October 2006
Special Focus

Estimating Sw with a volume measurement

With a large percentage of the world’s known hydrocarbon reserves located in carbonate reservoirs, the push to increase production has revealed a need to improve oil saturation calculations in carbonate transition zones. Numerous attempts have been made to modify the famous Archie equation’s a, m and n parameters to derive a saturation that more closely matches core saturation measurements, but these have met with limited success. Rather than continue this approach, a different model of the pore and fluid distribution has been proposed and field proven. The latest model is to change the saturation evaluation to a volume measurement and acquire that measure prior to significant invasion. Adding sigma measurement to LWD enables this. Using this approach, recent field examples show more hydrocarbons than indicated by resistivity or pressure gradient methods. PROBLEMATIC CONDITIONS Carbonate transition zones exhibit resistivity and pressure gradient phenomena that respond like water zones to resistivity and pressure gradient measurements.

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