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.