High-efficiency natural-gas storage method involving formation of gas hydrate in water/oil-cyclopentane emulsion

Abstract This study proposes a high-efficiency natural gas hydrate (NGH) storage method involving the formation of NGH in a water/oil emulsion with the addition of suitable thermodynamic additives. First, the CH4 hydrate formation conditions in a water/diesel emulsion with the addition of hydrate thermodynamic additives, including tetrabutylammonium bromide (TBAB), tetrahydrofuran (THF), and cyclopentane (CP), were measured for the first time. Interestingly, in contrast to the case of the aqueous solution, both the TBAB and THF could not promote the formation of CH4 hydrate in the water/diesel emulsion, and they exhibited a hydration inhibition effect. The CP substantially reduced the CH4 hydrate formation pressure in the water/diesel emulsion, and the resulting hydrate/diesel slurry exhibited good fluidity. The CH4 storage characteristics of the water/diesel-CP emulsion under hydrate formation conditions were systematically studied. At 276.15 K and 6 MPa, for a water/diesel-CP emulsion containing 30 wt% water cut, the CH4 storage volume (S) per unit volume water in the emulsion reached 161 m3/m3, which was significantly higher than that for pure water, a water-CP solution, and a single water/diesel-CP emulsion. S increased to 171 when the water cut decreased to 20 wt%; the entire experimental process lasted 90 min. An increase in the temperature from 274.15 to 282.15 K had a limited influence on S, indicating the wide range of application conditions of the water/diesel-CP emulsion for the production of NGH crystals. Furthermore, in situ particle video microscope observations and focused beam reflectance measurements confirmed the good flowability of the formed CH4 hydrate/diesel slurry. Continuous production of NGHs can theoretically be achieved using the water/diesel-CP emulsion.