Discussion on the effective stress coefficient in unconventional fracture-matrix gas transport

Gas transport in coal and shale matrices does not always fall into the continuum flow regime described by Darcy’s law. The contributions of Darcian and non-Darcian flows to the permeability are both involved and closely dependent on pressure evolution. It is found that a “U-shape” permeability trends, caused by flow dynamics and geomechanical effects, may appear in both modeling and laboratory works. The constitution of flow regimes related to different effective stress coefficients (Biot’s coefficient χ) is not fully understood by the current studies. In this study, multiple cycles of laboratory measurements of shale permeability with helium and CO2 (carbon dioxide) were conducted under hydrostatic conditions. “U-shape” permeability curve was observed in both positive and negative effective stress coefficients. Under negative effective stress coefficient, the permeability is dominated by the effective stress, which means the enlargement of the pore aperture is the controlling component of the dimension of the flow pathways. Thus, both negative and positive effective stress coefficients are necessary if a non-monotonic pressure-permeability evolution profile is to be built to explain the concurrent effect of diverse flow regimes at different pressure regions.