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.
