New Dual Purpose Chemistry for Gas Hydrate and Corrosion Inhibition

Abstract

Abstract An overlooked benefit of some gas hydrate inhibitors is their potential as corrosion inhibitors. In this paper, a family of molecules was evaluated as gas hydrate inhibitors by an autoclave technique. The chemicals were concurrently evaluated as corrosion inhibitors by linear polarization resistance. The results indicate that there are design criteria based on the chemistry of these systems, which can be used to optimize their performance as both gas hydrate inhibitors and corrosion inhibitors. Introduction and Background Formation of natural gas hydrates is a common problem during oil and gas production, and has been well documented(1). Gas hydrates are molecules of natural gas trapped in crystals of frozen water. They are metastable solids in which hydrogen bonded water molecules (hosts) encase the low boiling hydrocarbon molecules (guests) in a cage-like entity. The cage-like structures can totally enclose or trap a gas molecule. The mechanism of gas hydrate formation is believed to follow a two-step process. First, clusters of hydrogen bonded water molecules form around a non-polar core. This is followed by the joining of these clusters to form gas hydrates. Gas hydrate formation is usually favoured at temperatures closer to the freezing point of water. However, under sufficient pressure, gas hydrates will form at temperatures above the freezing point of water. The resulting solids can form plugs that restrict or block gas flow during oil and gas production. The most common Gas Hydrate Inhibitor (GHI) is methanol. Methanol functions as a thermodynamic inhibitor by shifting the equilibrium requirements for hydrate formation to lower temperatures and higher pressures. However, the high dosage requirements of methanol have created logistic and environmental problems. For this reason, Low Dosage Hydrate Inhibitors (LDHIs) have been developed. LDHIs have been classified as being either kinetic or anti-agglomerant. The Kinetic Inhibitors (KIs) work at low concentrations by delaying the nucleation and growth of crystals. KIs are cost effective, but fail to inhibit agglomeration of the crystals once nucleation occurs(2). KIs also have less sub-cooling potential than anti-aglomerants. For this reason, KIs are less desirable for situations with long shut-in periods. The Anti-Agglomerates (AAs) are believed to function at the gas-water interface preventing the formation of large hydrate crystals. AA mechanisms for gas hydrate inhibition are considerably different than those of kinetic inhibitors. It is believed that AAs have a dual mechanism of action. The first mode of action is believed to affect the structure of the growing hydrate. While the AAs allow hydrates to form, they limit the growth of these hydrates, thus minimizing the risk of pipeline plugging. The growth inhibition is believed to occur as a result of binding of the AA to the surface of the initially formed hydrate, thus altering the structure of the gas cage. AAs second mode of hydrate inhibition is achieved by their behaviour as dispersants. These molecules allow the previously formed hydrates to disperse in the oil phase and allow the transportation of the gas-water mixture through the pipelines(3).