Risk Management of Hydrate Reformation Using Synergistic Inhibition During Methane Hydrate Production

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. A risk management strategy for hydrate reformation is investigated by adopting under-inhibition with MEG along with the analysis of multiphase flow in the pipeline. The commercial software, OLGA, was used to predict the flow behavior of gas-water mixture as well as to estimate the amount of MEG needed to completely avoid the formation of a hydrate blockage. To understand the hydrate delay time in the presence of MEG, several experimental sets was carried out. The experimental results showed that MEG exhibited good performance in delaying the hydrate reformation in subsea flowlines. Considering the vertical flowline of 2160 m, the required delay time was 177.5 min to prevent the hydrate reformation during the transportation of gas-water mixture. Complete prevention of hydrate reformation can achieved with 35 wt% MEG concentration. 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. The risk of hydrate reformation also significantly increased when an undesired water breakthrough occurs. The MEG concentration becomes less than 10.0 wt% within 30 min. However, the addition of 0.1 wt% PVCap can be effective to delay the hydrate reformation when the MEG concentration is 10 wt%. The hydrate delay time from the synergistic inhibition will provide precious time for the transient operation at the offshore production platform.