Phase equilibria measurement and modelling of petroleum reservoir fluids containing gas hydrate inhibitors and water
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Understanding gas hydrate inhibitor distribution in hydrocarbon phases is essential for the economic design of process equipment. In order to build a clear image of the inhibitor’s distribution in various phases, three experimental investigations were devised; solubility in liquid and vapour phase as well as saturation pressure measurements. These data will contribute significantly to the understanding of the partitioning of these components as the data in the open literature are fairly limited. Aiming at filling the experimental gap found in the literature, the solubility of methane in pure methanol and ethanol as well as 70 and 50 wt% aqueous solutions at 238.15 – 298.15 K and 0.3 – 47 MPa were measured. The data from the ethanol/solution solubility measurements were used to optimise the methane-ethanol Binary Interaction Parameters (BIPs) of the CPA-SRK72 Equation of State (EoS). The model calculations showed an absolute average deviation of 5.3% over the full pure data range. To improve the CPA-SRK72 EoS predictions for aqueous solutions, new methane-ethanol BIPs were regressed showing significant improvement for both solubility and quaternary bubble point predictions. In order to determine the inhibitor loss to the vapour phase, the inhibitor content of methane was measured using Gas Chromatography (GC) between 0.7 – 62 MPa and 273.15 – 298.15 K. Additionally, a number of bubble point measurements were conducted for binary, ternary and quaternary systems containing methane, a liquid hydrocarbon phase (C7 – C12), methanol/ethanol and water. This was to investigate the effect of the inhibitor phase in the ternary, and the dominant excess water phase in the quaternary system, on the bubble point pressure as well as evaluating the CPA-SRK72 predictions. The saturation pressures were measured at 253.15 – 313.15 K. The solubility of CO2 in Mono-ethylene glycol (MEG), Di-ethylene glycol (DEG) and Tri-ethylene glycol (TEG) and their aqueous solutions (90, 60 and 40 wt%), at pressures and temperatures ranging from 0.2 – 43.4 MPa and 263 – 343 K, were measured. The solubility of CO2 in pure MEG, DEG and TEG were predicted using the CPA-SRK72 EoS, using a single binary interaction parameter, showing an absolute average deviation of 5.13%, 9.51% and 2.55% respectively. Correlations for the solubility of CO2 in MEG, DEG and TEG aqueous solutions, using aqueous solution regressed BIPs, showed an overall absolute average deviation of 17.5%, 18.2% and 25.16% respectively, a significant improvement from the non-aqueous solution BIP optimised predictions.