Controlled-source electromagnetic monitoring of reservoir oil saturation using a novel borehole-to-surface configuration

To advance and optimize secondary and tertiary oil recovery techniques, it is essential to know the areal propagation and distribution of the injected fluids in the subsurface. We investigate the applicability of controlled-source electromagnetic methods to monitor fluid movements in a German oilfield (Bockstedt, onshore Northwest Germany) as injected brines (highly saline formation water) have much lower electrical resistivity than the oil within the reservoir. The main focus of this study is on controlled-source electromagnetic simulations to test the sensitivity of various source-receiver configurations. The background model for the simulations is based on two-dimensional inversion of magnetotelluric data gathered across the oil field and calibrated with resistivity logs. Three-dimensional modelling results suggest that controlled-source electromagnetic methods are sensitive to resistivity changes at reservoir depths, but the effect is difficult to resolve with surface measurements only. Resolution increases significantly if sensors or transmitters can be placed in observation wells closer to the reservoir. In particular, observation of the vertical electric field component in shallow boreholes and/or use of source configurations consisting of combinations of vertical and horizontal dipoles are promising. Preliminary results from a borehole-to-surface controlled-source electromagnetic field survey carried out in spring 2014 are in good agreement with the modelling studies.