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Abstract
Numerical simulation has been used, as common practice, to estimate the CO2
storage capacity in depleted reservoirs. However, this method is time
consuming, expensive, and requires detailed input data. This investigation
proposes an analytical method to estimate the ultimate CO2 storage in depleted
oil and gas reservoirs by implementing a volume- constrained thermodynamic
equation of state (EOS) given average reservoir pressure and fluid
composition.
This method was implemented in an algorithm which allows fast and accurate
estimations of final storage, which can be used to select target storage
reservoirs and design the injection scheme and surface facilities. Impurities
such as nitrogen and carbon monoxide, usually contained in power plant flue
gases, are considered in the injection stream and can be handled correctly in
the proposed algorithm by using their thermodynamic properties in the
EOS.
Results from analytical method presented excellent agreement with those from
reservoir simulation. Ultimate CO2 storage capacity was predicted with an
average difference of 1.26 wt% between analytical and numerical methods;
average oil, gas, and water saturations were also matched. Additionally, the
analytical algorithm performed several orders of magnitude faster than
numerical simulation, with an average of 5 seconds per run.
Introduction
Greenhouse gas (GHG) emissions have been continuously increasing in the past 3
decades. More than 70% of these emissions are composed of CO2, which reached 30
Gt in 2009 (EIA 2011). Several environmental agencies and governments have
shown concern about this statistic and its potential relation with global
warming.
Coal consumption accounted for the release of nearly 14 Gt of CO2 during 2009,
almost 45% of worldwide carbon dioxide emissions (EIA 2011). Given that coal is
mainly used in power plants to generate electricity, these locations are large
sources of CO2 and become the most important target for carbon capture and
sequestration (CCS) processes. A large coal-fired power plant, generating 500
MW, emits approximately 2.9 Mt-CO2 per year or 55.2 BSCF of CO2 per year.
Metz et al. (2005), Dooley et al. (2006), and EPA (2011) defined carbon capture
as a process consisting first of removing the impurities from a CO2-based
stream to increase the CO2 concentration and improve the efficiency of the
final storage process; and secondly, compressing the gas stream to transport it
to a storage location, which can be geologic formations such as aquifers and
depleted oil and gas reservoirs, to achieve long-term isolation from the
atmosphere.
Geological storage of CO2 in aquifers and depleted oil and gas reservoirs
represent an attractive option to reduce carbon emissions to the atmosphere, as
it has been studied in the oil and gas industry for several years.
Particularly, interest now exists in using depleted reservoirs taking advantage
of the higher storage density in comparison with aquifers; additionally,
extensive knowledge of the reservoir’s static and dynamic properties, acquired
during the developing phase, are available to optimize the efficiency of the
project and increase the final storage capacity and profits.
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