Evolution of multiple stresses during in-situ coal seam extraction and its controlling mechanism on permeability
LIU Huihui1，2，YU Bin1，2，LIN Baiquan3，XIA Binwei1，2，LI Quangui1，2，ZOU Quanle1，2
(1. School of Resources and Safety Engineering，Chongqing University，Chongqing 400044，China；2. State Key Laboratory of Coal Mine Disaster Dynamics and Control，Chongqing University，Chongqing 400044，China；3. School of Safety Engineering，China University of Mining and Technology，Xuzhou，Jiangsu 221116，China)
Abstract：Single，first mining coal seam gas extraction are in situ extraction，which is the key and difficulty of coal mine gas extraction. Compared with the conventional gas reservoirs，the mechanical state response of coal seam during extraction is more complex due to gas adsorption，and its evolutionary law and control mechanism on permeability are still unclear. Therefore，in this paper，the evolution of in-situ coal seam permeability and mechanical state during gas extraction were studied under different stress field conditions，and the control effects of multiple stresses on permeability were discussed. Results show that：(1) The coal seam horizontal stress and vertical strain decline slowly at first and then rapidly during the process of gas extraction. The changes in mechanical state with respect to pore pressure satisfies a linear relationship，while this behavior related to gas adsorption can be well described using a Langmuir-like equation. On this basis，a mechanical state model of in-situ coal seam was established considering the coupling effects of geo-stress，pore pressure and gas adsorption. (2) The permeability does not change significantly with vertical geo-stress，but shows a significant downward trend under the action of horizontal geo-stress. This is mainly attributed to the fact that there is a direct mechanical effect between coal seam cleats and horizontal geo-stress，but not with vertical geo-stress. The mechanical state in the horizontal direction plays a major role in controlling the evolution of permeability. (3) During in-situ coal seam extraction，the permeability increases slowly first and then rapidly with the decrease of gas pressure，reaching the maximum value when the gas is completely extracted(final state). However，when only pore pressure is applied，the permeability shows a trend of slow decline. (4) Both pore pressure and gas adsorption control the evolution of permeability by changing the effective stress and the internal interaction between coal matrix and cleats，of these，gas adsorption plays a major controlling role. In the current stress-dependent permeability models，the effective stress is simply used to represent the coupling effect of pore pressure and gas adsorption on the coal seam permeability，but the internal interaction between the coal matrix and cleats is ignored.
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2023, Vol. 42 
