Study On The Geometric Shape Of Pores And Fractures And The Permeability Response Mechanism Of Loaded Water-bearing Coal Bodie

Coalbed methane is an associated resource of coal and a low-carbon clean energy source.The enhanced exploitation of CBM can effectively alleviate the energy crisis and contribute to the optimization of the energy structure and the " carbon peak and carbon neutrality" objective.In order to meet the growing demand for energy,coal mining is gradually moving deeper and deeper,where the pore-fracture structure is complex and dense,and the distribution is non-homogeneous.In addition,the high ground stress and high karst water pressure characteristics of the deep coal reservoirs lead to obvious low-permeability of coal,while the high ground temperature and strong mining disturbance characteristics exacerbate the uncertainty of gas hazards in low-permeability coal seams.Therefore,it is beneficial to investigate the mechanism of coal permeability evolution under the combined effects of multiple factors such as pore structure characteristics,stress and water,in order to facilitate safe,efficient and sustainable extraction of coal reservoirs as well as to accurately assess the economic benefits of coal reservoir development.This paper takes the geological background and physical characteristics of coal reservoirs collected from the data as the research background,and carries out permeability tests on the loaded coal under different water content for the raw coal samples of Zhaozhuang mine in Shanxi Province,using a combination of theoretical and experimental research methods to investigate the influence of multiple factors on the apparent permeability of different geometrical pore-fractures and to reveal the response mechanism between pore-fractures structure parameters and coal permeability.The main research progresses are as follows:(1)The permeability tests on loaded coal at different water contents are carried out.Under the same effective stress condition,the permeability of the coal decreases linearly with increasing water content.The coal permeability also decreases with stress loading under the same water content.Therefore,both the effective stress and moisture inhibit the high rate of gas transport in the pore-fractures channel.(2)Based on this,the model of apparent permeability of slit-like and circular capillary pore-fractures is modified by considering the effects of stress deformation,water-absorbing/gas-absorbing swelling,dynamic water film and slippage effect.And the reliability of the model is verified by experimental data.The effects of stress,water-absorbing/gas-absorbing swelling,dynamic water film and slippage effect on the permeability of coal are analyzed.The evolution mechanism of the apparent permeability of pore-fractures of different geometries is investigated.The results show that the water film thickness decreases in the order of circular,square and equieleteral-triangle pores.The permeability of the coal in different geometrical pores is in the opposite order of the water film thickness,while the slippage factor is greater in circular pores than in pores containing angles,and the difference between the slippage factors in square and equieleteral-triangle pores is smaller.(3)Interlocking coupling of factors within the real environment of a coal reservoir,the pore compressibility coefficient and dynamic water saturation are introduced to characterize the degree of inter-response between stress and water.Based on this,we quantify the dynamic pore spreading characteristics under the coupling actions of stress and water,establish a fractal apparent permeability model for coal,and validate the model using experimental data on the permeability of loaded coal with different water saturation to reveal the response mechanisms between structural parameters such as critical pore size and dynamic fractal dimension and the permeability of coal.The results show that the fractal dimension for pore-size distribution and the fractal dimension for tortuosity dominated by water had a positive effect on improving the permeability in coal.Conversely,the fractal dimension for tortuosity dominated by stress and the critical capillary radius had a negative effect.The effects of specific fractal dimension and dynamic fractal dimension on apparent permeability are consistent,but the effects on slip factor are opposite,providing a new perspective for understanding the evolution mechanism of coal permeability.