Characteristics of wellbore pressure change during shut-in after blowout

Key words:  drilling  drilling blowout  shut in  wellbore pressure  wellbore afterflow  gas slippage

Abstract:

The change of wellbore pressure during the shut-in period after blowout is an important issue for well killing design and affects the wellbore safety. Two main reasons that can cause the wellbore pressure change during the shut-in were accounted, including the wellbore flow after the shut in and the gas slippage due to the density difference between gas and liquid. A wellbore afterflow model was established considering the compressibility of gas and drilling fluid and the elasticity of the wellbore, and a gas slippage model was constructed based on gas-liquid two phase flow principles. It also considered the compressibility of gas and drilling fluid and the filtration of drilling fluid in the wellbore during the shut-in. The results indicate that the bottom hole pressure increases exponentially during the initial period of shut in, but it becomes linearly when the bottom hole pressure is greater than the formation pressure. The afterflow is essential to the bottomhole pressure during the initial shut-in period, while the gas slippage prevails after the bottomhole pressure is greater than the formation pressure.