Hydrogeological control and productivity modes of coalbed methane commingled production in multi-seam areas: A case study of the Bide–Santang Basin, western Guizhou, South China

Abstract

Coalbed methane (CBM) commingled production is an important measure to increase the CBM productivity in multi-seam areas. However, multiple superimposed CBM systems are widely developed in multi-seam areas and the difference in the hydrogeological conditions between CBM systems significantly influences the CBM commingled production. In this study, the Bide–Santang Basin in South China was used as an example and hydrogeological data analysis, production data analysis, and rock and coal sample tests were used to investigate variable hydrogeological conditions of superimposed CBM systems and their implications for the CBM production. The hydrogeochemical closed index (HCI) is proposed based on the ion compositions of the groundwater in the coal-bearing strata, which is a new parameter that can be used for the quantitative discrimination and description of superimposed CBM systems based on the determination of the groundwater environment. The hydrodynamics of the upper CBM system are stronger at a lower HCI value, and thus susceptible to shallow groundwater interference during the CBM production. The predicted daily water production based on the unit water inflow of the pumping test boreholes is generally lower than 1 m3/(d · m) when the median depth of the coal seam group exceeds 150 m. When the daily water production per unit drawdown (DWPPUD) of the producing fluid level in the CBM well is less than 1 m3/(d · m), high CBM production can be achieved in the well due to the absence of shallow groundwater interference in the Zhucang Syncline. Finally, three productivity modes of CBM commingled production wells in the Bide–Santang Basin were discriminated: shallow groundwater interference wells, high production wells, and fracturing fluid-contaminated wells. Numerical simulation shows that upper CBM system limits the production of middle and lower CBM systems. Suggestions are made for the CBM development in the Bide–Santang Basin based on the variable hydrogeological conditions and in situ stress of the superimposed CBM systems.