A comprehensive review of deep coalbed methane and recent developments in China

Over the past decade, the exploration and development of deep coalbed methane (CBM) resources in China have attracted significant attention for their immense potential. Typically found in geological conditions marked by high formation temperature, reservoir pressure, and in-situ stress, exploring and developing these resources present a multitude of challenges. As the coal seams extend to greater depths, adsorption behavior, pore-fracture structure, and mechanical properties of deep coal seams become increasingly intricate, impacting properties such as gas content, permeability, and fracability. Despite the presence of significant free gas in deep CBM reservoirs, production remains challenging due to the extremely low permeability of coal seams. Additionally, high in-situ stress and complex mechanical parameters of deep coal seams can significantly reduce hydraulic fracturing performance, further complicating the development of deep CBM. This comprehensive review provides a detailed analysis of the geological conditions and formation property variations at different depths for CBM reservoirs, illuminating the distinctions between deep and shallow CBM reservoirs. Furthermore, this review summarizes the pilot tests and research perspectives in China to address the challenges associated with exploring and developing deep CBM resources. By raising awareness of the particularities of deep CBM and offering practical and accessible references, this review aims to contribute to the better exploration and development of these valuable resources.