Hydrate Remediation in Deepwater Gulf of Mexico Dry-Tree Wells: Lessons Learned

摘要

Hydrate plugs were formed above the mudline in two dry tree oil wells in the Gulf of Mexico. The plugs were formed when trying to open the downhole safety valve with crude to return the wells to production after they were shut-in because of hurricane evacuation. Several unsuccessful attempts to melt the hydrate blockages included pumping methanol through the chemical injection lines below the plugs and lubricating in glycol above the plugs. As a last attempt, before using coiled tubing, injecting hot oil into the tubing-casing annulus was considered. Transient simulations were performed to determine the required injection temperature, rate, and time. Well integrity issues were mainly associated with the compatibility of the hot oil with the elastomers and possible as-phaltene or paraffin precipitation in the annulus. Sensitivity studies show that with a 1-bbl/min injection rate and 150°F injection temperature, the pressure-temperature condition inside the tubing located 3,000 ft below the sea level will come out of the hydrate formation region within 4 hours. However, as the section goes deeper, the warm up time increases and at some point the conditions will not warrant being out of the hydrate region even after several days of injection time. Hydrate plugs in two dry tree wells melted after 6 and 60 hours of injection time, respectively. A revised restart procedure has been implemented to eliminate the hydrate problem in future startups.