Abstract
The paper discusses the unrecognized issue of accelerated cracking and dissolution of stressed high strength dissolvable magnesium components at elevated temperature. A high strength dissolvable magnesium alloy was selected for inclusion in a frac plug designed for 125°C to 175°C service after thorough tensile, compression, and dissolution testing of the alloy. After a 125°C plug field test, the plug exhibited catastrophic, premature failure. Laboratory plug testing of two magnesium alloys for the slips replicated the failure at 140°C in tap water. Dissolution testing of coupons in a more aggressive media showed inadequate mass loss to compromise functionality. It was theorized that the passivating magnesium layer was unable to form due to the stress applied to the components with magnesium's inherent reactivity with water. Slow strain rate testing was used to study the potential mechanism causing stress corrosion cracking. Two high temperature high strength alloys, DM-1 and DM-2, were tested at 4 x 10^-6 in/in/s in 140°C tap water. DM-1 demonstrated a decrease in yield from 52.4 ksi to 33 ksi, a 37% reduction as well as a decrease in ductility from 10.9% to 0.8%, a 93% decrease. DM-2 demonstrated a decrease in yield from 59.3 ksi to 32.3 ksi, a 46% reduction as well as a decrease in ductility from 10.3% to 0.7%, a 93% decrease. A scanning electron microscope evaluation showed both materials possessed a highly developed secondary phase surrounding the grain boundaries. The development and subsequent investigation of an alternative magnesium alloy, DM-3, showed semi-continuous secondary phases and was investigated for a substitute at a component level. While the ultimate tensile strength decreased minimally, the ductility decreased by 36%. Laboratory testing of the plug in identical conditions with the DM-3 slips was still successful. It is imperative for high temperature magnesium plug material selection to ensure the alloy does not have highly interconnected secondary phases which may cause sudden failure during field operation.
