Experimental study on shale apparent permeability model under different stress conditions

To explore the influence of gas transport and stress coupling on shale apparent permeability in the process of shale gas extraction,this paper proposes a contains a variety of weighted factors coupling transmission mechanism,the adsorption of deformation and stress strain relationship of shale model to describe the gas flow,apparent permeability,the rationality of the model is verified by experimental data,and the influence of model-related parameters on shale apparent permeability were discussed. The results show that a new shale apparent permeability model considering fluid flow and stress coupling can reasonably describe shale gas flow,including considering the gas slippage effect under stress,pore structure and adsorption deformation,viscous flow of gas transmission and body phase flow(slip) and Knudsen diffusion. Under different confining pressures and pore pressures,the curves calculated by the new model are in good agreement with the measured values. With the increase of pore pressure and confining pressure,the apparent permeability of shale decreases exponentially,and the absolute value of the cleat compressibility Cf decreases as a whole. The lower the pore pressure,the higher the proportion of Knudsen diffusion. Under the same pore pressure,the smaller the pore size,the greater the contribution rate of Knudsen diffusion. The apparent permeability of shale is sensitive to the elastic modulus,and the larger the elastic modulus is,the smaller the matrix deformation caused by adsorption will be,and the corresponding increase of permeability will also decrease. The main control transmission mechanism of different shale pore sizes is different. With the decrease of pore sizes,pore sizes are comparable to the average molecular free path of gas,and slip flow occurs and permeability increases. The calculated values of apparent shale permeability with slippage effect taken into account are all greater than those without slippage effect,and are closer to the measured values. With the increase of pore pressure and confining pressure,the permeability change caused by slippage effect gradually decreases.