Abstract: Large-scale volumetric fracturing can generate complex fracture networks,leading to extensive redistribution of the in-situ stress field.Variations in the in-situ stress field may alter casing loads and induce casing deformation and failure.To solve this problem,a method was proposed to invert hydraulic fracture networks on the basis of microseismic monitoring data and thereby investigate casing damage.This method can characterize both the stimulated reservoir volume and the degree of rock fragmentation during fracturing.The traction-separation law of cohesive elements was adopted to describe the damage evolution of fractured zones.With the multistage volumetric fracturing of Well GS-HF as a case study,a finite element model describing the interaction among casing,cement sheath and formation was established and staged fracturing simulations were performed.The results show that the simulated casing deformation locations are generally consistent with field observations.During multistage volumetric fracturing,the original formation volume is fragmented,causing the fractured zones to“temporarily”lose confinement from the in-situ stress field and thus forming stress reversal regions,namely stress deficit zones.The asymmetric stimulated reservoir volume causes the in-situ stress field to exert lateral shear forces on both the casing and formation rock,resulting in a certain degree of radial or axial casing deformation.By progressively simulating the staged fracturing process with a finite element model and analyzing its effect on casing failure,this method can accurately identify high-risk zones of casing deformation,and thus provide a basis for timely adjustment of fracturing schemes and optimization of well pattern deployment in the field.

Key words: shale gas reservoir, casing deformation, hydraulic fracturing, fracture network, in-situ stress field, microseismic monitoring