A new model for the transport of gaseous hydrocarbon in shale nanopores coupling real gas effect, adsorption, and multiphase pore fluid occupancies

Abstract

A new gaseous hydrocarbon transport model, considering pore geometry, multiphase pore fluid occupancy, and porous deformation, has been developed to characterize the real gas flow in both nanoscale organic and inorganic porous media. To do so, Navier-Stokes equations were solved with modified second-order slip boundary conditions in nanocapillaries of organic matter (OM) and nanoslits of inorganic matrix (iOM). Due to gas adsorption, both surface diffusion and bulk gas flow affect the apparent gas permeability (AGP) of OM. Particularly, the Langmuir-slip model is used for the calculation of boundary velocity of bulk gas. In contrast, gas flow in a single nanoslit of iOM accounts for the impact of adsorbed water film with a certain thickness quantified by Li's model [15]. Additionally, porous deformation and real gas effect are included in both models of OM and iOM. As such, a unified AGP model of shale matrix is formulated based on the total organic carbon (TOC) content including contributions of both OM and iOM. Results show that, under depressurization condition, the AGP of OM/iOM with a large pore size distribution (PSD) presents a similar shape of “V”, while the AGP of iOM with a small PSD increases monotonously. The total flow capacity of OM is contributed by competitive mechanisms of surface diffusion, induced- and pure- bulk gas flow under various PSDs and pressure. For gas flow capacity in iOM with a small PSD, it can be weakened by adsorbed water film but compensated by nanoscale effect considerably at low pressures. Moreover, a higher relative humidity (RH) leads to a thicker water film resulting in a lower gas conductance. The AGP of shale matrix can be reduced or enhanced by the TOC content when accounting for gas adsorption and adsorbed water film. Besides, the real gas effect enhances flow capacity significantly in small pores with a high pressure. The proposed model provides a comprehensive understanding of the gas flow mechanism in shale nanopores under reservoir conditions.