RIASSUNTO
Abstract
A new thickener based on associative properties and its application in polymer flooding is discussed. The new thickener is an anionic, water-soluble copolymer containing pendant associative groups. These associative groups are based on a novel chemistry (patent application filed).
The viscosity of the new copolymer shows superior behavior compared to existing technologies like partially hydrolyzed polyacrylamide (PHPA), especially at elevated temperature and in presence of salt respectively divalent ions. A viscosity of 33 mPas at 60°C could be obtained by adding 900 ppm of the new copolymer to synthetic sea water. In comparison, with a high molecular weight standard partially hydrolyzed polyacrylamide (PHPA) a value of only 3 mPas was reached.
Moreover, the new polymer shows good resistance to shear during injection.
Core flood experiments were run to prove that the new associative thickening polymer is applicable in the field. These data show injectivity into a 2 Darcy core. Moreover the apparent viscosity of the polymer flood in the core outperforms the values found with commonly used polymers by far. The residual resistance factor indicates low adsorption onto sandstone.
Introduction
The use of polymers for enhanced oil recovery as water mobility control agents in polymer flooding is state of the art. Typically anionic, synthetic copolymers are used for this application. Most common is the use of copolymers of acrylamide and acrylic acid. These copolymers usually have a very high molecular weight in the range of up to 25 million g/mol. It is essential for this chemistry to have a high molecular weight in order to achieve the highest possible thickening efficiency per mass unit of polymer. The use of these acryl amide/acrylic acid copolymers - also known as partially hydrolyzed polyacrylamides (PHPA) - in field projects is documented in several papers (Koning et al. 1988, Putz et al. 1994, Dong et al. 2008, Morel et al. 2010).
Partially Hydrolyzed Polyacrylamide (PHPA)
PHPAs are cost-efficient thickeners for aqueous solutions. These polymers reach high viscosities in fresh water. Under these conditions the polymer backbone is stretched, because the negative charges of the acrylic acid moieties repel each other. This leads to a high viscosity yield. However, as soon as salt is present the negative charges are masked by cations from the salt. The Coulomb repulsion between the negative charges becomes less effective and the hydrodynamic radius of the polymer decreases. In other words, the polymer chains are now in a coiled state and not in a stretched form anymore. This leads to a lower viscosity of the polymer solution.
The presence of divalent ions may also cause precipitation. If the acryl amide moieties in the copolymer are hydrolyzed to acrylic acid - e.g. at elevated temperature or under basic conditions - more negative charges are formed. As soon as the amount of acrylic acid in the copolymer exceeds a certain level the polymer will form charge complexes with divalent cations like calcium and magnesium. These complexes are not soluble in water anymore and will precipitate from the solution. This precipitate can block the formation channels and additionally the viscosity of the polymer solution will drop heavily.
This effect can be reduced to some extent by incorporating certain functionalities into the copolymer like sulfonate or sulfate moieties. These functionalities coordinate with the divalent cations from the salt to a much lower extent. So the polymer chain stays soluble and these so-called sulfonated polyacrylamides (SPAM) are not precipitating anymore, although their thickening properties are still significantly reduced due to the charge screening.