To break or not to break: The role of secondary mineralisation on joint reactivation in sandstone

Discontinuities, including fractures and joint sets, modify the fluid transport properties and strength of rock masses. While open fractures and joints increase rock mass permeability and decrease rock mass strength, fluid flow within these structures can result in secondary mineral precipitation that complicates the effect of these discontinuities over time. Here we study the permeability and strength of joint-free and variably-sealed jointed sandstones to assess 1) whether deformation reactivates pre-existing joints, and 2) how permeability changes as a result of deformation. In undeformed jointed samples, well-sealed joints can act as barriers to fluid flow, but partially filled joints neither inhibit nor promote fluid flow with respect to their joint-free counterparts. All rocks in this study were deformed in the brittle regime under triaxial deformation conditions, and we found that the locations of the experimentally induced fractures depended on the extent to which joints are sealed. The mineralisation that fills well-sealed joints also permeates the surrounding sandstone matrix, locally reducing porosity and forming a cohesive bond between the joint-fill and the host-rock that increases rock strength: experimentally induced fractures do not exploit pre-existing joint surfaces in these samples. By contrast, strain is localised on the joint surface in samples containing partially sealed joints and the strength of these samples is lower than their un-jointed counterparts. The permeability of all samples increased after deformation, but permeability increase was largest in samples with pre-existing, poorly filled joints. We conclude that, in our rocks, partially sealed joints act as planes of weakness and that their reactivation can result in significant permeability increase. Well-sealed joints, however, may locally increase rock strength and never become reactivated during deformation: consequently, these joints may never re-contribute to the permeability. These observations provide insight into how fluid flow in sandstone rock masses may evolve following rock-fluid interactions, with important consequences for systems that rely on discontinuity-dominated permeable networks.