Abstract
Drilling into deepwater prospects of South East Asia have been very challenging especially in NW Borneo and subsurface hazards related to pore pressure and wellbore stability remain the greatest obstacle. A myriad of wellbore issues have led to failure of some wells to reach geological targets and the drilling cost as high as US$100 million (as of 2014) is not uncommon in the area. It is important to be able to understand the geological nature of pore pressure mechanism in deepwater Sundaland continental margin (DSCM) and thus redefine pore pressure prediction strategy in the area to better align with the regional characteristics.
The currently established empirical effective stress-based pore pressure prediction is dissected to reveal its pitfalls especially regarding its application to DSCM. Compaction mechanism assumptions are also revisited in depth. Three wells from a deepwater field in Northwest Borneo are used as a case study to demonstrate the distinct overpressure nature of the area.
This study confirms that DSCM not only exhibits overpressure profile typical to deepwater anywhere else, but also possesses distinct geopressure characteristics related to its unique geological setting, most notably the active compressional shortening of the mega thrust-fold belts accompanied by continuous rapid deltaic sediment deposition. As a result, unique geopressure-related challenges prevail in this area, such as: shallow top of overpressure, the failure of standard empirical compaction methods, and prominent centroid phenomenon.
It is successfully demonstrated that the shallow top of overpressure of DSCM is linked to rapid sedimentation of low permeability sediments; undrained tectonic shearing is the central cause of common underestimation of popular effective-stress based pore pressure prediction method; and that the dominance of compressional geological structures explains the high likelihood of centroid effect.
