Experimental study of deformation induced by high-pressure methane adsorption and desorption: Insights into anisotropy and hysteresis characteristics

Abstract

Adsorption deformation of the coal matrix significantly influences gas migration and enhances recovery in coal reservoirs. In deep coal seams, abnormally high fluid pressures complicate the accurate quantification of absolute adsorption using traditional models, affecting the assessment of adsorption deformation. To address this, this study conducted synchronous adsorption/desorption and strain testing on coals of varying metamorphic degrees under gas pressures up to 15 MPa. The results indicate that the simplified local density model effectively corrects the absolute adsorption amount. Compared to the smaller experimental errors in particle coal, cubic coal shows synchronized adsorption and strain changes, and the lower mean square error from the thermodynamic strain model fitting confirms its suitability for modeling adsorption deformation. As coal's metamorphic degree increased, the deformation modulus increased, indicating enhanced resistance to deformation. Replacing fugacity with pressure may also overestimate the deformation modulus. Further analysis of strain anisotropy and hysteresis during adsorption/desorption showed that anisotropy primarily arises from the macroscopic bedding structure and heterogeneous composition of coal. Anisotropy indices mainly range from 0.1 to 0.5 and gradually decrease as pressure rises during adsorption. Moreover, both adsorption hysteresis h_a and strain hysteresis h_s decrease with increasing pressure, while the overall hysteresis indices of adsorption and strain vary significantly due to irreversible deformation. For practical applications in coalbed methane extraction, incorporating strain anisotropy and hysteresis into constitutive and permeability equations is essential for optimizing multifield coupling models, thereby facilitating the efficient development of deep coalbed methane resources.