Polymer flooding most commonly uses partially hydrolyzed polyacrylamides (HPAM) injected to increase the declining oil production from mature fields. Apart from the improved mobility ratio, also the viscoelasticity-associated flow effects yield additional oil recovery. Viscoelasticity is defined as the ability of particular polymer solutions to behave as a solid and liquid simultaneously if certain flow conditions, e.g., shear rates, are present. The viscoelasticity related flow phenomena as well as their recovery mechanisms are not fully understood and, hence, require additional and more advanced research. Whereas literature reasonably agreed on the presence of these viscoelastic flow effects in porous media, there is a significant lack and discord regarding the viscoelasticity effects in oil recovery. This work combines the information encountered in the literature, private reports and field applications. Self-gathered laboratory data is used in this work to support or refuse observations. An extensive review is generated by combining experimental observations and field applications with critical insights of the authors. The focus of the work is to understand and clarify the claims associated with polymer viscoelasticity in oil recovery by improvement of sweep efficiency, oil ganglia mobilization by flow instabilities, among others.

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.

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This study focuses on the investigation of the total pressure drop with regards to the shear, elongational and frictional forces experienced by the viscoelastic EOR polymers during the flow through porous media. The main analysis is performed to these forces occurring at low Reynolds numbers. Single-phase flooding experiments were conducted in Bentheimer core plugs and micromodels. Moreover, observations at pore-scale level are included by streamlines visualization analysis. The overall approach can be summarized in the following sequence: 1) Single phase polymer flooding through Bentheimer core plugs 2) Analysis regarding the correlation between the pressure drop and the apparent flow behavior. This analysis also focuses on the contribution of shear, elongational and frictional forces to the pressure drop at low Reynolds number 3) Porescale streamline visualization experiments using micromodels 4) Analysis regarding the elastic instabilities or turbulences observed during the flow at low Reynolds number from streamline visualization experiments. The preliminary evaluation from core flooding experiments shows a significant additional increase in pressure drop during the viscoelastic EOR polymers flow through porous media. The analysis regarding the cause of the additional increase in pressure drop indicates that shear and frictional forces are not the main determinants during the flooding process. This leads to a strong indication that the elongational forces experienced by the EOR polymers while flowing through the pores are the primary reason for the additional increase in pressure drop. A correlation between elongational forces and flow instabilities during the experiments was observed. It was also observed that at a given shear rate the onset of elasticity occurs. The onset of elasticity was evaluated by the observation of the normalized data obtained by taking the ratios between apparent and bulk viscosity. Further evaluations from the porescale streamlines visualization experiments showed a clear occurrence of elastic instabilities during the flow at low Reynolds numbers in the form of vortices, crossing streamlines, and steadily changing flow directions of streamlines. These flow instabilities account for the additional increase in pressure drop. This study provides a novel comprehensive evaluation approach to characterize the pressure drop observed during the EOR polymers flow through porous media with regards to their viscoelastic behavior. It should help to understand porescale polymer displacement and the contribution of viscoelastic properties on additional oil recovery. Furthermore, this paper provides evidence of the flow instabilities through visualization experiments and detailed analysis.

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