Re-entrant transition of aluminum-crosslinked partially hydrolyzed polyacrylamide in a high salinity solvent by rheology and NMR Abduljelil Sultan Kedir, 1,2 John Georg Seland, 2 Arne Skauge, 1 Tormod Skauge 1 1 Uni Research, Realfagbygget, Centre for Integrated Petroleum Research (CIPR), All egaten 41, Bergen 5007, Norway 2 Department of Chemistry, University of Bergen, Realfagbygget, All egaten 41, Bergen 5007, Norway Correspondence to: A. S. Kedir (E-mail: abduljelil.kedir@uni.no) ABSTRACT: Linked polymer solution (LPS) is a nanoparticle polymer and designed by crosslinking a high molecular weight partially hydrolyzed polyacrylamide (HPAM) with aluminum (III). It has been applied in the oil industry to enhance oil recovery by improv- ing sweep efficiency and by microscopic diversion in porous media. To achieve good propagation properties, aggregates formed by intermolecular crosslinking and gel formation should be avoided. To our knowledge, there is no established method to distinguish between intra- and intermolecular crosslinking for high molecular weight (>10 3 10 6 Da), low concentration (<1000 ppm), polydis- perse solutions of partially hydrolyzed polyacrylamides in high salinity solvents (5 wt % NaCl). The high salinity solvent is relevant to represent for formation water in many oil reservoirs. The main objective of the present study is to establish an experimental method for determining phase transition of LPS from monomeric coiled state to aggregated state in a high salinity solvent. No single experimental methods are conclusive and we have therefore applied a combinatorics approach including two-dimensional NMR, dynamic rheology, and UV spectroscopy. The different techniques show similar trends, which allow overall interpretations of phase transitions to be made. The experimental results indicated that the LPS solution at high salinity solvent underwent a phase transition by chain re-expansion, called reentrant transition. The transition point was observed at addition of 100 ppm of Al 31 . Higher concen- trations of Al 31 suppressed the rate of reentrant transition, most likely because of intramolecular crosslinking of HPAM chains by Al 31 . Intermolecular crosslinking reaction was not observed at these conditions. V C 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43825. KEYWORDS: nanostructured polymers; oil and gas; spectroscopy; supramolecular structures; viscosity and viscoelasticity Received 28 January 2016; accepted 25 April 2016 DOI: 10.1002/app.43825 INTRODUCTION Interest in nanoparticle applications has been stimulated in the oil and gas industry in recent years. One of the applications is to use nanoparticles as flooding agents to enhance oil recovery. Linked polymer solution (LPS) is one of the nanoparticles (hydrodynamic radius 150 nm) that have been used as flood- ing agent to enhance oil recovery. 1 It is formulated by low con- centration of partially hydrolyzed polyacrylamide (HPAM) and aluminum citrate (AlCit). Injection of LPS into the porous media may improve oil recovery by increase the solution viscos- ity (improve sweep efficiency) and by temporary blocking of narrow pores (microscopic diversion). LPS has been imple- mented successfully in onshore fields in China. 2–5 Laboratory core floods experiments have shown increased oil recovery of 20–60%. 1 However, to improve application of LPS at various reservoir temperatures and formation water salinities, more detailed knowledge is needed with regard to conformational state and phase transitions of these nanoparticles. In this paper we limit the term “phase transition” to describe molecular chain expansion and contraction of the polymer. LPS experiences con- formational changes depending on polymer concentration, crosslinker concentration, salinity, pH, and temperature. Partially hydrolyzed polyacrylamide (HPAM) is polyelectrolyte macromolecule with flexible chain and good solubility in water. In deionized water, HPAM exhibit random and expanded con- formational state because of electrostatic repulsion between charged monomer units. However, in the presence of counter- ions the charge density of HPAM can be reduced as result of ion–ion and ion–dipole interactions. This interplay modifies the electrostatic interactions and restricts the randomness and flexi- bility of polymer chains. The degree of chain flexibility depends not only on electrostatic interactions, but also the counterion Additional Supporting Information may be found in the online version of this article. V C 2016 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM J. APPL. POLYM. SCI. 2016, DOI: 10.1002/APP.43825 43825 (1 of 11)