SPE Journal
Volume 17, Number 3, September 2012, pp. 805-816

Summary

Bulk-phase CO₂ injection into saline aquifers can provide substantive reduction in CO₂ emissions if the risk arising from aquifer pressurization is addressed adequately through mechanisms such as brine production out of the system (Anchliya 2009). While this approach addresses the risks associated with aquifer pressurization it does not address the problem of ensuring CO₂ trapping as an immobile phase and its accumulation at the top of the aquifer. The performance of bulk-CO₂-injection schemes highly depends on the seal-integrity assessment and presence of thief zones. The accumulated pocket of free CO₂ can readily leak through a breach in the aquifer seal. Ideally, the aquifer should be monitored as long as the free CO₂ is present, but if the CO₂ is not immobilized, it is expected to remain underneath the seal rock for more than 1,000 years. Therefore, long-term monitoring of the seal integrity and avoiding leakage will be very costly.

To minimize the free CO₂ below the caprock, we propose an engineered system to reduce aquifer pressurization and accelerate CO₂ dissolution and trapping. We achieve these objectives through effective placement of brine injection and production wells to facilitate the lateral movement (hence, residual and solubility trapping) of CO₂ in the aquifer and impede its upward movement. The simulation results for example engineered well configurations in this paper suggest that substantial improvements in immobilizing CO₂ can be achieved through increasing enhanced solubility and residual trapping that result from better CO₂-injection sweep efficiency. This approach has the potential to greatly reduce the risk of CO₂ leakage both during and 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 are used to provide guidelines for well spacing and flow rates depending on aquifer properties and the volume of CO₂ to be stored. Although it requires additional drilled wells, the active engineered configuration proposed for CO₂ injection significantly reduces the reservoir volume required to effectively sequester a given volume of CO₂, and the increase in the cost caused by additional wells is recovered by dramatic reduction in monitoring cost.

© 2012. Society of Petroleum Engineers

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History

• Original manuscript received: 3 February 2010
• Meeting paper published: 3 November 2009
• Revised manuscript received: 8 March 2011
• Manuscript approved: 24 August 2011
• Published online: 1 June 2012
• Version of record: 12 September 2012