Zonal isolation is facing new demands with advances in unconventional wellbore construction methods and a growing regulatory environment. New hydraulic fracturing techniques, directional drilling with longer laterals and deeper-set production strings are placing greater demands on isolation performance. Government regulation is increasingly involved in defining those performance standards over the life of the well.

A key zonal isolation objective of these applications and regulations is preventing the creation of microannuli in the cement and between the cement and the casing, which can allow gas migration. Sustained casing pressure resulting from gas migration is costly to remediate and can present a life-of-well maintenance problem. Groundwater contamination in naturally fractured formations further complicates the isolation issue and increases risk.

In many of these unconventional well applications the inflatable annular casing packer is providing a familiar solution, while swellable technology offers newer options.

Well construction isolation options

Inflatable technology. Inflatable packers have long provided a reliable tool for primary zonal isolation or as a cemented component of a zonal isolation system. These can be run individually to provide specific zone isolation or in conjunction with conventional primary cementing or stage cementing.

Most recently, annular casing packers (ACPs) have been applied to a broader scope of unconventional wells where challenges—including depth, lateral length and complex wellbore geometries—have made it a natural fit. In open- and cased-hole applications, inflatable packers provide a high expansion ratio that conforms to the uneven geometries and size of the borehole wall.

In these applications, an ACP can be a central component in an isolation system. Weatherford’s Bulldog ACP is used across a broad scope of applications, including water/gas shutoff, production segmentation, openhole testing, selective stimulation and acidizing, and plug and abandonment.

Run as an integral part of the casing string, the Bulldog ACP uses inflatable elements selected for wellbore fluids and temperatures to ensure performance over the life of the well. The inflatable element can provide isolation for temperatures up to 343 C (650 F).

A recent technical advancement to the design is a high-pressure valve prompted by performance requirements in longer and deeper wells. In shallow wells with long laterals, an ACP is often set in the vertical section. This produces high frictional pressure when cementing horizontal sections of more than 3,049 m (10,000 ft). Deep vertical wells have very high hydrostatic pressures that also put increased demand on the ACP valves. To address these conditions, a newly introduced valve allows functionality over 5,000 psi.

Swellable technology. Weatherford swellable products and services are engineered to meet the challenges of well construction by using a portfolio of tool designs and elastomer formulas. Issues range from short-term stimulation concerns to long-term microannulus isolation. The company incorporates oil swell, water swell or a customizable dual-fluid activated swellable hybrid elastomer technology in its range of tools to meet these challenges.

The swellable Micro-Seal isolation systems (MSIS) are a solution for controlling microannular fluid and pressure migration. The approach is based on swellable isolation seal units that are distributed along the cemented tubulars and provide a series of response points that can control fluid or pressure migration that results due to microannular crack formations within the cement sheath.

The annular swellable packers offer the reliability and cost benefits of conventional swellable packers with the advantages of customized swelling performance and proprietary design features. A patented anti-extrusion system can maintain higher differential pressure across the tool, resulting in a much shorter packer.

Regulatory drivers

Regulatory bodies on a global scale are defining that wells must be designed to prevent any interconnection between hydrocarbon-bearing formations and aquifers. This ensures that gas is contained within the well, associated pipework and equipment without leakage and that zonal isolation between different aquifers is achieved.

Also, usable quality water zones are to be isolated and sealed off to prevent contamination. Productive zones, potential flow zones and zones with corrosive formation fluids must be isolated to prevent vertical migration (including gases) behind the pipe.

Lost circulation presents a challenge because the problem zone cannot sustain a column of cement. An ACP run with a mechanical port collar or stage tool provides support for the cement column.

Shallow gas zones typically make primary cementing difficult due to gas migration. The ACP provides a pressure barrier to the gas flow while the cement hardens. The ACP is set just above the gas zone, and primary cementing through the shoe inflates the packer. For long-term assurance, multiple MSIS units placed in the cement can help prevent possible migration as a result of cement sheath damage due to fracturing or thermal stimulation.

Good industry zonal isolation practices include selection of casing hardware, including float equipment, centralizers, cement baskets, wiper plug stage tools, annular casing and swellable packers. Such equipment can help mitigate several potential zonal isolation issues, including inferior cement jobs, microannular gas migration and contaminated aquifers.

To comply with regulatory requirements, improved cementing technologies ensure proper casing depth and cement placement in the annulus above the ACP or swellable packer. The ACP provides a platform for cement placement to prevent potential gas channeling within the cement slurry before it sets. The swellable packer provides additional zonal isolation assurance.

These regulations present several additional challenges, including safety management and record keeping. Operators must maintain a comprehensive record for each well and ensure that contractors have a complete safety management plan. Weatherford uses an operational excellence and performance system to address safety, quality, reliability and training. Record keeping is enhanced by a post-job assessment, and best practices are reinforced through a performance-tracking database.

Effective isolation requires focused engineering, operational and technical support. The process must analyze the well, identify optimal equipment and install it so that well construction costs and risk are lowered.

Annular casing and swellable packers have provided a focused solution for traditional zonal isolation. With greater demands presented by deep wells and long laterals, zonal isolation in complex unconventional wells is also increasingly dependent on the specialized capabilities of these packer options.