We reformulate the original model of Hatchell and Bourne (2005) and Røste, Stovas and Landrø (2005) that couples fractional velocity change to subsurface strain via a fundamental constant R. The new model combines elastic compressibility of a dual porosity system for a sand-shale mixture with horizontal planes of inter-granular weakness. The majority of observed R-factor magnitudes from post-stack 4D seismic data in both the reservoir and overburden can thus be explained. R is predicted to depend strongly on lithology and also initial strain state. The model is also extended to predict the observed angle-dependence of time-lapse time-shifts from pre-stack data. An expression for the gradient of time-shift with incidence angle is obtained in terms of the background VP/VS, and also the ratio of tangential to normal compliance BT/BN representing loss or creation of inter-granular coupling. If accurately estimated from data, this compliance ratio can be used as an additional parameter to assess the post-production state of the overburden. It is concluded that whilst R remains the over-arching parameter controlling the magnitude of time-shifts measured from 4D seismic data, BT/BN is a subtler parameter that may also prove of future value.
- School of Energy, Geoscience, Infrastructure and Society, Institute for GeoEnergy Engineering - Professor
- School of Energy, Geoscience, Infrastructure and Society - Professor
- Research Centres and Themes, Energy Academy - Professor
Person: Academic (Research & Teaching)