The dead volume effect on the elastic moduli measurements using the forced‐oscillation method

Abstract

ABSTRACTIn low‐frequency laboratory experiments conducted on fluid‐saturated rocks, the boundary conditions of rock samples can significantly affect the results of the measurements of elastic moduli and acoustic attenuation. Generally, in laboratory measurements, the dead fluid volume associated with the fluid storage formed by the segments of the fluid line adjacent to a rock specimen cannot bе completely eliminated. Depending on the size, this fluid storage can significantly affect the elastic moduli measurements. We investigate the impact of the dead volume on (i) the quasistatic measurements of the elastic moduli of a fully fluid‐saturated rock specimen and (ii) the dispersion and attenuation measurements conducted on the same specimen at seismic frequencies. We present the results of the laboratory tests conducted at seismic frequencies on an n‐decane–saturated porous rock with the dead volume gradually changed from 2 to 260 cm3, as well as with the open‐pore‐fluid line. To take into account the dead volume, we developed a modified Gassmann theory and demonstrated that predictions of this theory are in good agreement with the measurements at lower frequencies. The Young's modulus dispersion and extensional attenuation observed at higher frequencies were explained on the basis of a one‐dimensional model considering the guided slow P‐waves in the rock specimen with boundary conditions specific to the experiment.