Experimental investigation of shear slippage behavior in naturally fractured carbonate reservoirs using X-ray CT

Shear slippage plays a critical role in forming complex fracture networks in unconventional reservoirs. Here we demonstrate experimentally the characteristics of slippage-induced fractures in 3D, which were not often clearly illustrated but certainly important for a better understanding of hydraulic fracture branching behaviors. The experimental characterization of slippage is realized following the design concept that most unpropped tensile fractures will close due to high confining pressure, while shear-failure fractures remain open because of unmatched rough surfaces generated during slipping, which can be detected using the CT-scan technique. In this study, the spatial distribution of natural fractures (NFs) in carbonate rock samples before fracturing are determined using X-ray computerized tomography (CT) and numerical reconstruction method. The NF angle between horizontal plane and NF plane ranges from 15 degrees to 90 degrees. Different hydraulic fracturing scenarios are designed and injection pressure curves show that the breakdown pressure of carbonate reservoirs can reach 20 MPa, even as high as 50 MPa under the confining pressure that vertical stress is 16.1 MPa, maximum horizontal stress is 7.5 MPa and minimum horizontal stress is 5.0 MPa. The 3D characterization of fracture distribution indicates four types of facture behaviors, namely: creation of new fractures, propagation of pre-existing fractures, connection of multiple fractures and closure of original fractures. Quantitative analyses of fracture length at different CT-scan slices demonstrate that most of the NFs are activated during testing, while limited numbers of fractures are squeezed closed under confining pressure. Finally analytical evaluation for the degree of shear slippage is performed and shows results consistent with lab experiments.