Transmissivity of aligned and displaced tensile fractures in granitic rocks during cyclic loading

• A review of experimental pressure dependent fracture conductivity measurements is given
• Permeability of aligned and displaced fractures in granites was determined
• Fracture permeability varies a few of magnitudes with displacement, rock, time and pressure
• Self-propped displaced fractures in granites allow sufficient fluid flow

The pressure dependent fracture permeability of four granitic rock samples was determined in a triaxial test cell under hydrostatic loading conditions at a temperature of 30 °C. Tensile fractures were generated in cylindrical samples by Brazilian tests. Confining pressures were cycled twice between 2 and 50 MPa. Permeability and strain responses were determined for two samples with aligned fracture surfaces and for two samples with a shear displacement of 1 mm. Fracture apertures were also determined optically before and after testing and the fracture surfaces were scanned. Permeability of the intact granite matrix is below 1e−18 m2 and sample permeabilities of fractured samples range between 1e−17 m2 and 1e−12 m2 depending on displacement and confining pressure. Fracture permeabilities of the aligned samples range from 1e−12 m2 to 2e−10 m2 and samples with displaced fracture surfaces have fracture permeabilities between 1e−11 m2 and 2e−9 m2. Without shear displacement, fracture permeability is reduced with time, even at low constant confining pressures of 2 MPa, while fracture permeability is constant at the same conditions for the displaced samples. With increasing confining pressure, permeability reduction is significantly less for the displaced samples compared to the aligned samples. Fracture permeabilities of displaced fractures in granites are similar to fracture permeabilities in sedimentary rocks using proppants. The results indicate that self-propped displaced fractures in granites may allow sufficient fluid flow for reservoir engineering purposes such as enhanced geothermal system development, without the use of propping agents.