UC Irvine

Fractures are often leakage pathways for fluids through low-permeability rocks that otherwise act as geologic barriers to flow. Flow of fluids that are in chemical disequilibrium with the host rock can lead to mineral precipitation, which reduces fracture permeability. When fracture surfaces contain a single mineral phase, mineral precipitation leads to fast permeability reduction and fracture sealing. However, the feedback between precipitation and permeability may be disrupted by mineral heterogeneities that localize precipitation reactions and provide paths of low-reactivity for fluids to persist over relatively long time-scales. In this dissertation, I explore the role of mineral heterogeneity on precipitation-induced permeability reduction in fractures. To do this, I use a combined experimental and numerical approach to test three hypotheses: (1) Mineral heterogeneity prolongs fracture sealing by focusing flow into paths with limited reactive surface area, (2) Precipitation-induced transport alterations at the fracture-scale are controlled by three-dimensional growth dynamics at the grain-scale, and (3) The effects of mineral heterogeneity become more pronounced as mineralogy and surface roughness become autocorrelated over similar length-scales.

Direct measurements of mineral precipitation using transmitted light methods in a transparent analog fracture show that mineral heterogeneity can lead to the progressive focusing of flow into paths with limited reactive surface area, which is in support of (1). In this experiment, flow focusing led to a 72\% reduction in the max precipitation rate; measurements of the projected mineralogy show that this was due to focusing of large dissolved ion concentrations into regions that contained 82\% less reactive surface area than the fracture-scale average. Results from a newly developed reactive transport model that simulates precipitation-induced fracture surface alterations as a three-dimensional process are in good qualitative agreement with these experimental observations. Comparison of these results with a reactive transport model that represents precipitation as a 1D alteration of the fracture surfaces show that this flow-focusing process is driven by lateral growth of reactive minerals across the fracture-plane, which supports (2). Lastly, results from simulations in fractures that contain varied degrees of heterogeneity show that precipitation leads to a competition between two feedbacks: (i) precipitation-induced reactive surface area enhancement, which increases precipitation rates, and (ii) precipitation-induced permeability reduction, which decreases precipitation rates. When surface roughness and mineral heterogeneity provide persistent paths of limited surface area, the reactive transport becomes very sensitive to local permeability reduction. Simulation results show that this prolongs the fracture-sealing process and can lead to a reduction in fracture-scale precipitation rate, which supports (3). Furthermore, the results presented in this dissertation demonstrate that predictions of fracture sealing by mineral precipitation can be easily misinformed by studies that ignore small-scale mineral heterogeneity and neglect the three-dimensional nature of precipitation-induced fracture surface alterations.