Study on the Change Law of Annular Outlet Flow Rate in New-Type Dual-Gradient Drilling under Gas Cut Condition

In order to accurately understand the change law of annular flow rate in new dual-gradient drilling under gas cut conditions, a gas-liquid two-phase flow model that considers density mutation has been established according to the theory of gas-liquid two-phase flow in the wellbore. They then analyzed the change of annular outlet flow rate under gas cut condition, as well as the influence of different factors on the change of annular outlet flow rate. The results showed that the change rate of annular outlet flow rate increased abruptly when the gas front reached the separator. Further, when the separator was located below the mud line, an abrupt increase of annular outlet flow rate would occur earlier than that of the gas at the bottom of the riser, which was helpful to the earlier identification of gas influx. Further, the influence degrees of density difference of light/heavy drilling fluids, gas influx, flow rate, separator position, well depth and wellhead back pressure on the change of annular outlet flow rate were reduced in turn. The gas-liquid two-phase flow model that considers the density mutation could accurately predict the change of annular outlet flow rate in the new-type dual-gradient drilling under gas cut condition,and provide a theoretical basis for early overflow monitoring in the new-type dual gradient drilling.