Coal body three-axis direction multi-point type pressure relief effect evaluation system and experimental method thereof

Abstract

The invention discloses a coal body three-axis direction multi-point type pressure relief effect evaluation system and an experiment method thereof. The method comprises the following steps: step 1, underground field investigation and coal sample preparation; 2, loading a coal sample and carrying out gas adsorption-desorption balance; 3, loading true triaxial stress; 4, monitoring a true triaxial stress state; the fourth step and the fifth step are started at the same time, and the fourth step is continued until the sixth step is finished. 5, coal sample fracturing operation and acoustic emission signal monitoring operation are carried out; sixthly, negative pressure extraction is carried out; and 7, evaluating the pressure relief effect in the three-axis direction. The coal sample stress redistribution and fracture expansion phenomena can be accurately monitored, and the coal sample fracturing and extraction rule can be better determined through the original real-time pressure attenuation ratio, the gas pressure attenuation ratio and the gas concentration attenuation ratio. The pressure gradient attenuation coefficient KDij, the gas pressure gradient attenuation coefficient KDgp and the gas concentration gradient attenuation coefficient KDgc are used for further evaluating the coal sample fracturing-extraction pressure relief range; and the influence of the fracture form on the pressure relief effect of the coal sample is evaluated by combining gradient attenuation coefficients of the near point, the far point and the average distance between the fracture propagation acoustic emission signal point and each acoustic emission measuring point and between the fracture propagation acoustic emission signal point and the extraction pipe interface, and the method plays a more accurate guiding role in the gas extraction operation of the fractured coal seam of the coal mine.

Description

Technical Field

[0001] The invention relates to the technical field of coal rock fracturing permeability enhancement and gas extraction, and in particular to a coal body triaxial direction multi-point pressure relief effect evaluation experimental method and device. Background Art

[0002] The geostress environment in deep mines greatly affects the safe extraction of gas and the normal safe production and mining of coal mines, and is very likely to cause accidents such as coal and gas outbursts.

[0003] In order to ensure the safe mining of deep coal mines, hydraulic fracturing, hydraulic punching and supercritical carbon dioxide are often used to relieve reservoir pressure and increase permeability, and to extract high-pressure and high-concentration gas in the reservoir.

[0004] During the fracturing and gas extraction process of coal rock mass, due to the damage to the coal body and the continuous extraction of gas, the ground stress field and pore stress field will be redistributed, thereby causing the ground stress, gas concentration and pressure in the local area to decrease, thereby achieving the purpose of efficient co-mining of coal and gas and safe production. At the same time, the extracted gas is also an efficient and usable clean energy.

[0005] At present, most of the coal fracturing and gas extraction experiments conducted in the laboratory use a constant pressure and constant flow pump to maintain a constant true triaxial state, but in this process the redistribution behavior of the true triaxial stress during fracturing and extraction is ignored.

[0006] The inventor's innovative idea is to introduce the consideration of triaxial stress redistribution behavior into the pressure relief law to obtain more accurate pressure relief evaluation results and thus better guide coal mine operation practices.

[0007] The study on the above-mentioned triaxial stress redistribution behavior will provide an important theoretical basis for determining (evaluating) the effective range of downhole pressure relief and permeability enhancement.

[0008] However, the laboratory currently lacks relevant true triaxial stress relief effect measurement methods. Therefore, there is an urgent need for methods and devices in the laboratory that can monitor and effectively measure the true triaxial state of the entire process of coal fracturing and extraction integration in real time. This will be conducive to the establishment and optimization of the underground regional pressure relief evaluation system, especially in the context of my country's coal mining gradually moving deeper and increasing ground stress.

[0009] In summary, the purpose of the present invention is to provide a method for inverting the stress state of the coal rock matrix around the entire fracturing-extraction process according to the triaxial stress and gas content during the integrated experiment of gas-containing coal fracturing-extraction. The method can set the experimental parameters such as triaxial stress loading and gas adsorption in proportion according to the underground ground stress, gas concentration and pressure conditions. The data can be obtained through experiments, and the actual underground pressure relief range can be more effectively evaluated under the condition of considering the redistribution of triaxial stress. The pressure relief effect range corresponding to the crack morphology generated by the fracturing technology can be quantitatively analyzed, thereby providing an effective means for pressure relief and permeability enhancement in deep mine areas, efficient extraction of coal and gas, and safe production. Summary of the invention

[0010] The purpose of the present invention is to provide a coal body triaxial multi-point pressure relief effect evaluation system for incorporating triaxial stress redistribution into coal seam fracturing pressure relief evaluation, thereby providing a basis for better coal seam fracturing pressure relief and gas extraction.

[0011] To achieve the above-mentioned purpose, the coal body triaxial direction multi-point pressure relief effect evaluation system of the present invention comprises a true triaxial stress loading device, which comprises a pressure chamber for accommodating a coal sample and applying triaxial stress to the coal sample; the pressure chamber comprises a frame in a cube shape, six faces of the frame are connected to loading plates, each loading plate is connected to a pressurized pipeline, each pressurized pipeline is provided with a valve, and each pressurized pipeline is connected to a high-pressure hydraulic system; One of the loading plates is connected to a fracturing pipe, which is used to selectively connect to an external gas source or an external fracturing medium source; a gas pressure sensor is provided on the fracturing pipe; Also included is an industrial control computer, which is connected to a signal line; A plurality of first pressure sensors for monitoring the stress redistribution state in a true triaxial stress experiment are distributed in a matrix array on the loading plate; A plurality of acoustic emission probes for monitoring the crack expansion state of the coal sample are arranged in a matrix array on the loading plate; the acoustic emission probes are arranged at intervals from each first pressure sensor; Each first pressure sensor and each acoustic emission probe are connected to the industrial control computer through the signal line, and a signal processing device is provided on the signal line between the industrial control computer and each acoustic emission probe, and the signal processing device includes a preamplifier and a data acquisition card; Each loading plate is provided with an extraction pipe interface, and a plug is provided on the extraction pipe interface; one of the extraction pipe interfaces is connected to an extraction pipe, and the extraction pipe seal passes through the loading plate and is connected to a gas collection bottle, and a gas concentration sensor and a gas pressure sensor are provided on the extraction pipe between the gas collection bottle and the loading plate; the extraction pipe passes downward through the top cover of the gas collection bottle and extends to the bottom of the gas collection bottle; The top cover of the gas collecting bottle is connected with a negative pressure tube, and the negative pressure tube is connected with a vacuum pump.

