Abstract: Shale matrix stores a larger amount of gas and provides a long-term gas supply for shale gas production. Therefore, matrix permeability variation with the change of pore pressure directly affects long-term productivity. Micro- and nano-pores are well developed in shale matrix. These organic and inorganic pores involve different mechanical properties and gas transport mechanisms, the variation of effective stress and flow regimes all affects their apparent permeability. Moreover, gas-adsorptioninduced organic matter swelling also influences the apparent permeability to varying degrees. Based on the poroelasticity theory, this paper establishes an apparent permeability model suitable for shale matrix with consideration of different mechanical properties and fluid flow mechanisms in organic and inorganic pores and gas-adsorption-induced deformation. The impacts of different pore properties are analyzed, and the controlling factors for apparent permeability evolution are figured out. Results indicate that shale matrix apparent permeability evolution is time-dependent during constant-pressure gas injection and depletion. The evolution process is mainly dominated by gas rarefaction effects, effective stress variation, and localized sorption-induced swelling or desorption-induced shrinkage near organic pores. These factors control apparent permeability at different times. The evolution process is also related to the ratios of the volume of organic and inorganic pores to the total pore volume.

Key words: shale, matrix pores, apparent permeability, sorption-induced strain, flow regime