Abstract
Water-Alternating-Gas injection is one of the most applicable EOR techniques in oil reservoirs worldwide. To optimize the WAG injection projects, numerical simulation is the best available tool to aid with such objective. In this paper, we are looking into the simulation process (from core-scale to reservoir-scale) in more details to understand the challenges and try to suggest best practices and guidelines.
The first part of this paper will discuss the role of hysteresis in WAG injection based on the results of several three-phase Water-Alternating-Gas injection experiments. Then the significance of three-phase hysteresis in simulating WAG injection will be highlighted. After establishing a good understanding of the role of three-phase hysteresis in WAG injection simulation, the second part will present sensitivity study at reservoirs-scale to identify the main differences in the simulation results. Finally, towards the end of the paper, WAG injection's best practices guideline will be suggested.
Currently, the common practice is to estimate three-phase hysteresis parameters (Land trapping parameter [C], secondary drainage gas relative permeability reduction exponent [alpha], residual oil modification factor [a], and three-phase water relative permeability [krw3ph]) from coreflood experiments. Based on several three-phase WAG injection experiments conducted at Heriot-Watt University, we came up with several conclusions. These conclusions are:
If the pressure drop (DP) during the three-phase water injection experiment is significantly higher than the DP during two-phase water injection, then a lower three-phase water relative permeability is essential to match the three-phase WAG injection experimental data.
If the pressure drop (DP) at the end of two-phase gas injection is lower than the DP at the end of three-phase gas injection, then alpha [α] is critical to match the three-phase WAG injection experimental data.
If the experimental results showed significant gas trapping [Sgt] during water injection after gas injection, then Land's trapping parameter is essential to model such trapping behaviour at any scale.
These parameters should be obtained from experiments starting with gas injection (GAW) to fit with the definitions accepted into the numerical simulations.
Many studies in the literature either ignores the effect of hysteresis in WAG injection or use invalid WAG hysteresis parameters. In this paper, we highlighted the role of hysteresis in WAG injection and clarified the confusion on the procedures to obtain the correct hysteresis parameters. Also, we provided an easy guideline to help the industry in making accurate predictions for WAG injection projects.
