Impact of Viscous Instabilities on WAG Displacement

Abstract

In earlier studies, we have investigated water-oil displacement at adverse mobility ratios, and the impact polymer may have to improve the oil recovery. This paper addresses gas-oil displacement which is inherently an unstable displacement process, due to the low phase viscosity and density of the gas phase. In this work, a systematic study of viscous fingering for gas injection and WAG is performed as a function of rock heterogeneity, viscosity ratio, and density difference. The results of these studies aim to improve the design of gas injection and WAG to optimize sweep and total oil recovery with least possible amount of gas recycling. The numerical modelling has been carried out using commercial reservoir simulators. The methodology for describing viscous fingering is similar to that presented by Sorbie et al. (2020), where a 4-stage approach was proposed; viz. (i) choosing the form of the fractional flow curve, (ii) from fractional flow deriving a set of relative permeability curves which gives the maximum total mobility function, (iii) establishing an appropriate random correlated permeability field and (iv) simulating the process with a sufficiently fine grid. Simulations have been performed with fine grid 2D, using variations in viscosity ratio and phase densities. The impact of heterogeneity has been studied by varying the local distribution of rock permeabilities. Here we use experimental data from different sources to determine WAG parameters. Horizontal gas injection gives viscous fingers, and the unstable flow leads to very early gas breakthrough. We studied the influence of gravity on formation and development of viscous fingers. By combining experimental data with simulations, we show that viscous instability of the gas-liquid front can be captured in simulation models based on relative permeabilities gained from gravity stable core floods. Numerical simulation studies confirm the recommendations and show the benefit of altering the core positioning. The simulation of generic core flow experiments was performed by using mm scale grid cells and heterogeneous permeability fields with rather short correlation lengths. In immiscible gas displacement, the finger pattern appears to be mostly dominated by the viscosity ratio. However, these observations will mainly apply in the viscous limit when the other forces, capillarity and gravity, are small. Trapped gas during WAG injection is found to dampen the gas fingers, even in a 2D cross-sectional case. In a full 3-D case the extent of the three-phase zone adds to the differences between the WAG and gas injection cases. Simulation studies show examples where stabilized flow relative permeability can model unstable displacement in fine grid models. The unstable horizontal oriented flow gives early gas breakthrough and viscous fingers dominate the flow. Shorter WAG cycles seem to be beneficial to optimize oil recovery and reduce gas recycling.