Production characteristics and the key factors in high-rank coalbed methane fields: A case study on the Fanzhuang Block, Southern Qinshui Basin, China

The temporal and spatial production characteristics of coalbed methane (CBM) wells in the Fanzhuang Block of the Southern Qinshui Basin, the first commercial CBM-producing basin in China, were studied to determine the dominant factors. The study indicates that gas production differs significantly in different wells adjacent to or located in different tectonic areas. In combination with the production characteristics in the temporal and spatial variations, the impact of seven factors (burial depth, thickness of coal, gas content, porosity/permeability, effect of fracturing, structural setting, and hydrogeological conditions) on the gas production was analyzed based on bivariate correlation analysis and gray system theory, which are suitable for solving the complex interrelationships between multiple factors and variables. The results indicate that hydraulic fracturing is an effective guarantee of high-production CBM wells in the first several months during a high-rank CBM field development. However, the effect of hydraulic fracturing on enhancing the gas production of CBM wells decreases with time because the initial reservoir permeability, lower than the hydraulic fracture permeability, limits the adsorption, diffusion and flow of CBM. This limitation inevitably leads to a slow gas production rate when the adsorbed CBM from the initial reservoir becomes the main gas source instead of the fracturing-affected zone. Gas content and permeability are two key factors that dominate CBM well productivity, because they perform elementary roles in controlling the volume of gas sources and conductivity, respectively. The structural setting and hydrogeological condition are also two important factors that dominate CBM well productivity because they can influence the spatial distribution of permeability and gas content as well as other factors (such as aquifers).

► CBM wells production performance in the Southern Qinshui Basin was introduced. ► GS theory and statistics were used to study the factors affecting gas production. ► Hydraulic fracturing determines the initial production performance of CBM wells. ► Gas content and permeability determine the long-run performance of CBM fields.