Experimental Data Acquisition and Modeling for the Study of Miscible CO2 WAG in Carbonate Reservoirs Under Oil-Wet Conditions

Abstract

Prediction of miscible WAG performance relies on proper calibration of thermodynamical and petrophysical models. Swelling, miscibility and stripping phenomena must be captured in the equation of state (EOS) and the oscillations of gas and water saturations require using history-dependent relative permeabilities. This paper presents data synthesis and numerical interpretation of a complete experimental program including bulk PVT measurements, horizontal coreflood experiments at reservoir conditions and a spontaneous imbibition test. Prior to flooding experiments, CO2-oil phase behavior was characterized through swelling test, slim tube, multiple contact test (MCT) and supercritical fluid extraction (SFE) to provide a reliable equation of state. Coreflood experiments were conducted using carbonate outcrop cores, carbonated water, undersaturated live oil from a Middle East field, and multiple contact miscible CO2. Produced gas and oil were experimentally separated at reservoir conditions. X-ray monitoring was carried out. The experimental data were matched using our in-house research reservoir simulator (IHRRS) and an optimization module. We used the classical and an extended version of Larsen & Skauge relative permeabilities model, coupled to our calibrated equation of state (EOS) slightly corrected with alpha-factors to impose the residual oil saturation. Spatial variations of remaining oil were observed, with a minimum achievable oil saturation of 7 %. The condensate produced by stripping and the oil produced by mechanical sweep were matched separately. Two-dimensional simulations were performed to take into account gravity segregation, evidenced by in-situ saturations measurements. We found an acceptable history-match for each three-phase experiment individually. Significant relative permeability hysteresis was observed for both gas and water phases. Some of the three-phase parameters were found dependent on saturations path and not compatible for simultaneous history-match of multiple experiments. This paper provides a robust methodology for miscible CO2 WAG experimental data acquisition and history match. The hypotheses and limitations of the relative permeability model were stated: for instance, trapped water saturation may be introduced. A new hysteresis model was developed allowing for water trapping, which improved the quality of the history match of the experimental observations.