Fundamental controls on kinetic hydrate inhibitor performance and polymer removal from produced waters

Abstract

Gas hydrate formation is one of the major concerns in the oil and gas industry, posing considerable risks to production operation when it is not controlled. Gas hydrates are traditionally avoided by injecting thermodynamic inhibitors (THIs) such as methanol or MEG, however over the past two decades, in response to economic and HS&E concerns associated with THIs, low dosage “Kinetic Hydrate Inhibitors” (KHIs) have seen increasing use in the industry as an alternative. Although KHIs use is now quite widespread and can offer considerable CAPEX/OPEX benefits, their hydrate inhibition mechanisms are still relatively poorly understood. In this thesis, a novel PVT phase behaviour/ crystal growth inhibition (CGI) method previously developed in-house has been used to study fundamental controls on KHI inhibition mechanisms in terms of gas and aqueous phase composition, pressure, polymer type and presence of other pipeline chemicals. Particular focus has been placed on gas composition, notably acid/sour gases, with results strongly suggesting that cage occupancy patterns play a crucial role in KHI inhibition performance as a function of pressure and presence of CO2 and H2S being a significant factor. In contrast, work on the effect of pH does not suggest pH reduction to be the main contributor to the observed behaviour in system containing CO2/H2S. In addition, extensive studies on KHI-THI mixtures for different KHI polymers in multi-component natural gas systems have revealed a potential synergistic effect of methanol up to a certain concentration, while proving a consistent ‘top-up’ effect for ethylene glycol, opening up options for novel combined KHI-THI inhibition strategies. While KHIs are gaining particular interest, there is the issue of handling/disposal of produced waters with the potential of polymer fouling problems. To address this problem, robust evaluation of a recently developed solvent extraction based polymer removal method shows this to have significant promise. Results also suggest that presence of other pipeline chemicals will not affect the removal effectiveness significantly. Work has also been expanded to examine whether the treatment chemicals themselves might offer a novel means to create “water immiscible KHIs” for certain applications. Results indicate that such a KHI formulation can work well, even though the bulk of the polymer is not in the aqueous phase but in an immiscible organic chemical. The treatment chemical extraction method also opens up options for potential KHI recovery and re-use.