Characterization of three-phase flow and WAG injection in oil reservoirs
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Large quantities of oil usually remain in oil reservoirs after conventional water floods. A significant part of this remaining oil can still be economically recovered by Water- Alternating-Gas (WAG) injection. WAG injection involves drainage and imbibition processes taking place sequentially, hence the numerical simulation of the WAG process requires reliable knowledge of three-phase relative permeability (kr) accounting for cyclic hysteresis effects. In this study, the results of a series of unsteady-state two-phase displacements and WAG coreflood experiments were employed to investigate the behaviour of three-phase kr and hysteresis effects in the WAG process. The experiments were carried out on two different cores with different characteristics and wettability conditions, using a low IFT (interfacial tension) gas–oil system. The first part of this study, evaluates the current approach used in the oil industry for simulation of the WAG process, in which the two-phase relative permeability data are employed to generate three-phase kr values using correlations (e.g. Stone, Baker). The performance of each of the existing three-phase relative permeability models was assessed against the experimental data. The results showed that choosing inappropriate three-phase kr model in simulation of the WAG experiments can lead to large errors in prediction of fluid production and differential pressure. While some models perform better than others, all of the three-phase kr models examined in this study failed to adequately predict the fluid production behaviour observed in the experiments. The continued production of oil after the breakthrough of the gas, which was one of the features of gas and WAG injection experiments at low gas-oil IFT, was not captured with these models. The second aim of this research was to develop a method for obtaining the values of three-phase relative permeabilities directly from WAG core flood experiments. For this purpose, a new history matching method was devised based on a Genetic Algorithm to estimate three-phase kr from unsteady-state coreflood experiments. Based on this methodology, a three-phase coreflood optimizer was developed that generates best kr values by matching the experimentally obtained production and pressure data. First, the iii integrity of the developed software was successfully verified by using two sets of experimental three-phase kr data published in the literature. Then, the program was used to determine three-phase relative permeability of various cycles of the WAG experiments performed at different wettability conditions. Two key parameters affecting the WAG performance, including the hysteresis phenomena occurring between kr of the different WAG cycles and the impact of wettability of the rock, have been investigated. The data have been used to evaluate the existing hysteresis models published in the literature. Some of the shortcomings associated with the existing methods have been revealed and discussed. In the latter part of the thesis, a new methodology is proposed for modelling of threephase relative permeability for WAG injection. This approach addresses the hysteresis effects in the three-phase kr taking place during the WAG process and attempts to reduce the inadequacies observed in the existing models. The integrity of this technique has been validated against the three-phase kr data obtained from our WAG experiments.