Interference Analysis of Methane Co-Production from Two Coal Seams in Southern Qinshui Basin

Gas co-production has been touted to be a cost-effective way to increase the recoverable coal-bed methane (CBM) resource in multiple coal seams. However, co-production in the southern Qinshui Basin has met with disappointing results due to interlayer interference. In this study, a reservoir model was established around one co-production well with high water production and low gas production; with respect to seven reservoir parameters, the interferences were analyzed quantitatively. The results show that, for a given aquifer communicated with the lower reservoir, it can not only restrain the output of water from itself but it can also reduce the effective water production from the upper reservoir, and thus, reservoir pressure cannot be reduced in both layers effectively. Moreover, the interference intensity, which is triggered by discrepancies in reservoir parameters between the two CBM systems, varies from strong to weak and is due to reservoir pressure, aquifer thickness and permeability, coal seam thickness, fracturing permeability, in situ permeability, and gas content. Finally, when progressive drainage from either layer is considered, drainage from the lower seam first is more advantageous than the other; besides, the placement of co-production well in the well-pattern was verified to be an effective approach to improve gas recovery. This study can provide a reference for the development of CBM resources in the study area and as basis for the theoretical study of gas co-production from coal measures.