Recognizing fracture distribution within the coalbed methane reservoir and its implication for hydraulic fracturing: A method combining field observation, well logging, and micro-seismic detection

Coal reservoirs are generally composed of complicated fractures and in situ stress systems in most basins, which significantly influence the production performance of coalbed methane. Based on field and underground joint observations, well logging, well tests, and micro-seismic detection during hydraulic fracturing in the southern Qinshui Basin, China, the distribution of natural fractures and their influence on hydraulic fracture propagation were systematically discussed. The results show that the field outcrop joints are mainly in the NE–SW and NW–SE directions, which was confirmed by imaging logging and underground joint observations. Coal is mainly composed of primary and cataclastic coal. The areas with shallower coal seams are dominated by the maximum horizontal stress (σH) > minimum horizontal stress (σh) > vertical stress (σv) or σH> σv> σh in situ stress conditions, while deep coal seams have mostly σv > σH > σh in situ stress distribution. The dominant direction of σh in all study areas is NW. Hydraulic fracture strikes are mainly N37°–55°W and N42°–70°E, with lengths varying between 62 and 157 m and heights of 20–70 m. The hydraulic fractures can be communicated, penetrated, or captured by natural fractures. The communicated positively influences well performance, which is characterized by high, stable gas production. The penetrated type limits fracture propagation, and the captured features do not contribute to gas production improvement. For fracturing fracture lengths >110 m, a common feature is that the primary structure coal is higher than 40% and the principal stress difference coefficient is higher than 0.2. This study presents an applicable method for natural fracture system identification and guiding hydraulic fracturing strategies for coalbed methane reservoirs.