Effect of Under-Inhibition with Methanol and Ethylene Glycol on the Hydrate Control Process
Authors: Yousif Majeed H. 
Published: October 2008

Hydrate control can be achieved by chemical injection. Currently, methanol and ethylene glycol are the most widely used inhibitors in offshore hydrate control operations. To achieve effective hydrate inhibition. a sufficient amount of inhibitor must be injected to shift the thermodynamic equilibrium condition for hydrate formation outside the pipeline operating pressure and temperature. Recently published field experiments showed that hydrate blockages form more readily in under-inhibited systems than in systems completely without inhibitor, A Iaboratory study is conducted to determine the effect of low concentration (1-5wt%) methanol and ethylene glycol on the hydrate formation process. The results show that, although these chemicals are effective hydrate inhibitors when added in sufficient quantities, they actually enhance the rate of hydrate formation when added at low concentrations to the water. Furthermore, the presence of these chemicals seems to affect the size of the forming hydrate particles.

Introduction

At favorable conditions of pressure and temperature, water in contact with natural gas crystallizes and physically entraps certain molecules of the hydrocarbon stream. The Natural gas components N2, H2S, C02, Cl, C2, C3, and iC4 are among the large number of molecules1, 2 that form clathrate hydrates with water. Larger molecules like n-butane, cyclo-pentane, and cyclo-hexane form hydrates only in the presence of a helper gas2. Hydrates can form from a hydrocarbon stream that is a single phase vapor or liquid, or from a two phase stream. Three crystal structures234, 5 have been identified for gas hydrates. These are structure 1, structure II, and more recently identified structure H. The hydrate structure is kept thermodynamically stable through van der Waals type interactions between the water molecules forming the cavities and the gas molecules occupying them. An extensive review of gas hydrates is given by sloan6.

For over 160 years hydrates remained a mere scientific curiosity. Their importance to the oil and gas industry was realized in the early 30's when Hammerschmidt7 discovered that the solid compounds that frequently plugged the gas transmission lines during cold weather were not ice but hydrates.

The interest in gas hydrates control has increased with the increase in the deepwater development of promising oil and gas prospects offshore the Gulf of Mexico, Brazil, the NorthSea, and other parts of the world. Unprocessed fluids produced from subsea completed wells may be transported for tens of miles to a main platform using subsea flowlines. Such flowlines are highly susceptible to hydrate formation and possibly plugging. The formation of hydrate blockages have been experienced in the Gulf of Mexico and the North Sea. Hydrate blockages in the gas and gas condensate tmnsportation lines can be a consequence of the failure of the chemical injection system, unscheduled shutdowns at the host platform, or inadequate inhibitor treatment during well startup.

Recently published field experiments showed that hydrate blockages form more readily in under-inhibited systems than in systems completely without inhibitor.