Gas transport characteristics in shale matrix based on multiple mechanisms

Gas transport characteristics in shale matrix can be represented by permeability. However, shale gas flow is influenced by multiple mechanisms, and existing matrix permeability models cannot comprehensively reflect these mechanisms. Therefore, current permeability models are reviewed first, and the multiple mechanisms are summarized based on previous achievements. Next, given the influence of adsorption on the gas-solid boundaries, the effect of shale gas slippage is corrected. Then, a new permeability model of shale matrix is derived based on rock and fluid properties, and its reliability is proved by using four methods. The proposed model contains 12 action mechanisms, including as many existing mechanisms as possible. Finally, the characteristics of matrix permeability are successfully analyzed by using this model and a simplified method of model application is given. The results show that, for action mechanisms, adsorption has the greatest effect on matrix permeability under small aperture at low pressure, and the influence of matrix stress sensitivity is greater than that of matrix shrinkage. Gas slippage in pores with adsorption capacity should take into account gas-gas interaction; otherwise, matrix permeability is underestimated. For flow mechanisms, the influence of viscous flow is important in all cases except for small apertures at low pressure. When the aperture is small and the pressure is low, surface diffusion and Knudsen diffusion become obvious, and the influence of surface diffusion even exceeds that of viscous flow. Pore size has a great effect on matrix permeability. The complex model can be simplified by ignoring special mechanisms, such as ratio of pores with adsorption capacity, adsorption, matrix shrinkage, and surface diffusion in most cases, even in nanopores.