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A Laboratory Study of the Effect of Boundary Conditions on the Elastic Moduli Measurements

Summary In this study, we demonstrate that the divergence of the boundary conditions from ideal can lead to a significant dispersion of the elastic moduli of a liquid-saturated rock measured in low-frequency experiments. In laboratory tests, the liquid-saturated rock sample cannot be completely sealed at its ends due to the presence of the fluid lines connected with the pore space of the sample. This pore-fluid storage, formed by the parts of the fluid line adjoined to both ends of the sample, determines the frequency dependence of the elastic moduli measurements. We present the results of the low-frequency measurements of elastic moduli carried out on a n-decane-saturated limestone sample with the pore-fluid storage volume varying from 2 ml to 260 ml, and also with the open fluid line (drained regime) at frequencies of 0.1 Hz to 120 Hz. In our experiments, the bulk moduli measured at 0.1 Hz are in good agreement with the Gassmann prediction when the dead volume does not exceed 30% of the pore space of the sample. We also found that the bulk modulus exhibits strong dispersion at frequencies above 20 Hz if the volume of the pore-fluid storage is larger than the pore space of the sample.