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ABSTRACT: Carbonate Reservoirs Evaluation with Advanced Well-Log Data

Olesen, Jean-Remy1, Dhruba Dutta1, K.M. Sundaram2
(1) ONGC-Schlumberger Joint Research Centre, New-Delhi, India
(2) ONGC Mumbai Regional Business Centre, Mumbai, India

In carbonates, fines are often composed of carbonate material rather than shale or silt. Conventional well log evaluation therefore fails to provide litho-depositional or textural information. Yet, this is essential information to better understand the geological environment, the permeability distribution throughout the reservoir and the level of irreducible water saturation. Another problem with the evaluation of carbonate reservoirs is the secondary porosity effect on resistivity logs, which often results in an over-optimistic hydrocarbon saturation estimation, unless the cementation exponent "m" is appropriately corrected to take secondary porosity into account.

These problems can be circumvented by the appropriate use of advanced well-log data. The NMR signal distribution is affected by the litho-deposition; obvious changes of the NMR signal distribution width are correlatable to textural changes observed by a formation micro-imager device. This is also confirmed against cuttings and core studies, as well as thin sections analysis. A secondary porosity evaluation technique making use of both compressional and shear acoustic velocities achieves a secondary porosity measurement independent of pore geometry and distribution, therefore, hydrocarbon saturation can be estimated with improved accuracy.

Furthermore, a method is developed to derive accurate NMR-based permeability and irreducible water saturation. The results obtained are compared to core permeability and irreducible water saturation data.

Meaningful prediction of test results is now possible from petrophysical evaluation. The reservoir evaluation derived from the petrophysical logs is compared to a wireline formation pressure gradient and well tests results over 11 reservoir levels.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia