The Application of Kinetic Inhibitors to Gas Hydrate Problems
Authors: Notz P.K. , Bumgartner S.B. , Schaneman B.D. , Todd J.L. 
Published: October 2008

Laboratory and field tests since 1991 have shown the feasibility of replacing the classical inhibitors (methanol and ethylene glycol) with kinetic inhibhors. Although the kinetic inhibitors have limitations, they have been shown to be eflkctive in preventing the formation of hydrate plugs in field situations. In many cases, there is an economic advantage to using the kinetic inhibitors. An analysisof the conditions in the production facilitiesmust be done to ascertain the applicabilityof a kinetic inhibitor. The data and tests which are required to determine the applicability of a kinetic inhibitor are discussed. The application of kinetic inhibitors to flow lines in producing fieldsis presented.

The cost to the Petroleum Industry to inhibit the formation of gas hydrates is greatly increasing with the development of offshore reservoirs. Gas hydrates are likely to form in subsea flow lines unless the water is removed down to the lowest dew point encountered, highly effective insulation is used or inhibitors are used. The first two possibilities are generally prohibitively expensive or the technology for subsea application does not exist. In many cases, kinetic inhibitors offer lower cost alternatives to casual inhibitors and are more environmentally friendly.

The results show that kinetic inhibitors can be economically applied to field production systems. This is an area where there is very little published Mormation at this time. In addition the data (both field and laboratory) which is required to verify the effectiveness of a kinetic inhibitor is illustrated.

This information will help producers with hydrate problems evaluate the use of kinetic inhibitors. Savings in the range of 50% of the chemical costs and reduced down time can be realized in many cases.

THE FORMATION OF NATURAL GAS HYDRATES

Natural gas hydrates are clathrate compounds in which natural gas molecules are encased in cages of water molecules to form a solid ice-like substance. The molecules commonly found in natural gas which occupy the cages are the alkanes through butane, plus carbon dioxide, hydrogen sulfide and nitrogen. Figure 1 shows the water cages which make up the two crystal forms of hydrates commonly found in petroleum production systems. Each form is a combination of the small cages plus either of the larger cages as shown in the figure. Of the twoforms, structure II is the most common because of the normal distribution of hydrocarbons in petroleum. Structure I would form with pure methane but the presence of as little at O.5 % propane in the gas would result in the formation of structure II instead. All of the cages do not have to be occupied by a guest hydrocarbon molecule in order for the crystal to be stable. If all of the cagesare occupied, the ratio of water molecules to hydrocarbon molecules would be 5.67 to 5.75 depending on which structure is formed. There are several books on the chemistry of natural gas hydrates and some articles on a recently discovered structure H gas hydrate which is formed withheavier petroleum compounds under conditions.1,5,7.