An embedded boundary approach for the simulation of precipitation and dissolution in sediments at the pore scale

Precipitation (or dissolution) of mineral grains modifies the geometry of the pore space in subsurface sediment with evolving solid-liquid boundaries. In turn, changes in the pore space alter the groundwater flow through the sediment, which ultimately affects the continuum scale reaction rates that are relevant for field applications such as carbon sequestration. Modeling provides a unique tool to understand and quantify the feedback processes between mineral precipitation (or dissolution) and flow at the pore scale. However, for modeling to accurately resolve the flow and reactive transport dynamics at the micrometer length scale in real porous media sediments, a method capable of representing complex solid-fluid and fluid-fluid boundaries in a high performance simulation framework is necessary. Here we present a modeling approach coupling flow and transport at the pore scale with multicomponent geochemistry that utilizes the embedded boundary method to characterize fluid-solid interfaces. The development is based on an adaptive, parallelized flow and transport software package, Chombo, and the geochemical code, CrunchFlow, providing powerful simulation capabilities. We demonstrate the approach in simulation of calcite dissolution in complex pore structures that are reconstructed from synchrotron-based x-ray computed microtomography (CMT) images. We apply high resolution techniques to track sharp gradients of concentrations that typically drive precipitation and dissolution reactions. We show that the approach is consistent with that used for moving fluid-fluid interfaces, and thus providing a robust and algorithmically consistent methodology that can be applied in multiphase flow problems. We use the model to examine the inter-dependence between continuum-scale dissolution/precipitation rates and flow patterns at the pore scale in different porous media geometries by using volume averaging methods.