Research on Wellbore Temperature and Pressure Prediction of Oil-Based Drilling Fluid System in Deep Water Drilling Under the Condition of Coexistence of Overflow and Lost Circulation

During deepwater drilling, the narrow pressure window frequently results in both overflow and lost circulation incidents. A transient coupling model, based on mass, momentum, and energy conservation equations, is developed to study multiphase flow, temperature, and pressure variations in deepwater oil-based drilling fluid systems during overflow and lost circulation coexistence. The model considers the effects of dissolution, migration, and expansion of intrusive gas components in the wellbore, along with overflow and lost circulation conditions. Results show that free gas precipitation mainly happens in the upper riser section, with the free gas section lengthening as gas invasion rates rise. At the same gas invasion rate, wellhead gas breakthrough times vary in this order: bottom hole gas invasion, upper gas invasion with lower lost circulation, and upper lost circulation with lower gas invasion. In overflow and lost circulation coexistence, with the same gas invasion and lost circulation rates, the annular temperature is slightly higher in upper lost circulation with lower gas invasion than in upper gas invasion with lower lost circulation. Below 500 m, the wellbore temperature under gas invasion and lost circulation coexistence is similar to that under lost circulation conditions. Above 500 m, the wellbore temperature significantly increases and approaches the temperature under the gas invasion condition.