Dynamic prediction and effectiveness analysis of co-production gas wells in coal-bearing formations

The juxtaposition of coal seams and tight gas reservoirs is a common geological structure in coal-bearing formations. Elucidating the production dynamics of coalbed methane（CBM） and tight gas from such overlaid formations holds significant importance for the synergistic development of coal-bearing resources. Initially, the study examines the gas and water production characteristics and reservoir average pressure evolution under separate development of CBM and tight gas. Subsequently, employing a wellbore node analysis method, it iteratively calculates pressure distribution characteristics under varying mass flow conditions, using wellbore pressure drop as a link to couple the mutual interference of pressure between coalseams and tight gas formations and their respective production characteristics. Finally, a dynamic gas production prediction model for co-production wells in the “coal seam above, tight below” juxtaposition relationship is established. The study analyzes the stable production capacity of co-production wells and the evolution of production contribution from each layer under different production systems, revealing that: ① the stable production period of co-production wells （averaging 2158 days） significantly exceeds that of CBM separate development. ② The cumulative gas production of co-production wells sharing a single wellbore differs by only 2.56% compared to multiple drilling for separate development, demonstrating the strong economic benefits of CBM-tight gas combined production. ③ Due to the typical characteristics of early-stage single-phase drainage of CBM, its contribution to gas production in co-production wells exhibits an initial zero phase followed by a slow increase in the later period.