The objective of this paper is to discuss the successful application of a cryogenic freeze plug to fulfill the requirements of dual shutoffs prior to removing the production tree. This operation is presented as a substitute when conventional means are not feasible.
The Saudi Aramco policy regarding the decompletion of wells with downhole packers requires two independent shutoff barriers for the tubing and tubing casing annulus prior to the removal of the production tree. These shutoffs must be independently and simultaneously operated. Moreover, each shutoff must be pressure tested separately. A dual shutoff is easily achievable with conventional means. That is, tubing plugs and tubing hanger back pressure valves, or a combination of the two.
Prior to removing the production tree from an oil-producing well, the production tubing status was found to have collapsed and parted. With the presented conditions, the only annular barrier was the kill fluid column. The well also had leaking tubing hanger seals, which were not holding pressure. As a result, introducing an additional shutoff barrier proved challenging.
An innovative practice was presented as a second shutoff application. The proposal was to utilize the cryogenic freeze plug as a solution to this event. The kill fluid column, filling the tubing and tubing casing annulus, was frozen by a cryogenic freeze plug method. This allowed the fluid to mimic the form of a second barrier.
This application serves as a temporary secondary barrier that would withstand the required pressure test with a full column of kill fluid in the tubing and tubing casing annulus.
This paper reviews the methodology, purpose, technique, and qualification of cryogenic freezing as a second barrier. It also supports the positive trend of technical reviews of new methods or alternative techniques for nonconventional isolation barriers or shutoffs.