(713c) Numerical Analysis of Hydrocarbon Flow in Shale Gas Reservoirs

The hydrocarbon recovery from unconventional gas and oil resources is rapidly increasing. The natural gas exploited from shale reservoirs is expected to account for 53% of total global gas endowment by 2040. Shale rocks have compact and heterogeneous microstructures with low porosity. The transport properties of hydrocarbons have a direct relationship with the size and geometry of pores. A significant portion of hydrocarbons is present in the physical adsorption state on the surface of the shale nano- and micro-fracture pores. Due to low permeability of matrix, Darcy flow has a minimal effect on gas flow in ultra-tight shale reservoirs and mass transfer in nano-pores is essentially controlled by non-continuum flow (Knudsen diffusion and slip flow). In this study, a numerical analysis is presented to investigate the dependency of apparent permeability in Knudson flow regime on certain physical parameters, namely the temperature, pressure, pore size, tortuosity and tangential momentum accommodation coefficient (TMAC). The results demonstrate that temperature and pore size have the most detrimental effect on apparent permeability. Decreasing the pore size, down to a few tens of nanometers, activates the slip flow and Knudsen diffusion. As a result, the apparent permeability far exceeds the intrinsic threshold represented by Darcy formula. While the large pores show little sensitivity to temperature change, the permeability of nano-pores is significantly enhanced by increasing the ambient temperature. This is due to the temperature-assisted gas desorption from the pore surfaces. Results also show that increasing the tortuosity and TMAC leads to the reduction of apparent permeability. Finally, the numerical analysis is used to simulate the transient pulse-decay experiments on a shale core plug, where the effects of gas adsorption and gas compressibility are taken into account. The simulation predicts that increasing the temperature leads to a pressure drop in gas flow.