Hydraulic fracturing is among the most costly oilfield operations. Costs include milling or drilling out downhole devices such as setting balls and plugs to recover the original-size fluid pathway for production. When the SPECTRE disintegrating frack plug was introduced at the recent Society of Petroleum Engineers Annual Technical Conference and Exhibition, it marked a nanocomposite industry first.

The new frack plug completely disintegrates downhole after fracturing to eliminate post-frack intervention requirements, accelerate completion times and leave behind an unobstructed fullbore production inside diameter (ID) for maximum flow area and simplified future access for recompletions.

The disintegrating frack plug represents the most recent application of a high-strength nanocomposite technology known as controlled electrolytic metallic (CEM) material. Developed and patented by Baker Hughes, CEM material owes its existence to scientists and engineers who applied both a fundamental understanding of materials science and engineering, and the courage and perseverance to defy previous assumptions about physical engineering to develop a breakthrough material that had never existed before.

Defying physical engineering
In 2010 materials scientists in the company’s advanced composites group were charged with a seemingly impossible task: develop a lightweight yet high-strength material that would disintegrate downhole and could be incorporated into various completion devices used in hydraulic fracturing. The ability for these devices to fully disintegrate after performing their required function would eliminate significant risk and operating expense.

However, no known metal composite material existed with the combined opposing properties required to create these downhole tools. Traditional materials that could easily disintegrate in wellbore fluids exhibited low mechanical strength. Conversely, high-strength materials usually were not able to disintegrate. Or, if they could disintegrate, the rate of disintegration was so slow as to be unsuitable for interventionless downhole tools.

The automotive and aerospace industries had already tried unsuccessfully to develop a material with similar opposing properties. The required material seemed to defy physical engineering.

With the key material properties identified and with an application goal in mind—setting balls for a new multistage fracturing system—the materials science team applied innovative thinking and processes to develop a new composite material with unique chemical and mechanical properties.

A magnesium-based alloy was chosen for the first generation of the new material because of its light weight and high specific strength. The material also is reactive, providing the foundation for a fast disintegration rate. At the time in the medical industry, magnesium—because it has the same density as human bones—was being considered for screws that could bind bones together and, after recovery, dissolve in the body’s own fluids.

However, because the magnesium is weak in mechanical strength, it disintegrated and generated byproducts too fast for the human body to adapt. And because its disintegration rate could not be adjusted or controlled, it was not deemed a viable solution. The company’s team discovered a similar disintegration rate control issue while researching its new material.

While attempting to strengthen the material and control the degradation rate, the team discovered that a nanostructured material was ideally suited for the desired operational outcome. The next step was identifying precision processing techniques that could reliably produce a homogeneous-looking composite.

The result was a highly engineered composite material that exhibits substantially continuous cellular metallic grains dispersed in the nanomatrix. The nanomatrix plays a dual role of providing reinforcement for high strength while having the unique chemical disintegration property that conventional materials do not provide. The composition and structure of the nanomatrix can be customized to applications or well conditions.

In the processed state, these nanostructures act as intermetallic adhesion promoters that yield metal composites that can withstand an impact of 95 bbl/min of fluid flow and 10,000 psi differential pressure. (As a comparison, typical flow rates in hydraulic fracturing are below 15 bbl/min.) At the same time, devices made from the CEM material can completely degrade in situ within a predetermined period of time when exposed to wellbore fluids. The designer material, which has been used in the fracturing of more than 80,000 stages, disintegrates fully at predictable rates based on temperature and salinity.

Applying the nanocomposite
The mechanical and chemical properties of nanostructured material provide high-performance interventionless operations in a variety of completions processes. The first product application of the material was in the IN-Tallic disintegrating frack balls used with the FracPoint multistage fracturing system.

The technology also was incorporated in a temporary barrier in gas-lift mandrels. The temporary plugs enable the operator to run production tubing and cement it in the same trip with gas-lift valves in place without compromising the integrity of the valve with cement. The plugs disappear after the cementing operations, saving the operator the expense of having to rig up wireline to replace the dummy valves with actual gas-lift valves.

Improved interventionless option for PNP completions
Plug-and-perf (PNP) completions with composite plugs account for an estimated 75% to 80% of new well completions in North America. The plugs enable flexible stage placement, provide fullbore access after plug drillout and allow treatment of stages as far as wireline and coiled tubing (CT) can reach while offering a long track record of field success.

Historically, however, when composite plugs have been used to isolate zones, production startup could be delayed for three or more days while the plugs and drop balls were drilled out using CT-conveyed milling tools and debris was circulated to surface.

Several new interventionless completion technologies have improved PNP completion effi ciency by eliminating time, cost and risks associated with post-frack drillouts. While these solutions might save some time upfront by eliminating post-frack intervention, they can complicate operations, both initially and later in the life of the well. Disappearing frack plugs that do not fully disintegrate leave behind partial or whole components that can damage equipment both downhole and at surface and cause problems during intervention. For example, standard frack plugs use cast-iron slips and rings or ceramic buttons coupled with a packer to create a tight seal and hold plugs fi rmly in place. Some of these high-strength materials do not degrade.

The fully disintegrating SPECTRE plug leverages CEM nanostructured material technology to alleviate these concerns. The plug offers the same flexible stage placement as composite plugs, but unlike other interventionless plugs, the entire plug—including the plug body, anchoring grip and packing element—fully disintegrates downhole after fracturing.

Much of the plug body is formed from the same CEM material used in the company’s fully disintegrating frack balls. A proprietary packing element system provides reliable stage isolation, and a specially engineered anchoring grip can reliably hold pressures of up to 10,000 psi during fracturing, enabling reliable diversion of stimulation treatments into the formation.

At the desired depth, the plug’s packing element expands and seals against the casing, and the slip system secures the plug fi rmly in place. Risk of premature degradation is effectively eliminated because the plug’s components only react with wellbore fl uids. After fracturing, the plug and frack ball completely disintegrate, leaving behind a debris-free fullbore ID, permitting future wellbore access without concerns around ID restrictions or debris-related tool complications.

Wireline conveyance allows well logging and optimized stage placement on the fl y throughout fracturing operations. Production can begin after fracturing because the plugs do not need to be removed with CT-conveyed milling tools. Additionally, development locations and plug setting depths are not constrained by the availability and/or fi nite horizontal reach of CT, enabling the treatment of more remote locations as well as longer laterals for increased pay zone access. The completely disintegrating frack plug has been run in the fi eld with positive results, and additional installations are planned in the near future.

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