

[0096] Where: is the length of the edge of the non-empty node that meets the density threshold at level i. i is the total level of the octree partition (i=1, 2, 3, ...n), and j is an edge of the node (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 level i. h is the number of non-empty nodes that meet the density threshold at level i (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 crack volume ratio K and evaluate the overall hydraulic fracturing effect

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

[0107]

[0108] (2) The degree of fracture development during the hydraulic fracturing process is evaluated based on the hydraulic fracturing fracture volume ratio, which is used to measure the overall hydraulic fracturing effect. 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. Estimation of the extension range of hydraulic fractures based on microseismic frequency kernel density

[0111] The above evaluation of the fracturing effect by calculating the fracturing volume and 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: ρ 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) Assessing the extension range of hydraulic fracturing cracks

[0117] The kernel density value of the microseismic event frequency during fracturing is calculated and a spatial cloud map is drawn, such as Figure 7 The figure shows that the crack extension range can be defined and the hydraulic fracture extension situation can be judged. The area with smaller frequency kernel density value is the sparse crack extension area, and the area with larger frequency kernel density value is the dense crack extension area.

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

[0119] The microseismic event frequency kernel density value can reflect the extent of fracture expansion, but it cannot reflect the intensity of fracture expansion, that is, the fracture opening. Therefore, the microseismic event energy kernel density value is used to evaluate the intensity of fracture expansion in different areas within the fracture range, assess the degree of fracture adequacy in different areas, and identify areas of insufficient fracture expansion.

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

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

[0122]

[0123] Where: ρ E is the energy density value of the microseismic event, (∑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 kernel density value of microseismic events during fracturing is calculated and a spatial cloud map is drawn, such as Figure 8 shown.

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

[0127] 7. Take local reinforcement and pressure relief measures for areas where fracturing is insufficient

[0128] Based on the frequency kernel density and energy kernel density cloud map of microseismic events, the insufficient fracturing area is identified, that is, the area with low 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. Figure 9 shown.

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

[0130] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above-described embodiments. The above-described embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of the present invention is defined by the appended 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 crack 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; Take local reinforcement and pressure relief measures for areas where fracturing is insufficient; Hydraulic fracturing creates a large-scale artificial fracture network system in coal rock. The generation of fractures during the fracturing process generates microseismic events of different energy levels. 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 the 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 along the x, y, and z dimensions. The first-level sub-region is the first-level node of the octree. Remove the empty nodes in the first-level sub-region, and do not contain microseismic events; select the non-empty nodes in the first-level sub-region, and equally divide the non-empty nodes in the first-level nodes into 8 second-level sub-regions along the x, y, and z dimensions. The second-level sub-regions after division are the second-level nodes of the octree. Again remove the empty nodes in the second-level sub-region, and do not contain microseismic events, and select the non-empty nodes in the second-level sub-region; (3) Set the target density threshold and perform conditional judgment. Set the target threshold for the spatial distribution density of microseismic events at non-empty nodes and judge the nodes at each level. When the microseismic event distribution density is less than the threshold, continue to divide the next child node; when the microseismic event distribution density is greater than the threshold, stop dividing and mark the node. (4) Perform 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, then stop the octree iteration and obtain the octree grid of all non-empty nodes of microseismic events; (5) Calculate the side 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 length of the three sides of the root node along x, y, and z is Each time the nodes of the next level are divided into eight equal parts, that is, they are divided into two equal parts along each side length. Then the side length of the non-empty nodes in 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, 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 level i 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 fracture expansion volume Vf: Based on the frequency kernel density and energy kernel density cloud maps of microseismic events, areas of insufficient fracturing are identified. That is, areas with low frequency kernel density and energy kernel density indicate that the fracturing cracks have little expansion and small crack opening. This area has not achieved the desired fracturing effect and requires local pressure relief measures, including blasting pressure relief and local hydraulic cutting.

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. Investigate the basic data for hydraulic fracturing in mines, including coal seam depth and coal seam inclination; collect hydraulic fracturing construction plans, including the location, thickness, and hardness of the fracturing coal and rock layers; collect microseismic data generated during the coal seam fracturing process. The microseismic data format must include time, x, y, z coordinates, and microseismic energy; and collect microseismic event location planes and cross-sectional base maps. The collected microseismic data are preliminarily used to locate the earthquake source in the cross-section and plan views; and 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 hydraulic fracturing construction plan, the total volume Vr of the theoretical fractured coal rock layer is calculated as: 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 and rock layer. If the rock layer thickness is less than the fracturing radius, the fracturing height is the coal and rock layer thickness, that is, h = m; if the coal and rock layer thickness 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: (1) Based on the theoretical fracture volume Vr and fracture expansion volume Vf calculated above, the hydraulic fracture 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, which is used to measure the overall hydraulic fracturing effect. 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; 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: The kernel density value of microseismic event frequency is used to evaluate the extension range of hydraulic fractures; (1) Calculate the frequency kernel density of coal-rock microseismic events and draw spatial cloud maps; Based on the collected microseismic events during fracturing, the frequency kernel density value is calculated as: Where: ρ 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) Evaluate the extent of hydraulic fracturing crack expansion; The frequency kernel density value of microseismic events during fracturing is calculated and a spatial cloud map is drawn. The spatial cloud map can be used to define the range 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.

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 energy kernel density value of microseismic events 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. (1) Calculate the microseismic energy density of 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: ρ E is the energy density value of the microseismic event, (∑E i ) j is the total energy of all 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) Evaluate the hydraulic fracture propagation strength; Calculate the energy kernel density value of microseismic events during fracturing and draw a spatial cloud map; The spatial cloud map can be used to identify the intensity of fracturing crack expansion. Areas with smaller energy core density values ​​indicate lower crack expansion intensity and insufficient fracturing; areas with larger energy core density values ​​indicate greater crack expansion intensity and sufficient fracturing.

Images