Evaluation and optimization of "engineering sweet spot" in deep shale reservoir: case study on Yongchuan and Dingshan areas in southern Sichuan

In the Sichuan Basin, geologic conditions are complex in deep layers. Burial depth of shale reservoirs is high. Gas well productivity is widespread low, and hydraulic fracturing confronts with extremely large challenges. In order to search for "engineering sweet spots" of deep shale gas layers, Yongchuan and Dingshan areas were taken as the examples. In-depth discussion was conducted on affecting factors of hydraulic fracturing results in deep shale reservoirs by drilling, core, imaging logging and seismic data, based on measured rock mechanical parameters, development degree of natural fractures, natural fracture effectiveness, horizontal stress difference and included angle between horizontal well track and the minimum main horizontal stress. Study results demonstrated that No.31 sublayer of the first member of Longmaxi Formation in Yongchuan and Dingshan areas, southern Sichuan had the best brittleness as the first-choice target horizon for hydraulic fracturing. Shorter distance from the faults, higher pump pressure and operational pressure, and smaller reconstructed volume. Good extension and turning capacity of the fractures could beguaranteed when included angle between hydraulic and natural fractures was roughly 55°. In silicon-rich shale and faulting zones, lower minimum main horizontal stress, easier-diffused hydraulic fractures. Higher included angle between horizontal well track and maximum main horizontal stress azimuth, better fracturing results. Finally, evaluation index system of "engineering sweet spots" for marine shale gas in complex structural areas of Yongchuan and Dingshan was constructed. Class Ⅰ geologic and engineering "sweet spots" were optimized in Yongchuan and Dingshan areas.