Study on Casing Deformation Mechanism and New Prevention Measure During Hydraulic Fracturing of Shale Reservoir

Abstract "Large-scale, high-intensity, precision-cutting"volume fracturing enables efficient shale reservoir development,but causes CD (CD) during fracturing, forcing abandonment of fracturing sections and increasing costs. In Sichuan Basin, a key "Gas Daqing" area, CD rates exceed 30% in Weiyuan and 60% in Luzhou blocks, severely impacting wellproductivity. Prevention measures like enhanced casing strength and reduced construction scale are unsatisfactory, making new CD prevention method a critical research hotspot in the industry.This study clarified that casing shear deformation caused by fracture slip is the main reason ofCD, and a three-dimensional fracture slipping finite element model integrating casing-cement ring-formation was developed. The influences of natural fracture characteristics, rock mechanical properties, and in-situ stress differences on CD were explored. Based on which, the non-cementing fracturing technology with outer casing packerwas proposed to prevent CD, with feasibility verified by finite element method.Results show that during fracturing, natural fractures are activated. As fracturing fluid is injected, fluid pressure within fractures increases, friction force decreases, causing shear slippage, resulting in local stress concentration and deformation of casing. When the angle between natural fracture and minimum horizontal principal stress is 45°, fracture slip risk and CD magnitude reach maximum.CD worsens with larger differences between horizontal principal stress, larger Young's modulus of rock, and higher fracture fluid pressure. Therefore, theNCF is proposed to reduce the force transmitted from rock to casing and prevent CD.By leaving a sufficient gap for fracture slip between casing and formation, direct contact between rock and casing after fracture slippage is prevented to controlCD. Simulation results show that compared with cementedcasing, this method reduces the maximumcasing load by 81.2%, alleviates local stress concentration, and reduces CDrisk.The Geological monitoring data indicated that CD occurred at well depth of 3090min Well HX-1 of W202 block during fracturing, and the maximumCD magnitude by logging was 31.8mm. The CD behaviors ofcementedcasing and NCF were simulated based on the data of this well. The maximum CD magnitude of the former is 30.58mm, with 3.8% error from logging date, indicating that the finite element model of fracture slip is accurate.The maximum CD magnitude of the latter is 0.22mm, which won't affect subsequent construction, showing CD prevention by NCF is feasible.Research canprovide reference for preventing CD caused by fracture slip.