Prevention of methane hydrate re-formation in transport pipeline using thermodynamic and kinetic hydrate inhibitors

Abstract

Methane gas produced from marine hydrate deposits will flow together with dissociated water in two phases, and the risk of hydrate reformation must be managed. This study used the OLGA multiphase flow simulation software to simulate methane transportation with dissociated water through a vertical 2160 m pipeline in order to determine the amount of monoethylene glycol (MEG) as a thermodynamic hydrate inhibitor. When the hydrate saturation ratio in hydrate deposits varies, a large amount of MEG becomes inevitable. Moreover, when undesired water breakthrough occurs, the MEG concentration would decrease to where hydrates may re-form before responding in the offshore platform. A risk management strategy for hydrate reformation is investigated by adopting under-inhibition with MEG and the addition of PVCap as a kinetic hydrate inhibitor (KHI). The experimental results in high pressure autoclave showed that PVCap exhibited a limited performance in delaying the hydrate formation in a high subcooling condition. Considering the vertical flowline of 2160 m, the delay time was not sufficient to prevent the hydrate re-formation in the transport pipeline. Complete prevention of hydrate re-formation can be avoided with 35 wt% MEG concentration, however decreasing the MEG concentration to 20 wt% was also feasible in order to avoid hydrate formation since it was not observed for more than 960 min. In the presence of 0.1 wt% of PVCap and 10 wt% MEG, the hydrate delay time was of about 311.5 min. This delay time is 76% longer than the residence time of methane and water mixture in the transport pipeline, thus this synergistic inhibition can significantly reduce the injection rate of MEG while preventing hydrate formation. This is the first work suggesting a risk management strategy for hydrate reformation in hydrate production system, and thus will provide insights to develop advanced hydrate production technologies.