Permeability evolution in variably saturated shales controlled by bimodal organic/inorganic pore structure and surface wetting characteristics

Moisture is distributed heterogeneously within shale with distinctive features in both organic and inorganic pores. Importantly, this influences gas flow, impacts reservoir dynamics, and affects the accuracy of production forecasts. We establish a variable pressure gradient-bimodal pore structure (VPG-BPS) model to capture these characteristics of methane transport in partially water-saturated shale. We determine permeability in both organic and inorganic pores by matching estimated to actual gas production. Dynamic production experiments are conducted on variably saturated shales from the Sichuan Basin to determine permeability. Methane adsorption capacities are categorized into three stages, delineated by two intermediate threshold moisture contents—lower (MLc) and upper (MUc) threshold moisture contents, respectively. We demonstrate that the moisture content significantly influences shale transport dynamics by controlling adsorption–desorption properties and thereby gas-water distribution and permeability. Ultimate gas production decreases with increasing moisture content (Mc). Gas permeability (Kin and Ko) and gas desorption rate coefficients (kads,in and kads,o) in inorganic and organic pores range from 1 × 10−8 μm2 to 1 × 10−7 μm2, and 1 × 10−4 to 1 × 10−3, respectively. With Mc increasing from 0% to 3.5% (MUc), Kin, Ko, and kads,in exhibit a consistent trend of initially increasing before decreasing although kads,in shows no definitive trend above the threshold peak (MLc). When Mc increases from 0% to 2.2% (MLc), the desorptive hysteretic effect gradually weakens and adsorption kads,in increases with increasing Mc, subsequently prompting an increase in the other three flow parameters. As Mc further increases to MUc, kads,in remains constant but the other three flow parameters decrease as the gas flow channel is occupied by the invading water. Even when moisture migration ignored, the VPG-BPS model provides superior predictions of time histories of gas production over existing models and thus provides a theoretical foundation for dynamic evaluation of shale gas production.