Modelling Approach to Predict and Mitigate Formation Damage Due to Halite Deposition During CO₂ Storage in Saline Aquifers

Evaporation of formation brine into the injected CO₂-rich stream during permanent CO₂ storage in hypersaline aquifers creates the risk of formation damage due to the deposition of halite arising from the "salting out" effect. Such deposition may block pores, negatively impacting porosity, permeability and injectivity, and put the entire project at risk. This paper demonstrates the impact of salt precipitation on formation damage and proposes remediation by using a low-salinity slug ahead of CO₂ injection. Calculations were performed using a reactive transport simulator to account for brine evaporation, halite precipitation, capillary pressure re-imbibition and gravity segregation. This entails including H₂O as a component in the Peng Robinson equation of state in two-phase flow simulation, accounting for CO₂ solubility in brine using Henry's Law, and modelling of aqueous components such as sodium and chloride and the resulting mineral reaction when the brine becomes supersaturated with respect to halite. 1D and 2D radial models with fine space discretization near-well and large outer blocks were used to achieve good resolution and pressure control, respectively. These calculations identify that a simple piston-like displacement of formation water away from the well during constant rate CO₂ injection, and subsequent evaporation of the "irreducible" water saturation, will not lead to significant formation damage. Indeed, the resulting increase in saturation of the CO₂-rich phase leads to an increase in CO₂ mobility. However, and very importantly, a combination of gravitational and capillary effects can lead to the replenishment of formation brine in the near well zones experiencing evaporation, leading to a continuous and ongoing process of salt deposition, which over time may build up to levels that can damage injectivity in sections of the well. In the 1D model, it was observed that due to capillary pressure re-imbibition, well injectivity index was reduced by about 85%, and porosity loss was 62%. The 2D model showed similar behaviour but with stronger localised effects of complete blockage in the lower half of the well. A 3,500m³ low salinity preflush slug (0.1Molar brine) was injected to displace formation brine away from the well. The impact was that the formation damage was significantly reduced and well injectivity maintained at almost original. Modelling the impact of halite-induced formation damage will be useful as a pre-assessment tool to decide the feasibility of CO₂ storage. It will provide a platform whereby remediation strategies such as low salinity brine flushes can be used to mitigate the effect of salt clogging and effectively maintain well injectivity. The knowledge gained will be necessary to optimize the size of the preflush to ensure an appropriate volume is injected, avoiding excessive cost.