Effect of monoethylene glycol and kinetic hydrate inhibitor on hydrate blockage formation during cold restart operation

Abstract

Hydrate blockage formation in offshore flowlines may induce production stoppage and operational hazards. Previous work suggested the risk of hydrate blockage became highest for water and decane mixture with 60% watercut, however the risk could be alleviated by adding a thermodynamic inhibitor along with the kinetic hydrate inhibitor. This work presents the effect of adding the hydrate inhibitors on hydrate blockage formation during cold restart operation, where the water and decane mixture stayed inside the hydrate formation region without mixing for 10h then executed mixing at constant stirring rates of 200, 400, and 600rpm. Depending on the mixing rate, liquid phase became stratified (200rpm), partial dispersing (400rpm), and full dispersing (600rpm). Without hydrate inhibitors, hydrates formed instantly upon mixing of liquid phase with fast growth rate. The stirrer was eventually stopped due to the formation of hydrate blockage within 13.9, 18.8, and 42.2min for stratified, partial dispersing, and full dispersing liquid phase, respectively. The resistance-to-flow could be estimated from the measurement of torque changes during the hydrate formation. Sever torque spikes were observed for the water and decane mixture without hydrate inhibitors. The hydrate growth rate decreases linearly as a function of hydrate fraction in liquid phase, then it drops upon torque spikes, possibly due to agglomeration and bedding of hydrate particles. Adding 20wt% mono-ethylene glycol (MEG) to the water phase could suppress the torque spikes while hydrate formation proceeds to the final fraction, suggesting MEG may prevent the agglomeration and bedding of hydrate particles for all flow regimes. However its performance was limited at 10wt% MEG concentration. The presence of kinetic hydrate inhibitor, Luvicap, was also found effective to suppress the hydrate formation for 155.0min at mixing rate of 200rpm, however soon lost its efficacy with increasing mixing rate to 400 and 600rpm. These results suggested that the hydrate formation mechanism during cold restart would be highly dictated by the mixing rate and corresponding flow regime, thus appropriate hydrate inhibition strategy must be developed to manage its risk. For 10wt%MEG and 0.1wt% Luvicap solution, hydrate formation initiated in interface between water and decane, then proceeded to gas phase without affecting the resistance-to-flow. The performance of hydrate inhibitors must be evaluated based on relevant data measurement and visual observation to better describe the hydrate formation mechanism.