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Abstract
CO2 capture, utilization, and storage (CCUS) in deep geological formations is regarded as a promising means of lowering the amount of CO2 emitted to the atmosphere and thereby mitigating global climate change. For commercial-scale CO2 injection in saline formations, pressure buildup can limit CO2 storage capacity and security. Issues of interest include the potential for CO2 leakage to the atmosphere, brine migration to overlying potable aquifers, and pore-space competition with neighboring subsurface activities. Active CO2 Reservoir Management (ACRM) combines brine production with CO2 injection to relieve pressure buildup, increase injectivity, spatially and temporally constrain brine migration, and enable beneficial utilization of produced brine. Useful products may include freshwater, cooling water, make-up water for oil, gas, and geothermal reservoirs, and electricity generated from extracted geothermal energy. By controlling pressure buildup and fluid migration, ACRM can limit interactions with neighboring subsurface activities, reduce pore-space competition, and allow independent assessment and permitting.

ACRM provides benefits to reservoir management at the cost of extracting brine. The added cost must be offset by the added benefits to the storage operation and/or by creating new, valuable uses that reduce the total added cost. We review potential uses of produced brine and conduct a numerical study of potential reservoir benefits. Using the NUFT code, we investigate CO2-injector/brine-producer strategies to improve CO2 storage capacity and minimize interference with neighboring subsurface activities. Performance measures considered in this study include magnitude of vertical brine migration and areal extent and duration of pressure buildup. We consider ranges of CO2-storage-formation thickness and permeability and caprock-seal thickness and permeability, comparing injection-only cases with ACRM cases with a volumetric production-to-injection ratio of one. The results of our study demonstrate the potential benefits of brine production to CO2-storage operations. The economic value of these benefits will require more detailed, site-specific analyses in future studies.

Introduction
Stabilizing atmospheric CO2 concentrations for climate change mitigation will require CO2 capture and storage (CCS) implementation being increased by several orders of magnitude over the next two decades (Fig. 3 of IEA, 2009). CCS in deep geological formations is regarded as a promising means of reducing atmospheric CO2 emissions (IEA, 2007). For widespread deployment of commercial-scale CCS to be achievable, several implementation barriers must be addressed. Previously identified barriers, such as CO2 capture cost, absence of CO2 transport network, legal and regulatory barriers, sequestration safety, and public acceptance are discussed in the Special Report on CCS (SRCCS) (IPCC, 2005). Implementation barriers receiving more recent attention are water-use demands from CCS operations and pore-space competition with emerging activities, such as shale-gas production (Court et al., 2011a). For commercial-scale CO2 injection in saline formations, pressure buildup can be a limiting factor in CO2 storage capacity, security, and safety. Primary issues for sequestration security and safety include the potential for CO2 leakage to the atmosphere, brine migration to overlying water-supply aquifers, and induced seismicity (Bachu, 2008; Carroll et al., 2008; Morris et al., 2011; Rutqvist el al., 2007). A key issue for storage capacity is pore-space competition with neighboring subsurface activities, including other CCS operations. A comprehensive review is presented by Court et al. (2011a) of progress, since the SRCCS, on several of these CCS implementation challenges: water management; sequestration safety; pore-space competition; legal and regulatory; and public acceptance.