Methane surface diffusion capacity in carbon-based capillary with application to organic-rich shale gas reservoir

Organic matter is widely distributed in organic-rich shale gas reservoir and has a large specific surface area to adsorb a significant quantity of methane molecules. In such nanoscale organic pores, the methane adsorbability and surface chemistries give rise to complex methane transport behaviours. Bulk state methane transports beyond continuum flow regime while surface diffusion for adsorbed methane plays a vital role in contributing to the total methane transport ability. In this study, we establish a methane surface diffusion model that considers the influence of confined pore space on methane adsorption, isosteric sorption heat and adsorbed methane coverage under high pressure. Grand canonical Monte Carlos simulations are carried out to estimate the adsorption isotherms of methane across a range of pore sizes and are applied to predict maximum methane concentration inside the adsorption layer volume. The contribution of surface diffusion on total methane transport ability in nanoscale confined pore space is investigated. Study results show that methane permeability for organic pores first decreases and then increases with the increase of pore size. Methane permeability for organic pore size less than 4 nm in relatively low pressure (<5 MPa) can be comparable to methane permeability for 20-25 nm inorganic pores. This can be attributed to the fact that the surface diffusion effect is enhanced in relatively low pressure and small pore sizes.