Carbon dioxide (CO2) is corrosive as a wet gas, dissolved gas in brines, or as a supercritical fluid with contaminants. Subsurface equipment made of nickel-based alloys has become well-established in hydrocarbon production; CO2 sequestration demands for identical alloys, despite the availability of potentially suitable stainless steels. In this paper, the author discusses results from complementary corrosion tests on metallic materials from tubular products and well equipment, including subsurface safety valves. The test program exposed corrosion-resistant alloys (i.e.,13Cr,15Cr, 17Cr, 22Cr, 25Cr, 925, 718, 716) to supercritical and near-supercritical fluids (dense phases) with elevated contaminant levels [i.e., water (H2O), oxygen (O2), hydrogen sulfide (H2S), sulfur oxide (SOx), nitrogen oxide (NOx), hydrogen (H2)] in addition to chloride-rich brines. The testing consisted of 4,000-to-4,500 psi (276-to-310 bar) autoclave tests at 70°F (21°C), 175°F (79°C), and 425°F (219°C), with test samples of each alloy immersed in the CO2-rich lighter fluids and the chloride brines. The materials were evaluated for mass loss, pitting, and crevice corrosion under pH values between 2.5 and 3.4. Overall, Alloy 718 was identified as a fairly complete alloy for CCS well equipment, while Alloys 25Cr and 925 continue to be attractive. For low temperatures and low contaminant levels, Alloys 17Cr and 22Cr can remain acceptable on a case basis, but are not recommended for typical subsurface equipment, because tubing metallurgy overmatching and manufacturing considerations weight heavily in favor of the nickel-based alloys.


Carbon capture and storage (CCS) represents to traditional industrial emitters a critical transitional technology to globally manage greenhouse gas emission, meet the latest carbon net-zero ambitions, and continue the commercial use of fossil fuels. In CCS, anthropogenic CO2 is captured near emission, treated, compressed, transported (usually by steel pipelines), injected underground through casings, tubings, and completion equipment, and finally permanently stored into saline formations, aquifers, depleted (abandoned) reservoirs, or un-mineable coal seams. [1] Depending upon emitters, the injected CO2 is either near or in a dense state (i.e., indistinctly including a supercritical phase and potentially liquids) having various impurities. These impurities, despite usually being at parts-per-million (ppm) levels, can influence the long-term well structural integrity, and therefore the well construction material selection. [2-4] At first, CCS wells look strikingly similar to basic hydrocarbon production wells, except that several characteristics trend in opposite directions, as pointed out by Table 1. Like hydrocarbon wells, CCS wells are designed and constructed from reliable and impervious well barriers that are intended to last decades. While pressure and temperature in CCS wells evolve opposite to hydrocarbon wells, corrosion in all its various forms does not, and therefore, corrosion must be thought of upfront in the well development cycle. Uniform (mass loss) corrosion, and particularly localized corrosion (pitting and crevice corrosion) are primary concerns of well-barrier equipment, such as subsurface safety valves, flow-control valves, packers, and subsurface monitoring hardware. When the metallic materials are carefully selected among, and as per NACE MR0175/ISO 15156 guidance, the risk of environmentally-assisted cracking become secondary. [5-6] Pure CO2 on its own is not corrosive; however, CO2 streams should be considered corrosive in CCS through mechanisms other than the traditional sweet corrosion of hydrocarbon production. [2] To reach its points of injection, the CO2 stream is usually compressed beyond its supercritical pressure (7.39 MPa, 1,071 psi), where after being geothermally heated, reaches a supercritical state (>31.1°C, 87.9°F), unless significantly contaminated. Beyond its critical point, pure CO2 is lighter than either water or brines, is non-viscous and gas-like, and is also often recognized as a powerful green solvent able to cause the degradation of metallic and nonmetallic materials. [1,7] Water and CO2, when combined to form various phases, can broadly and synergistically lead to various forms of material degradation, or corrosion.

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