CO 2 storage in the high Arctic: efficient modelling of pre‐stack depth‐migrated seismic sections for survey planning

• Published in: Geophysical Prospecting Vol. 66; no. 6; pp. 1180 - 1200
• Main Authors: Lubrano Lavadera, P., Kühn, D., Dando, B.D.E., Lecomte, I., Senger, K., Drottning, Å.
• Format: Journal Article
• Language: English
• Published: 01.07.2018
• ISSN: 0016-8025; 1365-2478
• DOI: 10.1111/1365-2478.12637

ABSTRACT The sequestration of CO 2 in subsurface reservoirs constitutes an immediate counter‐measure to reduce anthropogenic emissions of CO 2 , now recognized by international scientific panels to be the single most critical factor driving the observed global climatic warming. To ensure and verify the safe geological containment of CO 2 underground, monitoring of the CO 2 site is critical. In the high Arctic, environmental considerations are paramount and human impact through, for instance, active seismic surveys, has to be minimized. Efficient seismic modelling is a powerful tool to test the detectability and imaging capability prior to acquisition and thus improve the characterization of CO 2 storage sites, taking both geological setting and seismic acquisition set‐up into account. The unique method presented here avoids the costly generation of large synthetic data sets by employing point spread functions to directly generate pre‐stack depth‐migrated seismic images. We test both a local‐target approach using an analytical filter assuming an average velocity and a full‐field approach accounting for the spatial variability of point spread functions. We assume a hypothetical CO 2 plume emplaced in a sloping aquifer inspired by the conditions found at the University of Svalbard CO 2 lab close to Longyearbyen, Svalbard, Norway, constituting an unconventional reservoir–cap rock system. Using the local‐target approach, we find that even the low‐to‐moderate values of porosity (5%–18%) measured in the reservoir should be sufficient to induce significant change in seismic response when CO 2 is injected. The sensitivity of the seismic response to changes in CO 2 saturation, however, is limited once a relatively low saturation threshold of 5% is exceeded. Depending on the illumination angle provided by the seismic survey, the quality of the images of five hypothetical CO 2 plumes of varying volume differs depending on the steepness of their flanks. When comparing the resolution of two orthogonal 2D surveys to a 3D survey, we discover that the images of the 2D surveys contain significant artefacts, the CO 2 ‐brine contact is misplaced and an additional reflector is introduced due to the projection of the point spread function of the unresolvable plane onto the imaging plane. All of these could easily lead to a misinterpretation of the behaviour of the injected CO 2 . Our workflow allows for testing the influence of geological heterogeneities in the target aquifer (igneous intrusions, faults, pervasive fracture networks) by utilizing increasingly complex and more realistic geological models as input as more information on the subsurface becomes available.