Stress-State Dependent Elastic Properties: Experimental Investigation

We investigate the relevance of laboratory velocity-stress measurements to wireline data. A specific question is: if Vp and Vs are measured in a vertical borehole in a formation where the vertical stress differs from the horizontal stress, how close are these measured velocities from those measured in the lab under hydrostatic stress equal to the in-situ vertical stress? An example given here is for a low-tomedium porosity quartz-dominated sandstone subject to varying axial load at fixed confinement. We find that the axial elastic-wave velocities at the same total axial stress increase with increasing confining pressure. The respective differences are fairly small. These disparities in Vp are significantly smaller than in Vs due to the horizontal polarization of the latter. This means that where the in-situ disparity between the vertical and horizontal differential stress is small, hydrostatic-stress laboratory velocity data can be used as a robust proxy to wireline measurements in a vertical borehole, at least in the example presented.