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Values of irreducible water saturation calculated from conventional electric and porosity logs at Bayou Middle Fork field are commonly greater than 60%. Mercury injection curves reveal that the true water saturation averages about 40%. Scanning electron micrographs show oolitic grains that have been recrystallized and have rough irregular surfaces as well as intercrystalline porosity within the particles. Water adheres to these grain surfaces and occupies some of the intercrystalline porosity in productive zones, causing the calculated water saturations to read too high. Application of the Archie equation, the Guillotte et al technique, and the Rocky Mountain technique provides a different suite of water saturations.
Archie water saturations average about 75% when density-neutron porosity values are used. Density-neutron porosity values tend to read about 2 porosity units low when compared with core porosity values throughout the field. When core corrected porosity is used, the ARchie saturation averages about 60%.
The Guillotte et al technique employs a variable textural parameter "w" which is dependent on porosity and permeability. Mean water saturation with this method averages about 49%, which is in better agreement with the mercury injection data.
The Rocky Mountain ratio yields water saturations that are too high until the porosity balance technique is applied. Since Rt is assumed to read too low due to the excess immobile water, the Rocky Mountain derived porosities are too low. When Rt is increased to balance the porosity the water saturation averages about 43%, using Z = .025. When the Rocky Mountain Rta is used in the Archie equation, water saturations more consistent with the mercury injection data result. In addition the difference between the observed Rt and the Rocky Mountain derived Rta can be plotted as a function of porosity.
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