Study on the formation-wellbore coupling flow law during the drilling process of an oil-based drilling fluid system in deepwater fractured formations

A reliable and accurate formation-wellbore coupling flow model is essential for deep-water managed pressure drilling, facilitating key decisions during the complex drilling process. This study develops a wellbore-formation coupled flow model tailored for oil-based drilling fluid systems in deep-water fractured formations, explicitly addressing the coexistence of overflow and lost circulation. The model comprehensively considers the combined effects of formation fluid invasion, mud loss, fluid coupling flow within the wellbore, fracture deformation, and fluid compression. This study uses the model to explore the evolution of overflow and lost circulation conditions and to analyze the impact of various overflow and lost circulation parameters and engineering operational parameters on gas invasion and loss rates. The results indicate that the gas invasion rate increases sharply under coupled flow conditions due to the dissolution and precipitation effects of the formation's intrusive gas. The decrease in frictional resistance along the annulus results in a loss rate significantly lower than that observed under lost circulation conditions when not accounting for wellbore-formation coupling flow. Due to drilling fluid losses, under coexisting gas invasion and lost circulation conditions, the sudden increase in the gas invasion rate occurs 0.16 h later than under conventional gas invasion conditions. In contrast, the gas invasion rate under overflow and lost circulation is slightly larger than that under the conventional gas invasion condition. Compared to the lost circulation condition, due to the dissolved gas precipitation effect, the loss rate under the coexistence of gas invasion and lost circulation experiences a significant drop, with the loss rate being only 63.2 % of that under conventional loss conditions.