Synergism of thermodynamic hydrate inhibitors on the performance of poly (vinyl pyrrolidone) in deepwater drilling fluid

• PVP could prolong the induction time and lower the rate of hydrate growth.
• NaCl enhanced both the nucleation inhibition and growth inhibition strength of PVP.
• Glycol and glycerol were not the right synergists for PVP compared with NaCl.
• The synergism of NaCl and PVP was still strong at higher subcooling.

The influence of common thermodynamic hydrate inhibitors including NaCl, glycol and glycerol on the hydrate inhibition performance of PVP (poly (vinyl pyrrolidone)) in deepwater drilling fluid was experimentally studied, and the synergism of NaCl on the performance of PVP was further investigated. Results of tests demonstrated that as the representative of the first generation of kinetic hydrate inhibitors, PVP could prolong the induction time and lower the rate of hydrate growth. NaCl showed strong synergistic effect on the performance of PVP, while glycol and glycerol were not the right synergists in comparison with NaCl. The presence of NaCl enhanced both the hydrate nucleation inhibition and growth inhibition strength of PVP, thus giving a several time longer induction time with lower pressure drop rate. For deepwater drilling fluid containing 0.5 wt% PVP, the addition of 10 wt% NaCl could prolong the induction time from approximately 60 min to more than 1000 min at either the same water depth condition (2 °C, 10 MPa) or same subcooling (14.2 °C). At higher subcooling (16.4 °C), the synergistic effect was still strong, and the combination of 10 wt% NaCl and 1.5 wt% PVP could give a 600 min induction time. The long induction time is considerable for safe deepwater drilling at different water depth.