Borehole Controlled Source Electromagnetic (CSEM) Surveying for Monitoring HC-Saturation in the Bockstedt Oilfield (NW Germany)

Abstract

Abstract Main objective is to support reservoir monitoring in an EOR production scheme. Enhanced oil recovery calls for efficient monitoring of hydrocarbon saturation changes. In the onshore Bockstedt oil field there exists a substantial difference in resistivity at reservoir level: for initial oil saturation it exceeds 100 Ohmm reducing to 0.6 Ohmm once fully displaced by formation brine. Applicability of the controlled-source electromagnetic method to monitor such resistivity changes is investigated. Targets are Lower Cretaceous shallow marine clastics at approximately 1200 m depth. Finite difference forward modelling shows CSEM is sensitive to expected resistivity changes, but the effect is difficult to resolve with surface measurements only. Resolution increases significantly if sensors or transmitters are placed in observation wells closer to the reservoir. Behaviour of the vertical electric component (Ez) in shallow boreholes and/or use of complex source configrations look rather promising. In 2014, a borehole-to-surface CSEM configuration was deployed successfully across the Bockstedt oil field, whereby the current was injected via the metal casing of an abandoned production well. The set-up also allowed conventional magnetotelluric (MT) data acquisition when the transmitter is turned off at night. CSEM response functions are of good quality with high repeatability. Recordings from conventional and new borehole transmitters indicate different current distributions in the subsurface. Results of the tested source configuration are in agreement with predictions by numerical simulations. Preliminary 3D inversion is consistent with previous numeric simulations. A timelapse CSEM survey is scheduled for 2015, inclusive drilling of a shallow observation well for Ez component monitoring with a new borehole sensor. In parallel, a timelapse workflow for 3D CSEM inversion is developed.