The U.S. has experienced an unprecedented onshore energy boom over the past decade that has brought the country nearly to energy-independence. Much of that boom has been the result of a technology called hydraulic fracturing, often termed “fracing,” the process of artificially fracturing or perforating low-permeability rock strata in the vicinity of the drilled borehole with explosives and injecting pressurized solutions containing sand into those fractures, to facilitate oil and natural gas extraction.
According to the Society of Petroleum Engineers (SPE, Richardson, Texas), 60 percent of all new oil and gas wells globally undergo fracing and 2.5 million fracing procedures have occurred since 2012, with more than 1 million in the U.S. Considering that each wellbore requires anywhere from 10 to 40 multi-component tools called “frac plugs” (and accompanying “frac balls”) to stimulate multiple oil- or gas-producing layers, or “stages,” the demand for these downhole parts exceeds 20,000 per week, or more than a million annually, according to one oilfield composites expert. Demand is high for these critical parts, which are typically made with composites.
This booming market for frac plugs and balls, as well as other consumable downhole tools, means interest is high within the composites industry to supply this trend, as CW noted in an online blog a year ago (http://www.compositesworld.com/blog/post/composites-boon-from-hydraulic-fracturing). However, demand is growing for better, high-performance parts that can withstand the high temperatures and pressures that come with deeper wells. The high daily cost of drilling rigs and fracing procedures means oilfield operators need very reliable and durable frac plugs. The continued opportunity for composites in downhole applications requires better performance, says Stosch Sabo, a research and development engineer with Weatherford International (Baar, Switzerland and Houston, Texas), a major oilfield supply company: “The trend in the oil industry is deeper depths, higher temperatures and higher pressures, even beyond fracing. We need composite materials that can perform well for extended periods in severe conditions.”
Some history on downhole composites
The massive oil and gas industry, which employs more than 2 million people in the U.S. alone, can be viewed as a hierarchy with the major oil companies, such as Chevron, ExxonMobil, Shell, and BP, as well as many smaller independent companies, at the top. They are in turn served by drilling companies and oilfield service companies, who supply the equipment, tools and field services that ensure a successful well completion. In addition to Weatherford, service companies include Schlumberger (Houston, Texas), Halliburton (Houston, Texas), Baker Hughes (Houston, Texas, soon to become part of Halliburton), Fluor (Irving, Texas) and Saipem (Milan, Italy), among others. These companies supply the parts necessary for drilling, logging, completion, production and workover of oil and gas wells, including frac plugs and other downhole components like packers, bridge plugs or cementing tools. While Weatherford produces composite parts in-house, they and other big service providers still rely on a network of smaller, subcontracted composite part fabricators to make downhole-capable composite parts.
“It’s been something of an evolving landscape, without much regulation,” says Sean Reymond, chief technology officer of Impact Composites (Erlanger, Ky.), a producer of downhole parts made with thermoplastics. “If the company liked your part, and it performed as designed, you had a customer.” Weatherford’s Sabo agrees, pointing out that there are many different composite parts being manufactured, from frac plugs, to centralizers, and float equipment, and says “The have to function. But, there are few, industry-wide test requirements applicable to composites, and ASTM standards often don’t reflect the functional reality for these parts.” One industry expert says that while service companies like the drillability of composites, they get impatient with the often tedious, labor-intensive methods employed to make them.
Beginning more than 20 years ago, many small fabricators jumped into the downhole part fray, producing parts for the large service companies, but, says one insider, some struggled to meet demand. Parts were typically made with filament winding for the central body of the plug, combined with manually-machined filament-wound seals and slips. In higher temperature and pressure situations, standard glass/epoxy parts “just barely held up to the pressure,” this insider adds. Marginal qualifications, and occasional, very expensive onsite failures caused the oilfield companies to look for greater reliability from larger firms employing better technology.
Most composite fabricators and oilfield service companies consider parts highly proprietary, and protect their own designs with non-disclosure agreements. Manufacturers who currently supply composite downhole plugs, balls and other parts include Magnum Oil Tools (Corpus Christi, Texas), Forum Energy Technologies (Houston, Texas), Downhole Technology (Houston, Texas) known for its popular Boss Hog Frac Plugs, BR Oil Tools (Midland, Texas), Diamondback Industries (Crowley, Texas) and a wide array of smaller companies. Protek Systems (Grapevine, Texas) describes itself as a “boutique composite frac ball manufacturer;” Boedeker Plastics (Shiner, Texas) also produces frac balls. Additional oilfield composites suppliers include The Gund Co. (St. Louis, Mo.), Downhole Composites (Houston, Texas) and General Plastics & Composites (Houston, Tex.). CDI Energy Products (Humble, Texas) and Greene, Tweed (Kulpsville, Pa.) specialize in seals, sleeves and sealing solutions, as does Impact Composites. Aerospace composites firms more familiar to CW readers who also supply downhole parts include Automated Dynamics (Schenectady, N.Y.) and Exelis - Aerostructures (Salt Lake City, Utah), which produces the “Bear Claw” frac plug.
