Understanding the physics behind which carbon dioxide (CO2) and water displace oil in the porous medium is of the utmost importance for the success of any miscible CO2 water-alternating-gas (WAG) -injection project. This involves estimating parameters such as remaining oil saturation, trapped gas saturation, and three-phase relative permeability by carrying out and by interpreting several CO2 WAG experiments representative of flows inside the reservoir (i.e., pressure and temperature conditions, fluids, capillary number). This paper presents an experimental design, an apparatus, and a procedure to carry out such experiments at reservoir conditions with extensive and up-to-date data-acquisition techniques.
Two CO2/water cyclic injections were performed at reservoir conditions, on horizontal carbonate cores with light oil from a Middle East field, far above minimum miscibility pressure (MMP). Great care was given to the data-acquisition systems set for the experiments. Produced volumes were monitored to provide material balance of each phase at both reservoir and laboratory conditions with a clear separation of flashed oil, condensate, and gas. A full compositional analysis of the fluids produced was then conducted, with gas chromatography (GC) and liquid analysis of flashed oil and condensate. Differential pressure across the core was 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, highlight gravity segregation, and ensure that laboratory artifacts (end effects) do not influence interpretation conclusions.
Several material-balance methods were investigated to accurately calculate the average saturations and the recovery factor in the presence of strong CO2/oil interactions. With our experimental design, we were able to decouple thermodynamic effects (stripping) from three-phase flow effects (sweeping) in the raw-production data. Several hysteresis features were observed, such as reduction in gas relative permeability and variation in land-trapping parameter.