Overpressure Mechanisms in Deep Drilling in Western Offshore India

Kumar, Rajeev R.; Rao, Dhiresh G.; Parashar, Sarvagya; Singh, Hemant
DCS, Schlumberger Asia Services Ltd, Mumbai, India.

Accurate pore pressure prediction is one of the key factors for safe drilling of wells and casing design. There are two key over pressure mechanisms. One is based on disequilibrium compaction where log responses (seismic velocity, resistivity and sonic compressional slowness) are used to predict the pore pressure profile using the standard available methods e.g Eaton's method. Generally, higher porosities are associated with sediments underwent undercompaction. However, there are other mechanisms which are associated with post-depositional events like hydrocarbon maturation, fluid expansion, and vertical pressure transfer where higher geothermal gradients are present. The undercompaction methods are unable to predict these over pressure region.

This paper provides new insights into the nature, origin, petrophysical and drilling effects of the overpressure in western offshore, India. The log derived shale porosities show the signature of undercompaction at most depth intervals. At greater depths with higher geothermal gradients, sonic compressional slowness shows the signature of hydrocarbon maturation and vertical pressure transfer where no porosity anomaly is present. Sediments of Eocene and Oligocene Ages (Daman and Mahuva formations) show this characteristic more prominently. Formation pressure ranges to 13 to16 ppg in these formations. The compilation of extensive dataset ranging from formation pressure measurements to sonic, density, seismic velocity and porosity logs demonstrates the presence of compartmentalized over pressures in the region. At some depth intervals, shale section acting as seal is generating the overpressure in many wells.

A procedure is presented to identify the different high pore pressure regimes which can be used as one of input for well planning and mitigation of the drilling risks. Different log based methods are studied and tested using the available measured data to associate the mechanisms. Geological structural complexity and the associated role in the generation of overpressure are also discussed

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012