Remaining oil saturation, trapped gas saturation, three-phase relative permeability and injectivity are among the many parameters that play a major role in miscible CO2 water-alternating-gas (WAG) injection efficiency. In this work, we present a series of coreflood experiments designed to assess these parameters and investigate the microscopic efficiency of CO2 WAG injection on carbonated reservoirs, far above minimum miscibility pressure (MMP).

Experiments were carried out on intermediate-wet carbonate cores, initially saturated with light oil from a Middle East field, at reservoir conditions. Production was monitored at both reservoir and laboratory conditions providing material balance of each phase and separating the production of flashed oil, condensate and gas. A full compositional analysis was performed on produced volumes using gas chromatography (GC) and liquid analysis of flashed oil and condensate. Differential pressure across the core was also monitored for relative permeability estimation. Finally, a dual energy X-Ray scanner was used to measure three phase in-situ saturations, cross check material balance results and ensure that laboratory artifacts (end effects) do not influence interpretation conclusions. Coreflood data was complemented with PVT experiments including CO2-oil phase behavior characterization through multiple contact test (MCT) and supercritical fluid extraction (SFE).

CO2 WAG injections showed great performance with faster and better recovery than pure CO2 injection. After several cycles of WAG injections, high levels of differential pressure across the core were reached due to a reduction in both water and gas relative permeabilities with injection cycles.

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