Distribution And Origin Of Overpressure In The Nile Delta
Geoscience Honours Degree, 2011
University of Adelaide
The Nile Delta hosts significant commercial hydrocarbons. However, overpressures are commonly observed and can represent a significant drilling hazard. Understanding the distribution and mechanisms driving overpressure is essential for reliable pore pressure prediction, and thus avoidance of well control incidents such as blowouts. Furthermore, knowledge of regional pore pressure characteristics, such as overpressure onset and distribution, is valuable for both safe drilling and more accurate reservoir and production modelling. Gas and fluid behaviour can be more accurately predicted given the origin of overpressure. This study seeks to characterize the origin of overpressure through the use of sonic velocity-vertical effective stress analysis (Bowers‟ analysis). This analysis has not previously been undertaken in the region and will discern between overpressures resulting from disequilibrium compaction and fluid expansion.
The Nile Delta is deemed to be greatly compartmentalized given that overpressure distribution through the basin is highly heterogeneous. This is shown through the variability of depth to top of overpressure, and the sporadic nature of significant overpressure in the Pliocene sequences. Sonic velocity-vertical effective stress analysis of six overpressured wells revealed that fluid expansion mechanisms occur in five of the wells. Disequilibrium compaction represents an underlying trend, while fluid expansion is attributed to high overpressure variability.
This study postulates that fluid expansion overpressure within the Pliocene was not generated in situ. This has been inferred from the intermittent nature of fluid expansion overpressure, and the compartmentalization of the overpressures in reservoir sequences. I thus postulate that the primary overpressure mechanism is disequilibrium compaction, but that fluids and gases are also episodically transferred vertically up fracture conduits or reactivated faults to generate compartmentalized fluid expansion overpressure. In the recipient compartments of this excess volume, fluid expansion overpressure can contribute over 60% of observed overpressure.