Definition, connotation, formation and application of coalbed methane system: A case study on the Carboniferous-Permian coal seams in the Ordos Basin

Under the control of sensitive elements such as temperature, pressure and stress, coalbed methane (CBM) forms a coal-gas-water strong coupling system. Understanding this system is of great significance for accelerating the development of the CBM occurrence geological theories and promoting CBM exploration and development efficiently. In this paper, the basic concept of CBM system is defined, and the basic elements are determined. Then, the formation process and basic characteristics of CBM system are studied, and the reservoir pressure, formation temperature and groundwater chemical characteristics of CBM system and the occurrence state, composition, gas saturation and production performance of CBM are analyzed. Finally, the spatial distribution patterns of CBM system are identified with the CBM in the Ordos Basin as an example. The following results are obtained. First, the basic elements which should be considered in CBM system include coal seams and their associated strata in coal measures with similar formation pressure coefficient, caprock system, and CBM/ water fluid system with similar formation pressure coefficient. Second, the coal-gas-water occurrence states of CBM system at different depths are determined by the coupling of three major elements (reservoir, caprock and fluid) with the evolution of the system environment. Third, based on the occurrence state of CBM, the CBM system is divided into two zones, i.e., reworked zone and primary zone. The reworked zone corresponds to the shallow CBM in the usual sense, and the primary zone corresponds to the deep CBM. The primary zone is subdivided into main adsorbed gas zone (primary A zone), free gas-adsorbed gas zone (primary B zone), and adsorbed gas-free gas zone (primary C zone). The CBM in the reworked zone is dominated by a syncline controlling gas mode, and as for the CBM in the primary zone, especially in the primary B and C zones, attention shall be paid to the exploration of trap areas. What's more, the Carboniferous–Permian CBM system in the Ordos Basin transforms from reworked zone to primary A zone, then primary B zone and to primary C zone in turn from eastern and western flanks to the middle, and in the center of the basin, the CBM of primary B zone and primary C zone is dominant. © 2024 Natural Gas Industry Journal Agency. All rights reserved.