Study of Hydrate Dissociation by Methanol and Glycol Injection

Abstract 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 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 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 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.