Mechanisms of oil recovery via multi-contact miscible gas injection process within lensed systems

The recovery of oil by injection of miscible gas has been a subject of interest and research in petroleum engineering over the past 40 years. In a first contact miscible displacement, the injected fluid forms only single-phase mixtures with oil in place, so in theory, 100% recovery can be achieved. Unfortunately, many phenomena conspire to limit the efficiency of the miscible flooding process including viscous fingering and permeability heterogeneity. One type of important heterogeneities that occur at the small scale is the lensed system. This is one of the most common sedimentary structures found in sandstone reservoirs and is almost universal. This paper investigates the physics of gas-oil multi-contact miscible (MCM) displacements within lensed porous media using a combination of well-characterized laboratory experiments and detailed numerical simulation. The aims were to: quantify the recovery efficiency of MCM displacements conducted at different flow conditions, provide a set of benchmark experimental data for MCM displacements performed within lensed porous media, and validate conventional compositional simulation of MCM displacements The novel experimental studies were conducted in specially designed lens visual models packed with unconsolidated glass beads and used a two-phase three-component (IPA/water/cyclohexene) liquid system that exhibits an upper critical point at ambient conditions. First contact miscible (FCM), immiscible and MCM displacement experiments were performed. They were then simulated using a commercial compositional simulator with a single set of EOS parameters and without using history matching (all input data to the simulator were determined from independent experiments). The produced oil and gas in the experiments were found not to be in compositional equilibrium. This was observed in both homogeneous (as reported previously by Al-Wahaibi et al, 2005¹) and lens models. This indicates that nonequilibrium occurs at even smaller scales than the lensed structure. As a result, the oil recoveries and gas cuts predicted by compositional simulation for MCM displacements differed significantly from the experimental results, although an excellent match was obtained for the FCM and immiscible displacements. Further experiments investigating the effects of varying injection rates and gravity forces on the level of nonequilibrium observed in the produced fluids are described. Numerical simulation of these experiments provides additional insight into the physical mechanisms causing the nonequilibrium. The experiments performed in this study are the first of their kind as they report the significance of nonequilibrium on MCM process prediction and the influence of lens heterogeneities on MCM injection. This study has important implications for the correct interpretation of core data, and for scale-up processes to reservoir scale, particularly for handling gas/oil nonequilibrium when modelling MCM displacements.