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Overpressures (abnormally high fluid pressures) represent a significant geohazard and drilling problem. Prediction of overpressures is very important for well planning and safe drilling. However, accurate and reliable prediction requires an understanding of the origins and distribution of such overpressures. Petrophysical properties of the sediments are affected by different overpressure generation mechanisms and in turn help in understanding the types of such mechanisms. There are two distinct overpressure generating mechanisms, namely compaction disequilibrium (undercompaction) and fluid expansion (unloading), each of which have different petrophysical signatures and hence different prediction methodologies. The most common cause of overpressure generation in the majority of the sedimentary basins in the world is undercompaction, in which pressure increases due to rapid burial/loading of the sediments in an effectively sealed impermeable environment. This type of overpressure is normally associated with abnormally high porosities and shows up in changes in velocities. The secondary type of overpressure mechanism is fluid expansion. Thermal induced overpressure is the most common fluid expansion mechanism. This mechanism is very common in areas of high geothermal gradient and can result in significant overpressures. This mechanism, however, is not always present. Thermally induced overpressures result in decreasing effective stress in contrast to overpressure due to undercompaction where a constant effective stress is observed. Thermally induced overpressures are difficult to predict and require a different prediction methodology. Improved knowledge of overpressure generating mechanisms and distribution of pore pressure in a basin provides critical supporting information for the asset team in hydrocarbon exploration and production. This information not only has an immediate impact on drilling cost and safety but also provides insight to key elements in petroleum system analysis. This paper presents a study showcasing the geological control on origin and distribution of overpressure in a HPHT (high pressure, high temperature) field from offshore (water depth ~100-150m) South East Asia. Historically, the offset wells in the field were drilled through complex geological settings including high overpressure (~17-18 ppg), high temperature (170-185 deg C) and variable stress fields. The lithology is dominated by shales and most of the wells drilled in the area encountered drilling challenges with respect to high overpressure development. An initiative for a pore pressure prediction study was undertaken in a semi-regional scale involving ten offset wells in the study area. The main focus was to understand the overpressure mechanism and distribution in the study area vis-à-vis the geological setting and control. This was followed by predrill prediction for the planned wells, as one of the objectives of this study was also to aid in future development well drilling. Well planning based on the study results were done for two prospect wells which were located in similar shallow water. |
