Overburden 4D time shifts — Indicating undrained areas and fault transmissibility in the reservoir

Abstract

Production-induced geomechanical stress changes cause velocity changes in the overburden that might be detected as 4D seismic time shifts. The strength of the velocity changes depends on the degree of pressure changes and the elastic properties of the reservoir and overburden layers. Even small velocity changes (less than 1%) might accumulate into detectable seismic time shifts at the top reservoir, since the overburden thickness typically ranges from one to several kilometers. Reservoir pressure changes inducing seismic time shifts are observed in the overburden of the Snorre, Heidrun, and Statfjord fields, all located on the Norwegian Continental Shelf. The strong correlation between overburden time shifts, geomechanics, and reservoir pressure changes is used to indicate undrained areas and transmissibility across faults, which is useful information for increased oil recovery, well planning, and reservoir model updating. 4D geomechanical models are built with input from simulated reservoir pressures. Geomechanical strain and velocity changes are linked through a “dilation” factor, R. The Snorre, Heidrun, and Statfjord fields indicate an average R value of about 15 for the overburden, when combining modeled vertical strain with observed seismic time shifts. However, this study also shows strong vertical variation in R, implying that R might be layer dependent. For the Statfjord Field, seabed subsidence measurements from gravity and GPS monitoring are used to calibrate the geomechanical model. The Snorre Field results show that both reservoir pressure depletion and pressure buildup can be identified by the use of overburden time shifts. The properties of the reservoir formations and surrounding layers of the investigated fields are typical for many fields on the Norwegian Continental Shelf. This implies that pressure-induced time shifts might be expected for many producing fields, not only chalk or high-pressure, high-temperature reservoirs but also sandstone reservoirs close to hydrostatic pressure.