Core material from a North Sea reservoir has been investigated for strength and elastic properties in triaxial tests. The material behaves nonlinearly elastically, and the rock strength correlates with the static shear modulus in a data range previously little investigated. Dynamic elastic properties obtained by downhole measurements with full-waveform logs are compared with the laboratory-derived strength. Laboratory acoustic measurements show that the sonic velocities are strongly stress dependent, which is closely related to the previously mentioned elastic nonlinearity. Also, dynamic elastic moduli are found to exceed the static moduli considerably, which has to be taken into account when evaluating mechanical properties from logs. Because of the nonhydrostatic stress state downhole or in laboratory triaxial tests, an acoustic anisotropy is induced. On unconfined core materials, the pronounced observed acoustic anisotropy may, to a large extent, be caused by stress relief during and after coring. The possibilities of using this effect in in-situ stress determination are elucidated. Finally, the mechanical-properties logging practice commonly adopted is compared with experimental data, showing that the log interpretation approach must be modified to account for the data.

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