Review of the progress of geological research on coalbed methane co-production

Coalbed methane (CBM) co-production is an important way to improve the efficiency of CBM development in multi-seam areas, but the special nature of reservoir formation makes the co-mining method and production effect complex and variable, which presents challenges for efficient development. Experts in the field of CBM from China have carried out a lot of basic research and engineering practice on CBM reservoir formation and the feasibility of co-production in multiple seams, which have gained fruitful results, providing strong support for deepening the CBM geological theory and promoting industrial development. To provide reference for subsequent research, engineering implementation and industrial construction, this paper systematically analyzes and reviews the latest research progress in the field of CBM co-production geology in China from four aspects: reservoir formation theory of stacked CBM systems; co-production geological constraints; co-production feasibility identification method; and co-production reservoir damage. The main understandings can be summarized as follows. (1) The sequence gas control mechanism of the accumulation of the stacked CBM systems and the later modification effect of rock formation and ground stress are deepened. The hydrogeochemical closed index of coal-measure groundwater environment is constructed, which provides a new parameter to identify the stacked gas-bearing systems and hydrodynamic conditions, and three types of stacked geological patterns of gas-bearing systems (growth type, decay type and stable type) are identified by using fluid pressure profiles. The concept of stacked CBM system is further extended to the category of coal measure gas, and the theory and method system of “co-mining compatibility” of stacked gas-bearing systems based on coal-measure composite reservoirs is proposed and applied to the pilot demonstration project of coal-measure gas co-production, which has a chieved preliminary results. (2) The Carboniferous-Permian (Taiyuan-Shanxi Formation) in North China and Late Permian (Changxing-Longtan Formation) in Western Guizhou and Eastern Yunnan are the hotspot areas for CBM co-production research and engineering practice, and the fluid pressure system and permeability differences are the most concerned geological factors in co-production. The difference in hydrodynamic conditions and fluid supply capacity of Shanxi Formation and Taiyuan Formation is an important factor limiting the CBM co-production in North China. The maximum inter-seam span, cumulative thickness, coal body structure of the coal seam in co-production in Western Guizhou and Eastern Yunnan have received more attention, and interference from shallow groundwater is the key restricting the efficiency of CBM co-production in the Zhijin Block. (3) Productivity analysis, physical simulation, numerical simulation, and geochemical analysis of produced water are important methods to identify the feasibility and interference of CBM co-production. The basic idea, technical template and evaluation process for analyzing the produced water source and identifying the degree of fluid interference in the co-production wells based on the geochemistry of produced water, as well as the production layer contribution analysis method based on the peaking and identification of the gas-production curve, have been proposed. The continuous maturity and innovation of technical methods provide strong support for the optimization and efficiency improvement of CBM co-production engineering. (4) CBM co-production is more sensitive to geological conditions and engineering disturbances, and is prone to induce reservoir damage, involving Jamin effect and airlock damage induced by production layer exposure, as well as stress-sensitive and velocity-sensitive damage induced by the pressure system and permeability differences. Homogenized reservoir reconstruction, separated-pressure system development (separated-time or separated-space), and refined drainage design and control are effective ways to reduce reservoir damage.