When a major North Sea operator evaluated the potential of using through tubing rotary drilling (TTRD) to sidetrack a 20-year-old well in a UKCS field, it identified significant cost savings when compared to other solutions.

However, the design presented the operator with a completion challenge as traditional cementing wasn’t practical for zonal isolation. There would be difficulties in achieving a reliable cement job along the length of the horizontal section. After researching its options, the company determined that recently developed swellable elastomer packers from Swellfix could provide the required isolation requirements even in the tight restrictions of the planned well bore.

The basic principle of swellable
elastomers is natural and very simple. Add water or oil to the appropriate rubber-based

Figure 1. A Swellfix E-ZIP isolating a water-producing zone while allowing oil production from other zones. (Images courtesy of Swellfix)
compound and it will swell as it absorbs the liquid. There are no moving parts to fail. As long as the specific application is correctly engineered and deployed, there is very little that can actually go wrong. However, the inherent simplicity of swellable materials masks the real skill required to create a swellable packer solution for a well.

Swellable types
Although there are essentially only two main types of swellable elastomers used for downhole packers — oil or water swelling — thousands of different compounds have been researched and developed so that the swelling characteristics can be matched to the specific well conditions.

Water-swelling elastomers work on the principle of osmosis, a process that encourages the movement of water particles across a semi-permeable membrane, where there is a salinity difference on either side of the membrane. Oil swellable elastomers work on the principle of absorption and dissolution. The swelling rate and volume increases are directly related to the composition and characteristics of the oil. The specific gravity of the oil plays an important role, but other qualities of the oil can also affect swelling behavior.

Swellable packers combine the advantages of both cement and mechanical packers, without
Figure 2. S-ZIP for smart wells is in the foreground; E-ZIP as run in this well is seen in the background.
the inherent complications associated with the traditional technologies. The packers themselves are thin sections of swellable rubber that are vulcanized directly on to the 2 7/8-in. tubing — they swell when they come into contact with the appropriate fluid, water or oil.

Like cement, swellable packers adhere to the profile of the formation, and like conventional packers, they create a pressure-holding seal within the well bore. However, unlike these traditional methods of zonal isolation, swellable packers always have something in reserve in case of a washout in the future, or if water breakthrough occurs. In such cases the packer will swell further and adapt to the new structure until a seal is re-established.

Oil and water do mix
In the case of the North Sea well, prior to sidetracking and the installation of the swellable packer solution, it was producing a 70% water cut. The operator recognized that water cut was likely to arise again, so the packer solution had to be designed to cope with both current and anticipated future conditions.

The well design called for a kill
pill to be placed across the existing perforations in the main 51¼2-in. bore and a whipstock set some 300 ft (91.4 m) above. The side track was designed as a 27¼8-in. completion
within a 33¼4-in. hole.

In many cases swellable packer solutions are either oil- or water-swelling. However, the existing and predicted future conditions of the well led to a solution which combines both types of elastomers. The E-ZIP (External Zonal Isolation Profiler) swellable packer deployed is very simple in operation, and its slim running profile made it ideal for the sidetrack project.
In the well, the swellable packers were deployed in an oil-based mud (OBM). When the packer is immersed in the environment for which it was designed, the oil-swellable elastomer reacts with the OBM, and the packer begins to set. After a few days the packer expands to fill the available space, and an effective seal is created. If the annulus shape changes in future for any reason, the packer will expand further and the seal will remain intact.

The water-swellable elements in the system are designed to remain dormant in the well until they encounter water, at which point they will swell and provide additional sealing capacity. This design means that the well is now pre-equipped to handle a water intrusion if it becomes an issue at some point in the future.

With a detailed understanding of exactly how any of the myriad of swellable elastomer compounds developed by the company will react in the conditions it encounters, it
can accurately predict safe run-in time, the time to first swell and the amount of unused swell available to hold pressure.

In the North Sea well, the run-in time for the swellable packers was less than 24 hours, with a seal being set within six days. The sealing pressure required across the elements is a 1,500 psi differential.

Completion design
In total the operator ran eight packers deployed at depths of 13,500 ft to 15,300 ft (4,115.85 m to 4,664.63 m) in the horizontal section of the well. Each had 16 swellable elastomer elements. This type of configuration delivers a greater pressure holding ability and faster setting times. It also retains the flexibility of the joint, making it easier to deploy many packers in long-reach horizontal sections. At key points between the packers the operator introduced pre-perforated, pipe leaving blank pipe where water producing zones were suspected.

Should the well begin to water out in the future, the design configuration of blank pipe, packers and pre-perforated pipe will allow for a simple solution, an I-ZIP (internal Zonal Isolation Profiler) swellable packer to be applied. The small running diameter of the packer allows it to be deployed inside tubing. When its swellable elements are exposed to water they will begin to swell, in turn shutting off the water producing zone.

The operator also used swellable packers as the liner hanger in the horizontal section. In this location the 32 elements of oil and water packers provide a seal between the inside of the 4 1/2-in. tubing and the new completion string, with a sliding sleeve below the packers allowing communication to the old perforation below. The use of swellable packers as the liner hanger reduced complexity and the need for intervention. The swellable packers were simply dropped in place, self-centralizing as they set ensuring integrity at the liner hanger. The design life for the solution is for the life of the well, typically 20 years or more.

The well was completed on time and on budget, delivering the desired savings. When production commenced, the well flowed more oil than expected with negligible water being produced, and all the indications are that combining TTRD with swellable packers has proven to be a real success. The operator has now planned three more TTRD wells using swellable packers.