Coupled Confined Phase Behavior and Transport of Methane in Nano-sized Slits

Abstract:

The matrix in shale gas systems consist of mostly nano-sized organic pores. Phase behavior in these nanoconfined pores deviates from that of bulk phase due to significant interactions between gas molecules and wall surfaces. As a result, predictions of classical Equations of State (EOS) often fail to adequately predict phase behavior and thermodynamic properties of such fluids such as critical pressure and temperature values. Here, we present a modified Peng-Robinson (mEPR) EOS, which is motivated and guided by molecular dynamics (MD) simulations. Capillary pressure is account for via the introduction of a region-dependent parameter. The proposed mEPR-EOS captures the shift of critical properties as well as the density phase envelop of methane under different confinement scenarios. Moreover, we incorporate this mEPR-EOS in the lattice Boltzmann method to simulate gas transport in nano-sized slits. Results indicate that, compared to PR-EOS, the use of the proposed mEPR-EOS leads to a smaller fluid velocity and a lager fluid viscosity. This effect is likely due to strong interactions between fluid particles. Transport behavior is affected by the pressure of the system: in a system with a relatively low pressure, transport seems to be dominated by Knudsen diffusion, whereas in a system under a relatively high pressure, the contribution from viscous flow increases as the pore widens while the influence of Knudsen diffusion and surface diffusion diminishes.