Laboratory imaging of fracture contact and seismic velocity: insight into their correlation with permeability and resistivity

ABSTRACT: Subsurface fractures play a critical role in regulating the behavior of crustal fluids, which is essential for the development of enhanced geothermal systems. As electrical resistivity and seismic velocity are fracture sensitive, it would be advantageous to correlate them with fracture permeability. To date, the simultaneous changes in these properties have not been studied comprehensively. Consequently, we conducted simultaneous experimental measurements of these rock physical properties. Uniaxial compression tests up to ~50 MPa were performed while KCl (potassium chloride) solution was injected into the fractured specimens. During the tests, the fracture contact state was imaged using pressure sensitive film while the fractured specimens were subjected to uniaxial compression. We evaluated the spatio-temporal changes in P-wave velocity with increasing stress, along with permeability and electrical resistivity. The results indicated that seismic velocity and electrical resistivity increased with normal stress. The spatial distribution of P-wave velocity can be correlated with the visualized contact area. We also examined the relationship between permeability and electrical resistivity. This relationship changed with stress, at the threshold where the evolution of seismic velocity is nearly saturated (~15 MPa). These findings suggest that simultaneous monitoring of electrical resistivity and seismic velocity can predict changes in permeability during stimulation.