A THM Model for CH4-CO2 Transport in Fractured Coal

ABSTRACT: In the context of the global energy transition and the drive towards achieving net-zero carbon emissions, unconventional gas extraction methods have gained significant attention. This paper presents an advanced fully coupled thermo-hydro-mechanical (THM) model for enhancing methane (CH4) production and simultaneously storing carbon dioxide (CO2) in fractured coal seams. Our innovative approach integrates the complex interactions between thermal, hydraulic, and mechanical processes within the fracture network, addressing the dynamic evolution of matrix and fracture permeability ratios. By injecting CO2 into coal seams, methane is desorbed and produced more efficiently, while CO2 is securely stored within the fractures and pores of the coal matrix. The proposed model investigates the dual benefits of enhanced CH4 production and CO2 storage, offering a sustainable solution for energy production and greenhouse gas mitigation. Comprehensive simulations demonstrate the model's effectiveness in improving gas recovery rates and provide insights into the long-term stability of CO2 storage in geological formations. This study highlights the critical role of THM coupling in fracture networks and the dynamic permeability changes, ensuring a robust and efficient process for both gas extraction and CO2 sequestration, thereby contributing to the goals of the energy transition and net-zero carbon targets. 1. INTRODUCTION The burgeoning energy demands of rapidly developing nations such as China and India have propelled coal seam gas (CSG), primarily methane (CH4), to the forefront as a significant energy resource. Simultaneously, escalating concerns over climate change have underscored the urgency for effective carbon dioxide (CO2) sequestration methods. Injecting CO2 into coal seams emerges as a promising strategy that not only facilitates long-term CO2 storage but also enhances CH4 recovery, offering a dual benefit of energy production and greenhouse gas mitigation (Gunter et al., 1998; Luo et al., 2013). Despite the potential, the application of CO2 enhanced CH4 recovery and the simultaneous decarbonization faces substantial obstacles. Key among these are the complex multiphysics phenomena inherent in the process, including gas adsorption/desorption, diffusion, and the mechanical deformation of coal matrices (Bustin et al., 2016). The multiscale heterogeneity of coal seams—characterized by variations in porosity, permeability, and the presence of natural fractures—further complicates predictive modelling and optimization efforts. These challenges necessitate sophisticated modelling approaches to accurately simulate and manage the gas transport and storage mechanisms within coal seams.