The Effect of Bedding Plane Orientation on Crack Propagation and Coalescence in Shale

摘要

Hydrocarbon extraction from unconventional oil and gas reservoirs requires more accurate ways to describe fracture processes in shale. Fracture initiation, propagation and coalescence has been studied in many rock-like materials [1,2] and natural rocks [1,2,3]. However, shale is typically heterogeneous and anisotropic with naturally formed bedding planes. Natural bedding planes can be weak zones where fractures can initiate and propagate along [1,2, 3]. A series of unconfined compression tests were conducted on Opalinus clay shale with two pre-existing flaws and various bedding plane orientations (0, 30,60, and 90 degrees with respect to the horizontal). High speed and high resolution imagery were use to capture fracture initiation, propagation and coalescence between the flaw pairs. Distinct coalescence and cracking patterns were observed when compared to previously tested rocks. As the bedding angle increased,fractures initiating at the flaw tips tended to propagate more frequently along the bedding planes. Coalescence of cracks between flaws trended from direct-combined (tensile and shear cracks) to indirect as the bedding plane angle increased. Tensile crack initiation and coalescence stresses showed a characteristic U-shape profile with a minimum at 60 degree bedding plane orientation and a maximum at 0 degree bedding plane orientation.