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 wateruse 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; porespace competition; legal and regulatory; and public acceptance.