ENGINEERED PRECIPITATION OF CALCIUM CARBONATE IN POROUS MEDIA

Formation of mineral precipitates in porous media requires generating supersaturated conditions for the minerals through changes in physical parameters, by introduction of supersaturated solutions, or by mixing reactants in situ. In the latter case fluid flow, mixing and the physical distribution of precipitates are coupled. We illustrate how calcium carbonate precipitates can be generated by two mechanisms. In the first case, calcium carbonate is propagated as a wall along the interface between solutions containing calcium and carbonate ions flowing in parallel. Once formed, the reduced permeability of the carbonate barrier reduces further mixing between the solutions. Propagation of the carbonate barriers is illustrated in three types of experiments: 1) A simple small diameter column experiment, 2) A 2-dimensional cell where precipitate propagation can observed directly, can be studied in heterogeneous media, where relative flow rates can be varied over time to disperse the precipitate laterally, and where the mixing profile can be visualized using colored dyes, and 3) In an annular flow configuration where propagation of a calcium carbonate tube separating the two fluids is shown using X-ray tomography.

In situ generation of one or more reactants is another mechanism by which supersaturated conditions can be generated and subsequently lead to mineral precipitation. This approach was demonstrated in a column with immobilized urease enzyme that hydrolyzed dissolved urea in a CaCl2 + urea mixture to produce carbonate ions. Calcium carbonate precipitation was observed using X-ray tomography. The experiment using immobilized urease is also a simplified analog to a system where urea hydrolyzing bacteria can stimulate the production of calcite.

These simple systems are being used to demonstrate how impermeable or permeable barriers can be generated in subsurface environments, and may serve as prototype analogs for the engineered alteration of subsurface permeability by chemical precipitation or the production of biomass.