Anton P. Semenov

Study of the Effect of the Degree of Overcooling During the Formation of Hydrates of a Methane-Propane Gas Mixture on the Equilibrium Conditions of Their Decomposition

A study is made of the effect of the initial degree of supercooling on the equilibrium conditions of decomposition of hydrates of a model gas mixture comprised of 95.66 mol % CH4 + 4.34 mol. % C3H8. To ensure a high degree of conversion of water to hydrate, 0.1 wt. % sodium dodecyl sulfate (SDS) solution was used as the liquid phase in experiments performed in an RCS6 Sapphire Rocking Cell instrument. The hydrates were produced by cooling under isochoric conditions in a GHA350 gas hydrate autoclave and under isochoric-isothermic conditions on the RCS6 instrument. The hydrated samples that were obtained were decomposed by heating under isochoric conditions at the rate of 0.2 K/h. Analysis of the decomposition curves indicates that, depending upon the initial conditions, both mixed methane-propane hydrates of various compositions and methane hydrate are formed when the test gas mixture is hydrated. It is demonstrated that the proportion of the CH4 hydrate which is formed increases with an increase in the initial degree of overcooling during hydrate formation. The curve depicting the equilibrium hydrate decomposition conditions for the gas mixture has a more complex configuration than the simple exponential curve proposed earlier. The data that is obtained points to the existence of a multiplicity of equilibrium conditions for the hydrate decomposition of gas mixtures, with the exact conditions in a specific case depending on the degree of overcooling of the system during hydrate formation.

Kinetic Inhibition of Hydrate Formation by Polymeric Reagents: Effect of Pressure and Structure of Gas Hydrates

Kinetic inhibition of formation of methane hydrate (CS-I) and methane-propane (CH4 + C3H8 in 95.66 + 4.34 mole %) hydrate (CS-II) by the polymeric reagents Luvicap 55W and Luvicap EG is studied in the 40-120 bar pressure range. Cooling at the constant rate of 1°C/hr was used to assess the effectiveness of kinetic inhibition. It is shown that the kinetic hydrate formation inhibitors (KHI) Luvicap 55W and Luvicap EG in identical proportion of 5000 ppm are capable of inhibiting methane hydrate formation at a supercooling temperature twice as low (6-7°C) as in the case of hydrates of methane-propane mixture (13-14°C). In the presence of KHI, hydrates appear in the system in the form of visually discernible opacity of the initially transparent aqueous solution at a temperature that is 1-2°C higher than the temperature at the point of deviation of the P(T) curve from the straight line, i.e., they appear earlier than appearance of signs of gas absorption. Formation of such trace quantities of hydrate do not cause a marked deviation of the P(T) curve from the straight line and can be discerned only by more sensitive physicochemical methods. The inhibiting properties of Luvicap EG and Luvicap 55W with respect to methane hydrate differ insignificantly, but the former is more effective in inhibiting crystal growth. The experimental data indicate that Luvicap 55W is more effective than Luvicap EG in inhibiting nucleation and growth of methane-propane hydrate crystals.

Inhibiting Gas Hydrate Formation by Polymer–Monoethylene Glycol Mixture

Inhibition of formation of methane hydrate with cubic structure CS-I and methane-propane (95.66 CH4 + 4.34 C3H8 mole %) hydrate with cubic structure CS-II by isothermal method and method of cooling at the constant rate of 2°C/h, using 0.5% of a kinetic inhibitor (KIH) + 20.8% of the thermodynamic inhibitor (TIH) monoethylene glycol (MEG) is studied. It is shown that the synergic effect of increase in inhibiting capacity of a polymeric kinetic inhibitor (KIH) in the presence of 20.8% of MEG (TIH) is observed in the case of both methane hydrate and methane-propane hydrate inhibition. The synergy manifests itself in the form of increase in supercooling degree by 2.5-3°C that is attained in the KIH + TIH system before the initiation of hydrate formation as compared to a system that contains no TIH (MEG). The induction time is shown to depend on the degree of supercooling in the system while inhibiting CS-1 and CS-II hydrates with 0.5% KIH + 20.8% MEG. The obtained data indicate that KIH + MEG antihydrate reagents can be used to inhibit formation of technogenous gas hydrates at < 0C temperatures.

Polymer–Methanol Combines Inhibition of Gas Hydrate Formation

A study was carried out on combined inhibition by a solution containing 0.5% polymer kinetic inhibitor (KI) + 10.0% methanol as a thermodynamic inhibitor (TI) in the formation of methane hydrate (Class I) and the hydrate of 95.66 mol. % CH4 + 4.34 mol.% C3H8 (methanepropane mixture) (Class II). This combined inhibition was studied by an isothermal method and a method entailing cooling at a constant rate using a GHA350 autoclave. Methane was shown to have an adverse effect on the inhibiting properties of the polymer KI both relative to formation of methane hydrate and hydrates of C1–C3 hydrocarbons. The loss of inhibition by the polymer KI in the presence of methanol is expressed as the decrease in the extent of supercooling attainable in the system without adsorption of the hydrate-forming gas (1.5–2.5°C in comparison with the system without TI). The induction time is shown to depend on the extent of supercooling in the system during inhibition of formation of the Class I and Class II hydrates by the solution containing 0.5% KI + 10.0% TI.

Effectiveness of Hydrate-Formation Inhibitors Evaluated by a Polythermal Method

The effectiveness of hydrate-formation inhibitors is evaluated to select the best reagents for preventing hydrate formation in gas wells that are active-gas donors for conditions in the eastern part of Orenburg oil and gas condensate fields. A polythermal method that involves cooling at a constant rate of 1°C/h a system consisting of an inhibitor dissolved in simulated strata water and a gas mixture was used for the study. The inhibiting properties of the samples were characterized from the initial hydrate-formation temperature and the amount of gas turned into hydrate at the end of the test.

Effect of Pressure on the Effectiveness of the Kinetic Inhibition of Hydrate Formation by Polymer Reagents

We have studied the inhibiting properties of the reagents Luvicap 55W and Luvicap EG in the process of KC-I methane hydrate formation in the pressure range 60-120 bar. In order to evaluate the effectiveness of the kinetic inhibition, we used the method of cooling at a constant rate of 1 degree/hour. We established that both in a system containing the kinetic inhibitor and in a system without the inhibitor, the initial pressure does not affect the maximum degree of supercooling at which methane hydrate is still not formed. The reagents Luvicap 55W and Luvicap EG at a concentration of 0.5 wt.% can effectively inhibit methane hydrate formation for a degree of supercooling no greater than 6°C-7°C. For inhibition of KC-I hydrate formation for a greater degree of supercooling, the reagents Luvicap 55W and Luvicap EG should be used in a higher concentration or combined with appropriate thermodynamic inhibitors.