[0012] The present invention also discloses an experimental method using the above-mentioned coal body three-axis direction multi-point pressure relief effect evaluation system, which is carried out according to the following steps: The first step is underground field investigation and coal sample preparation; The second step is coal sampling and gas adsorption-desorption equilibrium; The third step is true triaxial stress loading; The fourth step is true triaxial stress state monitoring; the fourth step starts at the same time as the fifth step, and the fourth step lasts until the sixth step ends.

[0013] The fifth step is the coal sample fracturing operation and acoustic emission signal monitoring operation; The sixth step is negative pressure extraction; The seventh step is to evaluate the pressure relief effect in three-axis directions.

[0014] The first step, namely underground field investigation and coal sample preparation, is as follows: The ground stress, gas pressure and gas concentration parameters of the simulated coal seam are obtained by on-site underground measurements, and large coal samples are obtained from the underground site and processed into cubes that match the pressure chamber; after the first step is completed, the second step is carried out.

[0015] The second step is coal sample loading and gas adsorption-desorption equilibrium. The coal sample was loaded into a fluororubber sleeve and placed in a pressure chamber. Six loading plates were used to apply a pressure of 2-5 MPa to the coal sample, and then gas injection was performed first; The gas injection operation is: connect the fracturing pipe to an external gas source, adjust the outlet pressure of the external gas source to the same as the actual gas pressure of the simulated underground coal seam, and inject gas at a constant pressure into the coal sample through the fracturing pipe until the pressure in the fracturing pipe remains stable for more than 48 hours, then end the gas injection operation and carry out the pressure maintenance operation; The pressure maintenance operation is: the gas pressure in the fracturing pipe and the gas pressure in the extraction pipe are continuously monitored by the gas pressure sensor installed on the fracturing pipe and the gas pressure sensor installed on the extraction pipe; If the gas pressure in the fracturing pipe or the gas pressure in the extraction pipe fluctuates by more than 5% within 48 hours, return to perform the gas injection operation again; If the gas pressure in the fracturing pipe or the gas pressure fluctuation in the extraction pipe remains within 5% within 48 hours, it is regarded as the gas adsorption-desorption equilibrium of the coal sample; after the second step is completed, the third step is carried out.

[0016] The third step, true triaxial stress loading, is as follows: According to the actual ground stress conditions of the underground coal seam simulated by the experiment, the pressure parameters of the true triaxial stress loading device are set, the valves on each pressurized pipeline are opened, and the true triaxial stress is applied to the coal sample through each loading plate; after the true triaxial stress state is stable, the valves on each pressurized pipeline are closed, and after the true triaxial stress fluctuation range is within 5%, the fourth and fifth steps are carried out simultaneously.

[0017] The fourth step, namely true triaxial stress state monitoring, is as follows: The pressure changes on each loading plate are monitored in real time by a plurality of first pressure sensors distributed in a matrix array on each loading plate, and the pressure values ​​of the entire process of coal sample fracturing and extraction are recorded in real time; the fourth step is continued until the sixth step is completed; The coal sample fracturing operation in the fifth step is specifically: The fracturing pipe is selectively connected to an external fracturing medium source, and the fracturing medium is injected into the coal sample in a constant flow and pressure manner. When the coal sample pressure monitored by the first pressure sensor drops by more than 25% within two seconds, the external fracturing medium source is turned off, the fracturing process is stopped, and the sixth step, i.e., negative pressure extraction, is performed; The acoustic emission signal monitoring operation in the fifth step is specifically: The acoustic emission signal monitoring operation and the coal sample fracturing operation are carried out and ended at the same time; Each acoustic emission probe is used to monitor the dynamic response sound signal of the coal sample crack expansion process in real time. During the coal sample fracturing operation, the coal sample will release elastic waves when the cracks expand. The acoustic emission probe detects these elastic waves and converts them into acoustic emission electrical signals, which are transmitted to the industrial control computer through the signal line.

[0018] The sixth step, namely negative pressure extraction, is as follows: Open the plug on the extraction tube interface on one side of the loading plate, connect the extraction tube interface with the extraction tube, turn on the vacuum pump, extract the gas adsorbed by the coal sample into the gas collection bottle, and use the gas concentration sensor and gas pressure sensor to record the gas pressure and concentration in real time until the gas concentration is lower than the predetermined concentration of the experiment.

