Novel Low-Concentration Amphiphilic Inhibitors and their Application to Flow Assurance

Gas hydrate formation is a major problem in the field of gas and oil field exploitation, as it is known that hydrate plugs cause blockage of pipelines and extraction gear. It is therefore sought to inhibit hydrate formation, mainly by the use of chemical additives called inhibitors. Typical hydrate inhibitors (called thermodynamic inhibitors) are methanol and ethylene glycols, which are added in large quantities to oil and gas pipelines to lower the temperature and/or raise the pressure of hydrate formation within a particular oil or gas stream. The amounts of methanol or glycol needed may reach values up to 50 wt% or higher in certain production settings, thereby raising problems in terms of cost and environmental safety. More recent attempts to solve the problem of pipeline hydrate formation have focused on the use of particular molecules which exert their inhibiting effect at much lower concentration as compared to methanol and glycols. These compounds are known as low- dosage hydrate inhibitors (LDHIs), and are generally polymers (but also low molecular weight molecules) which act as either kinetic inhibitors, by increasing the hydrate induction time, or antiagglomerants, which prevent the growth by agglomeration of formed hydrate particles. In general, the (supra)molecular mechanism(s) underlying the inhibition effects of LDHIs are poorly understood, partly because of the differences in molecular structures among the various LDHIs found so far (vinylpyrrolidone- based polymers, polyacrylamides, poliacetamydes, alkylglucosides, onium salts, etc.). For a review, see Kell. Moreover, many references relating to novel hydrate inhibitors are to be found in the patent literature, where the efforts of setting out a rationale at the basis of the observed effects may be of secondary concern. A particular class of LDHIs is comprised of surfactants. Surfactants (surface active agents) are compounds whose molecules carry both lipophilic and hydrophilic moieties, i.e. they are amphiphilic in nature. Surfactant molecules in water tend to aggregate to form various species of supramolecular structures, such as spherical and rodlike micelles, multilayer structures, and complex biological membranes. To a first approximation, a surfactant solution in water is considered to contain isolated molecules, whose maximum allowed concentration is given by the critical micelle concentration (CMC), and, above that, fully formed micelles are formed. The onset of micellization is detected by sharp changes in such properties as surface tension, refractivity, conductivity (for ionic micelles), etc. Studies on the influence of surfactant micelles on physical properties of bound species and on chemical reaction rates and equilibria are described in several papers and monographs. To some extent, organic reactions at the micellar "pseudophase" mimic reactions in biological assemblies such as enzymes and cell membranes, and surfactants are increasingly used to bind DNA molecules for, e.g., transfection into living cells. It is worth noting that some surfactants are also hydrate promoters. Several papers and patents report on huge promoting effects exerted by anionic surfactants on the formation of natural gas hydrates, although the interpretation of the mechanisms of that promotion is still debated, particulary as relates to whether surfactant micelles have any promoting role, as compared to isolated surfactant molecules. The focus of the present paper is on the study and development of novel hydrate-inhibiting surfactants. These molecules were designed and synthesized based on previous literature reports and findings by the authors, that cationic surfactants - in particular tetra-alkylammonium salts - are good hydrate growth inhibitors (HGIs). Another structural feature that we wanted to introduce was a moiety which could partially fit into a hydrate cage (e.g., a cycloalkyl or heterocyclic moiety), to induce the formation of partially closed hydrate cages, and increase the strength of interaction between the surfactant molecule and the hydrate crystals. The study was originally designed to test the novel inhibitors directly in the field, by using an experimental field test facility which is being realized by Italfluid Geoenergy s.r.l., an Italian well testing company which is the sponsor of this work. Due to the recent geopolitical turmoil, however, the realization of this facility has been delayed. It will be comprised of a by-pass pipeline to a NG production well with a production output of ca. 106 m3/day, and a pressure drop from ca. 280 to 70 bar. The by-pass pipeline will be provided with a fiber-optic camera for visual monitoring, pressure and temperature probes, and a inhibitor solution-injection port/pump. Inhibition data were then analyzed according to a chemiometric approach to look for structural features of the inhibitor which could be isolated and optimized for further synthesis.