Evolution and geochemical characteristics of gas phase fluid and its response to inter-well interference during multi-well drainage of coalbed methane

The Fanzhuang block from the southern Qinshui basin was used as a monitoring area to determine the components, the methane carbon isotopes (δ13C1) and the methane hydrogen isotopes (δD) of the recovered coalbed methane (CBM) during the different production times. The spatial evolution characteristics of the CBM components, δ13C1 and δD and their response to inter-well interference during multi-well drainage in CBM production areas were analyzed visually. The results show that CBM components, δ13C1 and δD all fluctuated; the spatial distribution direction of CBM components, δ13C1 and δD also changed twice or more during sampling in the studied area. The fractionation of δD lags behind the fractionation of δ13C1. The spatial distribution of the CBM components is affected by the CBM components obtained from partial CBM wells; the spatial distribution of δ13C1 and δD of coalbed methane is affected by the fractionation of δ13C1 and δD from the produced CBM. The decrease in the CBM components and the δ13C1 and δD ratios reveals the effects of inter-well interference on the multi-well patterns. The variability in the spatial distribution direction of CBM components, δ13C1 and δD shows that inter-well interference may still be in an early stage or the degree of inter-well interference may be relatively weak. The incomplete consistency of the spatial evolution of CBM components, δ13C1 and δD is mainly related to the fractionation of CBM components and methane isotopes during drainage and the change in the CBM supply caused by the unstable fluid field under unbalanced inter-well interference. It was shown that inter-well interference during multi-well drainage increases and then begins to stabilize. The established geochemical response model and evaluation program of inter-well interference provide a method to determine the stage and the degree of inter-well interference in CBM production areas, not only in the southern Qinshui basin but also in other CBM basins with low permeability.