Characterization of flow parameters and evidence of pore clogging during limestone dissolution experiments

Rock dissolution induces changes in texture (porosity, pore-size distribution, or tortuosity) which modify multiphase flow and transport properties (permeability, diffusion coefficient, retention curve). Limestone dissolution will occur during CO2 storage or acid injection for well stimulation. Therefore, characterizing those changes is essential for understanding flow and transport during and after the CO2 injection because they can affect the storage capacity, injectivity, and trapping mechanisms. Yet, few published studies evaluate the changes of hydrodynamic properties due to fluid-rock interactions. We report seven dissolution experiments performed on four limestone samples by injecting water with pH ranging from 3.5 to 5.0. Sample porosity, diffusion coefficient, and pore-size distribution were measured before and after each rock attack, which was repeated twice on three of the samples. Permeability was monitored continuously and chemical samples were taken to evaluate calcite dissolution. We find that overall porosity increases over time as expected. But the increase is nonuniform along the sample. At the samples inlets, large pores increase significantly while small pores remain unchanged, which is consistent with wormhole initiation. However, the size of largest pores is reduced at the outlet, which we attribute to clogging by particles dragged from the inlet. As a result, the overall permeability is reduced. Particle dragging is unlikely during supercritical CO2 storage because head gradients are small, but may be expected in the case of dissolved CO2 injection or during well stimulation by acid injection. Our results imply that dissolution is highly localized, which will result in a significant increase in capillary trapping.