Linking complex geochemical and hydrological processes using streamlines for highly-resolved, reactive transport CO2 leakage scenarios

Simulating reactive transport and geochemical processes over large, finely resolved domains in three-dimensional space has traditionally been computationally infeasible. Subsurface physical heterogeneity and complex chemical processes along with the need to accurately resolve macrodispersion and mixing all contribute to computational expense. However, the ability to accurately model large-scale reactive transport has become increasingly necessary in order to evaluate potential groundwater contamination scenarios, such as those associated with CO2 leakage from Carbon Capture and Storage (CCS). Here we present a Lagrangian streamline approach where a large, heterogeneous three-dimensional flow field is reduced to a number of one-dimensional transport simulations. Each of these deconvolved, one-dimensional reactive transport problems are solved with an aqueous geochemical model (CrunchFlow) along a streamline, with the aim of representing complex geochemical reactive transport over a large domain in three-dimensional space. The streamline approach allows the mapping of these one-dimensional reactive transport simulations back onto a three-dimensional flow field, thus accounting for spatial heterogeneity within the aquifer and the complex aqueous geochemical processes. This methodology is demonstrated using a CO2 leakage scenario from a hypothetical CCS site. In this simulation, a plume of CO2 is intruded into the aquifer thereby lowering the groundwater pH and mobilizing metals from an existing mineral host-rock distribution. In this set of simulations, ensembles of correlated, Gaussian random fields are used to represent heterogeneity in hydraulic conductivity (K) for cases with increasing of variance in ln(K). Plume migration, related pH buffering and kinetic dissolution/precipitation processes within the aquifer are simulated under varying degrees of subsurface heterogeneity using this streamline-geochemical modeling approach. Metal concentrations and pH at groundwater pumping wells are then evaluated and linked to the degree of physical heterogeneity and geochemical processes. Results show that as the degree physical heterogeneity increases, the pH drops along source-to-well streamlines, lowering the overall observed pumping well pH compared to domains with lower variance. Thus through this novel modeling effort, a link is established between hydraulic heterogeneity and geochemical response. Furthermore, the complex interactions between geochemical processes and dynamic flow paths have implications for understanding the risks and observable outcomes to CCS implementation.