Use of acoustic logging technique for determination of intergrain cementation properties



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Tests have been performed in the laboratory which relate Archie's "cementation factor" to a theoretical exponent which can be determined from acoustic measurements. The current technique of determining the formation resistivity factor uses the empirical formula of Archie F = crm, for determining m, where high porosities, such as sandstones, have small m, and large values of m for low porosities, such as limestones. In Archies's formula, the exponent m is called "cementation factor". It varies with the degree of consolidation of the rocks, and ranges from 1.3 to values greater than 3. The value of m is chosen from the lithologic description of the rock of interest. The acoustic velocity of the dilatational and the shear waves in rocks increases as the hydrostatic pressure increases. However, the rate of increase of velocity is different for dilatational than for shear waves. For dilatational waves, the rate of velocity is a function of packing of the grains, while for shear, the rate of increase is a function of shear coupling. The shear waves are sensitive to the cementing material in the rocks, while the dilatational waves are mainly dependent upon the grains' properties. From the scope picture of the acoustic pulse, it is possible to determine the exponent, along with porosity and elastic properties of the rock in place. Knowing this exponent, better fracturing calculations could be performed.