Creep: A Neglected Phenomenon in Coal Permeability Evolution and Coalbed Methane Production

Abstract

Abstract During gas production, coalbed experiences compaction over time when effective stress increases as a result of pressure depletion triggered by escaping gas from micropores through desorption and diffusion. It is deemed that time-dependent compaction deformation (compaction creep) and deterioration of coal structure lead to a decrease in coal permeability. Alteration of compaction rate and permeability of coalbed depends on factors such as coal properties, pore pressure, time, and temperature. Thus far, little studies have been conducted to investigate the impact of compaction creep, viscoelastic, and viscoplastic behavior of coal on permeability. Creep may have such significant impact on coal permeability that its negligence may result in overestimation of Coalbed Methane (CBM) production. Appreciation of creep phenomenon, coal properties such as viscoelasticity and viscoplasticity, and their impact on the interaction of coal permeability and gas production borehole enables optimisation of CBM production. In this study, we aim to examin permeability models and highlight the significance of creep as a factor influencing the evolution of coal permeability and CBM production.