Effects of sodium hydroxide and calcium hydroxide on the phase equilibria of methane hydrates

Abstract

Alkalis such as sodium hydroxide (NaOH) and calcium hydroxide [Ca(OH)2] find lots of applications in oil and gas industry where formation of methane hydrates (and resulting flow hindrance) is a common occurrence. While many hydrate inhibitors have been identified to maintain flow assurance, effects of these alkalis on the phase stability of methane hydrate have not yet been explored in detail. In this study, the phase behaviour of methane hydrate has been investigated in the presence of aqueous solutions of NaOH and Ca(OH)2 with varying concentrations of 0.005, 0.01, 0.02, and 0.04 mass fractions in a high pressure stirred tank reactor. The phase equilibrium of methane hydrate in aqueous alkali solutions has been generated in the pressure and temperature ranges of 4.27–7.90 MPa and 276.60–283.22 K, respectively. Both the alkalis have found to exhibit inhibition of methane hydrate formation, and the inhibition effect becomes more pronounced at higher concentrations of the alkalis, with NaOH performing as a better inhibitor than Ca(OH)2. A plausible mechanism for the same has also been discussed. The heat of dissociation has been calculated to show that the presence of the alkalis does not influence the structure (sI) of methane hydrate. In addition, a simple model based on the van der Waals-Platteeuw (vdW-P) thermodynamic model consisting of single parameter has been used to predict the phase equilibrium of methane hydrate in the presence of aqueous alkali solutions. The absolute average relative deviations (AARD%) in pressure for the model were measured for the 40 experimental data points generated in this work. The predictions are well within 4 % of the experimental values, which suggests that the model can adequately predict the methane hydrate phase stability in the presence of alkalis. These alkalis are used in various operations during exploration and production of oil and gas. This study therefore will assist in designing an effective and economic thermodynamic inhibitor for methane hydrate based on alkalis for the oil and gas industry.