The Ascending Law of Gas Bubbles in a Wellbore Considering the Phase Change of Natural Gas Hydrates during Deepwater Well Shut-in

During the shut-in of deepwater wells, the formation of natural gas hydrate will significantly delay the migration of bubbles and affect the safe operation cycle of drilling. The bubble ascending dynamics considering the phase change of hydrate was studied. In the study, an indoor vertical cylinder was used to simulate the deepwater wellbore and to investigate the growth characteristics of hydrate film on the surface of methane bubbles.. A model was proposed which incorporated the hydrate lateral growth model and the hydrate film thickness prediction method considering natural convection heat transfer. The correlations between hydrate bubble deformation rate and Morton number, drag coefficient and Reynolds number were explored. A comprehensive prediction model of bubble ascending velocity in wellbore considering hydrate phase change under shut-in conditions was established based on the study, and the safe operation cycle of a well in the South China Sea was predicted and analyzed. The experimental and simulation results show that the newly established natural convection heat transfer model has higher prediction accuracy for the lateral growth rate and thickness of the hydrate film and that the deformation rate of hydrate bubble decreases with the Morton number. The drag coefficient decreases first and then increases gradually with the Reynolds number and the corresponding calculation formula was obtained through fitting. Studies suggest that the formation of hydrate on the surface of the bubbles can significantly reduce the ascending velocity of bubbles and prolong the safe operation cycle. However, the risk of hydrate blockage will increase as gas reaches the subsea wellhead, and pertinent well control measures should be taken according to the shut-in time.