Modeling of Viscous Instability in Compositional Gas and Wag Injections

P. L. K. Caetano Chang, A. Skauge, K. S. Sorbie, V. Rios, K. Kumar

Research output: Contribution to conferencePaperpeer-review


When a higher mobility fluid displaces a lower mobility fluid, viscous fingers will generally occur. The objective of this study is to include correct modelling of viscous instabilities as part of an improved description of gas injection and WAG processes. The approach has been applied to immiscible (IM), near miscible (NM) and miscible (M) situations.

The numerical modelling was made using a commercial reservoir simulator, GEM/CMG. The methodology of describing viscous fingering, like in Sorbie et al. (2020) , is a 4-stage approach: (1) selection of fractional flow to maximize total mobility; (2) derivation of the relative permeability; (3) establishing an appropriate random correlated permeability field; and (4) simulating the process with a sufficiently fine grid. Simulations have been performed with 1D (for calibration) and 2D models using fine grid and random Gaussian permeability field.

All three fluid models had the same composition (CO2 for the injected gas and 4 pseudo-components for the oil), phase viscosities and densities at initial pressure and temperature. The binary interaction parameters were modified to control the different miscible conditions.

Numerical 1-D cases were used as a first calibration of immiscible, near-miscible, and miscible, gas injection and WAG. The miscible case performed as expected, with approx. 100 percent oil recovery around 1 pore volume injected. The near miscible case showed earlier gas breakthrough, and the immiscible case a very early gas breakthrough due to the high mobility relative permeability supporting the viscous instability. In the 2-D model, viscous fingers occur in all cases (IM, NM, and M) both during gas injection and WAG slug injections (slug size 0.1 PV). For the IM and NM cases, gravity segregation of the gas phase combines with viscous instability to create a finger at the reservoir top and accelerate gas breakthrough. This, however, does not happen to the miscible case, where the viscous fingers are composed of rich CO2 oil, much less viscous, but similar in density with the original oil. Miscibility also slows the propagation velocity of the viscous fingers, to some degree, due to mass exchange effects. In all cases, hysteresis effects associated with WAG dampened the formation of viscous fingers and improved recovery.

The main contribution of this work is a new approach to include a more correct modelling of viscous fingers in gas and WAG injections at immiscible, near-miscible and miscible conditions. This is the first paper combining compositional WAG with the methodology for viscous fingering.
Original languageEnglish
Number of pages32
Publication statusPublished - 2 Oct 2023
EventIOR+ 2023 - The Hague, Netherlands
Duration: 2 Oct 20234 Oct 2023


ConferenceIOR+ 2023
CityThe Hague


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