Stress arching and its impact on 4D seismic amplitudes and traveltimes

Abstract

Pressure depletion during production from a reservoir can cause geomechanical changes in both the reservoir and overburden. Dropping the pore pressure in the reservoir increases the load on the rock matrix, resulting in compaction. In most cases, the magnitude of this compaction is negligible; however, when depletion pressures are large or when the reservoir rock is highly compressible, the compaction may be significant. As the reservoir pulls away from the surface of the earth, a stress arch forms in the overburden, with the vertical stress decreasing directly above the reservoir and increasing in the pillars of the stress arch. These stress changes can perturb the seismic velocities, which, combined with the changes in path length to the reservoir, can cause traveltime changes in the overburden. The unloading of the overburden directly above the reservoir shields the reservoir from seeing the full change in pore-fluid pressures and thus can have a significant impact on the amplitude changes in the reservoir between repeat seismic surveys. We describe a workflow for rapidly modeling these geomechanical changes and their associated seismic signatures. Using a set of simple synthetic reservoir models, we demonstrate the impact of reservoir aspect ratio (thickness to diameter ratio) and dip on the magnitude of the stress arch. Finally, we present a case study from the Gulf of Mexico that demonstrates the importance of including geomechanical changes in 4D modeling and interpretation.