In today’s drilling environment, whether it is horizontal wells, extended reach, brownfield development, subsea or deep water, the demands and functionality required of zonal isolation have never been greater. Gone are the days of only needing to know the pump time and compressive strength of the cement slurry. Now the requirements for a cement slurry include properties like fluid loss, free water, transition time, zero gel strength, bond strength and ultra-sonic strength development measurements.

Additionally, more and more wellbore construction activities require the cement slurry to have

Figure 1. Swell Technology has filled in the un-cemented area. (Images courtesy of Halliburton)
superior Poison’s Ratio, Young’s Modulus, shrinkage characteristics and the ability to withstand cyclic loading. The research and development departments of cementing providers work continuously to find admixtures. Nitrogen foamed cementing systems and polymer enhanced slurries are but two examples where the properties of standard Portland cement can be enhanced to meet demanding wellbore scenarios.

Despite these advancements in slurry design, slurry properties and the development of placement best practices, situations arise where cement does not get placed properly and the demands on the zonal isolation exceed its performance envelope. To address these needs, Swell Technology materials and services have been introduced as a companion to primary cementing. This approach takes certain functionality that would have been previously placed in the slurry in the form of an additive and strategically places it on the casing for use down hole as needed.

History of Swell Technology
It has been known for decades that certain rubber compounds swell when contacted with liquid hydrocarbons.

By adjusting the compounding components, rubbers can swell in water-based fluids as well as liquid hydrocarbons. The oil field considered this a nuisance and a characteristic to be avoided.

However, in 2001 Easywell introduced Swell Technology materials as a means to compartmentalize and control reservoir production. Since that time, the industry has installed literally thousands of swelling element packers under a variety of conditions. The technology has been successfully deployed to more than 33,000 ft (9,150 m) and 347°F (174.8°C) and can still provide differential pressure capacities upwards of 10,000 psi. The swelling of the rubber element can be achieved with only a few gallons of liquid.

Primary cementing
Late in 2005, when Swell Technology materials became readily available to the Halliburton organization, they began to be incorporated into the wellbore construction philosophy and ultimately became an applied technology. It has been proven in both the laboratory and in the field that Swell Technology materials can augment the zonal isolation process, particularly where there are concerns over cement placement issues, and to address micro-annulus issues.

Incomplete cement sheath
Situations arise where cement cannot completely encircle the casing. Many factors contribute to this condition, but the predominant factors include:
• Casing positioning that creates a wide side and narrow side;
• Poor mud properties;
• Inability to rotate and reciprocate the casing;
• Lost circulation zones; and
• Gas or water influx after placement.

Left unattended, the incomplete cement sheath (sometimes called a mud channel) can lead to cross flow between reservoirs, loss of production or increased production costs. If a remedy is attempted during the drilling phase, production will be delayed and casing integrity can be affected by the perforating and block squeezing used. Workovers to attempt to remedy these issues are costly, and they stop production and disrupt revenue streams.

Swell Technology materials are a potential remedy to this situation where cement does not
Figure 2. Closeup of rubber and cement interface.
completely encircle casing. The swelling element tool is run as an integral part of the casing string. Typical casing running procedures can be performed as no special handling, running precautions or actuating procedures are required. No specialist is required on location, as rig crews and casing crews can install and run the tool. The tool is strategically placed on the casing and spaced out for maximum effect. It should be landed on bottom where they separate two (or more) formations of different pressures and in locations where poor casing stand off is likely to occur.

Once the tool is in place, it utilizes in-situ resources like heat, wellbore fluids and flowing reservoir materials as the fuel to swell the element. The swelling takes place over days without any manipulation or intervention from the surface. The swell period does not interfere or impact the cement hydration process. The element will swell to fill in any uncemented area, incongruity or void space, thus establishing the necessary hydraulic seal to stop flow between reservoirs. The rubber compound can swell to more than twice its original size, which permits it to fill in significant volumes and still provide a differential pressure capacity. It has the ability to swell multiple times if need be. If formation changes or wellbore changes occur that might open a previously sealed region, the rubber element can swell time and time again to reestablish the seal. All that is needed is a fuel to feed the swelling element. Swell Technology materials are specifically designed to function with the cement sheath to provide a complete zonal isolation package.

Long-term sealing and micro-annulus development
After primary cementing, practically every wellbore event that follows will create pressure pulses that attempt to expand and contract the casing. All of these events are necessary for drilling operations, but they challenge the ability of the casing and set cement to stay bonded together. With enough pressure cycles of significant duration and magnitude, practically all casings and cement will de-bond. The micro-annulus created by de-bonding can be a conduit for reservoir fluids. Many wells are plagued with sustained casing pressure (SCP), where the micro annulus connects a subterranean pressure source to the wellhead.

When pressure is bled off, it quickly returns to sustain the pressure. SCP presents significant safety, environmental and regulatory risks, which can lead to shut-ins and the loss of production. SCP is more manageable for land operations but creates significantly more logistical, safety and environmental problems offshore.

SCP can be minimized by using Swell Technology to re-establish the seal once the casing and cement have debonded. It should be run as part of the original casing program. It will sit dormant in the well, entombed by the cement sheath. When the debonding occurs and the micro-annulus opens, reservoir fluids will attempt to flow through the micro-annulus, past the pre-positioned rubber element. The element will react to the fluid flow and swell.

North Sea
A North Sea operator wanted to access incremental reservoirs previously bypassed. The reservoirs could be drilled, but zonal isolation was critical to prevent production from flowing to depleted sub-pressured zones and to help prevent water from flowing into the productive interval from other adjacent reservoirs. High water cuts would challenge the project with the significant cost associated with treating and disposing the extra water along with the delay or total loss of production.

Once installed, the combination of primary cementing and swell technology provided a comprehensive seal that allowed the reservoirs to produce as expected without interference from nearby depleted zones. Each productive reservoir was protected above and below.

North America
An independent operator in North America had a problem with SCP appearing within 2 or 3 years of drilling the wells. Lost production, wellhead safety and environmental concerns prompted them to seek an alternative to their traditional cementing program. Swell Technology was recommended. Tools were assembled on the casing at the wellsite prior to running in the hole. The casing string was picked up and run as normal. The cement job was completed as planned. This well is on line and is being monitored daily. Offset wells provide a direct comparison to the baselines established from their years of production.

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