Enhancing mine safety and mitigating greenhouse gas emissions: A comprehensive analysis of fluid migration dynamics in coalbed methane co-production

Abstract

Coalbed methane (CBM) production confers a range of advantages such as improving coal mine safety, mitigating greenhouse gas emissions, diversifying the energy portfolio, and advancing towards carbon peak and neutrality targets. Understanding and delineating the migration patterns of CBM during the production process are crucial for ensuring process safety and environmental protection. This study employed a comprehensive research approach involving physical simulation experiments, mathematical modeling, and numerical simulations to investigate the reservoir-wellbore coupling control mechanism of gas migration during the coproduction of multiple thin coal seams. The results revealed that the gas pressure exhibits stage-wise response characteristics, with the low-pressure reservoirs showing a more pronounced evolution pattern. Increasing the wellbore pressure reduced the resistance to gas pressure expansion, weakened fluid disturbance effects, and led to a significant pressure drawdown funnel. Conversely, decreasing the wellbore pressure intensifies fluid disturbances, making pressure drawdown expansion more challenging and resulting in poor compatibility for co-production CBM. Based on the “dual-pore single-permeability” reservoir gas flow model, a mathematical model for reservoir-wellbore coupling control of fluid migration was established. When the wellbore pressure was relatively low, fluids in low-pressure reservoirs migrated from the coal seam center to the deeper parts, exhibiting centrifugal flow patterns, while the fluid migration in the wellbore was in the opposite direction. In high-pressure reservoirs, fluid migration patterns were centripetal, and the flow in the wellbore was in the same direction, prone to forming congestion effects. Increasing the wellbore pressure facilitates the migration and production of CBM, while decreasing the pressure amplifies the reservoir-wellbore property differences, which was detrimental to CBM production. This research elucidated the migration laws of CBM involving “desorption, diffusion, seepage, and fluid interference”. The findings aimed to provide theoretical guidance for reducing fluid disturbance and enhancing the efficient production of CBM.