Summary

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 if sensors or transmitters can be placed in observation wells e.g. measurement of the vertical electric field component and the use of alternative source configurations consisting of combinations of horizontal and vertical (energized steel casing) dipoles. In spring 2014, a borehole CSEM survey (4 transmitters, 25 receiver 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. 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 good agreement with predictions by numerical simulations.

Introduction

In an Enhanced Oil Recovery (EOR) pilot project Wintershall Holding GmbH, Kassel, is testing their newly developed biopolymer Schizophyllan in an onshore oil field in Lower Saxony, Germany (Leonhardt et al. 2011, 2013). Mixture with high saline formation water results in a polymer solution with much lower electrical resistivity than the oil within the Lower Cretaceous reservoir. To advance and optimize EOR techniques it is essential to know the areal propagation and distribution of the injected fluids in the subsurface.

At GFZ Potsdam, we investigate the applicability of the controlled-source electromagnetic (CSEM) method to monitor the distribution of the polymer solution and the change in oil saturation in the reservoir.

Previous 3D CSEM modelling results (Tietze and Ritter 2014, Tietze et al. in rev.) suggest that CSEM is sensitive to resistivity changes at reservoir depths, but the effect is difficult to resolve with surface measurements only. Resolution potential increases significantly, however, if sensors or transmitters can be placed in observation wells closer to the reservoir. In particular, observation of vertical electric field component Ez in shallow boreholes and/or use of novel source configurations consisting of vertical and horizontal dipoles appear promising.

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