Because of increased activity in deepwater and high-pressure/high-temperature (HP/HT) environments, the safe operation of corrosion resistant alloys (CRAs) of different types is important. Reservoirs can be subjected to different flow impairment mechanisms that can require the use of acidizing fluids. Titanium (Ti) alloys can be present in subsea completions and in the casing of geothermal wells, as these are able to withstand ultrahigh-pressure and temperature and corrosive conditions. The need to remove clays or silica fines obstructing fluid flow in deepwater wells and enhanced geothermal systems can pose difficulties to operators if the use of hydrofluoric (HF) acid fluids is necessary. At present, there is no HF-acid fluid compatible with Ti alloys because any contact of HF acid with a Ti alloy will result in its corrosion and dissolution. In this paper, the development of a corrosion inhibitor solution to protect Ti alloys against HF-acid attack is presented.
Different metallurgical specimens (Ti, chrome, carbon (low or high) steel) were exposed to an HF-acid fluid based on a sodium-containing aminopolycarboxylic acid (APCA-Na) chelating agent. The Ti specimens (Grade-1 and -29) underwent delamination or blistering at low temperature (140–250°F), depending on the inhibitor and type of alloy present. An HF acid modulating agent was used to help minimize corrosion and the dissolution of Ti. The steel alloys were tested at 260°F with the HF-acid/APCA-Na fluid, displaying minimal mass loss resulting from corrosion. The main effect observed was surface pitting. While suitable inhibitors exist against HF acid for CRA tubing, there are still limitations, such as in mixtures containing acetic acid, which lead to pitting, complicating the inhibitory process. Temperatures in excess of 100°C limit the exposure time to an HFacid fluid, further convoluting the corrosion protection and inhibitor optimization for multiple alloys. Use of the described HF-acid fluid containing APCA-Na in conjunction with the set of HF acid anti-corrosion agents can facilitate acidizing in wells whose completions contain HF-acid-sensitive alloys, including Ti.