This study focuses on the investigation of the flow behavior of viscoelastic polymer solutions in porous media, aiming to better understand three phenomena: Elastic turbulence, elastic flow instabilities and shear thickening. These phenomena are appointed lately by a few authors as the reason of oil recovery improvement. Qualitative and quantitative flow analysis in this work rely on streamline visualization and pressure differential obtained from polymer flooding experiments. Flooding experiments are performed using innovative Glass-Silicon-Glass (GSG) micromodels that resemble porous media.
Streamline visualization and flow pattern analysis of viscoelastic polymer solutions are based on videos and images taken during flooding experiments in GSG micromodels. Thus, 1 μm polystyrene tracers are added to partially hydrolyzed polyacrylamide (HPAM) solutions and injected subsequently. A state-of-the-art optical setup, consisting of an inverted epi-fluorescence microscope with a high-speed camera mounted on top, enables high-quality video and image acquisition. Micromodels are placed under the microscope and by steady illumination as well as using long exposure times, streamline images are obtained. The quarter-of-a-five-spot micromodel is connected with a syringe pump and a differential pressure sensor (0-30 bar) which enables an additional quantitative polymer flow analysis.
Comprehensive flooding experiments show that viscoelastic polymer solutions used in EOR exhibit clear flow instabilities in porous media. Implementation of an advanced particle tracing technology in GSG micromodels disclose the flow characteristics of viscoelastic polymers and their elastic turbulent flow behavior in detail. These characteristics can be seen at even low Reynolds numbers and can be described as: (1) vortices, (2) crossing streamlines near grain surfaces and (3) continuously changing flow direction of streamlines. In addition, it is shown that the so called elastic turbulence strongly depends on solutions mechanical degradation, polymer concentration and solvent salinity. Thereby, the strongest impact on polymer flow behavior was observed for changing salinity and the weakest for mechanical degradation. Also a dependency of polymer flow behavior on pore space geometry and injection rate is revealed. It was seen that elastic turbulence characteristics become stronger if injection or rather shear rate is increased. Additionally, streamline analysis shows that elastic turbulence especially occurs in wide open pore space geometry. Supplementary differential pressure monitoring during flooding experiments allowed to analyze rheological fluid properties and thus, supports qualitative flow characterization. Thereby, a distinct correlation between the onset of shear thickening and elastic flow instabilities was found.
This paper provides an improved understanding of polymer flow behavior and especially elastic turbulence in complex porous media. Since elastic turbulent flow is believed to contribute in an oil recovery enhancement, understanding of its mechanism is essential. Using particle tracing technology in GSG micromodels in combination with precise differential pressure monitoring during flooding experiments results in an improved evaluation of viscoelastic polymers used in EOR applications. Furthermore, the analysis results can be used subsequently to modify polymer solution properties in order to enhance EOR processes.