RIASSUNTO
Abstract
Commercial polymer floods have generally used partially hydrolyzed polyacrylamide (HPAM) as the polymer of choice, since HPAM generates high viscosities in relatively fresh brines at a reasonable cost. In high salinity or hardness brines, the anionic charges on the HPAM polymer chain interact with the dissolved cations, shrinking the hydrodynamic size of the polymer molecules. The smaller hydrodynamic radius generates lower viscosities and requires higher polymer concentrations. In order to reduce the polymer loading in saline brines, a series of hydrophobically modified biopolymers have been synthesized. The non-ionic nature of hydroxyethyl cellulose (HEC) provides an ideal polymer back bone since it does not interact with the dissolve cations. Hydrophobic groups (aliphatic hydrocarbon chains) are attached to the backbone of the polymer to impart associating properties between polymer molecules. The inter-molecular interaction between the hydrophobic moieties led to a significant viscosity increase in comparison to polymers without hydrophobic groups. Several screening steps were used throughout the product development to arrive at practical polymer formulations: 1) Filtration; 2) Sandpack tests; 3) Corefloods in the presence of oil. By varying molecular weight of the base HEC polymer and the amount of hydrophobic content, practical polymer products were developed for polymer flood applications. Coreflood tests showed that the hydrophobically modified HEC generated significant resistance factor in saline brines, outperforming the HPAM polymers.
Introduction
The poor sweep efficiency during waterflooding heavy oil leads to oil being produced at very high watercuts and large scale produced water recycling. The addition of water-soluble polymer in the injection brine generates higher viscosity in the displacing phase, reduces the water/oil mobility ratio, improves the sweep efficiency and accelerates displacement1. In recent years polymer flooding has been successfully implemented in western Canada for heavy oil reservoirs with oil viscosity up to 6,000mPa.s.
Partially hydrolyzed polyacrylamide (HPAM) is generally used as the polymer of choice in commercial polymer floods. Because of the anionic charges along the polymer chain, HPAM molecules are very sensitive to the ionic strength of the aqueous phase. In relatively fresh brine, the HPAM molecules are more extended thus generate high viscosity. In brines of high salinity and/or hardness, the anionic charges on the HPAM polymer chain interact with the dissolved cations and change the polymer molecules to more coiled conformation. The reduced hydrodynamic size of the polymer molecules generates lower viscosities and required higher polymer concentration in order to reach a target viscosity.
In order to reduce the polymer loading in saline brines, a series of hydrophobically modified biopolymers have been synthesized. The non-ionic nature of hydroxyethyl cellulose (HEC) provides an ideal polymer back bone for salt tolerance. Hydrophobic groups (aliphatic hydrocarbon chains) are introduced to the backbone of the polymer to impart associating properties between polymer molecules. The inter-molecular interaction between the hydrophobic moieties led to a significant viscosity increase in comparison to polymers without hydrophobic groups.