Pore–Scale Reactive Transport Modeling of Subsurface Water–Rock Interactions
Understanding of reactive transport is of fundamental importance to various applications in subsurface systems of energy and resources. Such subsurface formations as natural shales have complex compositions with carbonate, clay, and sulfide. In such systems, mineral–fluid interactions have critical impact on the fluid transport, as subsequently resulting in the porosity–permeability alteration, pore geometry alteration, and flow pathways evolution. In this regard, the present dissertation establishes the pore–scale reactive transport models to investigate the impact of mineral–fluid interactions on fluid transport. The present dissertation mainly focuses on three mineral–fluid interactions: carbonate dissolution, clay swelling, and iron precipitation from sulfide (pyrite). Here, we take quartz as a non–reactive mineral, and did not consider the impact of organic matters. These mineral–fluid interactions are investigated through the following specific application problems: calcite dissolution by hydrochloric acid injection (to investigate the carbonate dissolution), CO2–enriched brine injection (to investigate the carbonate dissolution and clay swelling), and hydraulic fracturing fluid injection into shale (to investigate the iron precipitation). In the first application problem, the developed pore–scale reactive transport model is applied to the calcite dissolution by hydrochloride acid injection on the ideal grain models, digital rock images adapted from Niobrara formation, and the synthetic models having natural fracturs with surface roughness. The numerical result has been compared with the benchmark experiment for the validation of numerical method. The effects of Damköhler number (DaII), Péclet number (Pe), the heterogeneity of the pore structure, and the mineralogy are investigated. The different dissolution and transport patterns are categorized with respect to the DaII and the Pe. By applying these results, we can choose the injection setting under chemical reaction–dominant range while avoiding diffusion to achieve efficient acidizing. In the second application problem, the reactive transport model is coupled with the momentum conservation equation for plastic solid to describe the clay swelling during the injection of CO2–enriched brine into the systems with mixed–minerals of calcite, clay, and quartz. The porosity–permeability relationships under mixed–mineral systems are investigated. In addition, the impact of fracture length, density, and connectivity on the fluid transport, porosity–permeability relationship, and evolution of pathways are studied. When the system contains clay, the permeability increase has been slowed by clay swelling, with respect to the porosity increase induced by calcite dissolution. In this regard, clay can be used to increase the sealing capacity for CO2 storage. In the third application problem of hydraulic fracturing fluid injection into shale, we first conduct the experiments with the pyrite samples to calibrate the reaction rate constants for the pyrite oxidation (at pyrite surface) and Fe2+ oxidation (in solution). The obtained reaction rate constants are utilized to establish the numerical method to track the pyrite surface oxidation and Fe2+ oxidation in solution. In the numerical simulation case 1, where the reactions of pyrite oxidation and Fe2+ oxidation occur, the transport patterns in the system are investigated based on the digital rock image model. For the numerical simulation case 2, the Level–set method is coupled with the reactive transport model to simulate the iron precipitation on the pyrite surface. The precipitation pattern on the digital rock image is investigated under different Damköhler numbers. Under the larger DaII, the precipitation has longer dendritic shape, and the precipitation pattern is highly random. The quantified pore–scale parameters obtained from this study are expected to improve current Darcy–scale models to accurately predict the long–term behavior of the subsurface water–rock interactions.