Experimental Study On Similar Materials Of Brittle Surrounding Rock In Pressure Relief Of Tectonically-deformed Coal During The Coalbed Methane Development

The research and development of similar materials in the top and bottom plates is the premise of indoor simulation test research on the development technology of coal caving gas in coal caving.This paper takes Liuzhuang Mine Field in Huainan Mining Area of Anhui Province as the research area,and collects the petrological characteristics of coal seam brittle surrounding rock samples by collecting wells 25#,16-1#,13-1#and 11-1#.The permeability of the original rock and the rock mechanics parameters were tested,and the characteristics of the brittle surrounding rock of the typical tectonic coal development zone in Huainan were obtained.Considering the factors such as raw material selection,proportion and molding conditions of similar materials,five-factor and four-level orthogonal test schemes were designed.The stress sensitivity and mechanical properties of similar materials were tested and based on the principle of stress similarity.The similarity ratio determines the optimal material selection and proportioning scheme for similar materials.The FLAC³DD software was used to simulate the stress,strain and displacement distribution characteristics of the brittle surrounding rock around the horizontal wellbore in the laboratory tectonic coal,and the rationality and safety of the similar materials developed in the indoor simulation test were evaluated.The research of the thesis has mainly achieved the following understandings and achievements:?1?The lithology of the briquette surrounding rock in the study area is mainly siltstone,fine-medium sandstone,fine-grained sandstone and medium-grained sandstone.The observation of rock flakes and XRD diffraction analysis indicate that the mineral composition of the original rock is dominated by quartz and feldspar.For dark minerals,mica,etc.The gas permeability values of siltstone,fine-middle-grain sandstone,fine-grained sandstone and medium-grain sandstone under the pressure of5 MPa are 0.004mD,0.012mD,0.0110mD,0.028mD,respectively,and the rock sample increases with effective stress.Permeability shows a downward trend.Under low effective stress,the permeability stress is sensitive,and as the effective stress increases,the permeability decreases.During the unloading process,as the effective stress decreases,the permeability gradually recovers,but there is a certain loss of permeability.Under the confining pressure of 15MPa,the compressive strength of the original rock is 133.19²23.58MPa,the elastic modulus is 10.99²2.72×10³MPa,and the Poisson's ratio is 0.16⁰.31.?2?Based on the basic principle of stress similarity,comparing the stress conditions of the original rock with the loading conditions of the simulated test platform,the similarity ratio C_?of the prototype to the model,the similarity ratio of the elastic modulus and the similarity ratio of C_E and compressive strength C_?c are determined.In addition,the Poisson's ratio of the brittle surrounding rock prototype to similar materials is required to be as close as possible.?3?Quartz sand is used as aggregate,gypsum and kaolin are used as mixed cementing materials,and borax is used as retarder.The effects of sand-to-binder ratio,water-to-binder ratio,ratio of stone to soil,molding time and curing time on permeability and mechanical properties of similar materials are considered.,carried out orthogonal design experiments.The results show that the permeability and compressive strength of the sample are mainly affected by the sand ratio,while the elastic modulus and Poisson's ratio are mainly affected by the water-to-binder ratio and curing time.The permeability of the sample decreases as the ratio of sand to rubber decreases,and the compressive strength increases as the ratio of sand to rubber decreases.The ratio of sand to rubber is similar to that of similar materials at 1:4 level,which can better meet the purpose of gas in closed structural coal.The compressive strength and elastic modulus of similar materials increased with the decrease of sand-to-gel ratio and water-to-binder ratio,and the Poisson's ratio increased with the prolongation of curing time.Considering the permeability of the sample and the variation characteristics of the rock mechanics parameters,it is considered that the optimum level of sand-to-gel ratio,water-to-binder ratio,ratio of stone to soil,molding time and curing time in the preparation of similar materials for structural brittle surrounding rock in the study area are 1:4,1:3.6,3:1,5h and 4d.?4?The FLAC^3D numerical simulation software was used to simulate the influence of horizontal borehole excavation on coal and rock in the 2m×2m×1.5m model?upper and lower layers of sandstone and middle layer of coal?,and it was found that the stress,strain and displacement were symmetrically distributed around the borehole.In the vertical direction,the stress gradually increases from the borehole to the circumference;in the horizontal direction,the stress gradually decreases from the borehole toward the periphery.The strain is decreasing from the center of the borehole to the circumference,and the compressive strain is mainly in the horizontal direction.The tensile strain is mainly in the vertical direction,and the displacement in the vertical direction is larger than the horizontal direction.