Kinetic and Thermodynamic Influence of NaCl on Methane Hydrate in an Oil-Dominated System

This experimental study reports the kinetic and thermodynamic inhibition influence of sodium chloride (NaCl) on methane (CH4) hydrate in an oil-dominated system. To thoroughly examine the inhibition effect of NaCl on CH4 hydrate formation, kinetically by the induction time and relative inhibition performance and thermodynamically by the hydrate liquid-vapor equilibrium (HLwVE) curve, enthalpy (ΔHdiss) and suppression temperature are used to measure the NaCl inhibition performance through this experimental study. All kinetic experiments are performed at a concentration of 1 wt % under a pressure and temperature of 8 MPa and 274.15K, respectively, whereby for the thermodynamic study, the concentration was 3 wt % by using the isochoric T-cycle technique at the selected range of pressures and temperatures of 4.0-9.0 MPa and 276.5-286.0K, respectively; both studies were conducted using a high-pressure reactor cell. Results show that kinetically, NaCl offers slightly to no inhibition in both systems with/without oil; however, the presence of drilling oil contributes positively by increasing the induction time; thermodynamically, NaCl contributes significantly in shifting the equilibrium curve to higher pressures and lower temperatures in both systems. In the oil system, the contribution of the THI to the equilibrium curve increases the pressure with a range of 0.04-0.15 MPa and reduces the temperature with a range of 1-3 K, which is due to the NaCl presence in the systems that reduces the activity of water molecules by increasing the ionic strength of the solution. At a high pressure of 9 MPa, the NaCl inhibition performance was greater than that at lower pressures <5.5 MPa because, at the high pressure, NaCl increases the activity of water, which means that more water molecules are available to form hydrate cages around gas molecules.