Production from oil fields requires monitoring of hydrocarbon saturation in the reservoir. In the Bockstedt oilfield there exists a substantial difference in resistivity between oil-filled (approx. 100-16 Ohmm) and brine-filled (0.6 Ohmm) reservoir. Electromagnetic method is chosen to test whether sufficient resistivity differences can be observed via surface measurements. The target is a Lower Cretaceous clastic interval located at an approximate depth of 1200 metres.

Forward modelling demonstrates that the expected resistivity changes at reservoir level cannot be resolved with a survey setup of only surface electrical sources and sensors. Therefore a borehole-to-surface technique has been developed, whereby the metal casing of an abandoned production well serves as input electrode. CSEM surveys were acquired in 2014 and 2015 as timelapse baseline and monitor for both Ex and Ey components. Forward modelling indicates that induction effects from metal objects like casings of production wells cannot be ignored in the EM modelling. A shallow observation well was drilled in 2015 to make collection of Ez datasets possible. A new downhole sensor was developed for this purpose. Numerical simulations suggests Ez data is more sensitive to the anticipated resistivity changes. Since Ez is two orders of magnitude smaller than the horizontal components, verticality is of great importance to avoid masking the Ez signal by interference from unwanted horizontal components.

Similar acquisition parameters are adopted for 2014 baseline and 2015 monitor surveys to facilitate the comparisons. The repeatability is good, generating comparable response functions. The earth model, retained so far by the inversion algorithm, confirms the main resistivity units seen by the resistivity logs in the calibration well. Incorporation of the metal casings in the EM modelling scheme increases the lateral continuity of inverted resisitivity bodies.

This study is a follow-up of work presented in the 2015 ADIPEC conference. In November 2016 a new acquisition campaign will be undertaken to collect a second Ex – Ey monitor and the first monitor survey for the Ez component. A limited time-lapse test has been performed in spring 2016 to monitor Ez with only 1 source station incorporating the borehole electrode Bo-23.

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