冷凍技術出新 Wild Well推出FreezeLITE流體

Freeze services have been used for many years to place a temporary ice plug barrier in tubulars (tubing, drill pipe, casing) when it is the only viable option to maintain pressure control while allowing remedial repairs to be made to surface equipment or tubulars. The cryogenic freeze service delivers an expedient, low-cost alternative to conventional well kill operations.

PROVEN TEMPORARY BARRIER

In recent years, Wild Well has worked with Texas A&M University to explore cryogenic freezing. Using liquid N2, or CO2, in combination with integral temperature monitors, an effective freeze can be safely and efficiently completed. The freeze process is monitored and temperature precisely maintained to ensure it is kept within recommended guidelines. Through the use of N2, or CO2, the freeze can be put into effect and held indefinitely to isolate inoperable tree components. Once the freeze is established, a positive and/or negative pressure test can be completed to confirm the temporary barrier is in place, allowing repair operations. Within certain parameters, cryogenic freezes can be set across multiple strings of casing resulting in an effective temporary barrier in the innermost tubular.

EFFECTIVE WHEN HYDROCARBONS ARE PRESENT

In order to establish a freeze barrier, there must be a freezable medium (brine, water, mud, etc.) in the ID of the component. If there is flow, any void, or gas or hydrocarbon-based fluid present in the freeze area, a freezable medium must be injected into the ID of the component. This can be through a hot tap to a tubular or possibly through the tree assembly. When liquid hydrocarbons are present, Wild Well’s newly developed FreezeLITE, a special water-based fluid, can be injected into the freeze zone. FreezeLITE will displace hydrocarbon-based fluids and stay suspended in the freeze zone above the hydrocarbon-based fluid. This allows a freeze to be put into effect without having to remove the hydrocarbonbased fluids in the well – saving thousands of dollars in terms of product and time. FreezeLITE is non-hazardous and safe for onshore and offshore applications. Cryogenic Freezing is proven to be a quick, cost-effective, and safe solution when a temporary barrier in the wellbore/component is needed for completing remedial work at the surface. Wild Well offers three proprietary freeze application systems: Flex Wrap, Coil Line, and Aluminum Canister Freeze.

FEATURES AND BENEFITS

Cost effective remedial solution, reducing downtime
??Eliminates expensive kill fluids
? Effective when hydrocarbons are present
? Applicable for tubulars up to 36-inch diameter, including multi-string casing
? Freezes can be held indefinitely
??Systems easily transportable

OFFSHORE WELL LOW-DENSITY FREEZE WITH HYDROCARBONS PRESENT

The wellhead of an offshore well scheduled for plug and abandonment malfunctioned with all valves stuck in the open position. This malfunction meant no vertical access to the well, and no traditional means of well entry using tubing or wireline was possible. With wing valves not operational, no means to connect to the well for injection existed either. This left the operator without any normal means of establishing the necessary mechanical well control barrier in order to repair the wellhead. Additionally, no reliable access point existed for bullheading kill fluid into the tubing. The tubing pressure was approximately 35 psi with seawater in the tubing. Height of the fluid column was not precisely known, and potential for liquid hydrocarbons on top of the seawater could not be ruled out. Gas hydrate formation had been an issue during previous well operations.

Solution

Engineering analysis indicated that the quickest, safest method of making the well ready for re-entry and abandonment was to set a freeze plug in the tubing below the top valve. With this flow barrier in place, the faulty top valve would be removed and replaced with two new gate valves. In order to reduce potential of liquid hydrocarbons or gas hydrate formation, a 15 bbls mixture of methanol and seawater was to be pumped into the well followed by ? bbl. freeze fluid formulated at a density of 6.3 lbs/gal. This low density ensured that the freeze fluid would float on the brine/methanol as well as any liquid hydrocarbon that might percolate to the surface. Thus, a stable volume of easily-freezable liquid would remain in the freeze zone. A wing valve on the wellhead was hot-tapped and a 2 in. connection installed. After successfully pressure testing to 4750 psi, injection was performed with nominal injection pressure of 3500 psi at ? bpm. After injecting methanol and seawater, the ? bbl. batch of the low-density freeze fluid FreezeLITE was mixed as described earlier. Injection completed at ? bpm, and the well was shut in. A freeze jacket was installed and N2 circulation started. After two hours circulation, temperature at the wellhead surface had cooled to -30 oF. At this point, engineering predictions indicated that the freeze plug had formed an adequate flow barrier. The wellhead and freeze plug seal were successfully pressure tested to 5000 psi. Pressure was bled to 0 psi, and the well was monitored for flow. No flow was detected, indicating the freeze plug barrier was holding. With the barrier in place, the top wellhead valve was removed, and replaced with two new valves. This new valve assembly then was successfully pressure tested to 5000 psi against the freeze plug barrier. Once the two new valves passed this pressure testing, the plug was thawed and abandonment operations initiated.

Conclusion

This unique, low-density fluid forms an effective medium for nitrogen freezing operations in challenging wells that may contain hydrocarbon-based fluids in the wellbore, which have been resistant to freeze operations using conventional methods. As a result, costly fluid kills can be eliminated and downtime minimized. At times, this method presents the only option to establish a flow barrier if no direct method for vertical entry exists. With a sufficient supply of the N2 or CO2 on hand and the ability to monitor the freeze process accurately in real time, freeze plugs can be installed and maintained indefinitely in a safe and controlled manner.