Subsurface mineralisation: Halite precipitation during CO2 injection
The injection of dry supercritical CO2 into brine aquifers has the potential to dry formation waters, due to evaporation effects1. Dry supercritical carbon dioxide has the ability to “evaporate” (or dissolve) small amounts of water. This could significantly impair injection rates, as has been noted in gas-storage reservoirs2. This work investigates the impact of this “evaporation” phenomenon in porous media, which can lead to severe increases in salinity and salt precipitation. This is of special interest in connection with CO2 storage in saline aquifers. Carbon storage in the subsurface is among the most promising immediately applicable climate change mitigation measures. CO2 can be injected as supercritical fluid into deep rock formations and be stored over centuries like natural gas and oil to reduce atmospheric carbon emissions.
The behavior of the flow system is as follows. As dry CO2 is being injected the formation water is “evaporated”. Deep formation water can have solids in solution, which are usually dissolved salts. As the water is removed into the flowing CO2 stream, salt concentration increases and eventually reaches the solubility limit, giving rise to precipitation of halite. The precipitated solids reduce the pore space available to the fluids and can block the pore throats in the sedimentary rock. The blocked pore throats do not permit fluid movement and hinder any further injection of carbon dioxide. The phenomenon occurs in and close to the borehole, where the dry CO2 enters the rock formation. Salt precipitation can lead to a positive skin effect and dramatically reduce injectivity. The physical processes involved are complex and include counter flow of aqueous and CO2-rich phases due to capillary effects, molecular diffusion of dissolved solids in the aqueous phase, and effects from increased density and viscosity of the aqueous phase at the evaporation front.
These phenomena were modeled with a reservoir simulator, TOUGH23, and the 1D results are presented in this report.