Sample boundary effect in acoustic attenuation of fluid-saturated porous cylinders

Abstract

It was shown previously, by using Biot theory, that there exists an attenuation in the low-frequency elastic modulus measurement on fluid-saturated porous rock cylinders caused by the open-pore boundary condition. This attenuation is associated with the energy dissipation of the viscous flow of the pore fluid in and out of an open-pore boundary of a fluid-saturated rock sample. For the extensional mode, the relaxation frequency of this attenuation is directly proportional to the permeability of the rock and inversely proportional to the viscosity of the pore fluid and the square of the sample radius. In this article, experimental studies that support the theoretical prediction of this sample boundary effect are reported. Experiments of varying saturation, sample radius, pore fluid viscosity, and rock permeability were carried out. The results, though consistent with the theoretical prediction, are not satisfactory in a quantitative sense because of the lack of an idealized sample. It was found, however, the procedure of jacketing the sample with an aluminum sleeve and measuring it at ambient pressure condition is not sufficient to remove the attenuation caused by the boundary effect completely.