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Abstract
When conducting enhanced oil recovery (EOR) projects, utilizing CO2 as a
solvent, there are three primary options for sourcing the CO2 injectant. These
are naturally occurring deposits, anthropogenic flue gas capture, or recycled
CO2 from an earlier stage of an EOR project. All of these sources may contain
impurities that will change the properties of the injectant, potentially
impacting all stages of the system. For instance, captured flue gas may contain
quantities of N2, while produced or recycled sources may contain CH4
and/natural gas liquids (NGL). Either or both may contain SO2 or H2S. Each of
these impurities has a different effect on the thermodynamic and fluid flow
characteristics of CO2; exhibiting either beneficial or detrimental impact on
miscibility, density and viscosity of the injectant and its interaction with in
situ hydrocarbons. Impurities can also modify compression capacity, corrosion
control and pipeline specifications. This paper describes the impact of CO2
impurities on all stages of an EOR project, from CO2 capture to flood
efficiency prediction.
Miscibility, density, and viscosity, along with rock properties, are the
primary determinants of flood efficiency in a given rock and fluid setting. A
screening tool has been developed to estimate EOR performance for different
crude oils. Screening is done for miscibility, density, and viscosity and other
parameters important to the efficiency of an EOR process. A key component of
the screening tool is a solubility-parameter-based approach to screening CO2
injectants containing impurities for miscibility against a range of crude oils.
This technology, combined with worldwide operating experience, has been used to
evaluate EOR potential in reservoirs across a full range of pressure,
temperature, and fluid compositions. By accurately evaluating the impact of
impurities in the injectant, it is also possible to estimate an economic
optimum solution for securing and utilizing an injectant supply without
incurring the additional costs of treating the stream to remove and dispose of
all impurities.
By way of a field example, the paper discusses the impact of miscibility,
density and viscosity on the efficiency of the CO2 miscible EOR process. It
also presents investigations aimed at extending current light oil recovery
projects into reservoirs with either residual oil zones (ROZ), viscous oils or
biodegraded crudes.
Introduction
Injecting CO2 for EOR has the dual benefit of increasing energy supply and
storing greenhouse gas emissions. A recent global estimate of incremental oil
recovery, with full implementation of CO2 EOR projects, is that 450-820 billion
barrels of additional oil could be recovered while storing 130 to 240 Gt of
CO2.1 The expectation is that the primary future supply of CO2 for injection
will be anthropogenic CO2. When a CO2 infrastructure develops there will
potentially be a blend of source streams containing different types and levels
of impurities. Therefore it is important to examine the potential impact of
these impurities on the full system of compression, transport, and
wells/reservoirs with EOR potential and the costs of purifying the streams at
source versus injecting a stream containing impurities.
Economic competition for use in EOR projects requires that CO2 from
anthropogenic sources be delivered to the field at price levels comparable with
naturally sourced CO2 supply. Currently, CO2 from naturally-occurring sources
is priced at <$2/kcf in the Permian Basin, USA whereas traditional
post-combustion amine based capture, compression, and transport costs
>$5/kcf. Several carbon capture technologies are under development aiming
at lowering the cost of anthropogenic CO2. Alternative capture sources will be
discussed in this paper with a focus on potential impurities in the CO2 and the
impact, if any, on future EOR projects. The impact of impurities in CO2 is
investigated for all stages of a potential EOR process: capture, compression,
transport, well injection, and EOR implementation.
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