Subsalt Pore Pressure Prediction From Spiral 3D VSP

Summary Subsalt pore pressure in the deepwater Gulf of Mexico is notoriously unpredictable, leading to costly side tracks and non productive time. The r elatively small investment in real-time VSP technology for pore pressure prediction has proven to be a cost effective method of mitigation, and the evolution to 3D acquisition constrains the uncertainty and ambiguity of interpreted pore pressure. Subsalt pore pressure was successfully predicted in a recent subsalt, deepwater exploration well, by estimating Vp/Vs ratio using a 3D spiral look-ahead VSP. The 3D spiral VSP was acquired in less than 18 hours with preliminary estimates of Vp/Vs ratio interpreted within a few hours of data reception. Predictions of Vp/Vs ratio, and thus pore pressure, were made from both raw shot gathers and migrated images. The use of the migrated image from the 3D spiral constrained the uncertainty of the predicted pore pressure, and eliminated ambiguity present on the shot gather and 2D AVA analysis. Completion of this operation marked the 11th consecutive single salt exit for GoM Exploration, and the first spiral 3D for pore pressure prediction.