Shear mechanical behaviors of ceramic proppant-infilled sandstone fractures under constant normal stiffness boundary conditions

The shear behavior of sandstone fractures filled with proppant has an important impact on tight gas production. In this study, the effects of normal stresses (1~6 MPa), normal stiffnesses (0~5 GPa/m), initial normal stresses (2~6 MPa) and proppant sizes (16/30 mesh, 20/40 mesh and 30/50 mesh) on the shear behaviors of ceramic proppant-infilled sandstone are revealed through the direct shear tests on two kinds of rough sandstone under the constant normal load (CNL) and constant normal stiffness (CNS) boundary conditions. The results indicate that after adding proppant, the peak friction angle, the shear stiffness and the peak shear stress of fracture decrease, the shear contraction increases and the shear dilation decreases. With the increase of the normal stress, the shear strength increases. Under the constant normal stiffness boundary, with the increase of the normal stiffness, the residual shear stress increases, but the peak shear stress, the final shear dilation and the apparent friction angle decrease. With the increase of the initial normal stress, the peak shear stress, the residual shear stress and the shear stiffness increase, and the variation range is higher than that of the normal stiffness. The increase of the initial normal stress inhibits the shear dilation of the fracture surface, and increases the apparent friction angle of the fracture. With the decrease of the proppant size, the shear strength increases and the dilatancy displacement increases.