How Does A Swellable Packer Work?

For most of the life of the oil industry, equipment manufacturers have spent time and money to develop elastomers that swell very little when exposed to oil or water. This is true for O-rings, packer elements, or seal stacks. Elastomers that swell in oil could result in a reduction of the designed performance. Over the course of time, new technology such as horizontal drilling, multi-stage fracturing, and reservoir management with the use of ICD's has required something different for annular isolation than cement. In the past, wells would be cemented "in place" regardless if they were vertical or horizontal. Cement can create an environment which reduces the productivity index by covering up most of the sand face on a horizontal well in a thin oil sand. Cementing in horizontal wells in some unconventional reservoirs could also make fracturing difficult by forcing fracture treatments to be pumped through perforations.

With the introduction of non-cemented completions came the necessity of an open hole packer. For many years there have been mechanical packers that can be hydraulically activated to create an annular seal between the open hole and the production casing. In some instances such as slotted liners, there is no way to pressure up to set the packers. This opened up the opportunity for swellable packers.

Natural rubber swells when exposed to hydrocarbon based fluid. The amount of swell is dependent on the chemistry of the oil and the temperature at which the exposure occurs. Oil is absorbed into the oil-swellable elastomer through diffusion. Through the random thermal motion of the atoms that are in the liquid hydrocarbons, oil diffuses into the elastomer. When the elastomer is wrapped on a piece of pipe, the result is an increase of the manufactured outside diameter of the oil-swellable elastomer. Oil continues to diffuse into the elastomer causing the packing element to swell until it reaches the inside diameter of the open hole. The swelling continues until the internal stresses inside the elastomer reach equilibrium. That is, the swell pressure increases until diffusion can no longer occur. At this point a differentially sealing annular barrier is created.

Operationally, the benefit of a swellable packer is simplicity. There are no moving parts required to work, through pipe manipulation or by applied hydraulic pressure. No special service personnel are needed. The packers are simply run to depth, similar to the casing, and allowed to swell before production or injection operations begin.

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The picture above illustrates a FREECAP® I packer. The elastomer is wrapped on API pipe with weight, grade, and connection specified by the well design. The seal length is determined by the required differential pressure, the well temperature, and application. Solid metal end rings are secured to the base pipe and protect the elastomer while the completion is run to depth. Critical to the function of the end rings is that they create an extrusion barrier. As differential pressure is applied across the packer, forces are created that want to extrude the rubber in the direction of the applied pressure. The end rings support the elastomer which results in a hydraulic annular seal. Swellable packers can have pressure ratings as high as 10,000 psi.

For low pressure applications there is a slip-on version of the swellable packer called a FREECAP® II. This packer works similar to a slip-on centralizer. They are often used to provide annular isolation for slotted liners, or flow barriers with sand screens which use inflow control devices.

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Elastomers have also been developed that will swell in the presence of water & water-based fluids. Swell is achieved by blending in a super absorbent polymer into the base elastomer compound. Once the packer is exposed to water, the water is absorbed by the polymer causing the packer volume to increase. This increase in volume corresponds to an increase in the packer diameter. Just like the oil-swellable elastomer, a seal is created once contact with the borehole wall is made. The water-swellable packer is capable of swelling in both monovalent and divalent cation brines. Using the super absorbent polymer creates a very stable water swell packer. If the water is removed and replaced with crude oil, the packer will maintain a seal.

Recently a water-swellable elastomer has been designed and installed in heavy oil wells which use steam to heat up the reservoir to cause oil to flow. The high temperature water-swellable packer presently has an upper operational limit of 575°F (302°C).

TAM International
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