Gas-oil non-equilibrium in multicontact miscible displacements within homogeneous porous media

This paper investigates the non-equilibrium observed between the compositions of the produced phases in a series of multi-contact miscible displacements performed in the laboratory. These used a two-phase, three-component (Isopropyl alcohol/water/cyclohexene) liquid system that exhibits an upper critical point at ambient conditions. Both vaporising and condensing drives were performed in well-characterised homogenous glass-bead packs. The use of analogue fluids and bead-packs enabled visualisation of the displacements as well as the usual measurements of effluent composition against time and recovery. Non-equilibrium was observed in the effluent from both the condensing and vaporising experiments. This increased with flow-rate but appeared to be independent of the permeability and the length of the bead-pack. Further experiments investigating the influence of gravity on vertical displacements indicated that non-equilibrium may also be a function of the viscous to gravity ratio. Detailed simulation using a commercial compositional simulator was unable to predict this non-equilibrium unless the results were tuned to the experimentally observed effluent profiles using alpha factors. This is despite the fact that all PVT data, relative permeabilities and other pack properties were taken directly from the experiments. However good results were obtained from a layered model with the permeability distribution previously adjusted to match effluent data from a unit mobility ratio miscible displacement in the same pack. These results are consistent with physical dispersion being the underlying cause of the non-equilibrium. Viscous fingering was discounted due to the low mobility ratio (~ 2) of the displacements.