Bulk phase CO2 injection can provide substantive reduction in CO2 emissions only if additional brine is produced out of the system to address relevant risk arising from aquifer pressurization (Anchliya 2009). This approach addresses the pressurization risk, but does not address the problem of CO2 accumulating at the top of the aquifer. The performance of bulk CO2 injection schemes highly depends on the seal integrity assessment and presence of thieve zones. The accumulated pocket of free CO2 would readily leak through a breach in the aquifer seal. Ideally, the aquifer should be monitored as long as the free CO2 is present, but the free CO2 is expected to remain for more than 1000 years. Long term monitoring of the seal integrity and avoiding leakage will be very costly.

An engineered system is proposed to avoid aquifer pressurization and accelerate CO2 dissolution and trapping (to avoid free CO2 below the top seal). This system would position a horizontal brine injection well above and parallel to a horizontal CO2 injection well with horizontal brine production wells drilled parallel to the CO2 injection well at a specified lateral spacing. Simulations showed that this configuration prevents CO2 accumulation at the top of the aquifer during injection and that 90% of the CO2 is permanently dissolved or trapped during injection after 50 years, including the 30 years of injection. This approach would greatly reduce the risk of CO2 leakage both during and forever after injection.

The controlled injection of CO2 with this technique reduces the uncertainty about the long-term fate of the injected CO2; prevents CO2 from migrating toward potential outlets or sensitive areas; and increases the volume of CO2 that can be stored in a closed aquifer volume during the CO2 injection period.

Field scale compositional simulation cases are discussed, and sensitivity studies provide guidelines for well spacing and flow rates depending on aquifer properties and the volume of CO2 to be stored. Although this technique requires additional drilled wells, the engineered case significantly reduces the reservoir volume required to effectively sequester a given volume of CO2 and the increase in the cost due to addition wells is compensated by dramatic reduction in monitoring cost.

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