Review and Outlook of Pore Structure Characteristics and Flow Mechanisms in Deep Coalbed Methane Reservoirs: Advances and Perspectives

The pore structure characteristics of deep coalbed methane reservoirs are complex, and the microscale and nanoscale effects are obvious in the development of coalbed methane reservoirs. Understanding the fluid flow mechanisms in deep coalbed methane reservoirs is essential for optimizing the development plan and enhancing the recovery rates of coalbed methane reservoirs. However, comprehensive studies on pore structure characteristics and flow mechanisms in deep coalbed methane reservoirs are lacking. In this review, we briefly illustrate the pore structure and permeability of deep coalbed methane reservoirs. A distinctive feature of deep coalbed methane reservoirs is their pronounced U-shaped pore size distribution, dominated by micropores and mesopores, in contrast to shallow coal seams. It analyzes the fluid flow mechanisms, including desorption, diffusion, slip flow, Darcy flow, and non-Darcy flow, and summarizes the latest research related to deep low-permeability reservoirs. It is revealed that research on desorption hysteresis in deep coal seams has shifted from “static adsorption” to “dynamic stage division and quantitative evaluation”. The main challenges and future prospects regarding the development of coalbed methane recovery in deep coalbed methane reservoirs are finally provided. The results further identify three core challenges hindering efficient development: unclear gas accumulation mechanisms, inadequate predictive models for complex seepage dynamics, and significant engineering difficulties, which demands multidisciplinary collaboration for resolution. Finally, we summarize the main conclusions about the pore structure characteristics and flow mechanisms in deep coalbed methane reservoirs. A more detailed understanding is required to evaluate the efficiency of coalbed methane recovery in deep coalbed methane reservoirs. This work can provide a basis and reference for future directions of fluid flow and numerical simulations during the development of deep coalbed methane reservoirs.