Salt Precipitation During Injection of CO2 into Saline Aquifers: Lab-on-Chip Experiments on Glass and Geomaterial Microfluidic Specimens
12 Pages Posted: 4 Apr 2019 Last revised: 24 Apr 2019
In a full-scale CCS, millions of tons of CO2 must be stored underground. Injection of dry or undersaturated (with respect to water) CO2 leads to dry-out of formation water and salt precipitation. Salt precipitation during CO2 injection into the geological formations causes reduced injectivity and negatively influences reservoir rock properties. It also may have the potential to block CO2 leakage pathways within the fractured caprocks. The present-day reservoir-scale models of salt precipitation consider mechanisms such as water evaporation into CO2 and capillary backflow of water into the dried zone. However, it has been suggested that salt precipitation due to these mechanisms fills only a fraction of the pore network and does not significantly impact the permeability. We report microfluidic experiments on glass-microchips and organic-rich shale specimens to provide insights into the physics and dynamics of salt precipitation at pore-scale and to find the possible explanations for the large-scale salt precipitation observed in the field operations. Moreover, we investigate whether salt crystals can partially or entirely clog fracture apertures and leaking pathways in the seal sequences. The experimental results introduce two interrelated phenomena –self-enhancing of salt growth and water film salt transport, which together remarkably intensify the rate and amount of precipitations. It is shown that salt crystals, although at different rates, grow in both aqueous and gas phases. The salt crystals precipitate in two distinct forms: (a) large single cubic halite crystals in the aqueous phase and (b) dense micrometer-sized halite crystals on the interface of rock and CO2 stream. The micrometer-sized crystals in the gas phase create a microporous medium with large capillarity that can strongly imbibe brine over long distances to the evaporation front via capillary connected water films. It is demonstrated that the CO2 phase states influence magnitude, distribution and precipitation patterns of salt accumulations.
Keywords: Salt precipitation; Saline aquifers; Fractured-shale; Injectivity; Caprock integrity; Microfluidic; CCS; CO2 storage
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