Study on Breathing Effect Under the Coupled Flow Between Formation and Wellbore During Deepwater Drilling

In deepwater drilling, due to the complex coupling mechanism between the wellbore and the formation, the breathing effect is easily induced. The formation of the breathing effect is closely related to the unstable flow between the wellbore and the opening–closing formation fractures. The breathing effect refers to the phenomenon where a portion of the drilling fluid enters the formation fractures during circulation and returns after circulation stops. Its characteristics are similar to those of a well overflow. However, confusing the two can lead to extremely serious consequences due to incorrect handling. Currently, research on the coupled wellbore–formation flow mechanism and the induced breathing effect is still limited, highlighting the urgent need for more refined techniques to identify the breathing effect. To address this issue, a numerical model of the wellbore breathing effect was established by combining the wellbore unsteady flow model and the fracture deformation model. This model comprehensively considers the effects of flow resistance, fluid compressibility, flow path expansion, fracture deformation, and the equivalent damage radius. The model was applied to a subsalt well in the deepwater region of Mexico, and the results showed that the model’s accuracy had an error of less than 10% compared to the field data. Simulations were conducted to analyze bottomhole ECD changes, mud loss during pump start, and mud backflow during pump stop under varying flow rates, which improved the accuracy of identifying the formation breathing effect. This study provides guidance for accurately identifying the breathing effect in the field.