Oil & Gas Services
Freeze services are typically used to place a temporary barrier (ice plug) in a tubular or vessel in order to maintain pressure control while allowing for safe mechanical repairs to be addressed involving possible valve replacement and general pipe work modifications. The freezing of wellheads and other pressure containing equipment is a time honored procedure which has proven itself as an inexpensive and safe alternative in well maintenance, and in some cases emergency situations.
It is a reliable method to control pressures with introduced primary and secondary barriers to facilitate the removal/replacement of wellhead, pipeline, BOPs and other auxiliary equipment which otherwise would, in some instances, be impossible to remove or repair safely. Freezing techniques have been successfully used in situations where the pressures have exceeded 10,000 Psi [70 Mpa/680 Bar].
Procedure
Through the utilization of liquid N2 and liquid CO2 along with state of the art temperature monitors, costly fluid kills no longer have to be performed and downtime is minimized. 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.
Factors to be considered in any freeze will be:
- Presence of freezable fluids
- Condition of the tubular or component that will be frozen
- Size of the tubular or component to be frozen as to time and product needed
The goal of the freeze technique is to form a sealed bridge between the pressure within the component being frozen and the equipment needing repair or replacement. The area selected to be frozen is displaced with a very thick slurry of uncontaminated bentonite gel. The gel plug is frozen in place by use of dry ice (solid CO2) contained within a cribbing and tamped manually to keep it in contact with the area to be frozen at all times. Once sufficient time has elapsed, the freeze plug can be pressure tested prior to removal of any equipment for repair. Once repairs have been completed, the newly installed or repaired equipment is again pressure tested. The pressures are then equalized within the system. The dry ice and cribbing system is removed and the area that was frozen is carefully thawed. Any remaining gel is then flowed out of the component.
The N2 process also allows for successful application of multi-string freezes across as many as five different annuli.
