Simulation study on multiphase flow considered seawater salinity in wellbore for deep-water bearing hydrate formation drilling

The gas-liquid-solid multiphase flow caused by decomposition of hydrate cuttings is a new problem facing the drilling of marine hydrate formations. In this paper, a gas-liquid-solid non-isothermal transient flow model is proposed considering the coupling relationships between hydrate decomposition, and heat and mass transfer in multiphase flow. The accuracy of the model in predicting gas void fraction, friction pressure and wellbore temperature is verified by using indoor experimental data and field measured data. Using this model, the influence of seawater salinity on the gas-liquid-solid flow behaviors is investigated. Numerical simulation results show that the decomposition area and decomposition rate of hydrate in the wellbore gradually increase with the increase of seawater salinity. Moreover, the higher the salinity of seawater, the greater the reduction in bottomhole pressure and the higher the decomposed gas void fraction reaching the wellhead. Increasing wellhead backpressure and lowering inlet temperature of seawater can effectively control the amount of hydrate decomposed in the wellbore. To ensure wellhead safety and reduce the rate of hydrate decomposition, the wellhead backpressure should be greater than 3.5 MPa. Compared with the wellhead backpressure of 2 MPa, the gas void fraction at the wellhead decreases 1.75 times when the wellhead backpressure is 5 MPa. When the seawater inlet temperature is in the range of 15 °C–30 °C, the seawater salinity control in the range of 8‰∼19‰ can manage the maximum gas phase volume fraction at the wellhead within 10%. Compared with the inlet temperature of 15 °C, the hydrate decomposition rate at the wellhead is 1.6 times higher when the inlet temperature is 30 °C.