Use of swellable elastomers to enhance cementing in deepwater.

Long, deviated open-hole sections of deepwater wells can make it extremely difficult to place a quality primary cement job. Eccentric casing in deviated hole sections causes asymmetric flow geometry, which can result in poor mud-filter cake removal during primary cementing and gradual failure of the cement sheath at shallower depths. This can lead to lost production due to thief zones, unwanted water or gas migration, and costly long-term well integrity issues.

Including swellable packers in the casing program, however, can help overcome many such challenges to achieving quality primary cementing.

A swellable packer is manufactured using absorbable elastomers that increase in diameter automatically when exposed to well fluids, which include drilling mud, space fluid, or produced/injected fluids. As the packer element increases in diameter, the elastomer makes contact with the borehole wall, further swelling to produce an interface seal line pressure between the elastomer and the borehole wall. Combined, the seal line pressure and the length of the elastomer provides hydraulic isolation in the areas where there is no cement. The packers should be positioned at depths where zone isolation is most critical.

Three basic factors are critical to assure a quality primary cement job. These include the cement slurry design, the position of the casing in the wellbore, and mud displacement ahead of the cement slurry. Of the three, mud displacement is the most critical and hardest to control.

No direct way exists to measure if the mud displacement during cement placement was effective. Poor mud displacement can leave mud channels or mud cake layers that will prevent zone isolation. The conditions most likely to lead to poor mud displacement are:

• Poor borehole quality, which lead to excessive wall rugosity, out-of-gauge sections, and poor flow dynamics
• Eccentric casing, which results in an asymmetric flow geometry
• Poor chemistry between spacer/mud or cement/mud interface
• Inability to rotate or reciprocate casing.

Failure of the cement sheath and/or creation of a microannulus are believed to be two failure modes that lead to loss of annular isolation. These failures typically do not manifest themselves until the well has been on production for some period of time. At some point, expensive production logs will evidence the most common signs suggesting there is a problem with annular isolation, i.e., an increase in water production, loss of production to adjacent zones, and/or sustained casing pressure. In most cases, operators choose conventional squeeze cementing in an attempt to regain zonal isolation, but for deep and ultra-deep water projects, the operating expense of repeated squeeze jobs is significant.

The authors build the case for use of a layer of elastomer between the casing and the cement sheath to protect the cement sheath in wellbore-critical areas. It can be achieved through the use of a swellable elastomer packer. With such a packer surrounded by cement, excessive stresses formed by pressure and temperature fluctuations can be absorbed by the compression of the elastomer.

The end caps of the swellable packer contribute to reduction of casing radial displacement. The end caps, being made of the same material as the casing, effectively increases metal thickness over the interval of the elastomer seal length, which decreases potential diametric displacement. While the end caps constrain the amount of displacement, the elastomer compresses and decreases in volume to accommodate reduced diametric displacement. The result is that the cement sheath around the swellable packer is not exposed to excessive forces that would create destructive stresses. If, for example, a swellable elastomer, 20-ft (6-m) seal length packer is included in the casing string design, there is assurance that if such destructive stresses do occur during the producing life of the well, there is a 20-ft (6-m) cement section that maintains its integrity.

And, say the authors, the swellable elastomer packer has the ability to swell and seal off a microannulus, should one be created by loss of the cement bond at the casing/cement interface.

Finally, swellable packers also can be used to ensure long-term well integrity in wells where liners will be used, which includes almost every deepwater and ultra deepwater well drilled today. In the Gulf of Mexico, for instance, several operators have recognized the potential risk of depending on the liner top packer to act as the primary – and only – seal. However, a swellable packer placed in the casing/liner overlap section provides a secondary or backup seal. After cement is pumped, this backup packer will swell in the fluid – usually cement spacer liquid – left in the overlap section.