Challenges and Key Technologies of Ultra-Deep Shale Gas Reservoir Fracturing for Wufeng-Longmaxi Formation in Southeast Sichuan

It has a large amount of ultra-deep shale gas in Wufeng-Longmaxi Formation in southeast Sichuan (TVD > 4500 m), which is the key field of exploration and development in the future. At present, a breakthrough has achieved for deep shale gas (3500 m < TVD < 4500 m). However, as the deep burial depth of ultra-deep shale gas formation, the ambient temperature and pressure, structural characteristics and rock brittleness characteristics become complex. Based on the analysis of geological conditions and fractured well for ultra-deep shale gas reservoir in southeast Sichuan, the challenges of ultra-deep shale gas fracturing are sorted out. The deep buried environment increases the rock strength and rock plasticity, in which the fracture tends to be in the single shear fracture mode. Meanwhile, the high horizontal two-direction stress difference limits the transformation of hydraulic fractures. These factors all affect the complexity of hydraulic fractures, which cause the SRV in ultra-deep shale gas reservoir smaller than that in middle-shallow and deep shale gas reservoirs. Also, the large married deep increases the fracturing pressure, which has exceeded the the pressure limitation of common equipment. The high pressure limits the displacement lifting during fracturing process, resulting in the problems of insufficient net-pressure in the fracture, small longitudinal sweep range, small fracture width and difficult proppant adding. What is more, high closure stress of ultra-deep shale gas reservoir leads to rapid loss of fracture conductivity, causing the insufficient long-term production capacity. In order to solve the above problems, several aspects are proposed to improve the fracturing efficiency of ultra-deep shale gas, such as improving the prediction of the “sweet spot” for ultra-deep shale gas reservoir, reducing the fracturing pressure, increasing the net-pressure to promote fracture complexity, and lifting fracture long-term conductivity. The research and understanding have important theoretical and practical significance for the exploration of ultra-deep shale gas in southeast Sichuan.