Experimental Study on the Mechanical Properties of Matrix and Laminae Planes in Shale

Abstract The mechanical properties of laminae planes have an essential effect on the nucleation and propagation of hydraulic fractures. Previous studies mainly focused on the strong anisotropic mechanical characteristics of laminae shale. However, the effective experimental data on the mechanical properties of matrix and laminae planes is generally limited. Therefore, a series of rock mechanics tests were conducted on laminae shale to study the difference in mechanical properties between matrix and laminae planes. The tested shale samples were subsurface full-diameter cores from the Fengcheng Formation in Mahu Sag. Specimens with different laminae orientations were drilled in directions perpendicular and parallel to laminae planes. The triaxial compression strength tests, direct shear tests, and Brazilian tests were carried out on the specimens. The results showed that the tensile strength, shear strength, cohesion, and friction angle of the specimens loaded parallel to laminae planes were lower than those of the shale matrix. It demonstrated that the laminae planes are weak planes of shale in terms of mechanical strength. However, the normal stiffness and shear stiffness of the laminae planes were higher than those of the shale matrix, indicating that laminae planes are not prone to elastic deformation. Two types of failure modes were observed in the Brazilian test. For the specimens loaded parallel to laminae planes, the fractures propagated along the loading diameter without branching fractures. For the specimens loaded perpendicular to laminae planes, the fractures deviated from the loading diameter and suppressed the propagation of main fractures, which resulted in secondary fractures along the laminae planes. It was concluded that the laminae planes, as the weak planes of shale, lead to the mechanical properties changing with the loading direction relative to the direction of laminae planes and also change the propagation path of the fractures. For fractures propagating along the normal or oblique direction of the laminae plane, the propagation pathways usually offset to the direction of laminae planes or direction parallel to laminae planes, which produces curved propagation pathways and branch fractures. The mechanical properties of laminae planes in shale provide necessary data support for numerical simulation of hydraulic fracture propagation, fracture geometry prediction, and fracture initiation pressure prediction.