Carbon Dioxide (CO2) has been recognized as a preferred miscible solvent for Enhanced Oil Recovery (EOR) methods. Miscible flooding by CO2 has proven its positive effectiveness through numerous laboratory and field projects. The main goals of this study are to provide microscopic reservoir rock characterization, and optimize the selection and the effect of CO2 slug size to be applied for carbonate oil field, U. A. E., on oil recovery by CO2 miscible flooding.

Scanning Electron Microscope (SEM) is used to provide accurate description of the reservoir rock surface topography. The SEM is used to attain the pore size distribution of the rock. Energy Dispersive X-Ray Spectrometer (EDS) and Inductively Coupled Plasma (ICP) are applied for detailed accurate analysis of elements constituting the rock.

Five flood experiments were performed to achieve the goals of this study. The first experiment was carried out using continuous CO2 flooding. Other three experiments used different CO2 slug sizes of 0.15, 0.30, and 0.45 hydrocarbon pore volume and chased by actual reservoir brine. The fourth experiment was carried out to evaluate the performance of waterflooding. All experiments were performed using actual crude oil injected into actual reservoir rock samples of almost 23 cm length and 10 cm diameter. These samples were saturated at initial-oil-in-place condition and then subjected to water and/or CO2 floodings. The minimum miscibility pressure (MMP) was calculated to be 3,700 psia using current correlations from literature. To secure good miscibility condition, all experiments were conducted under injection pressure of 4,200 psia and at 250 °F similar to the reservoir condition.

The results indicated that water flooding recovered 66 % IOIP and a continuous CO2 miscible flood produced 96 % IOIP. Furthermore, the increase of slug size increases oil recovery by CO2 miscible flooding, but the slug size of 0.15 HCPV is recommended since the increment in oil recovery using larger slugs does not represent significant addition to the oil recovery in comparison to applied smaller slug sizes.

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