Pressure distribution and fluid migration law of gas lift reverse circulation well washing flow field in drilling sinking

In order to reveal the pressure distribution and fluid migration law of the gas lift reverse circulation washing flow field in drilling method, the ø5m advanced drilling of the north wind well in Taohutu coal mine was taken as the research background. Based on the relevant theories of fluid mechanics, a pressure mathematical model of the gas lift reverse circulation washing flow field was established, and the theoretical analytical solutions of the fluid pressure in the drill pipe annulus, bottom hole and inside the drill pipe were given. Secondly, the CFD-DEM method (coupling computational fluid dynamics and discrete element method) was used to establish a numerical model of gas lift reverse circulation multiphase coupling well cleaning, verifying the correctness of the pressure model of the well cleaning flow field and obtaining the migration law of the fluid in the well cleaning flow field. Finally, based on the pressure mathematical model and theoretical analytical solution of the well cleaning flow field, the influence of factors such as gas content, mud density, mud viscosity, rock slag density, and slag content on the pressure of the well cleaning flow field was explored. The results indicate that: The average radial flow velocity of the horizontal bottom hole fluid of the truncated cone drill bit is greater than the tangential flow velocity and the axial flow velocity; Rock debris undergoes dynamic cyclic transport at the bottom of the well, including spiral downward movement, accumulation at the bottom of the well, propulsion and suspension, adsorption and lifting. It is transported in the form of “low flow rate, high density” and “high flow rate, low density” in the liquid-solid and liquid-solid gas sections inside the drill pipe, respectively. The fluid pressure inside the drill pipe decreases approximately linearly along the wellhead direction. At the gas injection end, the fluid pressure drops sharply, and the pressure difference inside and outside the drill pipe reaches its maximum; The fluid pressure at the bottom of the well shows a trend of “first slightly decreasing, then significantly decreasing” from the outside to the inside, with a greater decrease in fluid pressure and rate in the effective adsorption zone at the bottom of the well. The gas content, mud density, and fluid pressure difference inside and outside the drill pipe are positively correlated, while mud viscosity, rock slag density, liquid-solid and liquid-solid gas section slag content are negatively correlated. The sensitivity of various factors to the fluid pressure difference inside and outside the drill pipe can be summarized as follows: mud density>gas content>rock slag density>mud viscosity>liquid-solid section slag content>liquid-solid gas section slag content. The research results can provide useful theoretical references for solving the technical problem of low efficiency of gas lift reverse circulation well washing in Jurassic strata drilling method.