Abstract
It is not uncommon to encounter gas hydrate problems in the oil industry. Gas clathrate hydrates could form at low subsea temperatures and high pressure and serve as nuisance by blocking flowlines and reducing production capacity. It may form in subsea production facilities such as wellheads, jumper sections and risers during shut-in, start-up and steady-state conditions. Preventive methods employed include heating and chemical inhibition but these turn out to be progressively uneconomical, thus application of time-dependent kinetics is essential to manage production operations.
This paper seeks to remediate this challenge by using thermal transient analysis and nucleation kinetics to estimate the time of onset of gas hydrates in production systems. The induction time was estimated using empirical models and compared to flow loop tests results. The laws of heat transfer for an unsteady state system were applied to develop a temperature decline model specific to a subsea installation. The hydrate formation equilibrium condition was predicted from existing correlations that fit experimental results. A sensitivity analysis was done to study the effects of key parameters affecting the prediction model for the formation of gas hydrate in the field studied.
This technique was applied to a subsea system in an offshore field. The induction time estimate corroborated flow loop test results. However, the nucleation phase is random and probabilistic distribution was used to define confidence levels – induction time ~50 min (90% probability). The thermal transient model was modified by performing a history match at a prolonged shut-in period by linearization and fitting techniques and the resulting model was used to predict the period to hydrate equilibrium conditions at other shut-in periods of the same system. The results showed that hydrate formation could begin within 12 hours for an uninhibited system. The key parameters affecting the kinetics of the onset of gas hydrate formation during shut-in are system pressure, system temperature, initial temperature, cooling rate, as well as induction time. Hydrate formation can thus be avoided by reducing the operating pressure to less than its hydrate dissociation pressure (<13 bara) or prolonged by heating the flowline to increase the initial temperature before shut-in.
