Laboratory Investigations Of First Contact Miscible Wag Displacement: The Effects Of Wag Ratio And Flow Rate

Abstract

Abstract Miscible WAG injection has been implemented in a number of oil fields around the world and there are a number of numerical studies investigating the effect of rate, gravity, slug size and heterogeneity on WAG performance. However there are very few laboratory studies of WAG displacement efficiency reported in the literature. In this paper we report the results from a series of well-characterised WAG displacements through glass bead-packs. The aim of these experiments wasto investigate the impact of first contact miscible WAG injection on oil recoveryto clarify the physical processes during displacements.to provide benchmark data-sets to validate reservoir simulations. The use of bead-packs rather than cores enabled us to observe visually, for the first time to our knowledge, the fluid interactions during each WAG experiment. A series of secondary, miscible WAG displacements were conducted at WAG ratios of 1:1, 4:1 and 1:4. These were performed at a range of flow rates to investigate the influence of capillary number on recovery efficiency. Their behaviour was compared with that obtained from simple miscible injection and waterflooding. We show that recovery efficiency is a function of rate as well as WAG ratio. We also find that the water-oil and the water-solvent relative permeabilities are not the same for the analogue fluids used. This is despite the fact that the oil and solvent are first contact miscible. If this is true for reservoir fluids then clearly it will affect the prediction of WAG recovery efficiency. Introduction Miscible WAG injection has been implemented successfully in a number of fields around the world1. In principal it combines the benefits of miscible gas injection and water flooding by injecting the two fluids either simultaneously or alternately. Miscible gas injection has excellent microscopic sweep efficiency but poor macroscopic sweep efficiency due to viscous fingering and gravity over-ride. Furthermore it is expensive to implement. In contrast water-flooding is relatively cheap and is less subject to gravity segregation and frontal instabilities. However the residual oil saturation after water-flooding is relatively high. Injecting water with the miscible gas reduces the instability of the gas-oil displacement process due to relative permeability effects2–3, thus improving the overall sweep efficiency. It also improves the economics by reducing the volume of gas that needs to be injected into the reservoir. The optimum WAG ratio for simultaneous WAG injection in a homogeneous reservoir can be obtained by matching the advance rates of the water-oil and solvent-oil displacement fronts. Stalkup3 provided a method for calculating the optimum WAG ratio from the water-solvent fractional flow via graphical construction. However this method assumes that the water-oil and water-solvent relative permeabilities are the same. It also neglects the influence of capillary pressure on small scale displacement efficiency and the fact that relative permeability may change as a function of rate4–7. Numerical studies8 suggest that the optimum WAG ratio may be around 4:1, which is rather larger than the values typically calculated using Stalkup's method. The difference between theory and practice is normally attributed to the combined influences of reservoir heterogeneity and gravity, but may also be due to using inappropriate relative permeability curves in the calculation of the WAG ratio and the influence of capillary pressure. This is supported by the fact that the majority of WAG displacements have not recovered as much additional oil as was originally predicted1 by simulation studies. This is despite the fact that simulation models take into account reservoir heterogeneity and gravity.