[0019] The seventh step, i.e., the evaluation of the pressure relief effect in the three-axis direction, specifically includes three parts: pressure evaluation, drainage evaluation and data analysis. The pressure evaluation part of the seventh step is carried out simultaneously with the fifth step, and the drainage evaluation part of the seventh step is carried out simultaneously with the sixth step. Pressure evaluation: Calculate and process the pressure value detected by the pressure sensor on the loading plate in real time, and use the real-time pressure attenuation ratio in the three-axis direction to evaluate the pressure relief effect; The location of any first pressure sensor on the loading plate is called a pressure measuring point. The first pressure sensors are distributed in a matrix, so the pressure measuring point position is marked as ( i , j ); During the fifth step, the industrial control computer monitors the pressure curve of each pressure measuring point on each loading plate in real time, and records the pressure at any pressure measuring point as P ij ; The initial pressure of this pressure measuring point is P ij-0 , the real-time pressure is P ij-1 , then the industrial control computer calculates the real-time pressure attenuation ratio K of the pressure measuring point ij =( P ij-1 - P ij-0 ) / P ij-0 ; Extraction evaluation: Based on the gas pressure and gas concentration monitored by the gas concentration sensor and gas pressure sensor, the extraction impact of the decompression effect of the coal sample fracturing-extraction process is quantitatively evaluated; The industrial control computer monitors the gas pressure and gas concentration of the extraction pipe in real time during the sixth step, which is recorded as P CH4 and C CH4 , the initial gas pressure and concentration of the extraction pipe are P CH4-0 and C CH4-0 , the real-time pressure and concentration are P CH4-1 and C CH4-1 , the industrial computer calculates the gas pressure decay ratio as K gp =( P CH4-1 - P CH4-0 ) / P CH4-0 The industrial control computer calculates the gas concentration attenuation ratios as K gc =( C CH4-1 - C CH4-0 ) / C CH4-0 .

[0020] The data analysis includes: (1) Analysis of the influence of the hydraulic fracture near, far and overall on the stress field at each acoustic emission measuring point; this analysis is carried out simultaneously with the pressure evaluation; The location of any acoustic emission probe on the loading plate is called an acoustic emission measurement point. The acoustic emission probes are distributed in a matrix, and the location of the acoustic emission measurement point is marked as ( i , j); The industrial control computer records the distance between each acoustic emission measuring point of each loading plate and the acoustic emission signal point of the hydraulic fracture expansion based on the distribution of the acoustic emission signal points of the hydraulic fracture expansion monitored by each acoustic emission probe. d , where the maximum distance is d max , the minimum distance is d min , the average distance is d avg , define the pressure gradient attenuation coefficient KD of the acoustic emission measurement point ij =K ij / d ij , calculate KD ij-max =K ij / d ij-max , KD ij-min =K ij / d ij-min and KD ij-avg =K ij / d ij-avg , to achieve the purpose of qualitatively and quantitatively analyzing the influence of hydraulic fracture near point, far point and overall on the stress field of each acoustic emission measuring point; (2) Calculate the gas pressure gradient attenuation coefficient KD based on the distance between the extraction pipe interface and the acoustic emission signal point of the hydraulic fracture extension. gp =K gp / d gp and gas concentration gradient attenuation coefficient KD gc =K gc / d gc , perform quantitative calculation and analysis of the distance between the hydraulic fracture and each loading plate extraction pipe near point, far point and the whole at the same time as step S64; wherein, d gp It refers to the distance between the pressure measuring point and the acoustic emission signal point of the hydraulic fracture expansion. d gc It refers to the distance between the extraction pipe interface and the acoustic emission signal point of the hydraulic fracture expansion; (1) and (2) are performed simultaneously, followed by (3); (3) According to the pressure decay ratio K of each loading plate pressure measurement point ij Draw the stress attenuation surface of the coal sample in the three-axis direction and its average value K ij-avg , through the stress decay surface and K ij-avg The regional pressure relief effect of the whole process of coal sample fracturing and extraction is evaluated; combined with the K gp The pressure method is used to quantitatively evaluate the decompression area of ​​the coal sample. |K gp |≥(0.74MPa- P geo ) / P geo When the coal sample is completely in the pressure relief area, P geo is the original gas pressure in the well, which is measured in the first step; K combined with the extraction pipe interface gc The coal sample pressure relief area is quantitatively evaluated by the content method. When |K gc |≥|(0.01- C geo ) / C geo |, the coal sample is considered to be completely in the pressure relief area, where C geo is the original gas concentration underground, which is measured in the first step.

[0021] The present invention has the following advantages: The present invention uses the first pressure sensor array and the acoustic emission probe array to comprehensively and reliably monitor stress redistribution and crack expansion during coal sample fracturing, providing a basis for studying the laws of stress redistribution and crack expansion, and further more accurately designing coal seam fracturing parameters and extraction parameters, thereby improving fracturing effects and gas extraction effects.

[0022] The experimental method of the present invention has simple steps and can accurately monitor the stress redistribution and crack expansion phenomena of coal samples. Through the original real-time pressure attenuation ratio, gas pressure attenuation ratio and gas concentration attenuation ratio, the fracturing and extraction rules of coal samples can be better determined.

[0023] The larger the real-time pressure decay ratio, the more obvious the pressure relief effect. The applicant innovatively proposed the real-time pressure decay ratio as an indicator, and obtained the data of this indicator through the coal body triaxial direction multi-point pressure relief effect evaluation system, which intuitively and accurately reflects the fracturing pressure relief effect under true triaxial stress loading conditions, and intuitively and accurately derives the pressure relief effect and pressure relief area under specific fracturing parameters, which has more reliable guiding significance for guiding downhole pressure relief practice.

[0024] The data of gas pressure decay ratio and gas concentration decay ratio are convenient for evaluating the effect of the extraction system and predicting gas outburst. By comparing the gas pressure and concentration before and after fracturing, the effect of fracturing on gas extraction can be evaluated, and the fracturing parameters, extraction negative pressure and extraction pipe layout can be optimized in a targeted manner to optimize the extraction efficiency.

[0025] Calculate KD ij , KD ij-max、 KD ij-min , KD ij-avg , KDgp and KD gc After that, it is possible to qualitatively and quantitatively analyze the influence of the near point, far point and overall position of the hydraulic fracture on the stress field of each acoustic emission measuring point, so it has the following guiding significance for coal mine fracturing coal seam gas extraction operations: 1. Optimize fracturing parameters. By analyzing the distance between different acoustic emission measuring points and the acoustic emission signal points of crack expansion, the near point and far point of crack expansion can be determined, and then the fracturing parameters, such as the injection volume and injection pressure of the fracturing fluid, can be optimized to achieve more effective crack expansion and gas extraction. 2. Improve gas extraction efficiency. Since the influence of cracks on the stress field can be understood, the gas flow path can be predicted more accurately, and then the extraction drilling holes can be arranged more accurately in coal mine extraction to improve extraction efficiency. 3. Evaluate the influence of ground stress on crack expansion, more accurately predict the crack expansion direction and crack morphology, adjust the fracturing parameters, and provide a basis for fracturing design.

