Abstract

Interpreting and understanding overburden seismic changes are important to avoid health, safety, and environmental (HSE) accidents and well-drilling problems. Furthermore, overburden time shifts might indicate areas with reservoir compaction and depletion. In the Snorre sandstone field, 4D seismic time shifts of as much as 3 ms are observed in the overburden. The cumulative overburden time shifts captured at top reservoir correlate well with pressure-depletion areas in the reservoir, indicating that the time shifts might be related to changes in stress and strain because of reservoir compaction. For further investigation, a 2D geomechanical model is built in the area with the most prominent overburden time shifts. Based on the stiffness of the reservoir and surrounding rocks and simulated reservoir pore-pressure changes, the geomechanical model predicts a maximum downward displacement at the top reservoir of 0.4 m and maximum seabed subsidence of 0.25 m. By defining an appropriate strain-sensitivity parameter, the results from the geomechanical model are forward-modeled into velocity changes. For comparison, velocity changes are estimated from the time shifts. A good qualitative match is obtained between the geomechanical model and the observed time shifts, indicating that the time shifts are caused by geomechanical effects related to production. Other explanations of the observed overburden time shifts, such as fluid leakage, out-of-zone-injection, or seismic acquisition or processing problems, are found to be less realistic causes of the observed time shifts.

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