Stress, porosity, and failure-dependent compressional and shear velocity ratio and its application to wellbore stability

The ratio of compressional to shear wave velocities strongly depends on the rock lithology, porosity and mechanical integrity under the in-situ state of stress. When the rock properties or in-situ stresses are changed due to drilling or geologic perturbations, the compressional and shear wave velocities exhibit certain correlations with the generation of fractures and stress redistribution. We analyze laboratory experiments where acoustic measurements are made of rock samples under triaxial compressive testing. The relationships of lithology, porosity, rock failure, and the ratio of compressional to shear velocities (Vp/Vs) are established based on the experimental tests. The results indicate that the ratio of Vp/Vs increases significantly when the rock approaches its strength limit and after the rock fails. Due to the redistribution of stresses around the wellbore and the presence of fractures, the Vp/Vs ratio can reach several times its initial value. The abnormally high Vp/Vs ratio is primarily caused by deterioration of the rock's mechanical integrity brought on by geologic movement or borehole excavation while drilling. The creation of new fractures also increases permeability and facilitates fluid movement which can increase the rock porosity and further slow the propagation of shear waves. In both cases, the change in rock properties causes a significantly slower propagation of shear velocity and the resulting Vp/Vs ratio can be observed to detect drilling induced failure and naturally fractured rock zones.