(201a) Hydrate Risk Management during Cold Restart Operation Using MEG and Khi

Authors: 
Seo, Y., Seould National University
Park, K. H., Seoul National University
Hydrate blockage formation in offshore flowlines may induce production stoppage and operational hazards. Although there have been numerous application of KHIs and AAs in offshore oil and gas fields, MEG is still a reliable solution to avoid hydrate blockage formation in offshore flowlines. The required concentration of MEG in aqueous phase to avoid hydrate blockage may be more than 50 wt% in deep water or arctic oil and gas fields, therefore a reduction in the MEG concentration can result in significant investment saving. Our previous works suggested that the growth rate of hydrates can be controlled in the MEG concentration lower that the value required to fully avoid the hydrate formation, so called under-inhibition with MEG.

This work investigates the effect of adding the hydrate inhibitors on hydrate blockage formation during cold restart operation while mixing the fluids at constant stirring rates of 200, 400, and 600 rpm. Depending on the mixing rate, liquid phase became stratified (200 rpm), partial dispersing (400 rpm), and full dispersing (600 rpm). A high pressure autoclave was used in this study, which is equipped with a magnetic stirrer coupling and a two-blade impeller. A total liquid volume of 200 mL was loaded into the autoclave cell which had an internal volume of 510 mL. The cell was immersed in a temperature-controlled liquid bath connected to an external refrigerated heater. A platinum resistance thermometer monitored the temperature of the liquid phase inside of the autoclave with an uncertainty of 0.15 oC. The pressure was measured by a pressure transducer with an uncertainty of 0.1 bar in a range of 0 - 200 bar. To provide vigorous mixing of the liquid phase, a two-blade impeller on a solid shaft coupled with the motor was used. The impeller was located in the bottom of the shaft. A torque sensor with platinum coated connector measured the torque of continuously rotating shaft with an uncertainty of 0.3%. It used a strain gauge applied to a rotating shaft and a slip ring that provides the power to excite the strain gauge bridge and transfer the torque signal. To safely operate the motor and torque sensor, safety-lock was implemented in monitoring/control system to stop the motor once the torque value became higher than 50 N cm during the hydrate formation process.

For water and decane mixture without hydrate inhibitors, hydrates formed instantly upon mixing and leads to the hydrate blockage within 13.9, 18.8, and 42.2 min for stratified, partial dispersing, and full dispersing liquid phase, respectively. The resistance-to-flow was 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 was highest initially, but decreases with increasing hydrate fraction in liquid phase. When it reached the steady value, torque start to spike possibly due to agglomeration and bedding of hydrate particles. Adding 20 wt% mono-ethylene glycol (MEG) to the water phase could suppress the torque increase 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 decreased at 10 wt% MEG concentration, where surge of relative torque was observed although the resistance-to-flow was not enough to stop the fluids flow. The addition of 1.0 wt% Luvicap suppressed the hydrate onset for 155.0 min at mixing rate of 200 rpm, however soon lost its efficacy with increasing mixing rate to 400 and 600 rpm. The growth rate was also maintained low until hydrate fraction reached about 0.07, then catastrophic hydrate growth was occurred followed by increasing torque increase. 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. In this work, under-inhibition with MEG, adding Luvicap, and hybrid inhibition with MEG and Luvicap were tested. Among them, addition of 20 wt% MEG to the water phase was the best to manage the hydrate blockage risk. Luvicap was effective at stratified condition, however may induce blockage at high degree of turbulence. For MEG and 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.

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