Study of Wellbore Flow Behavior Under Gas Invasion in Large-Diameter Wellbores During Circulation Conditions

The deep reservoirs in Western China are rich in oil and gas resources and are widely distributed in onshore basins such as the Tarim Basin and the Qaidam Basin. Due to the complex geological conditions and deep burial of carbonate reservoirs in the Tarim Basin, along with a very narrow safety pressure window and large wellbore sizes, controlling wellbore pressure becomes challenging, leading to the occurrence of vicious accidents such as well kicks, wellbore losses, and simultaneous blowouts. Improper handling of these situations can escalate into blowout accidents, severely impacting drilling cycles. This article, based on the drift flow model, establishes a multiphase flow model of the wellbore after gas invasion in large-diameter wellbores and conducts numerical simulations to study the flow behavior of the wellbore under circulation conditions after gas invasion. The results show that under throttling circulation conditions after gas invasion, different hydraulic parameters have varying effects on the gas content in the wellbore. Among these parameters, drilling fluid density has the most significant impact, followed by wellhead backpressure, drilling fluid flow rate, and drilling fluid viscosity. The article also simulates the impact of different wellbore sizes on the gas content in the wellbore and concludes that different wellbore sizes have a relatively minor effect on gas content.