A Shut-in Pressure Estimation Method for Deepwater Drilling Considering the Thermal Expansion Behavior of Drilling Fluids

Deepwater drilling is conducted in structurally complex formations and harsh marine environments, which greatly intensifies the difficulty of maintaining well control. Reliable prediction of the shut-in pressure response in the wellbore is therefore essential for safe and efficient operations. In this work, published laboratory data are reanalyzed to derive empirical correlations that describe how drilling-fluid density changes with temperature and pressure. On this basis, the thermal expansion behavior of the drilling fluid is incorporated, and segmental temperature-field models are formulated for the seawater column, choke line, and formation intervals. A corresponding gas-expansion model and a shut-in wellhead-pressure prediction model suitable for deepwater conditions are then developed. Numerical examples are used to investigate the evolution of temperature, pressure, and fluid properties under different geothermal gradients. The simulations indicate that drilling-fluid density decreases exponentially with temperature and increases approximately linearly with pressure. In the choke line, the annular temperature continuously drops under the cooling effect of seawater, whereas in the formation interval it decreases in the upper part and increases toward the bottom, ultimately approaching the geothermal profile. When the thermal expansion of drilling fluids is taken into account, the geothermal gradient exerts a strong influence on the shut-in behavior: larger gradients enhance fluid expansion, leading to higher wellhead casing pressures and faster recovery of bottomhole pressure, which places stricter requirements on well-control design. The proposed method provides a useful theoretical reference for pressure prediction and safety evaluation during deepwater drilling shut-in.