Department of Geology and Geological Engineering Van Tuyl Lecture Series- Fall 2015 4:00-5:00 p.m. in Berthoud Hall Room 241 Thursday, October 1, 2015 Russell Detwiler Department of Civil and Environmental Engineering University of California, Irvine “Reactive Alteration of Fractured Rock: Can a Fractured Caprock Self-heal?” Abstract: The ability of geologic seals to prevent leakage of fluids injected into the deep subsurface is critical for mitigating risks associated with greenhouse-gas sequestration, waste disposal, and natural-gas production. Fractures and faults caused by tectonic or injection-induced stresses create potential leakage pathways. Fluids migrating through leakage pathways react with the host rock leading to mineral dissolution or precipitation, which may mitigate or exacerbate migration of injected fluids to shallow aquifers and the atmosphere; predicting which requires improved understanding of the coupled hydrologic, geochemical, and geomechanical processes that control the alteration of fracture permeability. I present results from laboratory experiments aimed at quantifying these coupled processes in a fractured dolomitic anhydrite caprock. During these experiments, preferential dissolution of anhydrite left a compacted layer of dolomite in the fractures. At lower flow rate, rock-fluid reactions proceeded to near equilibrium within the fracture causing preferential flow paths to develop over the 6-month duration of the experiment and a negligible change in permeability. At higher flow rate, the compacted layer of dolomite sealed the fracture resulting in a two-order-of-magnitude permeability decrease. This laboratory-scale observation of significant permeability reduction demonstrates a potential sealing mechanism for fractured porous caprocks composed of minerals with differing solubilities and reaction kinetics. However, scaling arguments suggest that at larger length scales the self-sealing process may be offset by the formation of distinct channels as observed at lower flow rate.