The main purpose of the primary cement job is to provide long-term zonal isolation for the life of the well. However, events in the life of the well can alter the integrity of the cement sheath, leading to undesirable and potentially dangerous effects. One of these effects, sustained casing pressure (SCP), continues to present a significant challenge to the oil and gas industry. Loss of pressure containment in localized areas of the well may also pose significant problems even if such loss does not lead to flow to surface as evidenced by SCP.

This inter-zonal communication down hole can significantly impair production as well as limit

Outside forces can cause an initially good cement job to debond. (Photos courtesy of Halliburton)
the effectiveness of stimulation treatments, making it equally problematic. To help combat loss of cement sheath integrity, Halliburton has developed a three-level defense strategy that provides intelligent zonal isolation for the life of the well.

Zonal isolation or pressure containment is typically provided in a well by the tubing or casing and the surrounding cement sheath. Four events can contribute to loss of pressure containment or SCP:
• Tubing/casing leaks;
• Ineffective mud removal during the primary cement job;
• Flow-through gas channels in the cement formed prior to set (due to loss of hydrostatic pressure); and
• Damage to the primary cement sheath after it has set (i.e., a micro annulus formation or sheath cracking caused by casing contraction and or expansion, or by temperature or pressure events).

The impact of SCP on operators varies according to each specific situation. The result could simply be the increased cost from monitoring, bleeding off pressure and reporting SCP. More severe consequences might include loss of production if the well must be shu-in or even a catastrophic loss of wells and surface facilities. Currently techniques to remediate SCP either involve costly workovers and the uncertainty of squeeze cementing or the use of techniques that remain largely in the experimental stage. In either case successful remediation cannot be assured. Therefore, significant efforts have been made by the industry to prevent the development of SCP and inter-zonal communication in the first place.

As a result of focused research, the service company has now made it possible to design and deploy intelligent systems during the well construction phase to help minimize the occurrence of SCP and inter-zonal communication during the life of the well. These intelligent zonal isolation systems deliver three levels of defense against loss of annular pressure containment.

The first level of defense requires the analysis, design and delivery of a cementing system that has been engineered to withstand the forces exerted on it by the various well
A rubber swellable packer can seal tightly against cement to maintain isolation integrity.
construction, completion, stimulation and production operations. Conventional industry practice focuses on the liquid cement slurry properties as well as compressive strength. However, the industry has recently recognized the importance of optimizing the mechanical properties of the set cement sheath in an effort to create a more resilient sheath that can withstand future well events without failure. The first level of defense should include determination of the cement properties under triaxial conditions and finite element analysis of the casing, cement and surrounding formation. The delivery of the engineered cementing system should then incorporate industry-recognized cementing best practices to maximize mud removal and minimize formation of gas channels through the unset cement. WellLife service models the integrity of the cement sheath and the risk of its failure as it is subjected to different well operations and thus has been used as the first level of defense to successfully optimize the construction of hundreds of wells globally.

Prior to designing the engineered cement system for placement in the well, certain well events are modeled and the effect of those well events on the sheath determined. Typical events modeled can include pressure testing and fracturing operations as well as temperature and pressure cycles. The cement sheath is then engineered to possess the necessary mechanical properties required to withstand these events without failure.
However, with the future being uncertain, it is possible that unpredicted events — natural and man-made — will occur during the life of a well. If well events exceed the parameters that the cement sheath was designed to withstand, even a correctly designed sheath may fail. Therefore, the second level of defense requires a display of intelligence by the in situ cement sheath. This intelligence is imparted to the cement sheath by means of incorporating certain additives in the cement that will swell in the presence of flowing gas or crude oil.

An initially competent cement sheath may fail (Figure 1) through debonding or shear deterioration resulting from unplanned events such as overpressuring, unanticipated formation subsidence or tectonic activity. Such failure can lead to the creation of pathways for gas and possibly liquid flow through or around the failed sheath. Such flow can cause the well to exhibit SCP. A cement sheath is now available that can react in the event of such failure and repair itself automatically, sealing the flow pathway without the need for intervention services. These new systems have already been field-proven in more than 50 wells in the Middle East, Asia, Europe, Latin America and the United States, both on land and offshore. Conventional cementing equipment can be used to place these cement systems.

In the third level of defense, a large reactive gasket — or swellable element — attached to the outside of the casing before it is run in the well provides an additional annular seal when required. One or more of these Cement Assurance tools can be used at critical points in the well. These tools use in situ temperature, wellbore fluids and flowing reservoir materials as the fuel to swell the element. When required by wellbore conditions, the element swells to fill in uncemented areas, incongruities or void space. The reacted element can establish the necessary hydraulic seal to stop flow between reservoirs or the reservoir and the surface (Figure 2). The rubber compound used in these tools can swell to more than twice its original size, allowing it to fill significant volumes while still providing a differential pressure sealing capacity. The gasket or element will remain dormant in an effectively placed cement sheath, ready to react in the event of failure in the cement sheath. The use of these swellable element tools is the third aspect of the intelligent zonal isolation system deployed in the well. These element tools are field proven, having been successfully deployed as an integral part of zonal isolation activities since 2005.

Sustained casing pressure and loss of pressure containment remain among the most serious challenges that occur over the life of the well. Therefore, proper planning for a critical cement job should involve both slurry properties and sheath properties, and the job should be executed according to established best practices in order to help achieve 100% mud removal. As part of the second line of defense, the slurry can be designed to automatically repair sheath damage that might occur from unplanned events. Swell Packer technology — the third line of defense in the total package — adds another method to help prevent pressure transmission up the annulus. By employing these three field-proven levels of defense, it is now possible to provide intelligent zonal isolation that can be expected to maintain an effective seal throughout the life of a well.