Effect of Elasticity on Displacement Efficiency: High-Concentration-Polymer Flooding
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 115315, "The Effect of Elasticity on Displacement Efficiency in the Laboratory and Results of High- Concentration-Polymer Flooding in the Field," by Haifeng Jiang and Wenxiang Wu, SPE, Daqing Petroleum Institute, and Demin Wang, SPE, Yeming Zeng, Shiguang Zhao, and Jun Nie, Daqing Oilfield Company, prepared for the 2008 SPE Annual Technical Conference and Exhibition, Denver, 21-24 September. The paper has not been peer reviewed.
New studies show that when the pressure gradient and interfacial tension (IFT) remain constant, viscoelastic-polymer flooding can achieve higher displacement efficiency (DE). Laboratory flooding experiments on heterogeneous cores with high-concentration-polymer fluid showed that the higher the concentration and viscosity of the driving fluid, the higher the DE.

Introduction
The most commonly accepted mechanism for polymer flooding is that the DE is determined by the viscous pressure gradient and the retention force on residual oil. The former is proportional to the viscosity, μ, and velocity, v, of the driving fluid, and the latter is proportional to the IFT, σ. The ratio is defined as capillary number, Nc=μv/σ; therefore, the Nc determines DE. In polymer flooding, the IFT between the polymer fluid and crude oil is near that between water and crude oil, and, for certain reservoir systems, the overall pressure gradient can be increased only slightly; therefore, the Nc of polymer flooding is near that of waterflooding. According to the commonly accepted mechanism, the DE for polymer flooding should be the same as that for waterflooding.
However, many test results in the laboratory and field show that the DE of polymer flooding is higher than that of waterflooding and that of viscous-fluid (e.g., glycerin) flooding. The theory that Nc determines the DE cannot explain the DE increase after polymer flooding. Because the Nc is the ratio of overall driving force to the retention force and because driving force is a viscous force, which reflects only the effect of the driving-fluid viscosity, the Nc theory is accurate in predicting the results of Newtonian-fluid flooding. For certain fluid/rock systems, the increase in viscosity cannot increase the DE when the overall pressure gradient remains constant. However, in viscoelastic-polymer flooding, the overall pressure gradient cannot explain the increase in DE, so the increase in DE is caused only by forces that do not change the overall pressure gradient (i.e., unrelated to viscosity), and these forces should be associated with the elasticity of the driving fluids.

Microforce
Visualization coreflood tests show that, when the oil saturation is high in viscous and viscoelastic-fluid flooding, oil blobs are pushed by the pressure gradient in pores, or become residual oil as the oil saturation decreases. However, in viscoelastic-fluid flooding, the residual-oil blob (oil droplet, film, or column) will have protruding portions that form mobile oil under driving forces. This phenomenon cannot be seen in viscous-fluid flooding, therefore, the driving forces in viscoelastic-fluid flooding are different from those in viscous-fluid flooding.