An essential element in any completion, the ubiquitous packer has been around for more than a century. Packers provide zonal isolation in liner completions, hydraulically separate producing zones and seal production tubing from the casing/tubing annulus. As companies increasingly return to mature fields to recover the last remaining oil, economics plays a key role, so they are systematically looking at any way to reduce costs. One way is by simplifying the packer.

For several years, Shell has been researching the use of swellable elastomers to provide the external seal. Swellable elastomers react with well fluids and swell significantly, enabling them to seal effectively to the inner diameter of tubulars or even to the borehole wall. According to the company, there are two types of swellable elastomers; one reacts to saline water and the other to aromatic hydrocarbons. The former absorbs saltwater through an osmotic process and has a typical operating range from 122°F (50°C) to 194°F (90°C). The latter swells by absorbing hydrocarbons and has a higher operating range, from 176°F (80°C) to 266°F (130°C). Swelling time varies, but its onset is delayed enough that the completion can be safely installed before the reaction begins. Once swelling commences, Shell has observed a wide range of swelling activity-periods lasting from as few as 5 days to as much as 50 days. But often it is not necessary to wait for the full period. In Malaysia, Shell has actually brought a well onstream before the swelling cycle was complete. There is a risk that premature operation could cause the packer to wash out, but in this case the company wanted to observe the packer's ability to shut off water production. Over a 10-day period, as the packer set, water cut decreased steadily from near 100% to less than 5%, finally reaching zero after 15 days. Perhaps most important is the packer's pressure rating. So far, Shell has tested its packer to 3,500 psi. According to the company, swellable packers complement installations where solid expandable tubulars (SET) are used. The first application was performed in 2002 and was designed as a cladding on SET to seal against an open hole to provide zonal isolation.

Although much remains to be learned about the technology and its applications, Shell is confident enough in its effectiveness that it has made more than 60 deployments in the Far East, Middle East, West Africa and the North Sea. Because of the relative newness of the technology, one unknown factor is service life, but since the swellable packers are not applied in critical situations where safety or the environment are at risk, the company is monitoring packer performance over time to determine its practical life-span.

Simple design

A swellable packer consists of two elements, a low-swelling outer layer surrounding a high-swelling core (Figure 1). Both are vulcanized to the outer surface of a mandrel, usually a joint of blank tubing or liner. The outer layer provides a wear-resistant skin that protects the packer element during run-in and also prevents premature swelling. During the absorption process, the saltwater or hydrocarbon in the well bore permeates the outer layer to react with the inner layer, high swellable compound to start the main swelling action. By their nature, swellable packers can be easily customized to suit a variety of completion scenarios. For example, because they can be installed onto any diameter mandrel, they can be made as long as necessary to achieve a seal and hold differential pressure. In one North Sea installation, external packers were installed on 27⁄8-in. liner joints and varied from 10 ft to 16 ft (3 m to 4.9 m) in length. Designed to seal in a 4.5-in. open hole, the packers' original outside diameter going in was 4.2 in. The packers can also be spaced out on a compound mandrel consisting of alternating sand screens or slotted liners and blank sections to span several producing layers in an openhole completion. They can also be used on tubing to isolate producing zones in a casing completion, or to seal off a water zone.

Significant cost reduction

Besides replacing costly mechanical packers, the swellable packers can eliminate cement jobs in many cases, particularly where the cement is being used to achieve zonal isolation (Figure 2). Wells previously completed conventionally using a cemented liner and subsequently perforated are now being completed using slotted liners interspersed with isolation packers - eliminating two costly operations. In addition, brownfield developments usually entail the use of small-diameter side-tracks and laterals. In the past, companies have experienced significant cementing problems involving considerable non-productive time when working with tight tolerances and small-diameter tubulars. One brownfield development involved several porous strata separated by thin shale beds. Some of the porous strata were oil-bearing, but most had watered out. Bed boundaries were well-defined and depth control was good. After making a few attempts to complete the zones conventionally with suboptimal results, the company built a custom completion string consisting of alternating joints of slotted liner separated by a dozen swellable packers. The water zones were successfully bridged over, and the packers achieved good seals in the interbedding shales.

To assure themselves that by forgoing a perforated completion they were not forfeiting significant production efficiency, Shell engineers modeled the conventional completion against the one with swellable packers. While perforations typically penetrate beyond the formation damage zone often encountered in wells drilled overbalanced, the brownfield developments consisted of re-entry jobs with sidetrack laterals, and formation conditions were well-known in advance. Accordingly, the company was able to drill the laterals using a mud system that produced relatively little formation damage. Subsequent modeling showed that the effect of differences in formation skin between a cemented and perforated completion and one done with swellable packers and slotted liners was relatively insignificant over time and, when balanced with the savings in completion costs, did not justify reverting to conventional completions.

Key issues when considering the use of swellable packers are summarized below:

• If packers are expected to seal in openhole, are the characteristics (hole size, hole shape, dogleg severity and formation permeability) of the proposed setting sites well defined?

• Are strata sufficiently separated, and are their depths and spacings well known so a completion can be reasonably installed?

• The packers need time to swell - is this acceptable?

According to Shell, the use of swellable elastomers to effect hydraulic isolation is viable and is experiencing rapid uptake. With the concept accepted, engineers are discovering more potential applications for the technology and are evaluating them carefully. While not a panacea, swellable packers can fill an important economic niche in specific situations.

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