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Abstract
Previous studies have shown that bulk CO2 injection in deep saline aquifers supplies insufficient aquifer storage efficiency and causes excessive risk due to aquifer pressurization. To avoid pressurization, we propose to produce the same volume of brine as is injected as CO2 in a CO2-brine displacement. Previous work showed that this increases the storage efficiency from 2% to 8%. However, this transforms the CO2 storage problem into a brine disposal problem. Therefore, we propose to desalinate the native brine and inject the saturated brine into the same aquifer while producing additional brine to maintain voidage balance.

A hypothetical case study using documented aquifer properties of the Woodbine aquifer in Texas indicates that the available volume is insufficient volume to store all of the CO2 being generated by power plants in the vicinity for more than 14 years. However, the CO2-brine displacement increases storage efficiency enough to store the CO2 produced for 84 years at the current rate of coal fired electric power generation. Using the reported brine salinity of the Woodbine aquifer, the energy requirements for CO2 transport and injection, brine production and transport, desalination, and saturated brine injection are estimated consistent with assumptions about the location of injection and production wells, the desalination unit or units, and whether desalinated water can be used by the power plant or for other uses.

While this approach may enable CO2 storage, the high energy cost ranging from 7.5% to 16% of the total power generation capacity is not insignificant, and comes with significant land use implications for injection and production wells, pipelines, etc. The importance of these results cannot be overstated.

Introduction
An average coal fired power plant with a capacity of 500MW will generate about 3 million tonnes of CO2 per year or about 8200 tonnes per day. While using CO2 in enhanced oil recovery (EOR) offers value, the amount of CO2 that can be used for this purpose is considerably less what is being emitted from coal fired power plants operating in the USA today. Deep saline aquifers offer considerably more potential pore volume, and typically they are located nearer to power plants than are EOR operations. Ehlig-Economides and Economides (2010) provided a simple model showing that when multiple injection wells are needed to enable bulk injection of the CO2 from one or more power plants into a given aquifer, the maximum aquifer storage efficiency is about 1%. That is, at least 100 aquifer pore volumes are needed for each pore volume of CO2 injected.

For a given aquifer, storage efficiency increases when more wells are used for the bulk injection, and the number of wells required is inversely proportional to the aquifer permeability thickness product. Because the aquifer pressure will rise under bulk CO2 injection, Ehlig-Economides et al., 2010, recommended regular pressure falloff testing in CO2 injection wells to monitor aquifer pressure behavior, and, in particular, to watch for evidence that CO2 may be leaking from the aquifer. Anchliya and Economides (2009) investigated producing the same volume of brine as is injected as CO2 to avoid pressurizing the aquifer and found that this increases the aquifer storage efficiency. This study also investigated brine injection above the CO2 injection as a way to keep the CO2 plume from rising to the top of the aquifer, thereby accelerating CO2 trapping and dissolution. A simulation applying this approach for the same aquifer properties as studied for bulk injection indicated a 4-fold increase in the aquifer storage efficiency. Furthermore this study indicated that almost 90% of the CO2 was rendered immobile as early as 20 years after the end of the CO2 injection period.