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Abstract
Geological storage of carbon dioxide (CO2) is one of the proposed strategies for mitigation of global warming. The CO2 injected into saline formations has less density than that of the resident formation brine. The injected mobile supercritical phase migrates upward and spreads under the sealing rock, due to its buoyancy. While CO2 is in the free mobile phase, there is always a risk of leakage through natural and artificial pathways. Once CO2 is dissolved into brine, it cannot migrate upwards other than by diffusion; and, it can then be retained with a minimal risk of leakage. Therefore, solubility trapping could enable more secure storage.

The objective of this study is the proposal of a CO2 injection well string to increase CO2 dissolution in brine and thereby reduce the risk of leakage of the injected CO2. The proposed well string configuration employs a tubing annulus system equipped with gas lift valves: The supercritical CO2 is injected through the annulus, and fresh brine is injected through the tubing. The designed flow rate of the injected CO2 through gas lift valves enhances dissolution of the CO2 into the injected brine.

A simplified single-phase mechanistic model is developed, and analytical solutions of the governing equations are obtained. Using the developed model, the dissolution of CO2 into brine is characterized by governing dimensionless numbers, such as the Peclet and Sherwood numbers. Scaling relations are presented and can be used to investigate the mixing performance under various operating conditions.

Introduction
Since the last century, an increase of 0.74±0.18°C has been observed in the global surface temperature (IPCC, 2007). This phenomenon, known as “global warming”, has resulted in an increase in the average temperature of the atmosphere and oceans since the middle of the 20th century.

Based on conclusions of the Intergovernmental Panel on Climate Change (IPCC), increasing concentrations of greenhouse gases, mainly water vapor, carbon dioxide (CO2), methane, nitrous oxide, and ozone, resulting from human activity, such as fossil fuel burning and deforestation, are the main reasons behind most of the observed temperature increase since the middle of the 20th century 0 2007).