Transient permeability in ductile rocks: the competition between deformation and healing

Fluid circulation in ductile rocks controls the deposition of critical resources such as copper and molybdenum, as well as the potential for deep, supercritical geothermal systems. However, the mechanisms that allow or hinder such circulation under high temperature and pressure conditions remain poorly understood.

In this study, we conducted two sets of healing experiments on thermally cracked Lanhelin granite, both water-saturated and dry, under high confining pressure and temperature. The first set of experiments was carried out under hydrostatic conditions (P_eff = 85 MPa) with increasing temperature (21–400 °C). The second set was conducted under triaxial conditions, in which specimens were deformed at P_eff = 85 MPa, temperatures ranging from 200 to 600 °C, and a strain rate of 10⁻⁶ s⁻¹. In both cases, permeability was continuously recorded throughout.

Under hydrostatic conditions, permeability remained roughly constant at room temperature and in dry samples, but decreased by up to an order of magnitude over 8 hours at 400 °C. Under triaxial deformation, water-saturated specimens were weaker and exhibited more ductile behavior compared to dry samples. Moreover, the more ductile the sample, the greater the increase in permeability observed leading up to failure.

Microstructural evidence supports chemical crack self-healing as the dominant healing mechanism in the hydrostatic experiments. In the deformed samples, post-mortem analysis revealed that the observed increase in permeability is associated with pervasive cracking throughout the bulk of the rock.

Overall, our study demonstrates the necessity of deformation to generate permeability in ductile rocks, while also highlighting the transient nature of this permeability.