Comparison and Application of Pore Pressure Prediction Methods for Carbonate Formations: A Case Study in Luzhou Block, Sichuan Basin

The Luzhou Block in the Sichuan Basin hosts a widely distributed high-quality shale gas reservoir. However, the overlying carbonate strata pose considerable engineering challenges, including severe risks of subsurface fluid loss and wellbore collapse. These challenges are primarily attributed to inaccuracies in pore pressure prediction, which significantly constrains the safety and efficiency of drilling operations in carbonate formations. To address this issue, this study systematically investigates and compares three classical pore pressure prediction approaches—namely, the equivalent depth method, the Eaton method, and the effective stress method—within the geological context of the Luzhou Block. A novel fitting strategy based on laboratory core experimental data is introduced, whereby empirical relationships between field-measured parameters and rock mechanical properties are established to improve model robustness in geologically complex formations. The optimized effective stress model is subsequently applied to the carbonate reservoir interval, and its prediction outcomes are evaluated against measured pore pressure data. The results demonstrate that the effective stress method achieves the highest prediction accuracy, with a maximum deviation of 8.4% and an average deviation of 5.3%. In comparison, the equivalent depth and Eaton methods yield average errors of 12.5% and 12.2%, respectively. These findings suggest that the effective stress method exhibits superior adaptability and reliability for pore pressure prediction in carbonate formations of the Luzhou Block, and holds significant potential for guiding mud density design and improving the operational safety of drilling programs.