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Technical Paper: Viscosity Reduction EOR with CO2 & Enriched CO2 to Improve Recovery of Alaska North Slope Viscous Oils

Society: SPE
Paper Number: 144358
Presentation Date: 2011
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Abstract

Carbon dioxide can be used as an injectant in Viscosity Reduction Water-Alternating-Gas (VRWAG) Enhanced Oil Recovery process for viscous oil reservoirs. The design of a VRWAG process with CO2 requires a series of laboratory data in order to properly understand the CO2 / reservoir fluid phase behavior and accurately model the reservoir performance. This study involved both laboratory experiments and reservoir simulation to investigate the effect of the phase behavior of the oil-CO2 system on the recovery of the Alaska North Slope viscous oils.

The laboratory experiments includes two four-step backward multiple-contact experiments (MCE) with pure and enriched CO2 added to Alaska North Slope viscous oil to investigate the phase behavior of the mixtures of CO2 and live reservoir fluid. The enrichment was achieved by mixing 85% of CO2 with 15% of natural gas liquids (NGL). Experiments were performed to determine the fluid compositions and other pressure-volume-temperature (PVT) properties such as saturation pressure, gas/oil ratio, liquid phase viscosity, and k-values under reservoir temperature and pressure conditions to simulate the dynamic behavior of the VRWAG process in viscous oil reservoirs. Precipitation of asphaltenes associated with the increase of carbon dioxide concentration and the number of contacts was also investigated.

The MCE experiments showed that pure CO2 could reduce the oil viscosity from 122 cp to 17 cp and that NGL-enriched CO2 could reduce the oil viscosity from 122 cp to 6 cp. These experimental data were used to verify a newly developed equation of state (EOS) which was then used in a numerical simulation model to predict the performance of CO2 flood process in viscous oil reservoirs. Simulation results showed that the VRWAG process could increase oil recovery factor by 10 percentage points with pure CO2 and 11 percentage points with NGL-enriched CO2, which represent an improvement of 44% and 50% respectively over the waterflood process.

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