Research On The Evolution Of Pore Structure Of Low Permeability Coal Seams In Guizhou Under CO₂ Fracturin

The coal seams in Guizhou have the characteristics of high gas content and low permeability.The low permeability,high geostress,and high gas pressure in deep coal seams make gas control and efficient gas extraction more difficult.Currently,CO₂phase change fracturing technology is widely used in the field of coal mine gas extraction in Guizhou due to its high efficiency,energy-saving,environmental protection,and pollution-free advantages.This technology can effectively transform the pore and fracture structure of coal reservoirs and improve their permeability.Therefore,studying the evolution of pore and fracture structure under CO₂fracturing is of great significance for further improving the fracturing and permeability enhancement mechanism,guiding gas extraction work,and promoting the application of CO2 fracturing technology.This paper focuses on low permeability coal seams in Guizhou.Firstly,the principle of CO₂phase change fracturing,stress wave propagation,and crack propagation mechanism were theoretically analyzed.Then,coal samples were collected before and after fracturing for experimental analysis,and the pore parameters of the coal samples were characterized in sections using mercury intrusion porosimetry,low-temperature nitrogen adsorption,and CO₂adsorption methods.The RHT constitutive model parameters of coal-rock were calibrated,and the expansion law of coal fractures under the action of liquid CO₂fracturing and the influence of geostress on fracture expansion were analyzed through numerical simulation of CO₂gas explosion.Finally,the macroscopic data after fracturing were applied in the coal mine field to verify the evolution law of pore and fracture structure under liquid CO₂fracturing.The main conclusions are as follows:（1）Based on elastic mechanics and fracture mechanics theories,we analyzed the mechanism of liquid CO₂-induced fracturing in coal-rock masses,including the formation of crushed zones,fractured zones,and vibration zones.The coal-rock mass is first compressed and broken by the shock wave generated by CO₂phase change,followed by the formation of tensile cracks under the action of stress waves.Continuous expansion of coal-rock fractures occurs under the influence of high-pressure gas.The crack propagation gradually stops as stress waves propagate and CO₂gas expands and dissipates.By using the principle of energy equivalence,we calculated that the energy generated by phase change when the storage tube of the MZL275-51/1200 fracturing device is filled with liquid CO₂is 2100 k J,corresponding to a WTNT of 0.4942 kg.（2）The main pore space in coal consists of macropores and mesopores,which account for more than 87.4%,with macropore volume occupying 66.3-82.7%.The specific surface area of micropores and mesopores in coal samples accounts for more than 97%,with micropore specific surface area ranging from 58.1-80.1%,indicating a large number of micropores providing space for gas adsorption and storage.The mesopores in the coal matrix are mainly 4 nm in size,with pores occurring as fissures and narrow cracks formed by sheet-like structures and plate-like particles.Fracturing expands the pore network,creating open pores,semi-closed/closed pores,and bottle-shaped pore structures.Fracturing reduces micropores while increasing mesopores and macropores,making gas more easily desorb from the coal matrix and promoting gas transport and diffusion,providing conditions for efficient gas extraction.In addition to micropore volume fractal dimension（D_S）increasing,the permeability pore fractal dimension(D_H1),diffusion pore fractal dimension(D_H2),surface fractal dimension(D_F1),and spatial fractal dimension(D_F2)of coal samples decrease,indicating that,except for micropores,the spatial structure and surface of other pores become simpler and smoother,and heterogeneity weakens under the effect of fracturing.Additionally,fracturing leads to a decrease in coal mechanical strength.（3）The RHT model can effectively simulate the tensile failure behavior and dynamic fracture propagation of coal.CO₂phase transformation fracturing can promote the generation,development,and expansion of coal fractures,which is beneficial for gas migration and extraction.Based on coal damage distribution maps and crack measurement results,CO₂gas explosion-induced fractures have radius around 1.1m under zero ground stress conditions.Moreover,the magnitude and anisotropy of ground stress significantly affect the coal failure mode.With increasing ground stress,the damage area and crack propagation are clearly suppressed.Under static hydrostatic pressure,the damage evolution and crack propagation are isotropic,while under non-static hydrostatic pressure,the coal damage zone appears elliptical,and the cracks mainly evolve along the direction of the maximum principal stress.Additionally,the free surface plays a guiding role in the evolution of damage and cracks,promoting their propagation towards the direction of the free surface.（4）Field test of carbon dioxide phase transition cracking was carried out in111204 fully mechanized mining face of Panzhihua Coal Mine.After 31 days of combined pumping induced by carbon dioxide phase transformation cracking and sealing holes,the gas permeability coefficient of coal in the cracking zone is2.86～3.63 times that of the original coal seam,the attenuation intensity of gas emission from boreholes is reduced by 27.7%～48.3%,the residual gas content is below the critical value of 8m3/t,and the cracking extraction rate is between 44.95and 54.31%.The gas permeability of coal seam is obviously improved after cracking.The extraction concentration and purity of the crack hole were significantly higher than that of the uncracked hole.The extraction concentration and purity curve of the fission-induced hole increased first and then decreased,and reached a stable state about 16 days after extraction,and the extraction concentration stabilized at about50%,which was 3.3 times of that without cracking hole.The extraction purity is stable at about 0.15m3/min,which is 15 times of that without cracking.The results show that the pore and fissure structure of coal body in the fissure region is improved obviously after the fissure,and the large pores and fissure increase,which makes it easier for gas desorption and migration and diffusion,improves the efficiency of gas extraction,significantly reduces the cycle of gas treatment,and ensures the safe and efficient production of the mine.