A Study of Polyacrylamide Based Gels Crosslinked With Polyethyleneimine

Abstract

Abstract Organically cross-linked gels have been used to control water production in high temperature applications. Most of these gels consist of a polyacrylamide-based polymer and an organic cross-linker. Polyethyleneimine (PEI) has been used as an organic cross-linker for polyacrylamide tert-butyl acrylate copolymer (PAtBA). Literature reported that PEI can also form ringing gels with polyacrylamide copolymers and simple polyacrylamide homopolymers (PAM). We report a comparative study of two water control gel systems, i.e., polyacrylamide tertiary butyl acrylate copolymer (PAtBA) and polyacrylamide homopolymer (PAM) crosslinked with polyethyleneimine (PEI). Several techniques were used in the present study, including: Gas Chromatography (GC), C13 Nuclear Magnetic Resonance Spectroscopy (C13 NMR) and steady shear viscometry. The gases produced during the reaction, structural changes and gelation time data were all integrated to provide further insights into differences between the two systems. Isobutene was identified at temperatures as low as 60oC. In addition, GC studies revealed for the first time the release of carbon dioxide (CO2) as a product of thermal decomposition of the tBA groups on PAtBA. Lower initial pH values were found to change gelation time of the two systems. Salts were found to increase the gelation time. This paper will summarize these results, and explain the main reaction mechanisms involved. It will also discuss how these new findings will impact application of these gels in the field. Introduction Excessive water production is a serious issue in maturing oil and gas reservoirs. It leads to additional costs in terms of constructing larger Gas Oil Separation Plants (GOSPs). Additionally, scaling, emulsification, bacteria and corrosion problems may arise due to high water cut. The oil industry worldwide spends billions of dollars to address the problem of excess water production every year.1 Various water control techniques exist. They include mechanical and chemical means. Among the available water control techniques, polymer gels have been widely used for lowering water relative permeability while keeping oil relative permeability unaffected through disproportionate permeability reduction (DPR)2–4 or completely block water flow from water producing zones.5 For a given well candidate, several factors including reservoir temperature, lithology and salinity of formation water affect the selection of polymer gel treatment needed. Polyacrylamides and their copolymers have been reported in literature as base polymers for most water control gels. Polymer gels can be classified in two categories, namely, inorganically and organically crosslinked gels, depending upon the crosslinker used. Inorganically crosslinked gels rely mainly on the ionic interaction between the positively charged trivalent cation (like Cr+3 and Al+3) and the negatively charged carboxylates on the partially hydrolyzed polyacrylamide (PHPA).6–8 This class of polymer gels is stable and can be used in reservoirs of low temperatures. When the temperature of the treated water producing zone increases, the effectiveness of inorganically crosslinked gels diminishes considerably due to the weakening of the ionic bonds described earlier.9 At high temperatures, organically crosslinked polymeric gels are preferred.10–15 The covalent bonding between the polymer and the crosslinker render these gels stable at high temperatures.9 Polyethyleneimine (PEI) has been used as the organic crosslinker to form gels with polyacrylamide tert-butyl acrylate (PAtBA)14,16 and mixtures of acrylamide, acrylamide and acrylamido-2-methylpropane sulfonic acid (AMPSA) and N,N-dimethyl acrylamide.17 A recent study by our group has indicated the possibility of crosslinking a simple polyacrylamide homopolymer (PAM) with PEI at high temperatures.18 The gelation kinetics of the PAtBA and PAM crosslinked with PEI was studied in detail at temperatures ranging from 80 to 140oC.18,19Fig. 1 shows the molecular formulas of PAtBA, PAM and PHPA. The crosslinking reaction between PEI and PAM is believed to occur between imine nitrogens on PEI and amide groups on the PAM (transamidation) as shown in Fig.2. On the other hand, the crosslinking of PAtBA with PEI is thought to heavily rely on nucleophillic substitution reactions of the tBA part as described in Fig. 3. Detailed studies on these mechanisms are reported in reference 15.