ABSTRACT

Reservoir production causes subsurface deformations and changes in seismic velocity. These deformations and velocity changes can be monitored using time-lapse seismic data. A fundamental challenge in the interpretation of time shifts observed in time-lapse data is the decomposition of the time delay into a spatial compaction component and a velocity change component. Several authors (Hatchell and Borne, 2005; Janssen et al., 2006) have published the application of pragmatic linear relationships between overburden stretching and velocity changes which, have proved applicable in a wide range of geological settings. In this study, we predict velocity changes through coupled reservoir and geomechanical modeling of the subsurface stress state and subsequent application of a rock physics model that relates changes in subsurface stress and strain to velocity changes. We expand on previously published work by investigating the anisotropy of the velocity changes and relate these to field observations of offset-dependent time-lapse time-shifts from the South Arne Field. Our modeling shows that in the overburden, vertical seismic velocities decrease over time, causing traveltimes in a monitor survey to increase compared to traveltimes in a base survey. We furthermore predict that horizontal seismic velocities in the deep overburden increase and for seismic waves propagating at intermediate angles (approx. 20° – 30°), the velocity changes are minimal. This suggests, that time-shifts between base- and monitor surveys are largest for zero-offset data and will gradually decrease as a function of offset. We test this prediction on a 4D field data set at South Arne, North Sea. For zero-offset data, we find a maximum traveltime increase in the overburden of 6ms. Time-shifts show a dependence on observation angle, with large offset time-shifts in the overburden being up to 50% smaller than near-offset time-shifts.

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