Dual-Porosity Apparent Permeability Models of Unconventional Gas Migration Based on Biot’s Porous Media

Stress-dependent permeability models are developed for the organic pores and inorganic cleats/ fractures in unconventional gas reservoirs, which are modeled as Biot’s porous media of dual-porosity. Further considering multiple flow mechanisms such as dynamic effects of gas flow and surface diffusion, apparent permeability models are obtained to investigate the characteristics of unconventional gas migration. Compared to the gas transfer in single-porosity reservoirs, the gas migration ability of cleats in dual-porosity stratums rarely changes while that of organic pores is greatly improved because cleats sustain major geomechanical shrinkage deformation when the pore pressure drops. Further, the mass flux of reservoirs is dominated by the mass flux of cleats, which has a lower peak value, but a much longer production term than those in single-porosity reservoirs due to the interaction between organic pores and cleats. Parametric analysis is conducted to identify key factors significantly impacting mass flux in unconventional reservoirs. Reasons for the mass flux variation are also explored in terms of gas migration ability and pore pressure distribution.