[0026] In the fifth step, the time difference between the acoustic emission signals reaching different probes can be used to determine the location of the cracks, and the expansion direction and shape of the cracks can be inferred by inversion. By analyzing and processing the acoustic emission electrical signals, the mechanical properties and fracturing process of the coal samples can be better understood, and the pressure relief area can be understood.

[0027] Pressure gradient attenuation coefficient KD ij , Gas pressure gradient attenuation coefficient KD gp and gas concentration gradient attenuation coefficient KD gc =K gc / d gc It is used to further evaluate the unloading range of coal sample fracturing and extraction, and to evaluate the influence of fracture morphology on the unloading effect of coal sample by combining the gradient attenuation coefficient of the near point, far point and average distance of the fracture extension acoustic emission signal point from each acoustic emission measuring point and the extraction pipe interface, so as to provide more accurate guidance for coal mine fracturing and coal seam gas extraction operations. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a structural schematic diagram of the coal body triaxial direction multi-point pressure relief effect evaluation system.

[0029] Figure 2 It is a structural diagram of the pressure chamber. DETAILED DESCRIPTION

[0030] like Figure 1 Figure 2As shown, the coal body triaxial direction multi-point pressure relief effect evaluation system of the present invention includes a true triaxial stress loading device, and the true triaxial stress loading device includes a pressure chamber for accommodating coal samples and applying triaxial stress to the coal samples; the pressure chamber includes a frame 1 in a cube shape, and six faces of the frame 1 are connected to loading plates 2, and each loading plate 2 is connected to a pressurized pipe, each pressurized pipe is provided with a valve, and each pressurized pipe is connected to a high-pressure hydraulic system; the pressurized pipe and the high-pressure hydraulic system are both conventional technologies, not shown in the figure.

[0031] One of the loading plates 2 is connected to a fracturing pipe and an electronic pressure gauge. The fracturing pipe is used to selectively connect to an external gas source or an external fracturing medium source, such as fracturing fluid or high-pressure gas for fracturing. The fracturing pipe is provided with a fracturing valve for controlling the on-off of the fracturing pipe and a gas pressure sensor for measuring the gas pressure in the fracturing pipe. The electronic pressure gauge and fracturing technology are prior arts. The figure does not show the electronic pressure gauge, fracturing pipe, gas source, external fracturing medium source, fracturing valve installed on the fracturing pipe, and gas pressure sensor.

[0032] It also includes an industrial control computer 3, which is connected to a signal line 4; A plurality of first pressure sensors 5 for monitoring the stress redistribution state in the true triaxial stress experiment are distributed in a matrix array on each loading plate 2; A plurality of acoustic emission probes 6 for monitoring the expansion state of coal sample cracks are arranged in a matrix array on each loading plate 2; the acoustic emission probes 6 are arranged at intervals from each first pressure sensor 5; in this embodiment, four acoustic emission probes 6 are provided.

[0033] The electronic barometer, the gas pressure sensor installed on the fracturing pipe, each first pressure sensor 5 and each acoustic emission probe 6 are connected to the industrial control computer 3 through the signal line 4. A signal processing device 7 is provided on the signal line 4 between the industrial control computer 3 and each acoustic emission probe 6. The signal processing device 7 includes a preamplifier and a data acquisition card; the preamplifier is used to amplify the acoustic emission signal collected by the acoustic emission probe 6, and the data acquisition card, as a signal processing unit, converts the amplified acoustic emission analog signal into a digital signal and transmits it to the industrial control computer 3. The preamplifier and the data acquisition card are both conventional technologies and are not shown in the figure.

[0034] Each loading plate 2 is provided with an extraction pipe interface 8, and a plug is provided on the extraction pipe interface 8; the plug is a conventional technology and is not shown in the figure. One of the extraction pipe interfaces 8 is connected to an extraction pipe 9, and the extraction pipe 9 passes through the loading plate 2 in a sealed manner and is connected to a gas collection bottle 10. A gas concentration sensor 11 and a gas pressure sensor 12 are provided on the extraction pipe 9 between the gas collection bottle 10 and the loading plate 2; the extraction pipe 9 passes downward through the top cover of the gas collection bottle 10 and extends to the bottom of the gas collection bottle 10; The top cover of the gas collecting bottle 10 is connected with a negative pressure pipe 13 , and the negative pressure pipe 13 is connected with a vacuum pump 14 .

[0035] The specific structure of each loading plate 2 is: the loading plate 2 is a composite structure in which two stainless steel plates are stacked and combined together, and grooves for placing each first pressure sensor 5, acoustic emission probe 6 and signal line 4 are provided on the joint surface of the two stainless steel plates, and a sealing structure such as a glue injection sealing structure or a sealing gasket compression sealing structure is provided at the end of the groove.

[0036] The present invention also discloses an experimental method using the above-mentioned coal body triaxial direction multi-point pressure relief effect evaluation system, which is carried out according to the following steps: The first step is underground field investigation and coal sample preparation; The second step is coal sampling and gas adsorption-desorption equilibrium; The third step is true triaxial stress loading; The fourth step is true triaxial stress state monitoring; the fourth step starts at the same time as the fifth step, and the fourth step lasts until the sixth step ends.

[0037] The fifth step is the coal sample fracturing operation and acoustic emission signal monitoring operation; The sixth step is negative pressure extraction; The seventh step is to evaluate the pressure relief effect in three-axis directions.

