Time-lapse CSEM: how important is survey repeatability?

• Published in: Geophysical Journal International
• Main Authors: Shantsev, Daniil V, Nerland, Elias A, Gelius, Leiv-J
• Format: Article in Journal/Newspaper
• Language: English
• Published: Oxford University Press (OUP) 2020
• Subjects: Geochemistry and Petrology; Geophysics; Snøhvit
• Online Access: http://dx.doi.org/10.1093/gji/ggaa452; http://academic.oup.com/gji/advance-article-pdf/doi/10.1093/gji/ggaa452/33792244/ggaa452.pdf; http://academic.oup.com/gji/article-pdf/223/3/2133/33885692/ggaa452.pdf

SUMMARY An important concern for time-lapse studies using the controlled-source electromagnetic (CSEM) method is repeatability of acquisition parameters for the base and monitoring surveys. We consider a challenging case when variations in source and receiver positions, conductivity of seawater, etc. will lead to differences in the recorded EM fields that exceed EM response due to production-induced changes in the reservoir resistivity. We show that even in that case, 4-D response can often be clearly resolved if the base and monitor EM data sets are inverted to produce 3-D resistivity distributions. More precisely, for a synthetic model of the Snøhvit gas field, changes in the inverted resistivity maps caused by such non-repeatability will be at least 10 times smaller than the time-lapse differences in the reservoir resistivity. By contrast, measurement errors and poor knowledge of background resistivity may reduce the resolution of 4-D CSEM to a much stronger degree. Analysis of field CSEM data from the Wisting oil field supports our conclusion about strongly relaxed repeatability requirements when 4-D effects are established by examining inverted resistivity volumes rather than by looking at raw EM data.