An analytical model for coalbed methane transport through nanopores coupling multiple flow regimes

Abstract

Understanding the complex transport behavior of methane in coal matrix pores is very important for coalbed methane exploitation. One important approach is to model the transport process of methane in coal matrix nanopores. In this work, we modelled the coalbed methane transport process in coal matrix nanopores, coupling gas adsorption, real (i.e. non-ideal) gas and gas slippage effects, and a new method is proposed to characterize the governing equation of transport flow regime. It was found that the proposed model agreed well with the published data on methane physical properties (viscosity and density) and coal permeability. We also focused on the influence of temperature and pore pressure on the flow regime, Knudsen number, surface diffusion and bulk gas flow in the model. The results indicate that under typical reservoir conditions, the real gas and gas adsorption effects have a significant impact on methane viscosity, density and apparent permeability. It was also found that the mass flow rate of methane by surface diffusion increases when pore pressure increases up to about 5 MPa, but tends to decrease when pore pressure increases above about 5 MPa. For any given pore, the flow mechanism of the bulk gas can change from one mechanism to another as the temperature and pore pressure conditions change. The governing equation of the transition flow regime can be regarded as a weighted superimposition of the slip flow equation and Knudsen diffusion equation. However, under reservoir conditions, the Knudsen diffusion regime and the transition flow regime are negligible. This work will provide a theoretical basis to assist with the effective development of coalbed methane.