Effect of residual guest concentration in aqueous solution on hydrate reformation kinetics

Abstract

Since the occurrence of gas hydrate mainly takes place at water–gas interface where having higher gas concentration, the gas concentration in the aqueous solution plays an important role in affecting the hydrate nucleation. Meanwhile, the aqueous solutions with dissolved gas are commonly found in oil / natural gas reservoirs, production wells and multiphase transportation pipelines, since these systems are always pressurized. This work investigated the reformation characteristics of gas hydrate in the aqueous solution with residual guest concentration, which was achieved by setting the dissociation (hold) pressure above the atmospheric pressure. The experimental results demonstrate that the induction time of hydrate reformation would significantly decrease with the increase of hold pressure, while barely change with duration time and reformation time. As the hold pressure can accelerate hydrate nucleation, an equivalent driving force model was established. This model exhibits more accurate and universal to represent the actual driving force of hydrate formation compared with using the subcooling. Based on the relationship between the induction time and the equivalent driving force, another model was established to predict the induction time of hydrate formation. When the difference of gas concentration in aqueous solution was eliminated by keeping the same hold pressure, the induction times of fresh water are higher than those of the decomposed water under all the higher driving forces. In addition, even though the hold pressures were always kept the same, a heat treatment in the hold pressure stage would cause the increase of the induction time of the subsequent hydrate reformation. All these findings prove the existence of some residual water structures in the solution after hydrate decomposition, resulting in the memory effect.