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.

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