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Evaluation of Potential Oil Producing Intervals in a Complex Carbonate Heavy Oil Reservoir - A Case Study from Wafra, PZ

Iqbal, Afzal *1; A. Rampurawala, Mansoor 2; Kelsch, Kenneth D.1; Hussain, Azhar 2; O. Remila, Gamal 1
(1) Saudi Arabian Chevron, Kuwait, Kuwait. (2) Schlumberger, Kuwait, Kuwait.

Estimating saturation, identifying movable versus non-movable oil and tar zone, evaluating rock quality and viscosity is complex and so the log based results may have high uncertainty in heavy oil reservoirs coupled with carbonate heterogeneity and low salinity challenges. This complexity sometimes leads to inappropriate well completion and hence results in poor production or unexpected high water cut.

We propose a comprehensive formation evaluation workflow, which combine conventional logs, 3D-NMR, induced-neutron capture gamma ray spectroscopy and image logs to address the multifaceted reservoir-description challenge. The workflow is comprised of four main steps: 1.Volumetric analysis using triple-combo and elemental capture spectroscopy log to evaluate porosity, lithology and resistivity-based saturation; 2.Analyze Magnetic Resonance Fluid (MRF) log data using restricted-diffusion model to evaluate NMR-based saturation and visible oil viscosity; 3.Integrate the results of first two steps to identify zone of movable oil and free water, and estimate volume of tar (oil non-visible to NMR) and viscosity, representing both the visible and non-visible oils; and 4.Review image log and combine with NMR to evaluate rock quality.

The new workflow has been applied in three Maastrichtian wells. The Maastrichtian reservoir is a 3rd shallow carbonate reservoir in Wafra field producing medium to heavy oil at high water cut. The reservoir attributes including medium to high porosity dolomite layers, complex lithology, presence of tar & organic rich shale and compartmentalization poses a challenge in understanding the production behavior. The presence of dissolution channels and fractures adds to this complexity. The main objective of this study is to provide improved log analysis results, based on which well completion and hence oil production could be optimized.

The outcome is the identification of potential oil producing intervals and estimation of oil viscosity and mobility. The integrated approach helped to reduce log analysis ambiguity resulting from carbonate heterogeneity, low/variable salinity and heavy oil. The production data and down-hole wireline formation test results in two wells, core data available in one well and PVT data available in another well is used to validate the log analysis results using the new workflow. We observed good correlation between those results and the actual production and the produced fluid analysis data.


AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain