Interest in degradable completion tools is on the rise, suggests Andy Rosenholm, the chief technology officer for Bubbletight LLC. “As refracturing becomes more prevalent, operators are looking for ways to ensure there is no obstruction left in the casing that could interfere with a second completion,” he says. “As a result, we are seeing a drive to make everything left in the hole degradable, including balls, plugs, seats and sleeves.”
Rosenholm says that goal will soon be achievable. He points out that degradable technology has proven successful in a range of applications, from frac balls to plugs for zonal pressure isolation.
Degradable polymer powders can be applied along with degradable diverter balls to plug existing perforations during a refrac, he adds. “The powder is pumped down with the fluid, where it packs off existing fractures for hours or days, then degrades to allow the fractures to re-open. This tool can increase the effectiveness of diverter balls, and the combination of the two may be less expensive than mechanical alternatives such as bridge plugs,” he says.
Degradable materials continue to get more reliable and versatile, Rosenholm says. “For example, Bubbletight has introduced a degradable composite polymer (DCP) that can degrade in lower temperatures than standard polyglycolic acid (PGA). This degradable composite dissolves in ambient freshwater, whereas PGA needs at least 180 degrees Fahrenheit to degrade.”
Rosenholm adds that DCP offers a tensile strength of 20,000 psi, some 4,000 psi higher than PGA.
“Tensile strength provides a useful reference value, but engineers should look at other qualities as well when evaluating degradable materials,” Rosenholm advises. “For example, with a frac ball, they also should look at modulus, which is somewhat synonymous with stiffness, because a ball that is too stiff will fracture, and one that is too soft will extrude. Elongation and shear are also important. If the shear strength is too low, as the ball is forced onto the seat, the seat will shear a ring off the ball’s diameter and cause it to pass through.”
Tensile strength generally correlates with all three qualities, but there are materials that have a high tensile strength but poor downhole performance, Rosenholm warns. He says the characteristics an operator should look for will depend partly on the application. “In a hot environment, the operator may want a product with a high modulus, because it may soften less quickly,” he illustrates.
For plugs, ductility/malleability and compression strength can be important, Rosenholm says. He recommends operators also look at the plugs’ sealing elements.
“To date, the Achilles’ heel of degradable plugs has been the elastomeric sealing element. No one has yet come up with an elastomer that degrades quickly enough to be practical,” he says. “In many cases, the elastomers are degradable, but the degradation may take two years.”
To provide faster degradation, Bubbletight has developed a sealing element made from a version of the plastic used to make DCP. “We can tailor the material to degrade anywhere from a day to a month, and the material can be molded and extruded net-shape,” Rosenholm reports.
For high-temperature or high-pressure applications, Rosenholm recommends using a metal-based degradable. “Good degradable metals cost more than polymers, but they do not soften and deflect as quickly, so they have better performance,” he explains.
Bubbletight manufactures degradable composite metals (DCM), which Rosenholm says are stronger than degradable alloys or cast products. “Our unique manufacturing process allows magnesium and other reactive elements to be combined without being re-melted. As a result, the DCM material has superior strength without compromising the degradation characteristics of the reactive elements,” he relates.
The world’s strongest metal that dissolves in brine and a new tough polymer that degrades in low-temperature water are coming to OTC 2015 and can be seen in Booth #729. The term “downhole degradables” is new, but the concept should be familiar with folks involved in well completions. Degradable balls used to divert fluid flow or activate downhole tools have been popular in the industry since 2013. The concept of using a degradable tool that doesn’t need to be retrieved or machined out needs not stop with diverter and activation balls. Bubbletight, LLC (Needville, Texas) will be exhibiting an industry first at OTC 2015 — a frac plug manufactured completely from the industry’s strongest brine-degradable composite metal. Another industry first will be shown in Booth #729 — balls made from a polymer that is tougher than nylon and degrades in ambient-temperature fresh water.
We hear a lot about hydraulic fracturing of shale rock to revive our energy industry by using pressurized liquid to aid in the extraction of natural gas and petroleum that is difficult to obtain by other means. You might be wondering what that has to do with the plastics industry. C. Andrew Rosenholm might be a familiar name to those who’ve been in the industry for many years. His family’s company, OAR Tool & Die, was a well-known mold manufacturer for 45 years, and as Rosenholm noted was a “toolmaker to the stars.”
OAR’s fortunes took a turn for the worst when the company began losing business to the lure of “inexpensive Chinese molds and cheap container shipping killed our customer base,” Rosenholm explained. In October 2009, OAR entered receivership. Luckily for Rosenholm, he had good connections in manufacturing, and he’d heard about something called “frac balls” through a vendor of a company he’d begun working for. “I was introduced to the oil and gas market in 2010 and the rest, as they say, is history,” he said.
Rosenholm went to OTC 2010, the premier trade show for the industry. While at the show he was struck by two thoughts—the first was oil and gas was a very vibrant industry, and second, there was a dearth of plastics engineers in oil and gas compared to other fields such as medical and automotive. “I enrolled in a plastics certificate program with UMass-Lowell to boost my understanding of plastics processing and my credibility in the industry,” Rosenholm said.
Today, Rosenholm has successfully combined his knowledge of plastics and moldmaking, and has developed a whole new business around making frac balls – balls that range in size from Ø 7/8″ to about Ø 5″. Frac balls are used to activate downhole tools and divert or stop the flow of fluid during the hydraulic fracturing process.