Microscale Analysis for Gas-water Two-Phase Flow in Dual-porosity Shale Matrix Based on Integrated Modeling of Transport Phenomena

Gas-water two-phase flow within shale matrix plays a critical role in shale gas production and directly affects gas recovery and fracturing-fluid flowback. This study develops a novel theoretical model for gas-water two-phase flow in shale matrix based on a dual pore-system framework that distinguishes organic and inorganic pore networks. The model integrates multiple fluid-transport mechanisms, accounts for the gas diffusion attenuation, incorporates the effect of irreducible water saturation, and represents pore-structure complexity using fractal descriptions. Model predictions are validated against multiple independent experimental datasets and are further applied to simulate microscale gas-water transport during shale gas production. Simulation results show that gas apparent permeability is higher at the later stage of recovery and near the boundary of the matrix, whereas water apparent permeability is higher at the early stage of recovery and in the central region of the matrix. Diffusion attenuation, irreducible water saturation, and capillary effects strongly limit fluid extraction from the shale matrix, leading to substantial retention of both gas and water. Neglecting diffusion attenuation in organic pores results in an overestimation of matrix gas production by approximately 80%. Increasing irreducible water saturation reduces the effective pore space and lowers the apparent permeability of both phases. Additionally, higher fractal dimensions of pore size distribution and pore tortuosity reduce gas-water apparent permeability, owing to decreased average pore size and increased flow-path length, respectively. This study provides a quantitative framework for describing gas-water two-phase transport in shale matrix and offers new insights into the coupled processes governing shale gas recovery.