The phase transition, from asubcriticalstate to a gaseousstate, of the natural gas with high H_2S content and thesolubility of the H_2S component in the drilling fluid will make the multiphase flow behavior very different from the pure natural gas-drilling fluid two-phase flow under the gas kick condition in a deep well. With consideration of the phase transition and thesolubility of the H_2S component in the natural gas, a multiphase flow model is established. Thesimulation analysis results indicate that, for a typical case of a well depth of 4 325 m, the density of the 100%-H_2S natural gas can be 4 times higher than that of the 0%-H_2S natural gas, and thesolubility of the 100%-H_2S natural gas is 130 times higher than that of the 0%-H_2S natural gas. These will make the detection of the gas invasion more difficult. While the invasion gas moves up along the wellbore to a certain position, the phase transition and the release of the dissolved gas may cause a rapid volume expansion, increasing the blowout risk. The calculation results alsoshow that the risks of a gas kick can be reduced by increasing the wellhead back pressure.
The hydrate phase transition presents new problems and challenges for the deepwater well control in the drilling processes.A simulation model is built for deepwater gas kicks with consideration of the hydrate phase transition.The model is based on the multiphase flow governing equations and the hydrate phase transition calculation equations.The influence of the hydrate phase transition on the gas kick hydraulics is investigated through numerical simulations.It is shown that the diameter of the gas bubbles can significantly influence the hydrate phase transition effect.The influence of the hydrate phase transition on the gas kick hydraulics increases with the decrease of the average gas bubble diameter.The hydrate phase transition adds a"hidden"nature for the well kick in deepwater and hinders the early detection of the gas kick.The influence of the hydrate phase transition on the gas kick hydraulics is also studied in the case when the hydrate inhibitor is added to the drilling fluid.
