Upscaling Saturation-Height Technology for Arab Carbonates for Improved Transition-Zone Characterization
Authors: Buiting J.J.M.. J.M. 
Published: January 2011

Much of the oil in Saudi Arabia is stored in giant and supergiant multireservoir fields. The Arab-D limestone is the most important of these and the most prolific. The large volumes, excellent porosity, and high productivity of these reservoirs do not mask the fact that these carbonates have complex pore systems. The problems associated with heterogeneous carbonate reservoirs pose significant and longstanding modeling complications that are not yet fully addressed by the industry. One important difficulty is the accurate modeling of the substantial transition zones present above the freewater levels (FWLs). In these giant fields, these transition zones hold large amounts of oil and are important commercial objectives. Commerciality requires accurate assessment of saturations and rock properties. Standard J-function methods are inadequate to model the well-log observed saturation-height behavior in the transition zones. It is necessary to characterize and account for the pore system variations and scale when modeling the saturation behavior of large rock volumes. The reservoir properties of geocells and wellbores must be reconciled with the measurements on core plugs. The measurements performed on these tiny pieces of rock need to be upscaled to represent the reservoir bulk properties.

Upscaling of core-plug-scale, laboratory measured porosimetry data and transport properties has been a general and persistent problem since the beginning of reservoir simulation. This critical step has been handled, over the years, using a wide variety of numerical computational schemes, approximations, and empirical methods. In this paper, we take the different and very specific approach of upscaling the capillary pressure data for the Arab-D limestone. We base the approach on the availability of a large amount of mercury (Hg) -injection data and statistical analysis thereof, obtained by fitting hundreds of individual core plugs to Thomeer functions.

For the Arab-D limestone, and similar carbonates, we derive a closed-form analytic expression for the upscaled capillary pressure function, which has significant implications for improving transitionzone hydrocarbon-volume estimates for this important petroleum system. The analytic expression also offers major efficiencies compared with other methods used by petroleum engineers, provided that the pore systems are adequately investigated and statistically characterized. A key result of the upscaled formalism is that reservoir cells, consisting of a large variation of pore systems, will start to fill with hydrocarbons much closer to the FWL than when using saturation-height functions based on simple averaged pore system parameter values. Therefore, transition zones for upscaled reservoir elements (and well-log volumes) are thicker than simple calculations based on data from single core plugs would indicate. The accurate upscaling of pore-system architecture is a major step toward the full understanding of the fluid-rock interactions of giant-field transition zones in the Middle East and is an industry technical milestone.