[0038] The first step, namely underground field investigation and coal sample preparation, is as follows: The ground stress, gas pressure and gas concentration parameters of the simulated coal seam are obtained by field measurement in the underground, and a large coal sample is obtained from the underground site and processed into a cube matching the pressure chamber; the size specifications of the coal sample and the pressure chamber can be: 500×500×500mm, 800×800×800mm, 1000×1000×1000mm, etc. In the present invention, mm means millimeter. After the first step is completed, the second step is performed.

[0039] The second step is coal sample loading and gas adsorption-desorption equilibrium. The coal sample is loaded into a fluororubber sleeve and placed in a pressure chamber. Six loading plates 2 are used to apply a pressure of 2-5 (including both end values) MPa to the coal sample. When the position of the coal sample needs to be adjusted, the position of the coal sample is adjusted by adjusting different loading plates 2 to apply different pressures. First, perform gas injection operation, connect the fracturing pipe to an external gas source (such as a high-pressure gas cylinder), adjust the outlet pressure of the external gas source to the same as the actual gas pressure of the simulated underground coal seam through a pressure regulating valve, and inject gas at a constant pressure into the coal sample through the fracturing pipe until the pressure in the fracturing pipe remains stable for more than 48 hours, then close the fracturing valve, disconnect the fracturing pipe from the external gas source, end the gas injection operation, and perform pressure maintenance operation; The pressure maintenance operation is: continuously monitoring the gas pressure in the fracturing pipe and the gas pressure in the extraction pipe 9 through the gas pressure sensor installed on the fracturing pipe and the gas pressure sensor installed on the extraction pipe 9; If the gas pressure in the fracturing pipe or the gas pressure in the extraction pipe 9 fluctuates by more than 5% within 48 hours, return to re-execute the gas injection operation; If the gas pressure in the fracturing pipe or the gas pressure fluctuation in the extraction pipe 9 is always kept within 5% within 48 hours, it is regarded as the gas adsorption-desorption equilibrium of the coal sample; after the second step is completed, the third step is carried out.

[0040] The third step, true triaxial stress loading, is as follows: According to the actual ground stress conditions of the underground coal seam simulated by the experiment, the pressure parameters of the true triaxial stress loading device are set, the valves on each pressurized pipe are opened, and the true triaxial stress is applied to the coal sample through each loading plate 2; after the true triaxial stress state is stable, the valves on each pressurized pipe are closed, and after the true triaxial stress fluctuation range is within 5%, the fourth step and the fifth step are carried out simultaneously.

[0041] The fourth step, namely true triaxial stress state monitoring, is as follows: The pressure changes on each loading plate 2 are monitored in real time by a plurality of first pressure sensors 5 distributed in a matrix array on each loading plate 2, and the pressure value of the whole process of coal sample fracturing and extraction is recorded in real time; the fourth step is continued until the sixth step is completed; The coal sample fracturing operation in the fifth step is specifically: The fracturing pipe is selectively connected to an external fracturing medium source (such as a pump for supplying fracturing fluid), and the fracturing medium is injected into the coal sample in a constant flow and pressure manner. When the coal sample pressure monitored by the first pressure sensor 5 drops by more than 25% within two seconds, the external fracturing medium source is turned off, the fracturing pipe is closed (such as closing a stop valve on the fracturing pipe), the fracturing process is stopped, and the sixth step, i.e., negative pressure extraction, is performed; The acoustic emission signal monitoring operation in the fifth step is specifically: The acoustic emission signal monitoring operation and the coal sample fracturing operation are carried out and ended at the same time; Each acoustic emission probe 6 is used to monitor the dynamic response sound signal during the expansion of coal sample cracks in real time. During the coal sample fracturing operation, the coal sample will release elastic waves when cracks expand. The acoustic emission probe 6 detects these elastic waves and converts them into acoustic emission electrical signals, which are transmitted to the industrial control computer 3 through the signal line 4.

[0042] The sixth step, namely negative pressure extraction, is as follows: According to the position and negative pressure parameters of the underground coal seam gas extraction pipe 9, the plug on the extraction pipe interface 8 on the loading plate 2 on one side is opened, the extraction pipe interface 8 is connected to the extraction pipe 9, and the vacuum pump 14 is turned on to extract the gas adsorbed by the coal sample into the gas collection bottle 10, and the gas concentration sensor 11 and the gas pressure sensor 12 are used to record the gas pressure and concentration in real time until the gas concentration is lower than the predetermined concentration of the experiment. Usually, the process stops when the gas concentration drops below the predetermined safety level.

[0043] The seventh step, i.e., the evaluation of the pressure relief effect in the three-axis direction, specifically includes three parts: pressure evaluation, drainage evaluation and data analysis. The pressure evaluation part of the seventh step is carried out simultaneously with the fifth step, and the drainage evaluation part of the seventh step is carried out simultaneously with the sixth step; data analysis is carried out after the pressure evaluation and drainage evaluation.

[0044] Pressure evaluation: Calculate and process the pressure value detected in real time by the pressure sensor on the loading plate 2, and use the real-time pressure attenuation ratio in the three-axis direction to evaluate the pressure relief effect; The position of any first pressure sensor 5 on the loading plate 2 is called a pressure measuring point. The first pressure sensors 5 are distributed in a matrix, so the pressure measuring point position is marked as ( i , j ); The industrial control computer 3 monitors the pressure curve of each pressure measuring point of each loading plate 2 in real time during the fifth step, and the pressure at any pressure measuring point is recorded as P ij ; The initial pressure of this pressure measuring point is P ij-0 , the real-time pressure is P ij-1 , then the industrial control computer 3 calculates the real-time pressure attenuation ratio K of the pressure measuring point ij =( P ij-1 - P ij-0 ) / P ij-0 ; Extraction evaluation: Based on the gas pressure and gas concentration monitored by the gas concentration sensor 11 and the gas pressure sensor 12, the extraction impact of the decompression effect of the coal sample fracturing-extraction process is quantitatively evaluated; The industrial control computer 3 monitors the gas pressure and gas concentration of the extraction pipe 9 in real time during the sixth step, which is recorded as P CH4 and C CH4 , the initial gas pressure and concentration of extraction pipe 9 are P CH4-0 and C CH4-0 , the real-time pressure and concentration are P CH4-1 and C CH4-1 , the industrial computer 3 calculates the gas pressure decay ratio as K gp =( P CH4-1 - P CH4-0 ) / P CH4-0 , the industrial control computer 3 calculates the gas concentration attenuation ratios as K gc =( C CH4-1 - C CH4-0 ) / C CH4-0 .

