Geological Controls on Multi-Seam Co-Production Optimization of Longtan Formation Coalbed Methane in Western Guizhou, China

The marine and continental transitional facies coal of the Longtan Formation in western Guizhou Province is a key area for coalbed methane (CBM) exploration and development in Southwest China and has numerous resources; however, the exploration and development effects have been average. The development status of the Longtan Formation is such that it presents prominent compatibility issues, challenging production layer combinations, and limited production capacity. On the basis of the “three-step method” of the multicoal seam coproduction combination method and considering the influence of effective gas content and coal seam modification on productivity, a new CBM production seam combination optimization method is developed in which acoustic logging is used to elucidate the coal structure of a coal seam. The combination optimization of production layers in 9 typical wells in the study area was carried out, and the influencing factors and geological insights were discussed. The following conclusions were drawn: Under the multiple influences of the in situ stress mechanism, temperature, and formation pressure, the effective gas content of the coal seam first increases and then decreases, with the highest value occurring at approximately 600 m, and the gas saturation continues to increase. The in situ permeability and transformation permeability first decreased, then increased, and then decreased, and the turning depths were 400 m and 600 m. The productivity potential of the combined mining combination tended to increase with increasing depth. At depths up to 1000 m, a 100 m interval is the optimal combined span, 550–750 m is the enrichment high-permeability window, and it is also the best depth for coal seam joint mining. At a depth of 1000 m, the permeability and effective gas content of the coal seam decreased, whereas the gas saturation and desorption pressure increased, which increased the development potential and research value of the CBM reservoir.