Frac plugs and balls: a composites niche
Downhole Technology’s research and process engineer Yanan Hou agreed to explain the design and fabrication of her company’s patent pending, trademarked “Boss Hog” plug (for 20-lb casing), shown in Figure 1. “An inner mandrel and the additional seven cylindrical elements that fit over the inner mandrel are wet filament-wound with E-glass and a proprietary epoxy resin as thick-section tubes, which are CNC machined to final dimension after an oven post-cure.” The company’s trademarked Seal-X seal segment includes both a flexible elastomeric part and a rigid (metallic) element, adds Hou. The completed Boss Hog plug is 44.5 cm long, with an11.1-cm outer diameter (OD), and is rated for downhole temperatures to 121°C/250°F (a higher-temperature plug is also produced, rated to 177°C/350°F, using a high-temperature epoxy) and is rated for a pressure of 69 MPa/10,000 psi.
Hou explains that when the “setting tool,” to which the plug is attached, reaches the desired depth in the hole, usually within the borehole’s horizontal portion, a charge inside the setting tool (triggered by the wireline or control cable from the surface) applies a differential force that drives the frac plug’s outer, movable elements downward; this in turn forces the composite and metal outer slips to move toward each other over their adjacent conical elements and “flower” (or expand radially outward) due to the applied compressive force, engaging the metal casing of the borehole to hold the plug in place. At the same time this setting force causes the Seal-X and the rubber seal element to compress and expand radially, blocking the annular space between plug and well casing to provide a full, radial pressure-tight seal. “The slips prevent the plug from moving axially in the borehole, and the elastomeric seal isolates the upper portion of the well,” asserts Hou. Once the plug is locked in place, the setting tool shears the composite threads, releasing the setting tool from the plug. The perforating guns are then pulled up to the desired zone. After that zone above the plug is perforated and the tools removed from the well, a composite frac ball is then dropped/pumped down the borehole, and engages or seats in the upper end of the inner mandrel, allowing the borehole to be pressurized. That additional hydraulic pressure forces the inner mandrel even further downward, resulting in higher compressive force, which Hou calls the “performance limit.” The fully-deployed plug with ball ensures a pressure-tight seal, directing the fracking fluid into the perforation clusters above the plug, which guarantees the success of the fracking process, she says. This process is repeated many times, for each frack stage, taking approximately one week, and the multiple plugs within the borehole are eventually drilled out into small pieces at the conclusion of the fracking operation: “The plug is designed to break up into fragments less than 1.6 cm in size, and can be drilled out in as little as six minutes,” adds Hou.
To ensure a high level of quality for its Boss Hog plugs, Downhole Technology employs a laboratory program that includes an engineered Frac Simulation System that it calls FCSS. The system is able to simulate the pressures and temperatures of an actual borehole situation, by placing a frac plug within a section of metal casing meeting American Petroleum Institute (API) 5CT pressure rating, filled with hot fluid. Hou reports that the system, with a frac plug in place, is pressurized several times, before increasing the in-casing pressure high enough to either cause the casing or the plug to fail. The tests show that as temperature increases within the casing, the setting force, or the load that shears the inner threads of the plug’s mandrel, was 132 KN at 93°C, but 190kN at room temperature. Hou explains that at room temperature (80°C), the plug’s performance limit actually exceeds the API 5CT casing’s pressure rating. Hou explains that elevated temperature does reduce the shear strength as well as the tensile or compressive strength of the plug’s thermoset composite material. “But,” concludes Hou, “the tests show that at full deployment, a Boss Hog plug is able to hold pressure over 10,000 psi up to 121°C.”
When asked if Downhole Technology might consider making a switch to a higher-temperature capable thermoplastic resin like polyetheretherketone (PEEK), Hou says that the company prefers thermoset resins and that epoxy is capable, for now, of meeting customer needs, as verified by the testing. She indicates that prepreg may be adopted in the future, in a towpreg-winding process, for better mechanical properties.
Frac balls are themselves highly complex, typically made from glass fiber/epoxy or phenolic resin, and are easier to mill out than metal versions. Composite versions can withstand higher pressures and temperatures than traditional neat plastic or dissolvable salt balls. Frac balls typically measure from 25.4 to 146 mm in diameter.
