Interlayer Interference Characterization and Optimization Model for Production Layer Combination in Superposed Coalbed Methane Systems

To reduce interlayer interference (INTI) during multilayer coalbed methane (CBM) coproduction and promote efficient development of CBM resources in multiseam areas, the Bide-Santang Basin in southern China is selected as the research area in this study. Using a self-developed physical simulation experimental system, gas–water two-phase seepage (GWTPS) simulation experiments for CBM coproduction were carried out under different geological models with various combinations of permeability and interlayer pressure difference (IPD). The results reveal the characteristics of GWTPS and the mechanism of INTI under CBM coproduction conditions. The effect of IPD on water production is more pronounced under high-permeability conditions than under low-permeability conditions. The “water-lock effect” in the low-pressure layer (LPL) is the primary cause of gas production limitation during coproduction; the growing-type upper layer suffers more severe impairment from the “water-lock effect” and more intense INTI than the decaying-type lower layer. Based on gas production contribution and INTI intensity, an INTI model for coproduction was constructed with permeability and IPD as variables, and the reliability of the model was verified via analysis of field well data. Under high-permeability conditions, the optimal production layer spans were 38 and 70 m in the growing type and decaying type, respectively. For low-permeability reservoirs, INTI is not the dominant factor restricting gas production from CBM coproduction, and productivity improvement hinges primarily on permeability enhancement. The growing type is more susceptible to INTI than the decaying type, which restricts its gas production rate during coproduction. The results provide a scientific basis for optimizing production interval combinations and improving the efficiency of CBM coproduction.