Time-Lapse Changes in Elastic Anisotropy: Applicability of Anisotropic Rock Physics Models

Abstract

Summary Depletion of a reservoir causes changes in triaxial stress state. As a result, time-lapse changes in reservoir elastic anisotropy occurs because of stress-induced anisotropy. An anisotropic rock physics model is therefore required to correctly model the 4D seismic response. We have investigated applicability of two anisotropic rock physics models to the prediction of time-lapse changes in elastic anisotropy of tight sandstone, and found that these models are very useful. The models require only two parameters to reasonably predict the time-lapse changes, hence simplifying the anisotropic modelling process. Moreover, these parameters can be estimated without using stress sensitivity of off-axis velocity and even without using that of S-wave velocities. This means that stress-sensitivity of anisotropy parameter δ, which is a difficult parameter to measure experimentally, can be estimated from stress sensitivities of only vertical and horizontal P-wave velocities. The number of required measurements can therefore be reduced, if the applicability of the models are confirmed using limited number of samples.