Adapting High Strength, Corrosion Resistant, Nanocrystalline Materials for Applications in Deep-Water and HPHT Hostile Environments
Authors: Chen Ting , Shenoy Ram , Wilkinson Christian , Zhou Jing , Roy Indranil 
Published: April 2020

It is well understood that strength to weight ratio is an important driver facilitating extended reach deviated wells, as such engineering corrosion resistant ultra-high-strength light-alloys (LAs) are an absolute necessity. High strength steels and nickel alloys, having yield strengths between 80 and 120 ksi, commonly used in oil & gas completions and production, have 3 to 5X higher specific gravity compared to LAs. In comparison, commercial coarse-grained polycrystalline light-alloys (CGP-LAs) have a much lower strength, typically less than 45 ksi yield strength and are not corrosion resistant. However, if strength of LAs can be increased to match that of steels and nickel alloys while simultaneously augmenting their corrosion and environmental assisted cracking (EAC) resistance through severe plastic deformation (SPD) routes, manifesting an ultra-fine-grained (UFG) microstructure, they will far surpass strength to weight ratios of steels and nickel alloys.

As such the quest for a perfect alloy with the right combination of strength, ductility, corrosion, and abrasion resistance is ever ongoing. The advent of nanotechnology and advancements in engineering nanostructures with high strengths and reasonable ductility has motivated researchers globally [1-7]. We have bridged a few of these technology gaps by addressing challenges relating to the compromised ductility of these novel high-strength nanomaterials, however there is limited data on their EAC susceptibility. EAC encompasses stress corrosion cracking (SCC) related events or catastrophic failures due to the loss of ductility of a metal exposed to acid gases, such as hydrogen sulphide (H2S) and/or through absorption of hydrogen.

To determine if grain refinement in a LA, and resultant Hall Petch strengthening, where strength of the UFG-LA surpasses that of commercial coarse-grained oil-grade steels, is instrumental in enhancing its EAC resistance, focused experiments were conducted. An aluminum-magnesium CGP-LA commonly used in offshore and marine applications was selected and subjected to SPD resulting in an UFG microstructure. Both CGP-LA and UFG-LA specimens were exposed to environmental conditions to compare their corrosion and EAC and resistance.

It was observed from polarization measurements and immersion tests that UGF-LA had better corrosion resistance than CGP-LA. Uniaxial tensile tests in a neutral halide environment for various holding times confirmed that strength and ductility was maintained for UFG-LA specimens while catastrophic failures of CGP-LA specimens were observed. It was also evident from orientation images that grain boundaries in UFG-LA were predominantly high-angle in contrast to CGP-LA. The improved corrosion and EAC resistance of UFG-LA was due to a large number of purer grain boundaries with nanocrystalline impurities distributed infrequently along them.

These results are very topical as they highlight the possibility of harnessing the enhanced strength to weight ratios of these unique alloys for designing buoyant thin walled flowable shells for deployment in high pressure high temperature (HPHT) oilfield environments, from vehicles being designed for interstellar gas giants to aggressive sour oilfield reservoirs.