Pore-scale Visualization of Oil Recovery by Viscoelastic Flow Instabilities during Polymer EOR

Abstract

This paper provides a new understanding of pore-scale polymer displacement processes, namely an additional oil recovery due to elastic turbulence. Using the potential of state-of-the-art GSG micromodels enables to conduct high-quality streamline visualization which is the key to an improved polymer EOR screening. Thereby enables to understand which properties of viscoelastic solutions contribute to oil mobilization. Moreover, this analysis can be used to optimized subsequently the fluid characteristics in order to achieve a higher recovery. Single and two-phase polymer EOR experiments were conducted in Glass-Silicon-Glass (GSG) micromodels that resemble porous media. The objective of this work is to investigate the additional oil mobilization associated to viscoelastic flow instabilities encountered during polymer flooding at pore-scale. To set a benchmark for non-viscoelastic flooding processes, polystyrene oxide experiments are presented as well. Experimental workflow consists of three steps: 1) Saturation of micromodel with a synthetic oil (10% silicon oil / 90% decane) with a viscosity of 25 mPas, 2) Displacement of synthetic oil by an aqueous polystyrene oxide solutions and 3) Displacement of remaining oil by a viscoelastic polymer solutions. All aqueous solutions are dissolved in a 4 g/l TDS brine. Additionally, viscosity of the polymer and polystyrene oxide solution are approximately matched. Furthermore, tracer particles are attached to the aqueous phase to enable high-quality streamline visualization using a high-speed camera mounted on an epi-fluorescence microscope. Here we show that viscoelastic flow instabilities are highly caused and influenced by fluid properties. It is also shown flow instabilities dependence on pore space geometry and Darcy’s velocity. Streamlines and pressure differential evaluations revealed a dependency of elastic turbulence on solutions’ mechanical degradation/pre-shearing conditions, polymer concentration and solvent salinity. Furthermore, two-phase flood experiments in complex pore-scale geometries have preliminary confirmed that elastic induced flow inconsistency provides a mechanism capable of increasing oil phase recovery by the viscoelastic aqueous phase. Thereby, a polymer flood under elastic turbulence caused 20% additional oil recovery, whereas a polymer flood under laminar flow conditions enhances the recovery by only 5%. Due to high-resolution particle tracing in the micromodels, the main causes of enhanced recovery can be described as: (1) vortices, (2) crossing streamlines, especially near grain surfaces and (3) steadily changing flow directions of streamlines. Thus by adding viscoelastic additives to injection fluids and considering a sufficient shear rate, even a low reynold numbers are able to further enhance the displacement process in porous media by its elastic instabilities.