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36 OCTOBER 2022 WorldWide Drilling Resource® Solving a 54-Year-Old Puzzle with Fractured Artificial Rock Adapted from Information by Princeton University Princeton researchers solved a 54-year-old puzzle about why certain fluids strangely slow down under pressure when flowing through porous materials, such as soils and sedimentary rocks. The findings could help improve many important processes in energy, environmental, and industrial sectors, from oil recovery to groundwater remediation. The fluids in question are called polymer solutions. These solutions contain dissolved polymers, or materials made of large molecules with many repeating subunits. Typically, when they're put under pressure, polymer solutions become less viscous and flow faster; but when going through materials with lots of tiny holes and channels, the solutions tend to become more viscous and gunky, reducing their flow rates. To find the root of the problem, scientists devised an innovative experiment using a see-through porous medium made of tiny glass beads. This transparent artificial rock allowed researchers to visualize a polymer solution’s movement. The experiment revealed the longbaffling increase in viscosity in porous media happens because the polymer solution's flow becomes chaotic, much like turbulent air on an airplane ride, swirling into itself and gumming up the works. "Surprisingly, until now, it has not been possible to predict the viscosity of polymer solutions flowing in porous media," said Sujit Datta, an assistant professor of chemical and biological engineering at Princeton and senior author of the study. "But in this paper, we've now finally shown these predictions can be made, so we've found an answer to a problem that has eluded researchers for over a half-century." "With this study, we finally made it possible to see exactly what is happening underground or within other opaque, porous media when polymer solutions are being pumped through," said Christopher Browne, a Ph.D. student in Datta's lab and the paper’s lead author. Browne ran the experiments and built the experimental apparatus, a small rectangular chamber randomly packed with tiny glass beads. The setup, akin to an artificial sedimentary rock, spanned only about half the length of a pinky finger. Browne pumped a common polymer solution laced with fluorescent latex microparticles into the faux rock to help see the solution's flow around the beads. He formulated the polymer solution so the material’s refractive index offset light distortion from the beads and made the whole setup transparent when saturated. Datta's lab previously used this technique to create seethrough soil for studying ways to counter agricultural droughts, among other investigations. "I was able to see and record all these patchy regions of instability, and these regions really impact the transport of the solution through the medium," said Browne when he zoomed in with a microscope to examine the fluid flow through each pore within the beads. As the polymer solution worked its way through the porous medium, the fluid's flow became chaotic, with the fluid crashing back into itself. As the polymers spread throughout the pore space, they stretched out, generating forces that accumulated and generated turbulent flow in different pores. This effect grew more pronounced when pushing the solution through at higher pressures. The Princeton researchers used data gathered from the experiment to formulate a way to predict the behavior of polymer solutions in real-life situations. Given that viscosity is one of the most fundamental descriptors of fluid flow, the findings not only help deepen understanding of polymer solution flows and chaotic flows in general, but also provide quantitative guidelines to inform their applications at large scales in the field. "The new insights we have generated could help practitioners in diverse settings determine how to formulate the right polymer solution and use the right pressures needed to carry out the task at hand," said Datta. "We're particularly excited about the findings' application in groundwater remediation." B e c a u s e polymer solutions Researchers developed a see-through medium to observe polymer flow. ENV Solving cont’d on page 38.