A suite of laboratory measurements have been conducted on Geysers metagraywacke and metashale recovered from a drilled depth of 2599 to 2602 meters in NEGU-17. The tests have been designed to constrain the mechanical and water-storage properties of the matrix material. Various measurements have been made at a variety of pressures and at varying degrees of saturation. Both compressional and shear velocities exhibit relatively little change with effective confining pressure. In all of the samples, water saturation causes an increase in the compressional velocity. In some samples, saturation results in a moderate decrease in shear velocity greater in magnitude than would be expected based on the slight increase in bulk density. It is found that the effect of saturation on the velocities can be quantitatively modeled through a modification of Biot-Gassmann theory to include weakening of the shear modulus with saturation. The decrease is attributed to chemo-mechanical weakening caused by the presence of water. The degree of frame weakening of the shear modulus is variable between samples, and appears correlated with petrographic features of the cores. Two related models are presented through which we can study the importance of saturation effects on field-scale velocity variations. The model results indicate that the saturation effects within the matrix are significant and may contribute to previously observed field anomalies. The results help to define ways in which we may be able to separate the effects of variations in rock properties, caused by phenomena such as degree of fracturing, from similar effects caused by variations in matrix saturation. The need for both compressional and shear velocity data in order to interpret field anomalies is illustrated through comparisons of model results with the field observations.

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