Abstract
Both vaporizing and condensing miscible gas floods are being conducted in a large number of reservoirs worldwide. The performance of these gas floods is usually determined via a combination of laboratory analysis and compositional simulation. However the ability of numerical simulation to correctly predict the progress of a multi-contact miscible displacement has not been fully verified
This paper investigates
the physical processes occurring during multi-contact miscible displacement
the ability of the Coats correlation to model the changes in gas-oil relative permeabilities with interfacial tension (IFT) and the associated inaccuracies in predicted oil recovery
the importance of gas/oil disequilibrium on the prediction of oil recovery and gas-oil ratio
These objectives are achieved by using a commercial compositional simulator to predict the behaviour of multicontact miscible (MCM) floods in well-characterised, bead-pack experiments. Both condensing and vaporizing drives are investigated using a ternary liquid system that exhibits an upper critical point at ambient conditions. Relative permeabilities as a function of IFT were obtained independently from displacement experiments. Glass bead-packs permitted the visualisation of the displacements whilst the effluent profiles in terms of composition and phase volume versus time were measured using a calibration with refractive index of the fluids.
Condensing and vaporizing gas-drive displacements were very efficient with about 90% of the oil in place recovered at one pore volume injected. Compared to immiscible displacements, breakthrough and total recoveries were increased by 8% and 20% respectively. However, in both condensing and vaporising drives, the compositional simulation (that assumes equilibrium conditions) over-predicted oil recovery. Further simulations identified that these errors are due to inaccuracies in Coats’ correlation describing the behaviour of relative permeabilities as miscibility is approached and/or the fact that the produced fluids were not in compositional equilibrium.
