Quantification of Oil Recovery Mechanisms during Gas Injection EOR Coreflood Experiments

Abstract

Abstract This paper presents results of gas injection enhanced oil recovery (EOR) core flood experiments to highlight mechanisms of oil production throughout the life of an EOR project. The relative contributions of vaporization and condensation/swelling production mechanisms are quantified by analyzing the compositional results of a series of gas flood experiments conducted with live fluids at reservoir temperature and pressure and a variety of gas injectants including CO2 and various hydrocarbon gases. The compositional information is useful for calibrating tuned equation of state (EOS) and gas flood simulation models, thus enhancing confidence in field-scale performance predictions. When analyzing results from such gas floods, care must be taken to properly account for changes in oil composition. Using the black oil assumption, the composition and formation volume factor (FVF) of the oil produced throughout the gas flood is assumed to be constant. The consequence of this assumption can lead to inaccurate estimates of oil recovery. To overcome the limitations of the black oil assumption, we introduce a rigorous methodology to account for changes in oil composition. A tuned 10-component EOS, is used to reconstruct the produced fluid compositions, measured at laboratory conditions, back to reservoir conditions. Conversion of detailed oil composition measurements to the 10-component compositions used by the EOS while honoring the weight fraction composition measurements is accomplished using a gamma probability distribution function. As expected, gases with lower minimum miscibility pressure (MMP) recovered a higher percentage of remaining oil in the core. In addition, the reconstructed reservoir-conditions compositional data revealed especially useful information for understanding the physical processes involved in oil recovery. The weight fraction of produced components as a ratio of the weight fraction of that component present in the original live oil was especially illustrative. Significant differences in the relative contributions of the vaporization and condensation/swelling mechanisms were observed depending on how close the reservoir oil was to its saturation pressure. Finally, results of history matching the gas flood experiments using compositional simulation confirm the results seen in the laboratory data.