Inhibition of methane hydrate re-formation in offshore pipelines with a kinetic hydrate inhibitor

Methane gas from marine hydrate deposits can be produced by one or a combination of three methods; depressurization, thermal stimulation, and the injection of hydrate inhibitors. Because residual hydrate structures known as hydrate precursors will exist in the liquid water phase after dissociation, the risk of methane hydrate re-formation has to be evaluated during the production and transportation of methane gas through offshore pipelines. New experimental procedures composed of three stages are designed to simulate the dissociation of marine hydrates and the transportation of produced fluids. The obtained experimental results have shown that methane hydrates are rapidly re-formed when the temperature of dissociated water falls into the hydrate formation region during the cooling down process. The subcooling for three different dissociation pressures of 80, 70, and 60 bar were 1.3, 1.2 and 1.6 °C, respectively. One viable option to avoid hydrate re-formation is injecting hydrate inhibitors before transporting the fluids through pipelines. Among various hydrate inhibitors, Poly(N-vinylcaprolactam) (PVCap) was selected as a possible candidate for a Kinetic Hydrate Inhibitor (KHI) and injected into dissociated water before cooling down the fluids. The concentration of PVCap was varied from 0.5 to 3.0 wt.%. With an increase in the PVCap concentration, the subcooling increased to 7.8 °C at a dissociation pressure of 80 bar, which suggests that the risk of hydrate re-formation can be reduced by injecting PVCap. Moreover it is observed that the subcooling increased to 8.8 °C at the PVCap concentration of 3.0 wt.% in the presence of NaCl in the water phase. Although the use of KHI in conventional gas production has become common, its applicability to methane hydrate production has not yet been studied thoroughly, especially in the presence of residual hydrate structures. In this work, the application of KHI to methane hydrate production is discussed.