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Layer spreading and dimming within the CO2 plume at the Sleipner field in the North Sea

Proceedings of the 10th International Conference on Greenhouse Gas Control Technologies, Amsterdam, Energy Procedia, Elsevier
Boait F, White, N., Chadwick, A, Noy, D & Bickle, M.
 This paper deals with the electrical resistivity monitoring of the Ketzin CO2 injection pilot (CO2ReMoVe EC
project) through time-lapse CSEM measurements. There, 3 boreholes about 800 m deep have been especially
designed for current injection at reservoir (sandstone) depth. CO2 is directly injected in a saline (~240 g/l) aquifer.
Prior modelling results indicated that the increase of electrical resistivity generated by the CO2 plume (gaseous and
liquid CO2 phases) supposed to be highly resisitive, would generate measurable changes in the EM fields on the
surface, when injecting current directly inside the reservoir. In order to highlight and follow these expected
resistivity changes, 3 CSEM surveys were performed in August 2008 (baseline prior to injection), June 2009 and
August 2010. Each time, 13 EM stations have been recorded during current injection of a square wave at 3
frequencies (0.125 Hz, 0.5 Hz and 4 Hz) in two configurations (“double mise à la masse” (2xMAM) and “mise à la
masse – surface” (MAM-Surface)). This paper only presents results of the 2xMAM configuration at 0.5 Hz.
In spite of a very noisy area (gas pipes, high voltage power lines), we measured signal amplitude 10 times higher
than noise amplitude. We show that EM fields vectors (both inphase and quadrature components) measured on the
surface are very similar to the forward modelling EM responses computed with COMSOL Multiphysics®. Models
also show that electric field spatial distribution is strongly affected by a thin and resistive layer (35 m - 200 Ωm) of
anhydrite above the reservoir, making E field diverging from the boreholes whereas a dipolar pattern was expected
for the dipole current injection used here. Moreover, while June 2009 survey highlighted the expected strong
increase of electric field (increase of resistivity), August 2010 survey showed electric field amplitudes similar to the
2008 baseline survey, revealing therefore major changes of the reservoir properties. Finally, the directional
sensitivity of the 2xMAM array is tested through modelling residuals computed for five CO2 plume spatial
distributions. Results show that a north-eastward migration of the CO2 plume is expected to fit field data.

© 2010 Elsevier Ltd. All rights reserved

Keywords: CSEM; CO2; Time-lapse monitoring; Mise à la masse; EM modelling; Ketzin
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