High pressure phase equilibria measurements and modeling of sour gas hydrates with inhibitors

Experiments were conducted to form mixed H 2 S/CH 4 hydrates with inhibitors to 30 MPa. • Comprehensive evaluations of inhibitors were performed with chlorides and alcohols. • Thermodynamic model was developed and validated, with model limitations identified. • H 2 S is preferentially incorporated into hydrate, which shows promise for sweetening. Sour natural gas containing high percentage of hydrogen sulfide (H 2 S) often experiences severe gas hydrate blockages. Accurate prediction of hydrate formation conditions is essential for flow assurance, yet reliable high-pressure data and robust models for H 2 S/CH 4 hydrates, particularly those containing thermodynamic inhibitors, are limited. This study employed the single-crystal phase boundary method to determine the three-phase equilibrium conditions (H–Lw–V) of H 2 S/CH 4 mixed hydrates in pure water and with inhibitors (NaCl, KCl, CaCl 2 , MgCl 2 , methanol, and ethylene glycol) at pressures of up to 30 MPa. Raman spectroscopy revealed that the H 2 S/CH 4 mixture forms structure I hydrate, with the H 2 S and CH 4 molecules occupying both large and small cages, and their Raman peaks shifting to higher wavenumbers with increasing temperature. We developed and validated a thermodynamic model integrating the PRSV equation of state for the vapor phase, the Chen-Guo model for the hydrate phase, and the Pitzer/UNIQUAC models for the aqueous phase. This model accurately predicted equilibrium pressures for pure water, methanol, KCl, and MgCl₂ solutions, with an absolute average relative deviation of less than 10%. However, it systematically underestimated pressures for NaCl, high-concentration ethylene glycol, and CaCl₂, revealing limitations in aqueous activity predictions. Assessment of inhibitor performance and economic feasibility indicated that methanol provides effective and economical inhibition for sour gas transport at ~10 MPa. This study provides essential high-pressure data and Raman spectroscopic insights and establishes a reliable thermodynamic modeling framework for H 2 S/CH 4 hydrates, supporting flow assurance management, inhibitor selection, and model optimization.