Thermal Expansion Effects in Deepwater High-Temperature High-Pressure Water-Based Drilling Fluids and Their Impact on Shut-in Pressure

Accurate prediction of water-based drilling fluid (water-based mud, WBM) density is of great significance for the precise calculation of hydraulic parameters and wellbore pressure, thereby ensuring drilling safety in deepwater high-temperature high-pressure (HTHP) wells. Based on actual WBM samples from a deepwater well in the South China Sea, experiments on the thermal expansion effects of WBM were conducted under conditions of 25–200 °C and 0.1–150 MPa. The variation trends of drilling fluid density with temperature and pressure were analyzed, and a characterization model for the thermal expansion effect of WBM was established. The model’s accuracy was verified using experimental data from Chen and McMordie. The relative error of the model-predicted drilling fluid density was less than 5%. Considering the influence of WBM thermal expansion characteristics on post-shut-in afterflow, a wellbore pressure calculation model for shut-in operations in deepwater HTHP wells was established. Comparison with data from a case well showed that the model’s calculated shut-in drill pipe pressure (SIDPP) achieved an accuracy of over 95%. The influence of WBM thermal expansivity on bottomhole pressure (BHP), SIDPP, and shut-in casing pressure (SICP) during the shut-in period was investigated. Results indicate that, compared to ignoring the thermal expansion of WBM, accounting for it leads to a slower BHP recovery rate, a longer required shut-in time for pressure stabilization, and higher final SIDPP and SICP values.