Stochastic modeling of carbonate mineral precipitation and dissolution in fractures

Abstract:

Carbonate mineral precipitation and dissolution can significantly alter permeability in underground fractures, and yet reactive transport modeling often neglects the underlying variability and uncertainty in reaction kinetics. Such uncertainty is especially relevant for precipitation reactions because of the limitations in experimental observations. We developed a stochastic representation of the kinetics of carbonate mineral reactions. We used the variability in literature parameters to construct probability distribution functions of kinetic parameters. The stochastic geochemical model was then applied to a wide range of solution chemistry conditions to produce probabilistic reaction rate values at different degrees of undersaturation or supersaturation with respect to calcite and dolomite. For dissolution rates, the model predicted variation mostly within an order of magnitude, whereas at high supersaturation (SI ~8) the model predicted variation in precipitation rates up to five orders of magnitude. We then developed a fracture reactive transport model that couples the stochastic geochemical model with a one-dimensional advection dispersion model. The resulting stochastic reactive transport model was used to simulate the profile of carbonate mineral precipitation and the reduction in fracture permeability. The results were compared to observations of carbonate mineral precipitates in a naturally-cemented fracture. The simulation produces a spatial mineral profile that mimics the observed heterogeneous profile of carbonate minerals along the length of the natural fracture. Furthermore, the stochastic approach produces a gradual reduction in permeability in contrast to the abrupt permeability alteration predicted by a conventional deterministic approach. This indicates that this novel approach is a promising tool to study the impact of heterogeneous carbonate precipitation reactions on permeability reduction of conductive pathways in the subsurface.