Experimental study and model improvement on coal permeability: The influence of effective stress, slip effect, and water content

Abstract

Permeability is a critical parameter in coalbed methane (CBM) recovery and received increasing attention in recent years. The slip effect and effective stress exert competing influences on permeability, with coal exhibiting varying sensitivities to effective stress depending on their pore structures. The presence of water further complicates these interactions, affecting both the slip effect and permeability. This study investigates the pore structure and permeability characteristics of four coal cores at varying water contents using low-field nuclear magnetic resonance (NMR) and pulse pressure decay (PPD) methods. An enhanced apparent permeability model was developed by incorporating water content, effective stress, and the slip effect. The dynamic variations of compressibility coefficient, slip coefficient, and intrinsic permeability for Cores C-F were theoretically examined based on the refined model, and the critical pore pressures at which the slip effect becomes significant were identified. The results indicate that cores with larger average pore sizes exhibit more pronounced changes in fracture compressibility coefficients as water content increases. Additionally, the slip coefficient decreases with increasing pore pressure and is notably lower at reduced water contents. Intrinsic permeability increases more significantly with pore pressure at higher water content, with cores having larger average pore diameters showing greater sensitivity to these changes. The critical pore pressure, where the slip effect becomes significant, increases with water content and is higher in cores with smaller average pore sizes. Finally, various coefficients are proposed to quantitatively assess changes in fracture compressibility, slip coefficients, intrinsic permeability, and critical pore pressures under varying water content conditions, enabling more accurate analysis of permeability behavior.