oa Forensic Mapping Of Spatial Velocity Heterogeneity In A CO2 Layer At Sleipner Using Time-Lapse 3D Seismic Monitoring

The Sleipner injection operation has stored over 17 Mt of CO2. Time-lapse seismic monitoring has provided high resolution images of CO2 plume development, constraining and verifying numerical flow simulations. Seismic velocity is a key diagnostic parameter for CO2 layer properties and we adopt a forensic interpretative approach to determine velocity variation in the topmost layer of the plume. The 2010 seismic dataset enables, for the first time, temporal thicknesses of the layer to be determined, taking into account interference-induced time-shifts. Combining these with CO2 layer thicknesses determined from structural analysis of the topseal topography allows layer velocity to be mapped. A marked spatial variation in velocity is evident across the layer with higher velocities (1630±103 ms-1) in the central part of the layer contrasting with lower values (∼1370± 122 ms-1) to the north. Recent published work has identified a north-trending channel in the topmost Utsira sand unit, which greatly improves history-matching of the topmost CO2 layer with numerical flow simulations. This channel correlates almost exactly with the low velocity area mapped from the seismic, the higher velocity area corresponding to less permeable overbank deposits. The seismic therefore provides key corroborative evidence of permeability heterogeneity within the reservoir sand.