Wellbore Dynamics of Kick Evolution Considering Hydrate Phase Transition on Gas Bubbles Surface During Deepwater Drilling

It is of high potential and risk to form gas hydrate along the wellbore in deepwater drilled-kick scenarios. Considering the transient mass transfer process that appears as the hydrate shell renewal at gas-liquid interface, we build a fully coupled hydrodynamic-hydrate model to describe the interaction of hydrate phase transition characteristics and wellbore multiphase flow behaviors. Through comparison with experimental data, the performance of proposed model is validated and evaluated. The simulation results show that the hydrate formation region is mainly near the seafloor affected by the fluid temperature and pressure distributions along the wellbore. The volume change and mass transfer over a hydrate coated moving bubble, vary complicatedly, because of the hydrate formation, hydrate decomposition and bubble dissolution (both gas and hydrate). Overall, hydrate phase transition can significantly alter the void fraction and migration velocity of free gas in two aspects: (1) when gas enters the hydrate stability field, a solid hydrate shell will form around the gas bubble, and thereby the velocity and void fraction of free gas can be considerably decreased; (2) the free gas will separate from solid hydrate and expand rapidly near the sea surface (out of hydrate stability field), which can lead to an abrupt hydrostatic pressure loss and explosive development of kick accident. These two phenomena generated by hydrate phase transition can make deepwater gas kick to be “hidden” and “abrupt” successively, and present challenges to early kick detection and wellbore pressure management.