Calcium carbonate precipitation along solution-solution interfaces in porous media

Biogeochemical processes, such as mineral precipitation, microbial growth, or filtration of biological or mineral colloids, can lead to localized solid deposition and changes in flow and permeability in porous media. The coupling between these processes and flow depends on instantaneous flow paths, dispersive or diffusional mixing, localized flow velocities and the kinetics of the biogeochemical reactions themselves. We have conducted experiments to help predict the outcome of this coupling and how solids are, or could intentionally be, spatially distributed. Initial experiments have used calcium carbonate precipitation in packed sand as a model system that has conceptual parallels to other biological and chemical processes that generate mass in porous media. (A second experiment involves precipitate promotion via in situ urea hydrolysis.) Parallel flowstreams of carbonate and calcium produce a mixing zone at the solution-solution interface by diffusion and dispersion. The flowstreams have been created in both an annular geometry in a column, and in a 2-D flow cell. X-ray tomography and color contrast methods are used to visualize the deposition process at the solution-solution interface, and tracer analysis is used to analyze the systems for the evolution of heterogeneities and changes in permeability. For the 2-D flow cell, continuum-scale modeling (using STORM, STOMP and Hydrogeochem codes) is used to simulate the reactant concentration profiles along the interfacial mixing zone behavior prior to precipitation, a saturation index profile, the changes in permeability that are induced as a consequence of precipitate deposition along the interface, and the subsequent changes in how reactive solutes mix along the interface. The modeling goal is to link the continuum-scale simulations to more detailed pore-scale simulations that are capable of representing the coupling between solute concentrations, nucleation kinetics, precipitate growth kinetics, how precipitates fill pore spaces and how flow paths changes with time. Smoothed Particle Hydrodynamics are being used for simulations at the pore scale. The intention is to experimentally observe, and ultimately to control, how precipitates can be distributed in porous matrices.