This paper investigates the additional oil recovery associated to viscoelastic flow instabilities encountered during polymer flooding. Single and two-phase polymer EOR experiments were conducted in micromodels that resemble porous media. To set a benchmark for non-viscoelastic flooding processes, Polystyrene Oxide (PEO) experiments are presented as well.
The experimental workflow consists of three main steps. First, saturation of the micromodel with a synthetic oil. Second, displacement of synthetic oil by an aqueous PEO solution. Third, displacement of the remaining oil by a viscoelastic polymer solution. For evaluation purposes, 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. The streamline data is gathered using a highspeed camera mounted on an epifluorescence microscope.
In this study we demostrate that viscoelastic flow instabilities are highly caused and influenced by polymer properties. It is also shown flow instabilities dependence on pore space geometry and Darcy's velocity. We have observed a dependency of elastic turbulence on mechanical degradation, polymer concentration and solvent salinity. Furthermore, two-phase flood experiments in complex pore-scale geometries have confirmed that elastic flow inconsistency provides a mechanism capable of increasing oil phase mobilization by the viscoelastic aqueous phase. Due to high resolution particle tracing in the micromodels, the main causes of enhanced mobilization can be described as: (1) Moffatt 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 creeping flow is able to further enhance the displacement process in porous media by its elastic instabilities.
This work provides an adittional understanding of pore-scale polymer displacement processes, namely oil mobilization due to elastic turbulence/flow instabilities. Using the potential of state-of-the-art 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 recovery. Moreover, this analysis can be used to modify subsequently the fluid characteristics in order to achieve an optimized process application.