Prediction and Analysis of Drilling Temperature and Pressure Field in Deep Water Gas Hydrate Formation

In the drilling process of deepwater gas hydrate formation, the gas–liquid-solid three-phase flow caused by the decomposition of hydrate cuttings poses a serious challenge to the accurate prediction of wellbore temperature and pressure field. In this study, the prediction model of transient temperature and pressure field is established considering the factors of hydrate cuttings migration and decomposition, hydrate phase change heat and interphase mass transfer. Based on the geological parameters and drilling data of the first depressurization production test of natural gas hydrate in the Shenhu area of the South China Sea, the influence of the migration and decomposition of hydrate cuttings on the wellbore temperature and pressure field during the drilling process of deepwater hydrate formation is analyzed with the help of this model. The results show that under the calculation conditions of this study, the annulus drilling fluid returns to the position 600–700 m from the sea level and the hydrate cuttings begin to decompose. The difference between the return temperature of drilling fluid with and without hydrate cuttings decomposition is 0.8–1.7 ℃. The gas content in the annulus from the decomposition position to the wellhead increases gradually. The calculation and analysis show that under the conditions of the drilling fluid inlet temperature of 22 ℃ and the mechanical penetration rate of 15 m/h, the gas content in the wellhead is about 8.9%, and the pressure drop at the bottom of the well caused by the decomposition of hydrate cuttings is about 0.16 MPa. The research results can provide reference for drilling design of deepwater gas hydrate formation.