Synthesis of Functionalised Polysaccharides for Hydraulic Fracturing Applications

Abstract

This thesis involves the chemical modifications of the highly reactive primary hydroxyl groups of 2-hydroxyethyl cellulose (HEC) to give rise to possible reaction sites with group(IV) metal-based crosslinkers, leading to the formation of viscous gel materials and possible applications in hydraulic fracturing. This is to offer an alternative to guar, a popular crosslinkable polysaccharide that is obtained from natural products and used routinely in fracturing applications, as its availability is often cast in doubt from year to year due to fluctuating crop harvests.

In Chapter 1, a general introduction to hydraulic fracturing is provided, including details on the types of polysaccharides, crosslinking agents and chemical additives utilised in fracturing fluids, along with possible mechanisms of crosslinking. This chapter also introduces Click chemistry and ring opening polymerisation (ROP) techniques used in this work.

Chapter 2 describes the use of trehalose, a disaccharide molecule, as a model compound for HEC as it has acomparable core skeleton structure, reactivity and solubility properties. The primary hydroxyl groups were converted to thiol groups via a three-step process. Each step was fully characterised by 1D and 2D NMR spectroscopy, and FT-IR spectroscopy. The thiolated trehalose was then utilised in a UV mediated thiol-ene Click reaction with a catechol-containing norbornene compound.

In Chapter 3, HEC is modified with small carbonyl-containing molecules via acid catalysed condensation reactions to impart diol or catecholic end group functionalities. The surface esterification of HEC via the ring opening of succinic anhydride using DMAP as a catalyst is also performed, leading to acid end group functionality. These materials were characterised using inverse-gated 13C NMR spectroscopy and FT-IR spectroscopy.

Chapter 4 describes the synthetic methods used to impart thiol functionality to HEC. This was initially based on the knowledge gained from Chapter 2, but ultimately was found to be unsuccessful when applied to HEC. However, an alternate one-pot synthetic route utilising triphenyl phosphine, carbon tetrabromide and a sulfur nucleophile of either sodium thiobenzoate or potassium thioacetate was successfully used. Thiolated HEC, with approximately 15% of the primary hydroxyl groups converted to SH, was then reacted with the aforementioned norbornene compound in a UV mediated thiol-ene Click reaction. Inverse-gated 13C NMR, DEPT 13C NMR and FT-IR spectroscopy were useful characterisation methods.

Chapter 5 investigates the utilisation of HEC as a macroinitiator in the anionic ring opening polymerisation (ROP) of the latent AB2 monomer glycidol. This will impart numerous hydroxyl groups within close proximity to each other, increasing the likelihood of participating in crosslinking reactions with group(IV) metal ions. The graft copolymers were purified by dialysis, then characterised by inverse-gated 13C NMR and DEPT 13C NMR spectroscopy, FT-IR spectroscopy, TGA and SEC. HEC-g-polyglycerol was then also utilised as a macroinitiator in the tin(II) ethylhexanoate-catalysed ROP of ε-caprolactone, to afford the novel graft copolymers HEC-g-polyglycerol-g-PCLn, where n = 10, 50, 100 or 250. Upon grafting, the hydroxyl end groups of the flexible PCL chains will be further away from the HEC backbone, thus reducing steric hindrance and increasing the likelihood of interacting with group(IV) metal ions of crosslinking agents. The two graft copolymers with the smallest PCL chains retained their water solubility, whilst the longer PCL chain graft copolymers were found to be hydrophobic. All of the PCL containing graft copolymers were fully characterised using 1D and 2D NMR spectroscopy, SEC, TGA, DSC and FT-IR spectroscopy. AFM analysis also determined the structural architectures of these graft copolymers, with a capillary-like network afforded when n = 10, and larger globules afforded when n = 250. It was calculated that each globule actually represents one individual macromolecule.

Chapter 6 describes crosslinking reactions carried out with guar and functionalised HEC materials. Triethanolamine-based zirconium crosslinkers containing different molar equivalents of water added during synthesis were shown to afford different delay times in the gelation of aqueous guar solutions. Crosslinking efficacy was shown to be dependent on the age of the crosslinker, with crosslinkers aged seven months affording much longer delay times. The six-coordinate alkanolamine ligand analogues N,N,N’,N’-tetrakis(2-hydroxyethyl)ethylenediamine (THEED) and N,N,N’,N’-tetrakis(2-hydroxypropyl)ethylenediamine (THPED) were used to synthesise the complexes [Ti(THEED)]2, [Zr(THEED)]2 and [Zr(THPED)]2. These complexes afforded no crosslinking ability with guar. The THEED-containing complexes were successfully recrystallised and single crystal X-ray diffraction crystallography was used to determine the structures of the complexes. These were found to be seven-coordinate dimers, with a binucleating oxygen atom from each ligand acting as a bridge to form a central M2O2 ring. ESI-MS data also helped confirm this. The [Ti(THEED)]2 complex was found to be a chiral species showing signs of disorder, whilst the [Zr(THEED)]2 complex was found to be centrosymmetric with a planar M2O2 ring. The structure of the [Zr(THPED)]2 complex could not be identified by X-ray crystallographic methods as single crystals could not be isolated, however, ESI-MS evidence supports it also being a seven-coordinate dimer. Aqueous solutions of the graft copolymers HEC-g-polyglycerol, HEC-g-polyglycerol-g-PCL10, HEC-g-polyglycerol-g-PCL50 and the functionalised polymer succinylated HEC were evaluated for their crosslinking abilities with a triethanolamine zirconate crosslinker and a α-hydroxycarboxylic acid zirconate crosslinker. All of the graft copolymers showed no signs of gelation with either crosslinking agent. Succinylated HEC showed no signs of gelation with the triethanolamine zirconate crosslinker. However, gelation of succinylated HEC was induced by the addition of α-hydroxycarboxylic acid zirconate crosslinker. Unfortunately, this was with a 3.0 wt.% polymer concentration, which is greater than the limits utilised in industrial fracturing applications.

In Chapter 7, general conclusions and future perspectives for the work are discussed.