ABSTRACT

Carbonate reservoir characterization continues to improve with advances in technology and synergistic techniques. Reservoir features previously undetected were found to have major impact on field performance. The combination of borehole imagery from horizontal wells with 3-D and/or 4-D seismic has revealed the presence and nature of unsuspected reservoir faulting and fracturing in many Middle East reservoirs. Geotesting of the open faults show them to be the cause of early water production. An equally important cause of early water production is the presence of unsuspected thin high-permeability layers defined by nuclear magnetic resonance (NMR) integrated with borehole imagery.

Both imagery and NMR together are revealing pore sizes and size distributions to better define effective porosity, even with an abundance of micropores present. It also has been possible to better evaluate complex reservoir facies through the integration of NMR and borehole electrical imagery with other logs and/or cores. This assessment included thin porosity layers, heterogeneous patchy or convoluted mixtures, complex Tertiary and Cretaceous karst fills, and vuggy and fractured facies in many formations. NMR defines pore size distributions, whereas electrical imagery reveals mega-pores and the surrounding matrix including the decimeter-scale porosity distribution. This approach reveals some thin, lower porosity layers to be the most permeable, while the highest porosity layers in one northwest Arabian reservoir are composed of micropores forming unsuspected porous barriers or baffles. Importantly, zones with very high-water saturation were correctly predicted to flow oil without water in Middle Eastern Arab and Thamama reservoirs, as well Paleozoic and Tertiary reservoirs elsewhere.

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