Multistage Approach on Pore Pressure Prediction—A Case Study in South China Sea

Abstract

Abstract A multistage approach on pore pressure prediction was successfully used for a drilling program in the South China Sea. The objectives were to optimize the casing-shoe position by predicting the overpressure profile in the formation and to select the optimal mud weight by forecasting the formation pressure variation. Before spudding the well, data from a surface seismic survey and log data from a nearby well were collected to build a pore pressure uncertainty model. This initial model had a pore pressure profile with relatively large uncertainties. While drilling, the model was updated in stages with additional measurements, such as those derived from vertical seismic profiles (VSPs), dipole sonic logs, gamma ray (GR) logs, etc. These additional data significantly reduced the uncertainty of pore pressure estimation. At each intermediate total depth (TD), wireline logs and VSPs were acquired. Pore pressure profile above the current TD was estimated using the wireline logs and VSP velocity. Pore pressure profile below the current TD was estimated using the look-ahead VSP inversion result. Above the current TD, a cross-check validation was made by comparing the pore pressure profiles estimated from wireline logs and VSP data. The predicted pore pressure model was calibrated with the actual mud weight and a well integrity test at 3505 m. At the well-planning stage, the maximum pore pressure was estimated to be 1.55 g/cm3. At intermediate drilling stages, the pore pressure was consistently estimated to be between 1.9 and 2.1 g/cm3 below a depth of 4100 m. This prediction was later confirmed by the formation test pressure value of 1.98 g/cm3 at 4226 m. All pore pressure prediction results were delivered within six hours after each intermediate data acquisition. The pore pressure prediction results were successfully used for the critical decision of setting the casing shoe and selecting the mud weight.