Accurate Calculation of Hydrocarbon Saturation Based on Log Data in Complex Carbonate Reservoirs in the Middle East

Tawfiq A. Obeida¹*, Faisal Al-Jenaibi², Sami Rassas¹, and Samy Serag El Din^1
1ADCO
²ADNOC
*[email protected]

To generate saturation functions (capillary pressure drainage curves or J-functions) to initialize complex carbonate reservoir simulation models always presents challenges to petrophysicists and reservoir engineers. This is especially the case when Special Core Analysis Laboratory (SCAL) data is neither available, nor rock-property trends such as the porosity-permeability relationship, which are used to assign a saturation function in 3-D models. As a start, the proposed method required the initial water-saturation distribution in the 3-D model. It can be calculated by log-derived J-functions if the permeability distribution is available. Otherwise, it can be calculated using a software program that is based on water-saturation log data. For each reservoir rock type (RRT), the capillary pressure calculated from the height function, was plotted versus log-derived water-saturation. Depending on the data scatter, ranges of water saturation can be determined in order to generate a capillary pressure (Pc) curve for each range. The Pc curves per RRT were based on water-saturation ranges regardless of the rock properties trends. To differentiate between such saturation functions, a script file was written to discriminate between these regions per RRT. Saturation numbers were then assigned for each region and these were used to initialize a massive and complex carbonate reservoir simulation models (Shu’aiba Formation) in the United Arab Emirates. The initial water-saturation profile from log data matched the water-saturation calculated by the dynamic model in 90% of the wells (more than 100 wells at initial water saturation). The difference in original-oil-in-place calculations between the static and dynamic models was less than 2%.

AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain