Effect of pressure and stress cycles on fluid flow in hydraulically fractured, low-porosity, anisotropic sandstone

Hydraulic fracture in deep rock masses is used across a variety of disciplines, from unconventional oil and gas to geothermal exploration. The overall efficiency of this process requires not only knowledge of the fracture mechanics of the rocks, but also how the newly generated fractures influence macro-scale pore connectivity. We here use cylindrical samples of Crab Orchard sandstone (90 mm length and 36 mm diameter), drilled with a central conduit of 9.6 mm diameter, to simulate hydraulic fracture. Results show that the anisotropy (mm-scale crossbedding orientation) affects breakdown pressure, and subsequent fluid flow. In experiments with samples cored parallel to bedding, breakdown pressures of 11.3 MPa, 27.7 MPa and 40.5 MPa are recorded at initial confining pressures at injection of 5 MPa, 11 MPa and 16 MPa, respectively. For samples cored perpendicular to bedding, breakdown pressure of 15.4 MPa, 27.4 MPa and 34.2 MPa were recorded at initial confining pressure at injection of 5 MPa, 11 MPa and 16 MPa, respectively. An increase in confining pressure after the initial fracture event often results in a significant decrease in flow rate through the newly generated fracture. We note that fluid flow recovers during a confining pressure “re-set” and that the ability of flow to recover is strongly dependent on sample anisotropy and initial confining pressure at injection.