Multicomponent seismic modeling and analysis for CO2 storage in the Redwater reef, Alberta, Canada

Taher M. Sodagar

The Devonian Redwater reef, in Alberta, Canada, is being evaluated for geological storage of CO2 for the Heartland Area Redwater CO2 Storage Project (HARP). The reef complex is one of the largest Devonian reefs in the Western Canadian Sedimentary Basin and is the third largest oil reservoir in Canada. It is located close to large sources of CO2 in the Redwater-Fort Saskatchewan-Edmonton region. The study characterized the Redwater reef, identified the reef margin, and mapped the facies variations within the reef. The seismic response of the reef to the CO2 saturation in the Leduc Formation was investigated. Fluid substitution and seismic modeling were undertaken to generate PP and PS synthetic seismic data to study the consequences of CO2 saturation on the seismic response of the various reef facies and formations below the reef, based on seismic attributes and character.

Common shot ray tracing and finite-difference modeling was undertaken to evaluate variations in the seismic response of the Redwater reef across the southern margin of the reef for CO2 saturation in the Upper Leduc interval. The input geological model was based on well data and depth-converted seismic data from the interpretation of legacy 2D seismic lines in the area. Seismic reflections display positive structure below the reef in time sections due to the lateral velocity change from on-reef to off-reef, but are corrected in the depth sections.

Terminations and the lateral position of the Upper Leduc and Middle Leduc events are clear on the pre-stack time-migrated sections and a modest improved on the depth-migrated sections. Higher amplitudes at the base of Upper-Leduc member are evident near the reef margin due to the higher porosity of the foreslope facies in the reef rim compared to the tidal flat lagoonal facies within the center of the reef.

The 2D and 3D time-lapse multicomponent seismic modeling predicted a significant amplitude difference for the seismic data before and after CO2 saturation, particularly for reflections from the Upper Leduc, the top of the reef rim, and the Mid Leduc member. The results show that it is feasible to map CO2 saturation of at level 40% within the Redwater Leduc Reef through multichannel seismic surveys.