Modeling of wellbore multiphase flow with free gas influx during horizontal drilling in marine hydrate reservoirs

The wellbore multiphase flow and managed annulus pressure are key issues during drilling in hydrate reservoirs. In this study, the intercoupling interactions of free gas, drilling fluid, cuttings, and hydrates are considered, and a gas–liquid–solid multiphase four-component flow model that incorporates the hydrate phase transition is proposed. The hydrate formation and multiphase flow submodels in the unified model were verified by comparing with the collected data from a hydrate formation experiment in a flow loop and a full-scale gas kick experiment, respectively. The multiphase flow behaviors and gas kick migration, as well as the effects of hydrate phase transition, were thoroughly analyzed. Simulation results show that the hydrate decomposition in the cuttings is sufficient to cause a pit gain of 1.96 m3, which is approximately equal to that of a 3 MPa bottomhole underbalanced pressure. Thus, failure to consider this factor in multiphase flow simulation will result in significant errors in the bottomhole and formation pressure inversion. The phase transition of hydrates delay the pit gain in reaching 1 m3 by 32 min. The prediction errors of the pit gain and outlet flow rate at the initial stage of overflow can reach up to 94.33% and 6.17%, respectively, when the hydrate phase transition is neglected. Using managed pressure drilling and increasing the wellhead pressure to 1.5 MPa in the case study can ensure that the gas void fraction in the annulus is within a controllable range. This unified model can provide insights into gas kick migration and wellbore pressure control for horizontal well drilling in hydrate reservoirs.