Petrophysical Reservoir Characterisation of a Complex Heavy Oil Carbonate Reservoir in North Oman

Al-Bulushi, Nabil (Petroleum Development Oman) | Al-Mjeni, Rifaat (Shell) | Said, Dhiya (PDO) | Al-Yaarubi, Azzan (Schlumberger Middle East SA.)



There have been a number of major heavy oil discoveries in Oman in recentyears. In order to devise efficient and cost effective recovery mechanismcareful and detailed subsurface understanding of these fields is critical. Tothis end, petrophysical understanding plays a critical role, as it represents abasic building block of the static and dynamic models. The field under study isa fractured carbonate reservoir with high viscous oil. It is believed that thisreservoir has gone through various cycles of drainage and imbibition. Thus, inaddition to the complex geology, understanding of fluid distribution and fluidmobility are among major challenges that detailed petrophysical evaluationneeds to address. Understanding these parameters will help determine thefeasibility of the recovery methodology to be adopted.

This paper details a novel petrophysical workflow that integrates 3D NMR, multiarray/multi frequency dielectric measurements, borehole images, and coreanalysis. The core analysis focused on capillary pressures, Dean-Stark, androck typing. Fracture studies included detailed image analysis and extensivefall off test for understanding the nature and distribution of the fracturenetwork in the reservoir. The wealth of well data coupled with geological anddynamic data reduced the overall reservoir properties and fluid distributionuncertainties.

Dielectric data provided resistivity independent saturations validated byDean-Stark data. Combining dielectric and 3D NMR data allowed better formationcharacterization and fluid type evaluation and their present day distribution.Additionally, this combination indicated that water is not at an irreduciblestate in the reservoir. This was supported by the core saturation heightfunction which indicated that present day saturation should be much higher ifthe reservoir was in drainage mode. These results were crucial to evaluatedevelopment options, underlying uncertainty/risks of this reservoir, and designoptimum future data acquisition requirements.