[0045] The larger the real-time pressure decay ratio, the more obvious the pressure relief effect. The applicant innovatively proposed the real-time pressure decay ratio as an indicator, and obtained the data of this indicator through the coal body triaxial direction multi-point pressure relief effect evaluation system, which intuitively and accurately reflects the fracturing pressure relief effect under true triaxial stress loading conditions, and intuitively and accurately derives the pressure relief effect and pressure relief area under specific fracturing parameters, which has more reliable guiding significance for guiding downhole pressure relief practice.

[0046] The data of gas pressure decay ratio and gas concentration decay ratio are convenient for evaluating the effect of the extraction system and predicting gas outburst. By comparing the gas pressure and concentration before and after fracturing, the effect of fracturing on gas extraction can be evaluated, and the fracturing parameters, extraction negative pressure and extraction pipe 9 layout can be optimized in a targeted manner to optimize the extraction efficiency.

[0047] The data analysis includes: (1) Analysis of the influence of the hydraulic fracture near, far and overall on the stress field at each acoustic emission measuring point; this analysis is carried out simultaneously with the pressure evaluation; The position of any acoustic emission probe 6 on the loading plate 2 is called an acoustic emission measuring point. The acoustic emission probes 6 are distributed in a matrix. The position of the acoustic emission measuring point is marked as ( i , j ); The industrial control computer 3 records the distance between each acoustic emission measuring point of each loading plate 2 and the acoustic emission signal point of the hydraulic fracture expansion based on the distribution of the acoustic emission signal points of the hydraulic fracture expansion monitored by each acoustic emission probe 6. d, where the maximum distance is d max , the minimum distance is d min , the average distance is d avg , define the pressure gradient attenuation coefficient KD of the acoustic emission measurement point ij =K ij / d ij , calculate KD ij-max =K ij / d ij-max , KD ij-min =K ij / d ij-min and KD ij-avg =K ij / d ij-avg , to achieve the purpose of qualitatively and quantitatively analyzing the influence of hydraulic fracture near point, far point and overall on the stress field of each acoustic emission measuring point; (2) Calculate the gas pressure gradient attenuation coefficient KD according to the distance between the extraction pipe interface 8 and the acoustic emission signal point of the hydraulic fracture expansion gp =K gp / d gp and gas concentration gradient attenuation coefficient KD gc =K gc / d gc , perform quantitative calculation and analysis of the hydraulic fractures near, far and overall distances from the extraction pipe 9 of each loading plate 2 in the same manner as step S64; wherein, d gp It refers to the distance between the pressure measuring point and the acoustic emission signal point of the hydraulic fracture expansion. d gc It refers to the distance between the extraction pipe interface and the hydraulic fracture expansion acoustic emission signal point. The position coordinates of the extraction pipe interface are stored in the industrial control computer. Combined with the position of the hydraulic fracture expansion acoustic emission signal point sensed by the acoustic emission probe 6, the distance between the extraction pipe interface and the hydraulic fracture expansion acoustic emission signal point can be calculated.

[0048] Calculate KD ij , KD ij-max、 KD ij-min , KD ij-avg , KD gp and KD gcAfter that, it is possible to qualitatively and quantitatively analyze the influence of the near point, far point and overall position of the hydraulic fracture on the stress field of each acoustic emission measuring point, so it has the following guiding significance for coal mine fracturing coal seam gas extraction operations: 1. Optimize fracturing parameters. By analyzing the distance between different acoustic emission measuring points and the acoustic emission signal points of crack expansion, the near point and far point of crack expansion can be determined, and then the fracturing parameters, such as the injection volume and injection pressure of the fracturing fluid, can be optimized to achieve more effective crack expansion and gas extraction. 2. Improve gas extraction efficiency. Since the influence of cracks on the stress field can be understood, the gas flow path can be predicted more accurately, and then the extraction drilling holes can be arranged more accurately in coal mine extraction to improve extraction efficiency. 3. Evaluate the influence of ground stress on crack expansion, more accurately predict the crack expansion direction and crack morphology, adjust the fracturing parameters, and provide a basis for fracturing design.

[0049] (1) and (2) are performed simultaneously, followed by (3); (3) According to the pressure decay ratio K of each loading plate 2 pressure measuring point ij Draw the stress attenuation surface of the coal sample in the three-axis direction and its average value K ij-avg , through the stress decay surface and K ij-avg Evaluate the regional pressure relief effect of the whole process of coal sample fracturing and extraction; combine the K gp and pressure method to quantitatively evaluate the decompression area of ​​coal samples, When |K is satisfied gp |≥(0.74MPa- P geo ) / P geo When the coal sample is completely in the pressure relief area, P geo is the original gas pressure in the well, which is measured in the first step; Combined with the extraction pipe interface 8 K gc The coal sample pressure relief area is quantitatively evaluated by the content method. When |K gc |≥|(0.01- C geo ) / C geo |, the coal sample is considered to be completely in the pressure relief area, where C geo is the original gas concentration underground, which is measured in the first step.

