Investigation of three-phase relative permeabilities and hysteresis effects applicable to water alternating gas injection

Large quantities of oil usually remain in oil reservoirs after conventional water floods. A significant part of this remaining oil can still be recovered economically by Water-Alternating-Gas (WAG) injection. Accurate numerical simulation of complex three-phase flow processes taking place in oil reservoirs under WAG injection requires reliable three-phase relative permeability (kr) as well as kr hysteresis that takes place due to cyclic injection of water and gas. In this paper we present a series of three-phase relative permeability obtained from core flood experiments of WAG injection into a water wet and a mixed wet core using a very low IFT oil/gas system. The objective of this study was to investigate hysteresis behaviour in WAG process under with different rock wettability conditions. The results of the WAG experiments revealed strong cyclic hysteresis of oil, water and gas relative permeabilities taking place in both water-wet and mixed-wet systems. These findings highlight serious shortcomings of the existing hysteresis models in which it is assumed that only non-wetting phase (usually gas) exhibits hysteresis effects. Comparison of the kr hysteresis observed for water-wet and mixed-wet WAG experiments demonstrates significant differences between the two-wettability conditions which highlights the impact of wettability on three-phase flow during cyclic injection of water and gas and the importance of performing these experiments under representative wettability of the reservoir. The measured data were also compared with the values predicted by some of the widely used 3-phase kr models available in commercial reservoir simulators. Large dissidences and disagreements were observed between the measured and predicted values as well as between predictions of different models. The results of this investigation clearly demonstrate some serious limitations of three-phase kr and kr hysteresis models available in commercial reservoir simulators and the need for developing improved methodologies and models for better quantification of these parameters for simulation of oil recovery by WAG injection.