PORE PRESSURE PREDICTION USING 3D SEISMIC VELOCITY DATA: A CASE STUDY, A CARBONATE OIL FIELD, SW IRAN

Pore pressure is an important parameter in hydrocarbon resource exploration and production. Accurate knowledge of the pore pressure is a key requirement for safe and economical planning of wells. Knowledge of formation pressure is not only essential for safe and cost-effective drilling of wells, but also is critical for assessing exploration risk factors including the migration of formation fluids and seal integrity. Pore pressure prediction based on seismic velocity is a common method for pre-drill pore pressure prediction, especially in sandstone reservoirs. In this method, pore pressure can be obtained from transformation of seismic velocity to pore pressure. But seismic velocities need to be derived using methods having sufficient resolution for well planning purposes. In this study, the velocity derived from pre-stack time migration (PSTM) was available in some parts of the field; however, in another part of the field the only available velocity field was the stacking velocity. This combined velocity field was calibrated with the velocities derived from sonic logs. They were then sorted on regular grid sizes using some geostatistical methods. The effective pressure cube was constructed using the Bowers equation and the calibrated velocity field. The pore pressure cube was constructed by computing the differences between the overburden pressure cube and the effective pressure cube, which was computed using the density cube. Finally the predicted pore pressure cube was calibrated with the measured pore pressures at the locations of 8 wells using geostatistical methods. In a large undeveloped oil field in southwest Iran, some carbonate formations encountered abnormal pressure zones. In the area of study, the combined velocity field was improved and calibrated; then, the pore pressure cube was generated accordingly. The predicted pressures show good agreement with the measured pressures at the 8 well locations.