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Abstract
Large volumes of CO2 will have to be stored in the subsurface for carbon capture and geological sequestration to have a significant impact on the reduction of carbon emissions. Injection of large volumes of CO2 into deep saline formations can lead to significant pressure increases within that formation. The increased pressure can be a limiting factor for injection rates; it can also drive vertical brine migration through leakage pathways (e.g., abandoned wells) that could contaminate sources of drinking water. Production of brine from the injection formation can reduce the pressure increase while also limiting the spatial extent of the pressure increase.

The impact of brine extraction is investigated using a hypothetical injection domain conditioned by parameters from the Illinois Basin. The domain contains one injection well and encompasses several aquifers connected through diffusive brine leakage. A vertically-integrated approach is used to model the injection formation and overlying aquifers. A set of production scenarios illustrates the impact of brine production on injection rates and vertical brine movement. The scenarios include production with surface disposal and production with reinjection into overlying formations (with and without desalinization).

The results show that brine production can reduce the pressure buildup in the injection formation, leading to an increase in injectivity and a concomitant reduction in fresh water contamination risk by reducing the area of potential impact. While reinjection of brine into an overlying aquifer solves the disposal problem, it also reduces the effectiveness of brine production by increasing the pressure. Injection of a smaller amount of more concentrated brine resulting from desalinization reduces the impact of reinjection and acts as an additional source of fresh water, but increases the cost of the injection operation.

Based on the results from these numerical experiments pressure management through brine production should be considered for industrial-scale CO2 injection operations, as it increases injectivity and reduces the size of the area of potential impact. However, the brine disposal problem needs to be solved for brine production to be a useful endeavor.

Introduction
Carbon capture and sequestration (CCS) is one of the options currently being discussed to reduce anthropogenic carbon emissions (Pacala &Socolow, 2004; IPCC, 2005; Meadowcroft & Langhelle, 2011). If CCS is to have a significant role in the carbon reduction strategy, large volumes of carbon dioxide (CO2) will have to be sequestered for long time periods. Deep sedimentary formations are being targeted as storage sites, because of their large accessible storage volumes (USDOE, 2007). Sedimentary basins often consist of a sharply defined sequence of alternating high and low permeability formations. CO2 is injected into formations with high permeability (e.g., sandstone) and the overlying formations with low permeability (e.g., shale) act as confining cap rock.