Online Journal for E&P Geoscientists

Al-Nasser, Kadhem ^*1; van der Graaf, Ap ²; Filippidou, Nikoletta ²
(1) South Rub Al-Khali Company Ltd., Al-Khobar, Saudi Arabia. (2) Shell International E&P, Rijswijk, Netherlands.

The South Rub Al-Khali Company (SRAK) is a joint venture of Saudi Aramco (50%) and Shell (50%), exploring for gas in the Rub’ al-Khali (‘Empty Quarter’) of Saudi Arabia. In July 2010, it completed its commitments for the ‘First Exploration Period’ and entered into the second exploration period on July 25th 2010.

Proper estimation of the hydrocarbon saturations in the reservoirs is crucial for the estimation of the in place volumes. Saturation Height Modeling (SHM) is fundamental in understanding the potential of fluid distribution in the reservoir with depth. It is an essential input to static and dynamic modeling in order to have a good estimate for total GIIP and characterize flow dynamics. Hence, getting a good estimate of gas volume in-place will result in crucial decisions for the ventures’ future.

Building a SHM using core data with proper rock typing has several advantages over Archie based saturation, especially in carbonate. Limitations of Archie/derived water saturation are mainly related to the input value uncertainties, such as Formation Resistivity Factor (mainly due to vugs), Resistivity Index (wettability effect), Formation Water Resistivity (absence of water sample in current study), resistivity tool resolution and physics.

Due to the uncertainties mentioned above, SRAK has built a Saturation Height Model for each rock type and used these for volume estimates through static modeling. The revised models have been generated using new SCAL data from latest cored wells and novel measurements (interfacial tension of sour gas) using reservoir fluids to arrive at a more accurate representation of the fluid-rock interactions.

Building good Saturation models require several steps, iterations and rigorous integration among subsurface disciplines. These steps can be summarized as: (1) Data QC and Preparation to ensure plug integrity and applying necessary corrections, (2) Rock typing, to ensure better model match, (3) Building the model by using Brooks-Corey model and petrophysical properties relations; (4) PVT Data significance to investigate fluid properties impacting on saturation, (5) K-Phi transform for each rock to enhance the fit, (6) Log-Core integration to compare the results and estimate free water levels if possible.

This presentation will explain these steps in detail and discuss results and conclusions.