Experimental Determination of the Stress Sensitivity of Elastic Wave Dispersion in a Fluid-saturated Cracked Rock

We investigate the effects of micro structure, stress, pore fluids and frequency of perturbation on the elastic response of a thermally-cracked Carrara marble.

The following data are reported at various effective pressure steps up to 50 MPa: (i) helium porosity and permeability under dry conditions; (ii) low-field NMR scanning under water-saturated conditions; (iii) low amplitude-high frequency (0.5 MHz) and low amplitude-low frequency (0.01 Hz) elastic moduli in dry and water-saturated conditions.

This data set suggests that: (1) the natural rock contains a small amount (<1%) of micro-porosity and is isotropic and homogeneous at the laboratory scale; (2) thermal cracking induces the appearance of micro-cracks located at the interface between calcite grains (∼300 micron); (3) the spatial and orientation distributions of these micro-cracks are uniform; (4) crack closure occurs around a threshold effective pressure of 30–40 MPa.

Below this pressure threshold dispersion in the thermally cracked and water-saturated rock between 0.01 Hz and 0.5 MHz reaches ∼25% for S-waves and ∼10% for P-waves; above this threshold, dispersion virtually cancels. Gas permeability at low pressure is larger in the thermally cracked rock than in the natural rock and decreases toward the value before thermal cracking with increasing effective pressure.