Stress sensitivity of elastic wave velocities in granular media

Abstract

Knowledge of the stress sensitivity of elastic-wave velocities in sedimentary rocks with different consolidation is important for several geophysical applications such as seismic pore pressure prediction, time-lapse seismic analysis and not least in seismic exploration. Since clays or shales are more compactable than sand or sandstones they often experience large plastic deformation causing both clay platelet orientation and porosity reduction of the order of 50% or more. The interplay between stress and porosity effects may mask the stress sensitivity of velocities in clays or shales. The porosity loss occurring in sandstones during mechanical compaction is generally slower and hence the stressinduced velocity changes are better preserved at least prior to the onset of grain cementation. Small amounts of contact cement, 1 to 2% of the total rock volume, can have a dramatic effect on velocities and their corresponding stress dependence. We demonstrate this reduction in stress sensitivity of elastic-wave velocity between sandstones of different consolidation through comparison of rock physics contact model-predictions of compressional to shear velocity ratio (Vp/Vs), with similar estimates obtained from time-lapse seismic amplitude versus offset (AVO) data from two oil producing fields.

Furthermore, modified contact models are proposed to investigate the role of the combined effects of intergrain contact friction, increasing differential stress and stress ratio in relation to the propagation direction of the elasticwaves in granular media. Model-predictions are compared with ultrasonic measurements obtained from experimental compression tests on unconsolidated sands.