ABSTRACT

Over a dozen commercial and new polymer, polymer-like carbon (PLC), and diamond-like carbon (DLC) coatings originally developed for the Oil & Gas industry show promising characteristics against geothermal carbonate scales. This paper discusses the major findings of both (a) calcium carbonate scale tests and (b) corrosion tests at two temperatures and with multiple brines, including a carbonate-forming brine and a high-chloride acidic brine. Coating performance Vs. characteristics such as thickness, surface roughness, and water contact angles, is also investigated. In general, thin coatings (less than 5 μm), whether polymer, PLC, or DLC, tend to be unfit for service due to an inadequate gas permeation resistance, even when the coatings are applied onto corrosion-resistant substrates such as Alloy 718. In contrast, the thicker polymer and DLC coatings (e.g., ∼15 μm) are found to be more fit-for-service, and more appealing when also strongly hydrophobic (i.e., with high water contact angles). This paper also shows that hydrophobicity, surface roughness, and a low carbonate scale deposition behavior are not noticeably correlated. Importantly, when scale formation is accompanied by strong carbon-dioxide degassing (e.g., 150°C), the more gas-impervious coatings have been found to resist better blistering and gas-induced removal or delamination.

INTRODUCTION

Adherent mineral or inorganic scales cause costly and persistent flow-assurance problems in hydrocarbon production, geothermal power generation, water treatment and transportation, and potentially in future novel subterranean fluid applications. The deposition of minerals from natural formation brines is known to create blockages in Oil Country Tubular Goods (OCTG) and hydrocarbon production equipment, resulting in (a) reduced production, (b) diminished well access to logging monitoring equipment, or (c) causing equipment failures such as electrical submersible pump overheating. 1-5 In geothermal power generation, scaling has also been reported to interfere with thermal exchanges and the proper functioning of steam turbines, while it always poses an omnipresent risk of crevice corrosion, precisely under scale deposits. 6-7 In response to new carbon-net zero objectives, combined geothermal-carbon dioxide sequestration developments have recently caught major attention. 8-9 As an example, Figure 1 describes a relatively new type of hybrid power plant wherein electricity is produced through conventional fossil-fuel combustion turbines as well as geothermal steam turbines. In such a power plant, wherein the exhaust carbon dioxide, or flue gas, is injected down a nearby well, the risk of scaling by calcium carbonates (e.g., the calcite allotropic form of CaCO3) may be concerning in the geothermal well some distance away. With new geothermal developments having potentially steam (water vapor) replaced by supercritical carbon dioxide, well-known to be a powerful solvent for many chemicals, fluid recirculation from the reservoir to the surface is also susceptible to induce scales. A general mechanism of scale deposition is also illustrated in Figure 1, where the greatest risk for calcium carbonate is created at ambient pressure with hot steam releasing carbonate and carbon dioxide. Overall, the risk of scaling is greater where mineral solubility is reduced by pressure and temperature drops.

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