[0050] Pressure gradient attenuation coefficient KD ij , Gas pressure gradient attenuation coefficient KD gp and gas concentration gradient attenuation coefficient KD gc =K gc / d gcIt is used to further evaluate the coal sample fracturing-extraction unloading range, and to evaluate the influence of fracture morphology on the coal sample unloading effect in combination with the gradient attenuation coefficient of the near point, far point and average distance of the fracture extension acoustic emission signal point from each acoustic emission measuring point and the extraction pipe interface 8, so as to provide more accurate guidance for coal mine fracturing coal seam gas extraction operations.

[0051] In the fifth step, the time difference between the acoustic emission signals reaching different probes can be used to determine the location of the cracks, and the expansion direction and shape of the cracks can be inferred by inversion. By analyzing and processing the acoustic emission electrical signals, the mechanical properties and fracturing process of the coal samples can be better understood, and the pressure relief area can be understood.

[0052] The above embodiments are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention is described in detail with reference to the above embodiments, those skilled in the art should understand that the present invention can still be modified or replaced by equivalents. Any modification or partial replacement that does not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A coal body triaxial multi-point pressure relief effect evaluation system, comprising a true triaxial stress loading device, which comprises a pressure chamber for accommodating coal samples and applying triaxial stress to the coal samples; the pressure chamber comprises a frame in a cube shape, six faces of the frame are connected with loading plates, each loading plate is connected with a pressurized pipeline, each pressurized pipeline is provided with a valve, and each pressurized pipeline is connected with a high-pressure hydraulic system; One of the loading plates is connected to a fracturing pipe, which is used to selectively connect to an external gas source or an external fracturing medium source; a gas pressure sensor is provided on the fracturing pipe; Features: Also included is an industrial control computer, which is connected to a signal line; A plurality of first pressure sensors for monitoring the stress redistribution state in a true triaxial stress experiment are distributed in a matrix array on the loading plate; A plurality of acoustic emission probes for monitoring the expansion state of cracks in coal samples are arranged in a matrix array on the loading plate; the acoustic emission probes are spaced apart from each first pressure sensor; Each first pressure sensor and each acoustic emission probe are connected to the industrial control computer through the signal line, and a signal processing device is provided on the signal line between the industrial control computer and each acoustic emission probe, and the signal processing device includes a preamplifier and a data acquisition card; Each loading plate is provided with an extraction pipe interface, and a plug is provided on the extraction pipe interface; one of the extraction pipe interfaces is connected to an extraction pipe, and the extraction pipe seal passes through the loading plate and is connected to a gas collection bottle, and a gas concentration sensor and a gas pressure sensor are provided on the extraction pipe between the gas collection bottle and the loading plate; the extraction pipe passes downward through the top cover of the gas collection bottle and extends to the bottom of the gas collection bottle; The top cover of the gas collecting bottle is connected with a negative pressure tube, and the negative pressure tube is connected with a vacuum pump.

2. The experimental method of the coal body triaxial multi-point pressure relief effect evaluation system of claim 1 is characterized in that Follow these steps: The first step is underground field investigation and coal sample preparation; The second step is coal sampling and gas adsorption-desorption equilibrium; The third step is true triaxial stress loading; The fourth step is true triaxial stress state monitoring; the fourth step starts at the same time as the fifth step, and the fourth step lasts until the sixth step ends.

3. The fifth step is the coal sample fracturing operation and acoustic emission signal monitoring operation; The sixth step is negative pressure extraction; The seventh step is to evaluate the pressure relief effect in three-axis directions.

4. The experimental method according to claim 2, characterized in that: The first step, namely underground field investigation and coal sample preparation, is as follows: The ground stress, gas pressure and gas concentration parameters of the simulated coal seam are obtained by on-site underground measurements, and large coal samples are obtained from the underground site and processed into cubes that match the pressure chamber; after the first step is completed, the second step is carried out.

5. The experimental method according to claim 3, characterized in that: The second step is coal sampling and gas adsorption-desorption equilibrium. The coal sample was loaded into a fluororubber sleeve and placed in a pressure chamber. Six loading plates were used to apply a pressure of 2-5 MPa to the coal sample, and then gas injection was performed first; The gas injection operation is: connect the fracturing pipe to an external gas source, adjust the outlet pressure of the external gas source to the same as the actual gas pressure of the simulated underground coal seam, and inject gas at a constant pressure into the coal sample through the fracturing pipe until the pressure in the fracturing pipe remains stable for more than 48 hours, then end the gas injection operation and carry out the pressure maintenance operation; The pressure maintenance operation is: the gas pressure in the fracturing pipe and the gas pressure in the extraction pipe are continuously monitored by the gas pressure sensor installed on the fracturing pipe and the gas pressure sensor installed on the extraction pipe; If the gas pressure in the fracturing pipe or the gas pressure in the extraction pipe fluctuates by more than 5% within 48 hours, return to perform the gas injection operation again; If the gas pressure in the fracturing pipe or the gas pressure fluctuation in the extraction pipe remains within 5% within 48 hours, it is considered that the coal sample is in gas adsorption-desorption equilibrium; After the second step is completed, proceed to the third step.

6. The experimental method according to claim 4, characterized in that: The third step, true triaxial stress loading, is as follows: According to the actual ground stress conditions of the underground coal seam simulated by the experiment, the pressure parameters of the true triaxial stress loading device are set, the valves on each pressurized pipeline are opened, and the true triaxial stress is applied to the coal sample through each loading plate; after the true triaxial stress state is stable, the valves on each pressurized pipeline are closed, and after the true triaxial stress fluctuation range is within 5%, the fourth and fifth steps are carried out simultaneously.

