Corrosion Mitigation of HF Acid-Containing Fluids for Ti Alloy in Marine Risers

Abstract

Abstract Titanium (Ti) is currently used in deepwater (DW) platforms in the GOM (GOM) because of its higher strength-to-weight ratio compared to steel and better corrosion resistance to the most common acids, except hydrofluoric (HF) acid. The presence of Ti Gr-29 alloy presents obstacles when acidizing fluids are used during well cleanup treatments, particularly fluids containing HF acid. This paper documents challenges and feasible solutions for using HF acid fluids in DW fields. An associated field case history is also presented. Currently, no established testing standards exist to help guide the fluid selection process of HF acid blends for the conditions discussed here. A testing protocol is delineated that discriminates the effects of the various chemical additives and fluids used during a sandstone HF acidizing stimulation treatment, specifically managing the corrosive effects of HF acid on Ti Gr-29 as well as other susceptible alloys. The following challenges needed to be addressed: (i) spent fluid composition, (ii) corrosion inhibition for Ti Gr-29 of fluoride-containing fluid, (iii) solubility and stability of inhibitor composition, (iv) compatibility of inhibitor composition with stimulation and other operational fluids, and (iv) temperature and pressure effects. The method used relied on standard corrosion static tests as well as fluid-fluid compatibility under relevant temperature conditions. The spent HF acid fluid was generated by flow through a packed clay/sand column or in a batch reactor. The corrosion-determining properties of the spent fluid tested here were pH and initial spent fluoride concentration, in conjunction with temperature and exposure time. The total inhibitor concentration was optimized for maximum protection of the Ti alloy in addition to compatibility with fluids containing aluminum and silicon fluorides, calcium, methanol (up to 45%), and pH of 1 to 7. The inhibited spent HF acid fluid contains methane gas hydrates inhibitor. The inhibitor composition prevented corrosion of Ti for up to 24 hours (120 or 140°F), maintaining a corrosion mass loss below detectable levels (<0.000 mpy or 0.0005 lbm/ft2). The optimum inhibitory effect was above a pH of 2 using an inhibitor concentration (w/v) of 3 to 10%. Ti Gr-29 experienced corrosion and surface delamination, or even cracking, when a concentration of 3% inhibitor below a pH of 2 was used. Results indicate that concentrations of more than 3% corrosion inhibitor effectively protected Ti against a spent HF acid containing a concentration of 1.5% HF acid, up to 140°F. Application of the inhibitor in a high-temperature sandstone reservoir is documented. The identification of crucial factors impacting corrosion inhibition of Ti alloy supported the feasibility of protecting the surface of sensitive components, such as a Ti stressed joint, against spent HF acid fluid corrosion and managing the use of water-based fluids within the window of gas hydrates formation. A treatment job designed to inhibit spent HF acid fluid in deepwater well was successfully implemented.