Formation of gas hydrates have been known to cause severe problems of blockages in natural gas pipelines, wellbores and natural gas processing units. Methanol and glycols are commonly used as hydrate inhibitors to control or prevent formation of gas hydrates, due to their ability to lower hydrate formation (or dissociation) temperatures considerably. For the same reason, they are very effective hydrate dissociation stimulants for enhancing gas production from hydrate reservoirs as demonstrated by a production from hydrate reservoirs as demonstrated by a field study in the Messoyakha gas hydrate reservoir of the Soviet Union (Makogon, 1981). Effect of these hydrate inhibitors on the thermodynamic phase behavior of gas hydrates has been well established, however, no experimental data exist on effect of inhibitors on the rate of hydrate formation or dissociation. Hence, the current methods of prevention of hydrate formation by inhibitors have relied upon the thermodynamic data rather than kinetic data.
In this study, the characteristics of hydrate dissociation process during methanol and ethylene glycol injection were Investigated. After formation of methane hydrates in the synthetic cores, hydrates were dissociated by injection of inhibitor solution of known initial concentration, at a constant rate. The pressure was held constant during dissociation and gas production, inhibitor solution temperature and dissociating hydrate front position were monitored continuously. Experimental position were monitored continuously. Experimental results show that the instantaneous rate of hydrate dissociation is function of inhibitor concentration, inhibitor injection rate, pressure, temperature of inhibitor solution and hydrate-inhibitor interfacial (contact) area. Based upon the unsteady state dissociation data, empirical correlations for the rate of hydrate dissociation in presence of inhibitors are developed. The correlations can presence of inhibitors are developed. The correlations can be used to compute the degree of enhancement in the rate of hydrate dissociation by inhibitors in applications such as hydrate prevention or production of natural gas from hydrates.
METHANOL AND GLYCOLS AS HYDRATE INHIBITORS
Gas hydrates are clathrate compounds in which each water molecule forms hydrogen bonds with it's four nearest water molecules to build a solid crystalline lattice structure, that encages gas molecules in it's Interstitial cavities. In 1934, Hammerschmidt determined that these solid gas hydrates form during transportation of natural gas and cause severe problems of blockages in pipelines. Since then, several methods of prevention of formation of gas hydrates were developed. The most commonly employed industrial methods include: removal of moisture content of natural gas by dew point lowering method, heating of a section containing hydrate plug to raise it's temperature above hydrate dissociation temperature, depressurization of a hydrate plug simultaneously from both ends at a slow rate to a pressure below hydrate dissociation pressure, and injection of chemicals which act as hydrate inhibitors into the gas flow stream, Makogon (1981) and Sloan (1990) provide comprehensive reviews of these processes.
The term "hydrate inhibitor" is used for those chemicals which have ability to lower hydrate formation temperature (or shift hydrate equilibria). These chemicals include: methanol, glycols, ammonia, salts such as chlorides of sodium, potassium, calcium and magnesium. Several studies have been reported in literature which provide experimental data on effect of inhibitor provide experimental data on effect of inhibitor concentration on the thermodynamic phase equilibria of gas hydrates. Makogon (1981) and Sloan (1990) provide good review of these studies and a discussion on effectiveness and screening of these inhibitors for hydrate prevention. In general, the degree of inhibition, for hydrate prevention. In general, the degree of inhibition, for hydrate lowering of hydrate formation temperature) is function of the type of inhibitor, inhibitor concentration, pressure and composition of hydrate forming gas.
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