Abstract
Miscible gas injection is a proven enhanced oil recovery method for medium-light oils. Miscibility is typically assessed with the slimtube experiment, which aims at identifying the minimum pressure (MMP) above which the displacement process is multi-contact miscible and leads to low microscopic residual oil saturation. During implementation of a miscible gas injection scheme, there will often be areas close to producing wells where the reservoir pressure is below the MMP and the question then arises whether the process can still be considered miscible. Another complicating factor occurs in reservoirs with significant lateral fluid composition and pressure variations, where it is not intuitively clear how the MMP varies aerially.We employ 1D compositional reservoir simulation to investigate the impact of flowing bottom-hole pressures below MMP on the development and propagation of a miscible front. For systems with lateral fluid variation, we find that MMP is controlled by the reservoir fluid composition near the injection well as the injection gas will never actually contact the original fluid near the producer. We also find that a miscible displacement can be maintained even if the average reservoir pressure is slightly below MMP, as long as the local pressure at the gas oil front exceeds MMP. Lastly, we define the minimum density difference as the overall minimum over time of the difference between oil and gas density observed at a fixed point in the system and we demonstrate that it extrapolates towards zero linearly versus pressure for the fluid systems investigated thereby enabling determination of the dispersion-free MMP with a maximum of three pressure iterations only.
NSTL
Elsevier