High Efficiency Non-Polymeric Gas Hydrate Inhibitors
Authors: Pakulski M. 
Published: December 2007

Laboratory and field results have confirmed the advantages of kinetic gas hydrate inhibitors compared to classical thermodynamic inhibition methods. Kinetic inhibitors being used currently are high molecular weight water soluble polymers and they require a polar carrier solvent. In situations where the solvent is allowed to evaporate, i.e. hot spots in a gas pipeline, the solid polymer will plate out of the solution and may cause problems. This paper describes new low molecular weight kinetic gas hydrate inhibitors. These newly developed products, being liquid, will not plate out of solutions, can be used either in a polar solvent or a hydrocarbon carrier. Simulated hydrates laboratory testing has been done on a tetrahydrofuran/salt water system in a simple and unique testing loop. Comparable laboratory results were obtained for experimental and established kinetics gas hydrate inhibitors. Positive laboratory results were followed with successful field testing. Laboratory and field results indicate the experimental inhibitors have potential commercial applications in gas hydrate prevention technology.

Introduction

Gas hydrates form when water molecules crystallize around guest molecules. The water/guest cocrystallization process has been recognized for several years, is well characterized and occurs with sufficient combinations of temperature and pressure. Significant amounts of natural gas are deposited in the form of hydrates and are considered as future energy sources.

Light hydrocarbons, methane-to-heptanes, nitrogen, carbon dioxide and hydrogen sulfide are the guest molecules of interest to the natural gas industry. Depending on the pressure and gas composition, gas hydrates may build up at any place where water coexists with natural gas at temperatures as high as 80 F. Long gas transmission lines are particularly vulnerable to being blocked with hydrates during extended cold weather conditions. Underwater pipelines and equipment may be exposed to hydrate forming conditions all the time. Extensive studies on gas hydrates were conducted by B. Dendy Sloan Jr. and coworkers at the Colorado School of Mines (CSM). These studies sponsored by several oil and gas companies included both the theoretical and practical aspects of hydrates and methods of hydrates inhibition. Gas hydrates are being studied worldwide at several research laboratories.

There are few methods of preventing gas hydrates formation. The obvious ones like removing the water component, heating the system above the temperature of hydrate formation and lowering the pressure, are often not practical. Another method, the addition of large amounts of ethylene glycol or methanol, decreases the hydrate stability and effectively lowers the temperature of hydrate formation. The above mentioned methods are called thermodynamic inhibition because they destabilize water/gas clathrates by changing the composition or conditions. Thermodynamic inhibition with chemicals requires relatively large amounts of methanol and/or glycol; hence it is quite expensive.

Kinetic methods of gas hydrates prevention have been developed by CSM and other laboratories. Kinetic inhibitors prevent a growth of hydrate nuclei to form larger crystals. Kinetic inhibitors are usually water-soluble copolymers. They are effective at concentrations typically ten to one hundred times less than the effective concentrations of ethylene glycol or methanol. With thermodynamic inhibitors increasing in price, the kinetic ones are becoming economically favorable. Known kinetic inhibitors like poly(N-vinylpyrrolidone (PVP), poly(N-vinylpyrrolidone/N-vinylcaprolactam/N, N-dimethylaminoethylmethacrylate (VC-713), poly(N-vinylcaprolactam) (PVCap) or poly(N-vinylpyrrolidone/Nvinylcaprolactam) (VP/VC) inhibit gas hydrates formation by coating and commingling with hydrate crystals nuclei, thereby interfering with agglomeration of small particles into large ones which would result in plugging the gas pipeline and equipment.