Weatherford says its erosion-resistant frac balls are five times stronger “than anything on the market,” and it is apparently seeking even higher performance with a November 2013 patent application titled, “Filament Wound Composite Ball.” According to Sabo, an industry trend in research has been toward “controlled degradation.” Meanwhile, General Plastics & Composites’ 2012 patent uses “three dimensionally woven reinforcement . . . to alleviate the interlaminar shear limitations found in balls manufactured by typical composite laminates.”
On the horizon are new, dissolvable frac balls, made from even stronger materials. Baker Hughes’ IN-Tallic frac balls are made from controlled electrolytic metallic (CEM) nanostructured material that is reportedly lighter than aluminum and stronger than some steels, but disintegrates when exposed to brine fluids. Terves Inc. (Euclid, Ohio) is developing TervAlloy magnesium and aluminum nanocomposite frac balls to withstand 15,000 to 20,000 psi/1034 to 1379 bar but disintegrate (turn to powder) when exposed to an electrolyte fluid or electrical or thermal stimuli. Both systems are designed to work with current frac sleeves and composite frac plugs, but they eliminate the milling out of frac balls and the potential for debris clean up, reducing well completion time and cost. However, Terves claims TervAlloy offers disintegration in hours versus the 1.5 days reportedly required for IN-Tallic frac balls.
More technology advancements
Impact Composites’ Reymond says “the corrosion resistance of thermoplastic resins such as PEEK and PPS [polyphenylene sulfide] certainly offer huge advantages. But TPs won’t displace epoxy entirely, since the part geometry sometimes limits the use of very long or continuous fiber reinforced thermoplastics.” Automated Dynamics’ business development manager Brett Kimball relates that his company’s technology for automated fiber placement of thermoplastic prepreg has led to several downhole and offshore applications. Automated Dynamics focuses on specific components of downhole tools, and not entire tool assemblies. He emphasizes that high-performance thermoplastics offer downhole service temperatures from 149°C to 232°C and can withstand pressures up to 30,000 psi.
Another trend is compression molding of components as an alternative to filament winding. One expert notes that a matched molding process, employing either thermoset or thermoplastic molding compounds reinforced with long fibers, enables better process control and reduces the chances of part failure. Greene, Tweed is a well-known supplier that molds seals, bearings and cylindrical elements for downhole parts, employing compression molding, combined with part design, tool design and construction, and quality control.
Along with the trend of higher-performance materials, the next wave of fracking technology may actually reduce the number of composite frac plugs and balls significantly, with the goal, as always, to cut rig time and cost as the price of oil continues to slide. Baker Hughes offers a new flow-through frac plug, which it has trademarked the SHADOW, which is designed to be left downhole during production, completely eliminating the plug drillout phase of plug-and-perf completions. The plug features a large flow-through inside diameter (ID) and uses Baker Hughes IN-Tallic disintegrating frac balls so production can flow with the plugs in place, reportedly shaving days off of completion times says the company. The IN-Tallic frac balls hold pressure during fracturing operations, then completely disintegrate in the well when exposed to produced fluids to prevent production blockages and eliminate debris.
Weatherford’s ZoneSelect fracturing completion system has been used in the North American Bakken shale oil reserve to access 25 to 30 separate frac zones within 24 hours compared to a typical 6- to 8-day installation for a more traditional composite plug and perforate (plug-and-perf) operation. Weatherford also claims this sliding sleeve system can cut water use by 30 percent via not having to push plugs downhole. The “stimulation balls” used by the ZoneSelect system can be composite, although aluminum and dissolvable balls are also an option.
Will this market grow?
Over the past decade, demand for hydraulic fracturing crews and downhole composite equipment has exceeded supply, and oil and natural gas production has surged. But oil’s recent and precipitous price drop may spell a slowdown in this lucrative composites market sector. And, some of the new strategies discussed above may also reduce the number of downhole parts needed.
Weatherford’s Sabo reiterates that as the well intervention and completion process becomes less complex and more commodity-driven, composite material suppliers and fabricators need to step up as well, with temperature- and pressure-capable materials that can handle any onshore or offshore application: “Moving forward, the oil industry is going to be tackling deeper depths, with hotter temperatures and higher pressures, and downhole materials need to perform.” Time will tell, and CW will continue tracking this interesting application.
To see a video about how frac plugs work, visit this YouTube video produced by Magnum Oil Tools:
To see a video that shows the operation of a Baker Hughes Shadow Frac Plug in operation, visit www.bakerhughes.com/products-and-services/completions/well-completions/multistage-hydraulic-fracturing/shadow-frac-plugs.