Immiscible fingering in porous media under different wetting conditions and its role in polymer flooding

Abstract

Immiscible viscous fingering occurs when a low viscosity fluid immiscibly displaces a high viscosity fluid. In the field of geoenergy, this is typically a major problem whether in gas storage or in oil recovery. When water is injected into the reservoir to aid recovery, it can finger through a viscous oil, leaving large volumes bypassed and giving early water breakthrough – neither of which is ideal from an economic or carbon footprint viewpoint. Three major questions present themselves with regard to viscous fingering in such systems: how can fingering be modelled correctly?; how can fingering be evaluated in the laboratory?; and how can it be remedied? These are the 3 main areas of research that will be addressed in this thesis. A novel simulation methodology is used to directly model viscous fingers using standard, commercial numerical simulators. In this work, this approach is validated against literature experiments at a range of unstable viscosity ratios (μo/μw ~400 to 7,000). It is then applied to model conventional core flood experiments, conducted as part of this thesis, where μo/μw = 100. The simulation method is then used to upscale the core flood results using scaling theory to a series of conceptual and sector models of the Captain reservoir, which is currently undergoing polymer flooding in the North Sea. The same numerical method is used to demonstrate how laboratory scale unstable displacement experiments are sensitive to the suppression of viscous fingering by capillary dispersion. This is then shown to occur even under extremely weak wetting conditions. Using scaling theory, it is then shown how fingering “remerges” as the system size is increased towards the field scale. These observations are then further supported by carrying out laboratory 2D slab flood experiments under different wetting conditions for an unstable immiscible displacement with viscosity ratio μo/μw = 100. The systems studied include a weakly water-wet case which shows an apparently stable front, while the equivalent weakly oil-wet system is highly fingered. By applying scaling theory, it is demonstrated that capillary forces must be made negligible at the laboratory scale in order to maintain the same viscous-capillary force balance which applies at the field scale system. Finally, the well-established enhanced oil recovery technique of polymer flooding is re-evaluated in the context of these findings. It is demonstrated both by simulation and experiment that the principal increased recovery mechanism of the polymer is through viscous crossflow. This mechanism is shown to be responsible for the large and very rapid response in oil recovery on polymer injection – even in highly viscous systems (>2,000 mPa.s) - as bypassed oil crossflows into established water channels (fingers). This mechanism is evident in the laboratory when viscous fingers are allowed to form (viscous-dominated) and supports the conjecture that both polymer flooding and water flooding are best examined without the stabilising effect of capillarity. In addition, the findings of this thesis cast doubt on the conventional methods of “measuring” relative permeability in the laboratory for application in adverse viscosity ratio immiscible displacements in the field.