Reactive geochemical flow modeling with CT scanned rock fractures

Obtaining quality three-dimensional geometries of fractures in a natural medium, such as rock, is a non-trivial task. This paper describes how several geothermal fractured rocks were scanned using computed tomography (CT), the reconstruction procedure to generate the three-dimensional (3D) geometry of the fractured rock, and the methodology for isolating the fracture from the CT scan. A conversion process to capture the relevant geometric features of the fracture is then discussed. The scanned aperture distribution was then used to simulate the reactive flow and transport processes using a reactive transport code CrunchFlow. The accurate use of CT images in fluid flow models within complex structures allows detailed understanding on how the aperture distribution affects mineral dissolution and fracture property evolution during the EGS process. Our preliminary simulation results show the formation of the preferential flow in zones with larger apertures, which led to higher calcite dissolution rates and even larger aperture size over time in these zones. Because calcite only occupied 10% of the solid phase, its dissolution did not completely open up the aperture because other relatively non-reactive minerals (clay and quartz) remained. The traditional measure of mechanical aperture could not take into account the partial increase in void space in the rock matrix and underestimated the increase in average aperture. The chemical and hydraulic apertures, which explicitly take into account changes in mineral volumes in the rock matrix, relate better to the overall change in the effective permeability of the sample.