An experimental investigation of the effect of gas and water slug size and injection order on the performance of immiscible WAG injection in a mixed-wet system

Water-alternating-gas (WAG) injection is a widely used enhanced oil recovery (EOR) technique. The performance of WAG injection is affected by a number of design parameters; including the size of the gas and water injection slugs as well as their order of injection. Since the prediction of the impact of these parameters by numerical simulation lacks reliability, experimental data are needed to optimize the performance of WAG injection. In this work, we will present a comprehensive series of experimental data in which we investigate the impact of these design parameters on the performance of WAG injection in terms of oil recovery and injectivity. A set of coreflood experiments have been performed in which the performance of WAG injection and the effect of pertinent parameters have been systematically investigated. To minimize laboratory artefacts, long and large cores were used to experimentally investigate various gas and water injection strategies, including WAG and SWAG injection, WAG injection with different slug size and order of gas and water injection. The experiments were performed on a mixed-wet rock using an immiscible gas/oil system. Comparison of the results of WAG injection with different slug sizes shows a better efficiency and higher recovery performance for short water and gas slug injections compared to the large cycle injections. Reducing the size of the injected WAG slug enhanced oil recovery over that of the large slug size WAG injection, by 6.5 % (IOIP %). WAG injection produced more oil than water or gas injection alone. For the conditions of our experiments, WAG produced 9.0 % and 14.0 % more oil compared to waterflood and gas injection, respectively. The results also revealed that the order of fluid injection was important. When WAG injection started with a gas injection period followed by water injection, 6.0 % additional oil was recovered compared to WAG injection started by a water injection period. However, the large gap of oil recovery observed between LS-WAG (large slug WAG beginning with water) and LS-GAW (large slug WAG beginning with gas) became narrower after reducing the size of the injected slugs. Reducing the injected slug size appears to lead to higher oil recovery and approach the trend of oil recovery by SWAG injection. In other words, the amount of oil recovery observed for SWAG injection seems to present the upper limit of the small slug WAG tests. A novel and new set of experimental data are reported in this paper. WAG injection is a special case of three-phase flow, which is known to be difficult to reliably simulate by the current reservoir simulators. Therefore, reliable experimental data are needed for better understanding of the complex physics involved in the process of oil recovery by WAG injection and for developing improved models and methodologies for predicting the performance of oil reservoirs under WAG injection.