Research progress on enhancing recovery of medium-shallow coalbed methane through surface well development: mechanisms, main controlling factors, and main technologies
China’s coalbed methane (CBM) research focuses on the development of deep CBM. However, medium-shallow CBM resources are abundant and highly proven. Currently, the main challenge is the difficulty in effectively utilizing a large amount of adsorbed gas. If the remaining adsorbed gas can be effectively utilized, the production scale of medium-shallow CBM could achieve a significant breakthrough. To clarify the development direction of surface well development technology for enhancing recovery in medium-shallow CBM, this study conducts an in-depth discussion based on a chain mass-transfer model of “in-situ desorption-matrix diffusion-cleat seepage-fracture conductivity” and reviews the definition and connotation of recovery efficiency in medium-shallow CBM. Among these processes, matrix diffusion, as the key link connecting microscopic desorption and macroscopic seepage, is identified as the core factor restricting the overall recovery efficiency. Furthermore, the influencing factors are systematically classified into two categories: (1) the main seepage controlling factors affecting pressure drop sweep efficiency, such as fracture conductivity, well pattern layout, and pressure drop rate; (2) the main diffusion controlling factors affecting desorption efficiency, including matrix block size, gas diffusion coefficient, water saturation, and temperature and pressure conditions. On this basis, this study systematically reviews the mainstream technologies for enhancing recovery, such as well pattern optimization, gas injection displacement (CO2 flooding, N2 flooding), negative pressure extraction, desorption agent injection, physical field energy enhancement (acoustic wave, microwave, electric field), microbial stimulation, and hydraulic slotting. Their applicable conditions, field application effects, and failure mechanisms are also analyzed. It is found that although these technologies can increase production under certain conditions, they generally have the common limitations of difficult matrix entry, easy energy dissipation, and limited stimulation effect. In other words, external energy cannot effectively act inside the matrix, the pressure drop sweep range is limited, the desorption efficiency improvement is insufficient, and the stimulation effect rapidly decreases over time, making sustainable development difficult to achieve. For medium-shallow CBM development, the mobilization rate of recoverable reserves (i.e., the kinetic process) is more practically significant than the absolute amount of recoverable reserves (i.e., the thermodynamic state). Rapid and efficient mobilization of adsorbed gas within the matrix is the key to increasing production and enhancing recovery efficiency. Therefore, surface well development technologies for enhancing recovery should focus on three directions: increasing mass-transfer driving force in the matrix, reducing matrix block size, and enhancing the gas-phase flow capacity within the matrix. Among these, reducing matrix block size is currently the most readily achievable breakthrough direction. Increasing the matrix mass-transfer driving force has limited effectiveness, while enhancing the gas phase flow capacity within the matrix requires long-term research and development to achieve a breakthrough.