Geological evaluation and effective fracturing technology of Longtan Formation CBM in Nanchuan Block, Southeast Chongqing, China

The development of China’s coalbed methane (CBM) industry is unbalanced between the south and the north. As an important replacement resource in unconventional fields in southeast Chongqing, The beneficial exploitation of Longtan coalbed methane is restricted by the selection of existing geological sweet spots and reservoir fracturing reconstruction technology. Therefore, the Nanchuan block in southeast Chongqing is selected as the research object. Based on seismic, drilling, logging, analysis and testing data, the geological conditions of Longtan coalbed methane Formation are analyzed, the geological sweet spot area is optimized, and the targeted fracturing technology is proposed and applied to the field practice. The results show that: ① The coal-bearing strata in this area are characterized by multiple coal layers, large cumulative thickness and good gas bearing capacity. However, the basic reasons for the failure of coalbed methane breakthrough in this area are the strong heterogeneity of each coal seam and large horizontal stress difference coefficient, leaving conventional fracturing inadequate to form effective propping fractures over long distances. ② A geological sweet spot evaluation system of CBM enrichment and high-yield has been formed with three core elements of “material basis, preservation conditions and fracability”. The C25 coal seam was selected as the longitudinal sweet spot segment. The middle and deep sweet spot area of 500-2000 m was 219.2 km² with 385.3×108 m³ of resources. ③ It proposed an effective fracturing reconstruction idea with the core of “strong scale reconstruction + large particle size support + multiple rounds of construction”. This theory has been successfully applied in well D1, with a stable daily gas output of more than 6 200 m³ in a single directional well, achieving a major breakthrough in CBM exploration in southeast Chongqing. To sum up, the integrated idea of geological engineering, which combines the geological sweet spots evaluation based on the theory of CBM controlled by three factors with the effective support fracturing process, provides technical support for fully releasing CBM resources in favorable areas of southeast Chongqing and realizing overall benefit development.