Polymer induced permeability reduction: The influence of polymer retention and porous medium properties

Abstract

Polymer injection is broadly applied for enhanced oil recovery (EOR) in high crude oil viscosity and heterogeneous reservoirs. Although polymer flooding is very effective to decrease the mobility ratio towards favorable values and improve sweep efficiency, some side effects have to be considered including the risk of a severe polymer induced permeability reduction (PIPR) resulting from polymer/rock interaction either near the wellbore of in-depth of the reservoir. Many factors affect the degree of PIPR such as rock mineralogy, oil saturation, temperature, formation water salinity, rock pore structure, polymer type, polymer molecular weight, shear rates in the porous medium, and quality of the injection water (solid particles and oil droplets content). Accurate assessment of PIPR is very important for polymer selection to define the relative merits of polymer flooding for a certain reservoir and avoid polymer injectivity loss during field operations. A severe PIPR can be due to excessive polymer retention inside the reservoir rock, fluid incompatibility of polymer with formation fluids and rocks, clogging of pore throats (accumulation of large polymer molecules at the pore throat), shear thickening due to high shear rate in the near-wellbore area, Improper preparation of the polymer solution, and bad quality of the injection brine used to prepare the polymer solution.In this paper, coreflooding as an assessment technique for PIPR is reviewed and a new interpretation technique is presented to correlated the polymer retention to permeability reduction measured from coreflooding experiments. Coupling the polymer retention and rock quality index was very effective in matching the permeability reduction for data presented in the literature for sandstone and carbonate rock samples.PIPR should be considered as an important design characteristic during any polymer selection workflow. Although different techniques are presented earlier, most of them lack the ability for upscaling. An accurate assessment of PIPR using experimental data permits critical evaluation of polymer flow deeper in the reservoir and prediction of any injectivity problems as well as improving project design.