Exploration and Practice of Composite Temporary Plugging Fracturing Technology for Deep Shale Gas Horizontal Wells: A Case Study from X2 Well in Southeastern Sichuan

Abstract

Abstract The deep shale gas formation that has a burial depth of more than 3,500m covers nearly half of the area of entire Wufeng Formation-Longmaxi shale gas formation in the Southeastern Sichuan Basin, and more than 50% of the shale gas resources are reserved in this burial depth range. Deep shale gas is buried deep with high temperatures, high formation stress, and strong rock plasticity. It is difficult to form and maintain a complex fracture network underground. The deep shale gas fracturing technology is important to promote the overall development of shale gas in the Sichuan Basin. Well X2 is a pre-exploration well in the Southeastern Sichuan Basin. The maximum vertical depth of this well is 4,343.8 m, the average clay mineral content of the high-quality shale interval is 26.2%, the average siliceous mineral content is 48.1%, the average salt mineral content is 14.9%, the elastic modulus is 45.6 GPa, and the maximum and minimum horizontal principal stress of the reservoir is 115.5 MPa and 98 MPa, respectively. Then the horizontal stress difference is 16.8~18.5 MPa. The reservoir is characterized by a high horizontal stress contrast and a low brittleness, which severely restricts the formation of an effective complex fracture network. A composite temporary plugging fracturing of inner-fracture + inlet fracture is proposed, and the fracture net pressure is increased to improve the fracture network complexity and balance the propagation of multiple fractures by adjusting the inlet flow rate of each fracture to achieve high-efficiency fracturing. The 1,503 m horizontal well interval is hydraulic fractured 30 stages with 3 clusters in each stage, cluster spacing is 8~10 m, and the average stage length is 47.33 m. The total injected fracturing fluid volume is 89,736.6 m3 (fluid strength 63.19 m3/m), and the total amount of proppant injected was 3,477 m3 (sand strength 2.45 m3/m). The average overall sand-fluid ratio is 3.88%, the pump rate is 14~18 m3/min, and the daily test production of this fractured deep shale well reached to 41.2×104 m3. The fracturing performance is remarkable. We proposed a deep shale gas fracturing technology with "more stage & less cluster, dense cluster perforation, large fluid volume, high pump rate, medium proppants amount, double plugging, and variable viscosity slippery water alternating injection". It makes an exploration breakthrough of the deep shale gas buried more than 4,000 m and promotes the development of deep shale gas fracturing technology in China.