Effect of CO₂ Phase States and Flow Rate on Salt Precipitation in Shale Caprocks-A Microfluidic Study

Fracture networks inside the caprock for CO₂ storage reservoirs may serve as leakage pathways. Fluid flow through fractured caprocks and bypass conduits, however, can be restrained or diminished by mineral precipitations. This study investigates precipitation of salt crystals in an artificial fracture network as a function of pressure-temperature conditions and CO₂ phase states. The impact of CO₂ flow rate on salt precipitation was also studied. The primary research objective was to examine whether salt precipitation can block potential CO₂ leakage pathways. In this study, we developed a novel microfluidic high-pressure high-temperature vessel to house geomaterial micromodels. A fracture network was laser-scribed on the organic-rich shales of the Draupne Formation, the primary caprock for the Smeaheia CO₂ storage in Norway. Experimental observations demonstrated that CO₂ phase states influence the magnitude, distribution, and precipitation patterns of salt accumulations. The CO₂ phase states also affect the relationship between injection rate and extent of precipitated salts due to differences in solubility of water in CO₂ and density of different CO₂ phases. Injection of gaseous CO₂ resulted in higher salt precipitation compared to liquid and supercritical CO₂. It is shown that micrometer-sized halite crystals have the potential to partially or entirely clog fracture apertures.