An experimental apparatus to investigate fluid-assisted long-term recovery of fractured rocks

Seismological observations show that earthquakes produce significant changes in the elastic and transport properties of active faults, with co-seismic drops in seismic wave velocities consistently followed by a slow post-seismic recovery, over months to few years. Such variations occur in volumes up to several hundred metres thick that correlate well with the dimensions of fault damage zones. This suggests the existence of a damage-recovery cycle within active fault zones, with the recovery phase possibly driven by a range of fluid-assisted re-strengthening “healing” mechanisms in the fractured medium and/or stress relaxation. Understanding how and how rapidly fractured rocks seal, regain their stiffness, and drive fluid flow in fault zones is fundamental to comprehend the mechanics of the brittle crust and for geo-engineering applications such as geothermal energy, ore deposits, the deep disposal of radioactive waste and CO2 sequestration.

At the Department of Geosciences of the University of Padua, a “percolation cell” apparatus has recently been installed to study long-term fluid-rock interaction under hydrostatic conditions with a maximum confining and pore pressure of 100 MPa and a maximum temperature of 250°C. Such apparatus is equipped with two syringe pumps and a back-pressure regulator that allow to monitor permeability evolution through time and a set of high-temperature P- and S- ultrasonic transducers to track changes of rock elastic properties in-situ. In addition, the pore fluid inlet circuit can flow into a stirred autoclave to pump solutions with controlled chemistry up to 20 MPa pore pressure and 200°C temperature through an externally heated pipe. Such an experimental apparatus allows to study both diffusion- and advection-dominated regimes within conditions representative for the upper crust.

Together with the experimental setup, here we present some preliminary long-term percolation tests in which de-ionized water was flowed at 25°C through rock cylinders of micritic limestones with mated and non-mated single fractures under 20 MPa confining pressure and 5 MPa pore pressure. The temporal evolution of permeability and elastic properties were monitored together with the fluid-chemistry at the outlet. Mechano-chemical processes along the fractures were also investigated through X-ray microtomography and SEM analyses.