Polymer Flooding Definition / Meaning
Polymer flooding is a chemical enhanced oil recovery method that thickens injection water with a water-soluble polymer to improve the efficiency of a waterflood. Raising the viscosity of the injected phase lowers its mobility, produces a more stable displacement front, and sweeps a larger fraction of the reservoir than water alone. It is the most widely applied chemical EOR technique, and it is most valuable in reservoirs with viscous oil or marked permeability contrast.
How Polymer Flooding Works
The controlling parameter is the mobility ratio, the mobility of the displacing fluid divided by that of the displaced oil. Where this ratio is unfavourable, injected water fingers through the oil and channels to the producers, leaving large volumes unswept regardless of how much water is injected. Adding polymer thickens the water and lowers its mobility, pushing the ratio toward or below unity so the flood advances as a coherent front.
A second effect reinforces the first. Polymer adsorbs onto the rock and reduces its permeability to water — the residual resistance factor — without equally reducing permeability to oil. The result is improvement in both areal and vertical sweep efficiency. Polymer flooding does not reduce residual oil saturation in the swept zone; it enlarges the swept zone. That distinction matters, because it sets what the method can and cannot deliver.
Key Polymers Used
| Polymer Type | Common Examples | Key Characteristics | Typical Concentration |
|---|---|---|---|
| Synthetic polymers | Partially hydrolysed polyacrylamide (HPAM) | High viscosity per unit cost; sensitive to salinity, hardness and temperature; degrades under shear | 500 to 2,000 ppm |
| Biopolymers | Xanthan gum, schizophyllan | Rigid structure gives tolerance to salinity and shear; vulnerable to bacterial attack; higher cost | 500 to 1,500 ppm |
| Modified synthetics | ATBS-containing copolymers | Engineered for higher temperature and harder brine than conventional HPAM | Varies with conditions |
Screening Criteria
- Oil viscosity: generally favourable from roughly 10 up to several hundred centipoise. Below that, waterflood already sweeps well; far above it, thermal methods usually win.
- Reservoir temperature: HPAM hydrolyses and loses viscosity as temperature rises, particularly in hard brine. Temperature is often the binding constraint.
- Salinity and hardness: divalent ions collapse HPAM coils and destroy viscosity, which is why make-up water chemistry is a design decision rather than a detail.
- Permeability: must be high enough for the thickened fluid to be injected at economic rate without fracturing the formation.
- Heterogeneity: moderate permeability contrast is the sweet spot. Severe fracturing or thief zones channel polymer as readily as water and waste it.
Polymer Degradation
Retaining viscosity from the mix plant to the reservoir is the central engineering problem. Polymer is degraded mechanically by shear through pumps, chokes and perforations; thermally at reservoir temperature over time; chemically by oxygen and iron, which drive free-radical attack; and biologically in the case of biopolymers. Facility design therefore favours low-shear pumps and careful oxygen exclusion, and injection quality is monitored continuously, since a solution that has lost its viscosity is simply expensive water.
Usage Example
“The pilot injected HPAM at 1,200 ppm and cut water cut from 92 to 78 percent over eight months. Viscosity at the sandface came in below design, and the review traced it to shear across the injection chokes rather than to thermal degradation.”
Field Implementation and Economics
Polymer flooding requires a mixing and hydration plant, filtration, and injection facilities built for viscous fluid, and it demands enough remaining oil in place to justify that outlay. Chemical cost scales directly with the volume injected, so the economics turn on polymer consumed per incremental barrel rather than on recovery factor alone. Response is measured in months to years, which makes the method a poor fit for short licence tails. Large-scale applications at Daqing in China and Mangala in India, and extensive use in Canadian heavy oil, have established it as a mature technique rather than an experimental one. See enhanced oil recovery (EOR).