Borehole Controlled–Source Electromagnetics for Hydrocarbon–Saturation Monitoring in the Bockstedt Oil Field, Onshore Northwest Germany

Abstract

Summary Production from oil fields requires the monitoring of hydrocarbon saturation in the reservoir. In the Bockstedt oil field there exists a substantial difference in resistivity between an oil-filled (approximately 100–16 Ω·m) and a brine-filled (0.6 Ω·m) reservoir. Controlled-source electromagnetics (CSEM) is chosen to test whether sufficient resistivity differences can be observed by means of surface measurements. The target is a Lower Cretaceous clastic interval at an approximate depth of 1200 m. Forward modeling 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 an input electrode. CSEM surveys were acquired in 2014 and 2015 as a time-lapse baseline and monitor for both horizontal electric (Ex and Ey) components. A four-transmitter and 25-receiver configuration was deployed for these surveys. Forward modeling indicates that induction effects from metal objects, such as casings of production wells, cannot be ignored in the electromagnetics (EM) modeling. A shallow observation well was drilled in 2015 to make the collection of vertical electric field (Ez) data sets possible. A new downhole sensor was developed for this purpose. Numerical simulations suggest Ez data are more sensitive to the anticipated resistivity changes in the reservoir. Because 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. A new, computationally efficient approach considers the effect of the metal casings, and is implemented in the 3D modeling and inversion codes. Preliminary resistivity models obtained from 3D CSEM inversion explain the observed data, and are in agreement with results from a 3D-seismic survey and resistivity logs in the calibration wells. Incorporation of the metal casings in the EM-modeling scheme increases the lateral continuity of inverted resistivity bodies. In November 2016, a new time-lapse acquisition campaign was undertaken to collect a second Ex/Ey monitor and the first monitor survey for the Ez component.