CSEM Monitoring of Reservoir Oil-saturation Using a Novel Borehole-to-surface Configuration

To optimize enhanced oil recovery techniques it is essential to know the spatial propagation of the injected fluids in the subsurface. We investigate the applicability of controlled-source electromagnetic (CSEM) methods to monitor fluid movement in a German oilfield (Bockstedt). Injected brines have much lower electrical resistivity than oil within the reservoir. 3D modelling suggests CSEM is sensitive to resistivity changes at reservoir depths, but only if sensors/transmitters are positioned closer to the reservoir, e.g. using observation wells. In particular, the vertical electric field component in shallow boreholes and/or use of alternative source configurations consisting of combinations of vertical and horizontal dipoles are promising. In spring 2014, a borehole CSEM survey (4 transmitters, 25 MT stations) was conducted across the oil field using a new horizontal-vertical source. The current was injected via the steel-casing of a 1.3 km deep abandoned oil-well. Redeployment tests of transmitters demonstrate high repeatability of CSEM measurements. First comparison of recordings from conventional and the new borehole transmitters indicate differing current distribution in the subsurface. Results of the new source configuration are in general agreement with predictions by numerical simulations.