Gas-Oil Non-Equilibrium in Multicontact Miscible Displacements within Homogeneous Porous Media

Abstract

Abstract Compositional simulation is usually used to predict the performance of multi-contact miscible (MCM) recovery schemes. One key assumption in most such simulations is that of instantaneous compositional equilibrium is achieved between phases in each grid block. This is despite the fact that most grid blocks are tens of metres long and at least a metre thick. This paper investigates the non-equilibrium observed in series of multi-contact miscible displacements performed in the laboratory. For simplicity a two-phase, three-component (IPA/water/cyclohexene) liquid system that exhibits an upper critical point at ambient conditions was used. 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 experimental 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 experiments. However good match was obtained from a layered model with the permeability distribution obtained 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 is discounted due to the low mobility ratio (∼2) of the displacements. Introduction One key assumption of current compositional simulator formulation is that of complete vapour/liquid equilibrium in each grid block.[1] This allows the calculation of different phase saturations from the composition of the different hydrocarbon components in that grid block. However in many field scale simulations the size of the grid blocks (>100m) and the time-step between solutions (days) is such that the validity of this assumption is questionable.[2–4] Viscous fingering, gravity over¬ride and channelling through sub-grid heterogeneities are all possible reasons why there may not be compositional equilibrium within field scale grid blocks. Numerical dispersion within the simulator may also result in incorrect phase behaviour predictions.[2] Furthermore, previous experimental studies[5–8] have shown that even on the laboratory scale produced fluids may not be in equilibrium. Although a variety of methods have proposed to model this lack of equilibrium,[9–11] there has been little investigation of the physical mechanisms causing it. In this paper we investigate the lack of equilibrium observed in previous experiments performed by Al-Wahaibi et al.[6–7] Multi-contact miscible displacements in homogeneous and cross-bedded glass bead packs were performed. A three component fluid system was used that exhibited an upper critical point at ambient conditions. They observed that even in homogeneous bead-packs, the produced phases were not in equilibrium (see figure 1 for some sample results). This was irrespective of whether a condensing or vapourizing drive was studied. Moreover the non-equilibrium observed in the cross-bedded bead-packs could be predicted by a commercial compositional simulator[12] using alpha factors[10] tuned to the effluent profiles measured in the homogeneous packs.[7,8] This suggested that non-equilibrium was not caused by heterogeneity in the bead-packs.