Hydrates are crystalline water compounds that may form in deepsea pipelines and have the ability to quickly and efficiently plug flow. Given this issue, gas hydrate formation tests have been conducted and reported in a number of published articles. However, most published data has focused on continuous "steady-state" operations and a gap exists for the more complicated, yet interesting transient shut-in/restart scenarios. Motivation to study such systems are further exacerbated given that industrial fields can operate outside the gas hydrate formation region during continuous production. Hydrates become a major concern especially during unplanned shutdowns and restarts. Therefore, this work focuses only on hydrate transient results. Industrially relevant scenarios are investigated by looking into gas hydrate formation and its effect on fluid transportability through visualization and pressure drop during shut-in and restart in a laboratory-scale flowloop.
The effects of parameters such as water content and dispersion state, shear, chemical injection, and oil properties on hydrate formation under transient conditions are reviewed and general agreement is shown between the studies. Overall, the general consensus is that lower water contents, dispersed water, high shear, thermodynamic inhibitor injection, dosing of anti-agglomerants or kinetic hydrate inhibitors, higher oil viscosities, and oils with natural chemical surfactant properties can be favorable for ensuring successful restart of hydrate forming systems.
Flowloop tests in a gas-dominated system show that film growth over and around the liquid pool at the bottom of the pipe could lead to plugging in a significant portion of the flowline. This could continue to build up over time (with the addition of fresh hydrate forming materials in an actual system) and inhibit gas flow. Further tests on an oil-dominated system with an intermediate water content demonstrate how shut-in conditions could potentially increase the risk of plugging compared to dispersed lower water content systems. In the former case, water accumulation to low spots in the flowloop allowed for localized areas of high water concentration, where rapid hydrate bed formation in partially dispersed oil-water sections controlled the overall fluid transportability.