Modeling Reactive Transport Processes in Fractures

Fractures are ubiquitous and important features in the Earth subsurface (Berkowitz 2002; Pyrak-Nolte et al. 2015). They are created as a result of rock failure when the critical stress (i.e., fracture toughness) is exceeded, or in the case of subcritical crack growth, when cracks propagate under stress conditions below fracture toughness, facilitated by chemical reactions (Atkinson 1984). The necessary conditions for fracture growth can be created by natural unloading from land erosion (Engelder 1987), tectonic events (Molnar et al. 2007), and crystal growth in presence of fluids supersaturated with respect to solid phases (Royne and Jamtveit 2015). Fractures can also be artificially created for enhanced energy recovery, through excess fluid pressure and change of thermal stress, as in the case of geothermal energy extraction and unconventional oil and gas production (McClure and Horne 2014; Lampe and Stolz 2015). Fractures can be observed by surveying rock outcrops, inferred by fluid flow and geochemical measurements, and detected using geophysical techniques (Berkowitz 2002; St Clair et al. 2015; Walton et al. 2015).When open, fractures act as preferential flow pathways because of their high permeability, and thus typically control fluid migration and solute transport in fractured rocks. For this reason, fractures are avoided when siting and designing geologic isolation systems, such as for nuclear waste and CO2 storage, in order to prevent undesired fluid and chemical migration (Kovscek 2002; Lewicki et al. 2007; Birkholzer et al. 2012). In the Earth’s critical zone, fractures control the availability of water for rock weathering and hence the development of the regolith layer (Brantley et al. 2017). It has also become accepted that weathering itself typically controls fracture permeability in hard rock aquifers (Lachassagne et al. 2011).