Seismic modeling of CO2 fluid substitution for the Heartland Area Redwater CO2 Storage Project (HARP), Alberta, Canada

Abstract

Abstract The Devonian Redwater reef is being assessed for geological storage of CO 2 for the Heartland Area Redwater CO 2 Storage Project (HARP). The reef complex is one of the largest Devonian reefs in the Western Canadian Sedimentary Basin. It has an area of about 600 km 2 and lies at depth of approximately 1000 m, and has a thickness of up to 300 m. The reef is of Devonian age and is made up of the Lower, Middle and Upper Leduc Formations. It is underlain by the Cooking Lake carbonate platform and overlain by the Ireton Shale which forms the caprock to the oil-bearing leg on the updip side of the reef as well as for the proposed CO 2 storage. A shale embayment occurs around some parts of the reef margin at the Mid-Leduc level. In order to assess seismic monitoring potential for the CO 2 storage project, fluid substitution seismic modeling was undertaken. Gassmann fluid substitution modeling was undertaken using on-reef wells which penetrated the Cooking Lake platform below the reef. Porosity of the Leduc Formation was calculated from the density logs and ranged from 4% to 6%. Synthetic seismograms were calculated from the log data before and after fluid substitution with supercritical CO 2 replacing brine. Gassmann fluid substitution calculations were computed for the entire thickness of the Leduc Formation (over a depth range from 1086 to 1324 m) for both on-reef wells. A P-wave velocity decrease of 4% is predicted to occur between 0% to about 40% CO 2 saturation, assuming uniform CO 2 saturation. For CO 2 saturations above 40%, the P-wave velocity will increase slightly. In comparison, the S-wave velocity is calculated to increase almost linearly with CO 2 saturation by a maximum of 0.6%, yielding an increase in Vp/Vs of about 4.5%. Seismic modeling was based on the changes in Leduc acoustic impedance predicted to occur between the wet in-situ reservoir and after replacement of pore fluids with supercritical CO 2 . From this modeling, a time delay of 4.3 ms is observed for reflections from the base of the Lower Leduc Formation, assuming that the entire reef interval has at least 40% CO 2 saturation. An integrated surface seismic and vertical seismic profile time-lapse seismic program has been designed to monitor CO 2 injection for HARP, planned to start by 2011.