numerical study on casing integrity during hydraulic fracturing shale formation

Abstract

Due to complex sedimentation environment, shale formations are usually characterized as highly heterogeneous and anisotropic in in-situ stresses. The injection process for reservoir stimulation changes the local in-situ stresses in a quick and significant manner. Activation of existing geo-features is frequently observed by researchers. The large magnitude of matrix displacement potentially deforms the casing which cut through the discontinuous plane, weakly bonded interfaces, and bedding planes. Casing deformations during hydraulic fracturing have been observed in the Southwest China Sichuan basin, which impeded shale gas development. This research analyzed this phenomenon and discuss practical engineering solutions. The theoretical geomechanics studies indicate water-induced shear will activate shale formation. This study used the finite element model to study the casing-cement-formation geostatistical conditions under hydraulic fracturing. And studied risks for hydraulic fractures intersecting with casings in different aspects. It discussed major mechanisms that govern this interaction behavior. Fluid flow rate and in-situ stresses are two main parameter that induced anisotropic stresses onto casing-fault. A high injection rate (0.005m/s) can apply over 600 MPa unbalanced stress on casing on the top part of casing-fault intersection. Anisotropic in-situ stresses can apply over 100 MPa stress on casing during hydraulic fracturing. This type of casing damage can be reduced by improving stimulation design, adjust the perforation location, and improve the strength of the casing-cement system.