Borehole-to-Surface Time-Lapse CSEM Measurements Across a Producing Oil-Field: Repeatability, Vertical Electric Fields, 3D Inversion Including Steel-Casings

Summary Controlled Source Electromagnetics (CSEM) measurements were acquired across an onshore oilfield in Northern Germany between 2014 and 2018 comprising up to 5 transmitters, 29 surface and 3 borehole receivers. Repeatability of data is an essential prerequisite for reservoir monitoring. Our results suggest that repeatability of CSEM measurements depends on source-receiver distances, source-polarisation, and relocation errors, in particular at sites close to the source. Best repeatability was observed for receiver stations at 2–4 km distance from the source and frequencies <20 Hz. At these stations, phases and amplitudes usually agreed within ±1° and ±5% between repeat measurements. The vertical electric field (Ez) was measured with a newly developed receiver chain, suspended in a 200 m deep observation borehole. Although amplitudes of Ez are about one to two orders of magnitude smaller than amplitudes of horizontal electric fields, Ez data are stable and show excellent repeatability within <±2° and <±5 % during the 3 years. For 3D inversion of the field data set, we developed a new methodology which accounts for first order effects of steel-cased wells in the oil field. We demonstrate that both energised and passive well casings can strongly influence the outcome of 3D inversion.