A numerical study on optimization of multilateral horizontal wellbore patterns for coalbed methane production in Southern Shanxi Province, China

China's southern Shanxi Province contains thick, gassy coals of anthracite rank. These high-methane-content (higher than 500 ft3/ton) coals necessitate pre-mining methane drainage to enhance the health and safety of underground coal workers. Both traditional vertical wells and multilateral horizontal wells, which were shown to perform better, have been drilled in the area in order to effectively reduce the gas content of coals. However, optimization of well drilling patterns is crucial to improve the gas drainage by boosting well performance and resulting in significant additional recovered gas in a cost-effective manner. In this study, exploration borehole data from an undeveloped coalbed methane field in China's Qinshui Basin was obtained where permeability is expected to range from 0.1 to 10 md.Reservoir modeling conducted in this study concludes that in the presence of low permeability coals (less than 1.0 md) well patterns can be optimized to maintain high-methane production rates. In this work, relationships to determine combinations of horizontal lateral spacing and permeability that yield similar production curves were developed in order to maximize production. Pinnate wells with laterals spaced at 600-ft intervals in reservoirs with 2.0 md permeability performed similar to those with 200-ft lateral intervals in 0.5 md reservoirs. Additional similar relationships were drawn for reservoirs ranging from 0.1 to 8.0 md. Furthermore, because of the Qinshui Basin's potential high stress regime, Pitchfork and Pinnate horizontal well patterns were analyzed based on their potential for catastrophic wellbore failure at well junctions and producing well branches. Analyses concluded that Pitchfork wells had a significantly lower risk for significant wellbore loss due to borehole collapse, while providing the opportunity for higher gas recovery in reservoirs exhibiting directional permeability.

Research highlights
► A high risk of catastrophic well loss exists with Pinnate type drilling patterns.
► Low permeable coals can produce ample volumes of gas by adjusting drilling patterns.
► Pinnate patterns are less sensitive to cleat orientation than Pitchfork patterns.