Evolution of Water-Gas Relative Permeability with Effective Stresses in Coal and Sandstone

Abstract Relative permeability is an important parameter in reservoir engineering, playing a significant role in understanding and forecasting fluid flow behavior within rock formations. Relative permeability is affected by numerous factors, such as rock type, fluids wettability, pore size and fracture aperture, which was determined by effective stress and fracture stiffness that changes during the reservoir production. Nevertheless, when conducting reservoir simulation, it is a common practice to assume that relative permeability curves are fixed and do not vary with reservoir depletion, leading to substantial uncertainty in simulation outcomes. In this work, a suite of laboratory measurements was conducted to characterize the gas-water two-phase flow in coal and sandstone under varying confining stresses, and the results were then used to refine and update the existing relative permeability curves. Finally, this study conducted a comparative analysis of the two-phase flow curves in sandstone and coal, delving into the distinctions in the characteristics of two-phase flow within fractured porous media (coal) and non-fractured porous media (sandstone). Our results show that as confining pressure increased, water mobility reduced, leading to its retention in the coal sample and resulting in a higher residual water saturation. These changes were reflected in a rightward shift in coal relative permeability curves. Additionally, the relative permeability of both water and gas increased with the increasing confining stress in the coal sample. In contrast, the sandstone sample showed only a slight increase in relative permeability with increasing confining stress, differing from the coal sample. Furthermore, the relative permeability of gas changed more obviously with water saturation when increasing confining stress for both coal and sandstone samples. The findings offer insights into the dynamic behaviour of permeability under varying effective stress conditions, enhancing our understanding of fluid flow in both coal and sandstone reservoir rocks.