7. The experimental method according to claim 5, characterized in that: The fourth step, namely true triaxial stress state monitoring, is as follows: The pressure changes on each loading plate are monitored in real time by a plurality of first pressure sensors distributed in a matrix array on each loading plate, and the pressure values ​​of the entire process of coal sample fracturing and extraction are recorded in real time; the fourth step is continued until the sixth step is completed; The experimental method according to claim 6 is characterized in that: the coal sample fracturing operation in the fifth step is specifically: The fracturing pipe is selectively connected to an external fracturing medium source, and the fracturing medium is injected into the coal sample in a constant flow and pressure manner. When the coal sample pressure monitored by the first pressure sensor drops by more than 25% within two seconds, the external fracturing medium source is turned off, the fracturing process is stopped, and the sixth step, i.e., negative pressure extraction, is performed; The acoustic emission signal monitoring operation in the fifth step is specifically: The acoustic emission signal monitoring operation and the coal sample fracturing operation are carried out and ended at the same time; Each acoustic emission probe is used to monitor the dynamic response sound signal of the coal sample crack expansion process in real time. During the coal sample fracturing operation, the coal sample will release elastic waves when the cracks expand. The acoustic emission probe detects these elastic waves and converts them into acoustic emission electrical signals, which are transmitted to the industrial control computer through the signal line.

8. The experimental method according to claim 7, characterized in that: The sixth step, namely negative pressure extraction, is as follows: Open the plug on the extraction tube interface on one side of the loading plate, connect the extraction tube interface with the extraction tube, turn on the vacuum pump, extract the gas adsorbed by the coal sample into the gas collection bottle, and use the gas concentration sensor and gas pressure sensor to record the gas pressure and concentration in real time until the gas concentration is lower than the predetermined concentration of the experiment.

9. The experimental method according to claim 8, characterized in that: The seventh step, i.e., the evaluation of the pressure relief effect in the three-axis direction, specifically includes three parts: pressure evaluation, drainage evaluation and data analysis. The pressure evaluation part of the seventh step is carried out simultaneously with the fifth step, and the drainage evaluation part of the seventh step is carried out simultaneously with the sixth step. Pressure evaluation: Calculate and process the pressure value detected by the pressure sensor on the loading plate in real time, and use the real-time pressure attenuation ratio in the three-axis direction to evaluate the pressure relief effect; The location of any first pressure sensor on the loading plate is called a pressure measuring point. The first pressure sensors are distributed in a matrix, so the pressure measuring point position is marked as ( i , j ); During the fifth step, the industrial control computer monitors the pressure curve of each pressure measuring point on each loading plate in real time, and records the pressure at any pressure measuring point as P ij ; The initial pressure of this pressure measuring point is P ij-0 , the real-time pressure is P ij-1 , then the industrial control computer calculates the real-time pressure attenuation ratio K of the pressure measuring point ij =( P ij-1 - P ij-0 ) / P ij-0 ; Extraction evaluation: Based on the gas pressure and gas concentration monitored by the gas concentration sensor and gas pressure sensor, the extraction impact of the decompression effect of the coal sample fracturing-extraction process is quantitatively evaluated; The industrial control computer monitors the gas pressure and gas concentration of the extraction pipe in real time during the sixth step, which is recorded as P CH4 and C CH4 , the initial gas pressure and concentration of the extraction pipe are P CH4-0 and C CH4-0 , the real-time pressure and concentration are P CH4-1 and C CH4-1 , the industrial computer calculates the gas pressure decay ratio as K gp =( P CH4-1 - P CH4-0 ) / P CH4-0 The industrial control computer calculates the gas concentration attenuation ratios as K gc =( C CH4-1 - C CH4-0 ) / C CH4-0 。 10. The experimental method according to claim 9, characterized in that: The data analysis includes: (1) Analysis of the influence of the hydraulic fracture near, far and overall on the stress field at each acoustic emission measuring point; this analysis is carried out simultaneously with the pressure evaluation; The location of any acoustic emission probe on the loading plate is called an acoustic emission measurement point. The acoustic emission probes are distributed in a matrix, and the location of the acoustic emission measurement point is marked as ( i , j ); The industrial control computer records the distance between each acoustic emission measuring point of each loading plate and the acoustic emission signal point of the hydraulic fracture expansion based on the distribution of the acoustic emission signal points of the hydraulic fracture expansion monitored by each acoustic emission probe. d , where the maximum distance is d max , the minimum distance is d min , the average distance is d avg , define the pressure gradient attenuation coefficient KD of the acoustic emission measurement point ij =K ij / d ij , calculate KD ij-max =K ij / d ij-max , KD ij-min =K ij / d ij-min and KD ij-avg =K ij / d ij-avg , to achieve the purpose of qualitatively and quantitatively analyzing the influence of hydraulic fracture near point, far point and overall on the stress field of each acoustic emission measuring point; 2) Calculate the gas pressure gradient attenuation coefficient KD according to the distance between the extraction pipe interface and the acoustic emission signal point of the hydraulic fracture expansion gp =K gp / d gp and gas concentration gradient attenuation coefficient KD gc =K gc / d gc , perform quantitative calculation and analysis of the distance between the hydraulic fracture and each loading plate extraction pipe near point, far point and the whole at the same time as step S64; wherein, d gp It refers to the distance between the pressure measuring point and the acoustic emission signal point of the hydraulic fracture expansion. d gc It refers to the distance between the extraction pipe interface and the acoustic emission signal point of the hydraulic fracture expansion; (1) and (2) are performed simultaneously, followed by (3); (3) According to the pressure decay ratio K of each loading plate pressure measurement point ij Draw the stress attenuation surface of the coal sample in the three-axis direction and its average value K ij-avg , through the stress decay surface and K ij-avg The regional pressure relief effect of the whole process of coal sample fracturing and extraction is evaluated; combined with the K gp The pressure method is used to quantitatively evaluate the decompression area of ​​the coal sample. |K gp |≥(0.74MPa- P geo ) / P geo When the coal sample is completely in the pressure relief area, P geo is the original gas pressure in the well, which is measured in the first step; K combined with the extraction pipe interface gc The coal sample pressure relief area is quantitatively evaluated by the content method. When |K gc |≥|(0.01- C geo ) / C geo |, the coal sample is considered to be completely in the pressure relief area, C geo is the original gas concentration underground, which is measured in the first step.

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