Polymer Injectivity : Experimental Studies of Flow in Porous Media for EOR Polymers

Abstract

Despite the maturity of the polymer flooding process that is applied in enhanced oil recovery (EOR) technologies, the core of the process, which is the flow of the polymer in porous media, is poorly understood. The types of most applied polymers in EOR are partially hydrolyzed polymers (HPAM), which exhibit non-Newtonian flow behavior. That the changes of polymer flow properties with flow velocity are a challenge for the description of the polymer flood process since the velocity changes from ultra-high near the injection well to very low further into the reservoir. At high velocities, the polymer behaves as a shear thickening fluid, i.e., the viscosity increases with velocity. This limits the injectivity of the polymer.

High flow rates in porous media impose a high shear on the polymer, which may lead to mechanical degradation. The shear degradation can be both beneficial and detrimental to the polymer flooding process. It is beneficial in the way that it reduces the viscoelasticity of the polymer, and this property is the prime factor for the reduced injectivity. However, it may be detrimental if there is a corresponding loss of shear viscosity, which is the most important factor for oil mobilization deep in the reservoir. The objective of this thesis is to improve the characterization of flow in porous media by mechanically degraded polymer solutions at high and low flow velocities with the aim to find an optimum between improved injectivity and loss of viscosity.

The thesis characterized polymer flow in porous media by performing core flood experiments at different conditions. These conditions were: different degrees of mechanical degradation, variation of polymer molecular weight and concentrations, and variation in porous media properties, i.e., permeability and wettability. The results show that there was an optimal degree of pre-degradation of the HPAM polymer, which reduced the viscoelasticity to significantly improve injectivity and, at the same time, only leads to a small reduction in viscosity. This suggests that mild pre-degradation can be used to improve polymer flood design in field applications. It was also shown that mechanical degradation increased with propagation distance in the porous media. This is contrary to the more common belief that mechanical degradation occurs only at the point of highest shear, but not after subsequent exposure to lower shear. Another key finding is that the presence of oil in the pores greatly reduces the shear thickening, suggesting the injectivity is underestimated in core flood experiments with only water present.