关键词: CO₂-ECBM, 多相多组分流体, 流体连续性过程, 吸附-解吸, 扩散-渗流, 数值摸拟

Abstract: CO₂-enhanced coalbed methane (CO₂-ECBM) recovery and sequestration offers dual benefits of greenhouse gas mitigation and enhanced methane production, which is of significant strategic value for supporting China’s "dual-carbon" strategy and ensuring energy security. Multiphase and multicomponent fluid flow mechanisms constitute the theoretical foundation for overcoming the engineering bottlenecks of CO₂-ECBM, enabling efficient CO₂ injection, secure sequestration, and enhanced CH₄ production in coal reservoirs. This paper systematically elaborates on the multiphase and multicomponent fluid permeation mechanisms in CO₂-ECBM, along with the key influencing factors. It further reviews advancements in corresponding experimental methods, multiscale modeling and numerical simulation techniques, highlighting their practical applications in engineering field, while identifying current theoretical challenges and potential solutions. The research results show as follows. (1) Due to its microscopic-level competitive advantages including higher quadrupole moment, stronger adsorption heat, and smaller kinetic diameter, CO₂ preferentially occupies high-energy adsorption sites on the coal surface; fluid diffusion occurs through a coupled mechanism involving bulk diffusion, Knudsen diffusion, and surface diffusion; low-velocity non-Darcy flow is prevalent in coal reservoirs, while fluid continuity processes are characterized by multiphysics, multiphase, multicomponent, and multiple scales. The sequential coupling of the adsorption-diffusion-seepage processes is critical to ensuring displacement efficiency. (2) The pore-fracture structure is a critical factor influencing multiphase and multicomponent fluid flow. Meanwhile, the cooperative transport mechanisms within multiscale pores and fractures and their spatial connectivity significantly influence fluid migration processes. The Knudsen number is an important parameter for characterizing flow regimes. CO₂ injection causes matrix swelling and an increase in effective stress, leading to a reduction in permeability. Geochemical effects, such as mineral dissolution, enhance pore permeability and facilitate fluid migration. Injection pressure and rate are key engineering parameters that control the advancement of the flow front, and the proper configuration of these parameters is essential for improving displacement efficiency. The coupled interactions and competitive relationships between multiple physical fields also have a significant impact on multiphase and multicomponent fluid migration. (3) Characterization of coal pore-fracture structures and fluid occurrence primarily relies on observation techniques, radiation detection techniques, gas adsorption and fluid intrusion methods, and integrated multiscale approaches. Adsorption-desorption behaviors are revealed through isothermal and competitive adsorption experiments. Diffusion-seepage experiments, supported by nuclear magnetic resonance and computed tomography (CT), provide dynamic insights into fluid migration under reservoir conditions. Displacement-drainage-storage physical simulations elucidate the key mechanisms of pore structure evolution and permeability changes under geological conditions. (4) Multiscale modeling and numerical simulations enable the establishment of a molecular-pore-engineering hierarchical research framework. Molecular modeling and dynamic simulations reveal the micromechanisms for the competitive adsorption and diffusion of CO₂ and CH₄. Pore-scale techniques, such as pore network modeling (PNM) and computational fluid dynamics (CFD), combined with X-ray computed tomography (X-CT) and focused ion beam-scanning electron microscopy (FIB-SEM), enable the analysis of fluid flow and mass transfer in complex pore networks. Core-scale numerical simulations serve as a crucial link between experimental results and field applications, while engineering-scale coupled multiphysics simulations provide dynamic assessments of injection-production processes, storage capacity, and safety. This integrated approach offers valuable support for well pattern optimization and parameter design. Currently, research into multiphase and multicomponent flow mechanisms in CO₂-ECBM is transitioning from single-process analyses to an integrated approach involving multiple technologies and scales. Future research should focus on key scientific issues such as pore-fracture evolution, multiphase-multicomponent fluid coupling, and phase transition mechanisms. Efforts should focus on experimentally replicating in-situ reservoir conditions, developing dynamically calibrated three-dimensional geological models, and integrating artificial intelligence and big data to achieve breakthroughs in critical technologies. This approach facilitates comprehensive integration of theory, experiments, and simulations, ultimately enhancing the CO₂-ECBM displacement efficiency and ensuring storage safety.

Key words: CO₂-ECBM, multiphase and multicomponent fluids, fluid continuity process, adsorption-desorption, diffusion-seepage, numerical simulation