A gas apparent permeability model in coal micro/nano-pores considering the poroelastic effect and its application in gas extraction
The poroelastic effect in micro/nano-pores of coal significantly affects gas production.However,it is currently rare for the poroelastic effect to be considered in gas apparent permeability model for micro/nano-pores.In order to investigate the influence of the poroelastic effect on gas flow mechanisms in micro/nano-pores,a gas apparent permeability model considering the dynamic evolution of pore size is constructed based on the poroelastic effect and multiple mechanism flow model in micro/nano-pores.The dominant flow mechanism of the dynamic apparent permeability model is determined and the contribution of slippage effect in micro/nano-pores to gas recovery production isevaluated.The results show that the poroelastic effect impacts the contribution of the slippage and the surface diffusion effects to the apparent permeability by influencing the evolution of pore size.As the pore pressure increases,the dynamic apparent permeability ratio(the ratio of the apparent permeability affected by the poroelastic effect to the initial apparent permeability)is controlled by the slippage effect and the poroelastic effect in turn.Additionally,the contribution of the slippage effect to the apparent permeability ratio decreases rapidly at lower pressure but decreases more gradually at higher pressure as the pressure increases.In the process of gas recovery from low-permeability coal seams,the contribution of the slippage effect to gas recovery production initially increases rapidly,then decreases gradually,and eventually reaches a slow increase.A smaller average pore size in micro/nano-pores leads to a greater contribution of the slippage effect to gas recovery production.
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