Gas Transport Through Nanopores of Shale Gas Reservoirs: Coupling Pore Diffusion and Surface Diffusion

Abstract

The understanding for gas transport in nanopores is the basis for accurate numerical simulation, which has important implications for the economic development of shale gas reservoirs (SGRs). Gas transport mechanism in SGRs is significantly different from that of conventional gas reservoirs, which is mainly caused by the nanoscale phenomena and adsorbed gas. Firstly, a model for bulk gas transport in nanopores was established, which was based on slip flow and Knudsen diffusion. The total gas flux in bulk phase should not be a simple summation of slip flow flux and Knudsen diffusion flux, but a weighted summation. Secondly, a model for adsorbed gas surface diffusion in nanopores was established, which was based on Hwang model and considered the gas coverage effect at high pressure. Finally, the combination of the two models established, a unified model for gas transport in nanopores was established and validated through molecular simulation and experimental data. Results show that: (1) slip flow has a great contribution to gas transport under the condition of meso-macropores (pore radius>2 nm) and high pressure, while it can be ignored under the condition of micropores (pore radius≤2 nm) and the pressure lower than 1 MPa in SGRs.; (2) Knudsen diffusion has an important contribution to gas transport under the condition of macropores (pore radius>50 nm) and the pressure lower than 1 MPa, while it can be ignored in other cases; (3) surface diffusion coefficient is comparable with pore diffusion coefficient, and gas transport is always dominated by surface diffusion in all over the range of pressure in micropores (pore radius≤2 nm), while it can be ignored under the condition of pore radius greater than 25 nm and pressure higher than 1 MPa; (4) Temperature and pressure dependence of surface diffusion are weaker compared with those of pore flow; and (5) surface diffusion contribution decreases, while silp flow and Knudsen diffusion contributions increase with an increasing isosteric sorption heat. The unified model can capture the physical mechanism of the interaction between bulk gas transport and adsorbed gas surface diffusion, and calculate the corresponding fluxes for different gas transport mechanisms and their contributions. Results innovatively indicated that surface diffusion is very important and even dominates gas transport, which leads to the additional recovery of 35% and it is likely to play positive role during the production of SGRs.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016