Solubility trapping of CO2 in deep saline aquifers

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Carbon capture and storage is one of the major global challenges. Long-term storage of carbon dioxide (CO2) in deep underground saline aquifers is a key opportunity that offers the possibility of sustaining access to fossil fuels such as oil sands in Canada while reducing emissions. However, prior to implementation, associated risks of CO2 leakage need to be addressed to ensure safety of storage. This requires development of tools in order to assess all the factors involved. CO2 storage takes place by several mechanisms. These include dissolution in brines, trapping in the reservoir rocks in form of residual or through mineralization. However, there is a knowledge gap in how these mechanisms contribute to and affect the long-term (century-to-millennium time scales) fate of CO2. Also, the storage capacity of these underground reservoirs is uncertain. Studies have shown that the Alberta basin is well suited for CO2 storage with a dissolution capacity of 4000 gigatons assuming that all the pore water could become saturated with CO2, which is not likely. Our recent studies have shown that in the short-term (~30 years) only 8% of CO2 can be trapped in brines. The rest may remain in the free phase, which is prone to leakage. However, long-term mechanisms such as convective dissolution may increase this limit and reduce the risk of leakage. The injected CO2 in deep saline aquifers is less dense than the resident brines. Driven by density contrast, it will spread under a sealing rock as a free phase while there is risk of leakage. However, CO2 slowly dissolves into the underlying brines. The resulting CO2-saturated brines are slightly denser than resident ones, making them sink to the bottom of the aquifer. Enhanced dissolution of CO2 by continuous sinking of CO2-saturated brines as well as upward flow of fresh brine from bottom to the top reduces the volume of the free phase CO2 and thus reduces the risk of leakage. The main point is that once CO2 sinks to the bottom of the aquifer, it cannot migrate upwards easily and it may be retained with minimal risk of leakage. Therefore, dissolution trapping could enable secure storage. This research program expands our knowledge and provides required methodologies by performing studies that help secure and safe implementation of geostorage. Currently, large-scale modeling of dissolution trapping using reservoir models poses a great challenge and requires prohibitively expensive computational resources and time. Therefore, efficient models that can accurately simulate the process at geological scale are required. In particular, scaling laws, which can be incorporated into reservoir simulators, are of great importance and will be a key and novel contribution of this proposal. The proposed research will provide a venue for incorporation of new formulations into numerical reservoir simulators leading to a computationally less expensive and efficient field scale simulations.