A kinetic inhibitor is a polymeric chemical that, when added to a production stream, will not change the hydrate formation temperature but will delay the growth of hydrate crystals. These chemicals are polymericand include N-vinylpyrrolidone (5 ring), saccharides (6 ring}, and Nvinylcaprolactam (7 ring).
An anti-agglomerator is an alkyl aromatic sulphonate, a quaternary ammonium salt, or an alkyl glycoside surfactant. When added to a production stream with a continuous oil phase, this chemical will minimise hydrate crystals from agglomerating or growing in size. The continuous oil phase provides a medium to transport the hydrate crystals through the piping system while crystal growth is delayed. This chemical and its physical reaction with hydrate crystals is not dependent upon the amount of subcooling and therefore has a wide range of pressure-temperature applications.
The use of kinetic inhibitors and/or anti-agglomerators in actual field operations is a new and evolving technology. These are various formulations of chemicals that can be used in a mixture of one or more kineticinhibitors and/or anti-agglomerators. At the current time, to get an “optimum” mixture for a specific application it is necessary to set up a controlled bench test using the actual fluids to be inhibited and determine the resulting equilibrium phase line. As the mixture of chemicals is changed, a family of equilibrium phase lines will develop. This will result in an initial determination of a near “optimum” mixture of chemicals.
To determine the appropriate injection rate, a field test should first beperformed at one of the industry-sponsored full-scale loop test facilities. The “optimum” mixture, its injection rate, and location of injection points will be a function of flow geometry, fluid properties, pressuretemperature relationships, etc., that will be encountered in the actual field application. The appropriate injection rate and location of injection points can be determined from this test by observing pressure increases, which indicate that hydrate plugs are forming.
Application of this “optimum mixture of chemicals” in the actual field installation would begin with the design injection rate of the mixture and by adding a sufficient concentration of a thermodynamic inhibitor to inhibit the free water that may exist in low spots in the piping system. After the system has reached an operating equilibrium, the volume of the thermodynamic inhibitor is then decreased in stages with a time period between each stage to ensure that no hydrate plug is formed. Some small amount of thermodynamic inhibitor will continue to be used to inhibit free water in low spots. In this manner the lowest chemical cost that will provide the necessary flow assurance is eventually reached.
