Fluid saturation effects on dynamic elastic properties of sedimentary rocks

Abstract The influence of saturation by water, oil, gas, and mixtures of these fluids on the densities, velocities, reflection coefficients, and elastic moduli of consolidated sedimentary rocks was determined in the laboratory by ultrasonic wave propagation methods. Twenty rock samples varying in age from Pliocene to early Devonian and in porosity from 4 to 41 percent were tested under uniform pressures equivalent to subsurface depths of 0 to 18,690 ft.Fluid saturation effects on compressional-wave velocity are much larger in low-porosity than in high-porosity rocks; this correlation is strengthened by elevated pressures but is absent at atmospheric pressure. At a frequency of 1 MHz, the shear-wave velocities do not always decrease when liquid pore saturants are added to rocks as theorized by Biot; agreement with theory is dependent upon pressure, porosity, fluid-mineral chemical interactions, and the presence of microcracks in the cementing material. Experimental results support the belief that lower compressional-wave velocities and higher reflection coefficients are obtained in sedimentary rocks that contain gas.Replacing pore liquids with gas markedly reduces the elastic moduli of rocks, and the effect is enhanced by decreasing pressure. As a rule, the moduli decrease as the porosity increases; Poison's ratio is an exception to the rule. Liquid and gas saturation in consolidated rocks can also be distinguished by the ratio of compressional and shear wave velocities (V p /V s ). The potential diagnostic value of elastic moduli in seismic exploration may stimulate interest in the use of shear-wave reflection methods in the field.