Factors controlling high-yield coalbed methane vertical wells in the Fanzhuang Block, Southern Qinshui Basin

Whether a coalbed methane (CBM) well achieves high yield is controlled by various factors. Structural and hydrological control models of CBM productivity in vertical wells were built using a combined investigation of basic tectonic settings and hydrological conditions, as well as drainage parameters of the coal reservoir in the Fanzhuang Block, Southern Qinshui Basin. This study indicates that gas/water production differs significantly among wells with different structural settings or hydrogeological conditions. Low and unstable gas production rates in individual wells are the primary problem for the CBM development in the Fanzhuang Block. The impacts of some geologic and engineering factors on gas production were analyzed and estimated based on comparative analysis, quantitative analysis, and gray system theory. The results indicate that the critical reservoir ratio (CRR), liquid column height (LCH), and gas content are the most important factors in determining well productivity, followed by the equivalent drainage radius (EDR), the volume of frac sand, and the decline rate of working fluid levels during initial production. High-yield wells in the Fanzhuang Block always have the following conditions: gas content > 20 m3/t; burial depth of 500–700 m; CRR > 0.7; LCH > 400 m; volume of frac sand > 40 m3; EDR of 30–60 m; and a decline rate of working fluid level lower than 2 m/day during the initial production stage.