Effect of porous media properties on the onset of polymer extensional viscosity

• Rock micro-structures affect the onset of extensional viscosity.
• De is sufficient to capture the onset of extensional viscosity.
• Extensional viscosity occurs at lower Darcy velocity in more tortuous rock.

Based on experimental results, it has been observed that, although the applied polymer solutions in EOR generally demonstrate shear thinning properties in rheometer, in porous media above a critical flow rate, apparent viscosity increases. This behavior is interpreted as the consequence of both elastic properties of polymer solution and rock properties. In this study, elastic properties of polymer solution is imported into numerical simulation by using modified form of the Carreau model and only effect of rock properties on the onset of extensional viscosity are studied. Results proved that for a more tortuous rock sample, extensional viscosity happens at a lower Darcy velocity. To define the onset of extensional viscosity, Deborah number explanation (De=ετr) is applied. A linear correlation is proposed as the relationship between stretch rate and Darcy velocity, in which the linearity coefficient is related to the tortuosity of rock sample. By calculating the stretch rate using this modification, De at the onset of extensional viscosity is the same for all rock samples. Furthermore some microscopic properties of pore geometries such as aspect ratio and length of pore throat are studied by using simplified 2D contraction–expansion channels. Initially the simulation is well validated by using experimental results reported by Chauveteau (1981). By increasing the aspect ratio, the onset of extensional viscosity occurs at lower Darcy velocity as polymer molecules are being exposed to larger stretch rate values. On the other hand by increasing the length of contracted part, the onset of extensional viscosity happens at higher Darcy velocity. This is due to the fact that polymer molecules have more time to be relaxed after being deformed due to the contraction channel.