Pore-scale Simulation of Reactive Transport in Micro-CT Images

Chemical reactions induced by the injection of CO2-rich fluids into carbonate reservoirs may cause petrophysical alterations in the host rock with possible consequences for reservoir development and monitoring. X-Ray microtomography is a very powerful technique to characterize and model flow and transport phenomena at the pore-scale. More recently, it became possible to image reactive flow (solid-fluid interaction) in rocks at reservoir conditions. In this work we present a particle-based method to simulate reactive transport in carbonate rocks directly on the voxels of 3D micro-CT images. We use images from a dynamic imaging experiment performed at high temperature and pressure conditions to validate our model. Rock-fluid interaction is modeled using a three step approach - solute advection, diffusion and reaction. Particle advection is done using a novel pore-scale streamline method that is consistent with pore-scale boundary conditions - no-flow at the pore-solid walls. Diffusion by random walk is superimposed and reaction occurs when the solute particles flow across the pore-solid interfaces. For the specific case of carbonate dissolution it is possible to derive from first principles a relationship between the local particle flux (a function of the particle concentration and of the diffusion coefficient) and the calcite dissolution rate from batch experiments. Our method is capable of predicting the evolution of petrophysical properties (porosity and permeability) in carbonate rocks subjected to CO2 injection. To validate our simulations we use a set of 3D images from a dynamic imaging experiment where carbonate samples were imaged during CO2-equilibrated brine injection. The image-calculated increases in porosity and permeability are predicted within large degrees of accuracy. This is the first time a pore-scale simulation is used in conjunction with such kind of experiment to study carbonate dissolution. We will also present and discuss the various dissolution patterns in heterogeneous carbonates that may arise from different flow regimes and describe how this methodology can be applied to model diagenetic phenomena in sedimentary rocks. We conclude showing how such changes in the rock frame impact the elastic properties of the reservoir rocks.