Experimental study on methane adsorption and time-dependent dynamic diffusion coefficient of intact and tectonic coals: Implications for CO2-enhanced coalbed methane projects

Abstract

CO2-enhanced coalbed methane (CO2-ECBM) technology can be applied not only to extract coal seam gas, but also to geologically store a large amount of CO2 produced in various industries, thus reducing the CO2 concentration in the atmosphere and protecting the earth’s environment from the adverse effects of climate change. For CO2 injection and methane recovery from coal seams, the differences in the gas adsorption and diffusion characteristics due to the properties of the coal seam are crucial. In this study, first, intact and tectonic coals were collected to conduct a proximate analysis and high-pressure methane adsorption/desorption tests. Subsequently, the adsorption characteristics of the intact and tectonic coals with different particle sizes were analyzed. A time-dependent dynamic diffusion coefficient was introduced to investigate the diffusion characteristics of the two coals. The results showed that the intact and tectonic coals have higher adsorption capacity for CO2 than methane under the same particle size, demonstrating the potential for storing a large amount of CO2 in coal seams. The time-dependent dynamic diffusion coefficient decreased significantly in the initial stages and then tended to stabilize as the desorption progressed. To better explain the gas diffusion characteristics of the coal, a gas diffusion mechanism model was established to analyze the variation in the diffusion coefficient over time. In addition, a new method for reducing the pressure difference was proposed to maximize CO2 storage and methane recovery. Finally, the environmental implications and safety of CO2 storage in coal seams during the implementation of a CO2-ECBM project was analyzed.