Effect of pressure and stress on water transport in intact and fractured gabbro and granite

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Abstract
New laboratory data are reported on the effect of confining pressure (to 60 MPa), pore‐water pressure (to 30 MPa), and stress difference (to 0.88 of the fracture stress) on permeability of intact and fractured White Lake gneissic granite, Westerly granite, and Creighton gabbro. Permeabilities as low as 10−24 m2 (10−12 darcy) have been measured using a transient technique. Fracture displacement, electrical conductance, compressional velocity, and pulse amplitude are determined simultaneously. The loads applied to the 0.15‐m‐diameter by 0.28‐m‐length test sample are controlled automatically, and most data are taken by microprocessor. Tests on the intact gneissic granite indicated permeabilities of 10−22 to 10−24 m2 that appeared to be unaffected either by effective pressure or by stress. The granite yielded permeabilities of 4×10−20 m2 that decreased by a factor of 2 as effective pressure increased to 25 MPa and varied by a factor of 2 as stress was increased to 0.5 of the fracture stress. Permeability of the gabbro linearly decreased from 2×10−22 to 8×10−24 m2 with effective pressures to 25 MPa. Loading of the gabbro up to 0.88 of the fracture stress increased permeability by a factor of 7. The introduction of a throughgoing fracture increased the apparent permeability by 106 to 109 over the intact values in both granite and gabbro. When compared to the initial value, compressional velocities increased by 5% with pressure to 30 MPa in the gneissic granite. For granite, pressurization from 2 to 25 MPa increased the velocity and pulse amplitude by 5 and 30%, respectively, and decreased the conductance by 50%. Velocity, amplitude, and conductance were weakly dependent on pressure in gabbro. The addition of stress decreased velocity and amplitude while increasing conductance markedly in both granite and gabbro. All data on both intact and fractured rock are consistent with crack closure and dilatancy with pressure and stress. Conductance and amplitude exhibit the best potential for monitoring changes in permeability and joint behavior in situ.

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