Interference mechanism in coalbed methane wells and impacts on infill adjustment for existing well patterns

Abstract

Inter-well interference, a superimposed effect of pressure propagation from adjacent wells, can enhance the dynamics of methane desorption in pores and gas transport in fractures through energy migration. To better understand the variation pattern of seepage efficiency, the inter-well interference mechanism of different types of reservoirs and productivity changes after infilling coalbed methane wells were investigated. The results showed that the average productivity of old wells increased by 627 m3/d after infilling a horizontal well in the high-permeability reservoirs without surrounding rock water supply (HP-WW); however, the newly infilled horizontal well produced gas at 4996 m3/d. The infill well in HP-WW reservoir resulted in 11％-39％permeability increment compared with initial permeability (the average permeability changes from 1.58 to 2.18 mD). While, much higher permeability increment was observed (varies from 29％-50％) in relative low permeability coalbed formation due to infill well. After the formation of interference, increased confining pressure increases effective stress, which is smaller than the positive effect formed by the fracture expansion and the increase in the effective diffusion area. Additionally, well interference promotes coupled superposition of the pressure drop funnel, which effectively suppresses the influence induced by velocity sensitivity, water sensitivity, and water lock, resulting in an increased pressure difference between the reservoir and the wellbore. This enhances the dynamics of methane desorption from the pore surface and gas migration from the fracture. For gas diffusion, the mean free path of gas molecules decreases with well interference, implying that the main diffusion resistance changes from collisions between gas molecules and the pore wall to collisions among gas molecules. Therefore, the findings of this study can help for better understanding of the response mechanism of inter-well interference to efficient CBM production.