| Microseismic monitoring is an important means to detect the stimulated researvoir volume,especially to observe the hydraulic fracture propagation in real time.At present,microseismic monitoring is introduced to evaluate the hydraulic fracturing of coal seam in underground coal mine.However,it is not mature yet that the understanding of microseismic response mechanism of hydraulic fracturing in coal seam,including the effective monitoring distance.Especially,microseismic source location becomes difficult due to the underground confined space.Therefore,it is of great significance to carry out the research on the microseismic characteristics and the source location method in confined space for the accurate implementation of hydraulic fracturing in coal seam and efficient gas drainage.In this thesis,microseism of hydraulic fracturing in underground coal seam are systematically studied with the methods of theoretical analysis,numerical simulation,physical simulation and engineering experiment.Firstly,the research on the response law of hydraulic fracturing microseism and the propagation characteristics of elastic wave in coal measure strata is carried ou.Secondly,the three-component polarization location method of hydraulic fracturing microseism source in coal seam is proposed,the theoretical model of three-component polarization location is established,and the experimental verification research is carried out.Thirdly,the lithology identification method of hydraulic fracturing fractured strata is explored to correct the source location.Finally,the research results are summarized after that,the industrial application of hydraulic fracturing microseismic monitoring in underground coal seam is studied.The main research results are as follows.(1)The mechanism of microseismic response of coal and rock hydraulic fracturing is revealed,and the multi frequency response characteristics of acoustic emission of coal and rock hydraulic fracturing are found.Different local fractures of coal and rock mass will produce different resonance frequencies,release different frequency acoustic emission signals,showing the characteristics of multi frequency response.Multi frequency response index(MFRI)can characterize the formation of macro cracks.Sandstone hydraulic fracturing experienced micro fracture,micro fracture aggregation and macro fracture stage;soft coal hydraulic fracturing experienced primary fracture connection,primary fracture aggregation and macro fracture stage.The hydraulic fracturing of sandstone is a mixture of tension and shear fracture,while that of soft coal is mainly shear fracture.In the macroscopic fracture stage,the sandstone shows the whole tensile fracture,while the soft coal shows the whole shear fracture.(2)This thesis reveals the propagation characteristics of elastic wave in coal measure strata,puts forward the calculation method of microseismic monitoring distance based on the combination of energy and frequency parameters,and obtains the effective monitoring distance of hydraulic fracturing in underground coal seam.The effective monitoring distance of microseism is related to source energy,frequency and propagation medium.The research shows that under the condition of water injection pressure of about 25 MPa and water injection flow of about 10 m~3/h,the effective monitoring distance of hydraulic fracturing microseism in underground coal seam is 200m.(3)Three component polarization positioning method of microseismic source for hydraulic fracturing in underground coal seam is proposed.The key technology of three component microseismic polarization analysis is formed.The three component microseismic monitoring system is developed and a large-scale microseismic monitoring experimental platform is established.The inversion results show that the absolute positioning error of the three-component polarization positioning method is only 1.75 m,while the error of the traditional arrival time-based method is 100.52 m,which verifies the effectiveness of the three-component polarization positioning method and solves the problem of microseismic source positioning in confined space.(4)An automatic lithologic identification method for acoustic emission signals of coal and rock hydraulic fracturing is proposed,and a horizon correction model for source location is established.The lithologic identification results of long-term and short-term memory network,perceptron network and convolution neural network show that the accuracy of acoustic emission lithologic identification of coal and rock hydraulic fracturing reaches 90%,and the identification results can be further used to correct microseismic source location.(5)The industrial application research of microseismic monitoring shows that:the frequency of microseismic in hydraulic fracturing of underground coal seam is mainly around 150 Hz;the time of microseismic occurrence is mainly concentrated in the stage of water injection volume of 0～50 m~3;the microseismic sources are mainly distributed along the direction of adjacent hole and primary fracture zone,and the microseismic events in the upper part of coal seam dip are more than those in the lower part;the distribution of microseismic sources is consistent with the evolution of disturbance stress.The radius of hydraulic fracturing range in Hongyang No.2 mine is 26～100 m,and the gas drainage concentration after hydraulic fracturing is 61%higher than that before hydraulic fracturing.The findings of this thesis provide theoretical support and technical basis for the microseismic monitoring of hydraulic fracturing in underground coal seam.The research results have important significance for standardizing the hydraulic fracturing and promoting the safe and efficient gas extraction. |
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