Theoretical Modeling of the Impact of Salt Precipitation on CO2 Storage Potential in Fractured Saline Reservoirs

Abstract

Deep saline reservoirs have the capacity to hold large volumes of CO₂. However, apart from the high brine salinity, which poses an injectivity challenge, a high percentage of saline reservoirs are also fractured. The mechanisms of drying and salt precipitation and the resulting impact on CO₂ injection are unique in fractured reservoirs. Analytical models were developed to investigate the impact of salt precipitation on CO₂ injectivity and storage capacity. Two types of fractured saline reservoirs were considered: type I fractured reservoirs, where storage capacity and injectivity are contributed by only fractures, and type II fractured reservoirs, where both fractures and the adjacent rock matrix blocks contribute to CO₂ storage and injectivity. We found that, depending on the initial brine salinity, salt precipitation could severely impair CO₂ injectivity and reduce storage capacity. Salt precipitation had a fourfold impact on CO₂ injectivity compared to storage capacity. Type I reservoirs with high irreducible brine saturation were less susceptible to salt clogging in the fractures. The results also suggest that fractures with rectangular aperture were less likely to be plugged by salt compared to elliptical fractures. Contrary to previous reports, some fractured deep saline reservoirs may not be suitable for CO₂ storage. Generally, type II fractured reservoirs were found to be more suitable for CO₂ storage in terms of susceptibility to salt clogging. The findings provide valuable understanding of the mechanisms and effect of drying and salt precipitation on CO₂ storage potential, making a strong case for CO₂ storage in naturally fractured deep saline reservoirs.