3D Inversion of Controlled-source Electromagnetic Data in the Presence of Steel-cased Wells

Abstract

Controlled-source electromagnetic (CSEM) methods are investigated for their applicability to monitor fluids in a German oilfield, where injected brines have much lower electrical resistivity than oil. In spring 2014, a borehole CSEM survey (4 transmitters, 25 MT-stations) was conducted across the oil-field including a new horizontal-vertical source using the steel-casing of a 1.3km deep abandoned oil-well for current injection. The survey was repeated in autumn 2015, expanded by measurement of the vertical electric field with a newly developed sensor in a 200m deep observation borehole. Steel-cased wells in the oilfield influence the propagation of electromagnetic fields in the subsurface and cannot be neglected in CSEM modelling. Since horizontal dimensions of well casings are very small compared to their vertical extent, discretising boreholes as conductivity anomalies of the subsurface becomes computationally prohibitive. To tackle the modelling problem, we extended an integral equation method and describe the influence of steel-casings by equivalent source currents which generate a secondary primary field. The new approach allows to include steel-cased wells into our modelling codes, including interaction between multiple wells. We demonstrate the effect of steel-casings on CSEM responses and sensitivity of 3D inversion. Finally, the new inversion method is applied to the field data.