Simulation Study of Wellbore Three-Phase Flow After Gas Invasion in Large-Displacement Wells Drilled in Seabed Hydrates

During the drilling of natural gas hydrate reservoirs, gas invasion may occur, which has an adverse impact on the upward return process of cuttings and the control of bottom-hole pressure. A multiphase fluid CFD simulation method based on the Eulerian model was optimized. With the help of Fluent 23R1 software, simulations of the three-phase flow of drilling fluid–cuttings–invaded gas in the vertical section, build-up section, and horizontal section of the complex annulus multiphase flow in the hydrate drilling wellbore based on particle dynamics were carried out. The characteristics of multiphase flow in the wellbore after gas invasion were revealed, and the impacts of gas invasion on the deposition and migration of cuttings and the control of bottom-hole pressure were analyzed. The research shows that after gas invades the wellbore, it has little impact on the upward return of cuttings in the build-up section. However, in the vertical and horizontal sections, with the increase in the flow rate of the invaded gas, the slip and upward return of cuttings are promoted. The gas invasion velocity has a more significant impact on the cutting migration in different well sections compared to the gas invasion volume fraction. A faster invasion velocity accelerates the upward return of cuttings in the wellbore annulus. When gas invasion occurs, it has little impact on the pressure in the build-up section, decreases the pressure gradient in the vertical section, and increases the pressure gradient in the horizontal section. At the same time, there is a high linear correlation between the pressure gradient in each well section and the density of water-based drilling fluid. By reasonably controlling the density of drilling fluid according to the gas volume fraction detected at the wellhead, the aggravation of gas invasion can be significantly prevented. The research methods and calculation results are helpful in providing a reference for the subsequent drilling process of hydrate reservoirs.