Impact of Effective Stress on Coal Relative Permeability: Experiment and Model Development

Coal seams are typical fractured porous reservoirs and serve as one of the key sites for coal seam gas (CSG) or coalbed methane (CBM) production and CO₂ geological sequestration, during which the reservoir pressure changes within coal seams, leading to alteration in cleat characteristics, thereby affecting coal relative permeability. However, direct experimental studies providing sufficient data for modelling the impact of effective stress on relative permeability are limited. Additionally, existing relative permeability models primarily expressed relative permeability as a function of saturation, cleat and/or pore size, and overlooked the impact of the effective stress changes, which can introduce significant uncertainties in estimating gas–water two-phase flow under varying reservoir conditions. This study conducted a suite of gas–water two-phase flow experiments on coal cores under different effective stresses using steady-state method to quantify the impact on relative permeability curves. Our results demonstrated that increased effective stress enhances both gas and water relative permeability in coal. Based on the laboratory results, an improved relative permeability model was developed, incorporating the effective stress impact, which offers greater reliability for assessing coal seam gas well productivity and CO₂ injectivity. The findings provide valuable insights for investigating gas–water two-phase flow in fractured rock formations.