Mechanical properties of marine shale and its roof and floor considering reservoir preservation and stimulation

The mechanical differences between reservoir rock and its overlying (roof) and underlying (floor) rocks have significant influence on oil/gas preservation and hydraulic fracture propagation, which is still lack of adequate research in marine shale gas formation. In this work, homogeneous intact specimens of roof siltstone, reservoir shale and floor limestone were prepared from representative outcrops, and the corresponding siltstone-shale-limestone composite samples were also fabricated with strong/weak interface cementation properties. Mechanical parameters, fracture morphology, brittleness, and failure mechanism of marine shale and its roof and floor in tri-axial compressive state were comparatively investigated. Results show that reservoir shale had relatively low compressive strength, Young's modulus, internal frictional angle, and cohesive strength, making it more susceptible to rupture, compared with the roof and floor. The floor limestone had relatively high strength and low brittleness, which were beneficial for maintaining its integrity and the preservation of shale gas during tectonic movement. Considering the high brittleness of roof siltstone, more attention should be given to the integrity of the roof, where fractures induced by tectonic movement could provide as a pathway of gas escape. Interface cementation property played a key role in the fracture propagation behavior at lithologic boundaries. Interfaces with weak cementation tended to arrest extending fractures, which was advantageous to gas preservation or fracture height control in reservoir stimulation. Whereas fracture would probably cross over strong cementation interfaces, leading to the threat of oil/gas leakage or incontrollable hydraulic fracture propagation in vertical direction.