We will present results from a field trial of onshore time-lapse EM in a production environment, with special focus on the influence of well casings and pipelines on the acquired data, and lessons learned for future application of 4D EM.

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Controlled-source electromagnetic (CSEM) data are sensitive to the subsurface resistivity distribution, but 3D inversion results are ambiguous, and in-depth interpretation is challenging. Resolution and sensitivity analysis as well as the influence of noise on resolution have been used to quantify 3D inversion performance. Based on these numerical studies, a land-based CSEM survey was designed and carried out at the Schoonebeek oil field, the Netherlands. The acquired data were processed and subsequently inverted for the resistivity distribution. The 1D and 3D inversion of horizontal electric-field data show the reservoir at the right depth, matching well-log data without using a priori knowledge about the actual reservoir depth. We used a 1D model with fine layering as a starting model for 3D inversion. Synthetic data inversions and sensitivity tests demonstrate that resistive or conductive bodies inside the reservoir zone may be well-detectable with our limited acquisition geometry. Spatial variations in the reservoir resistivity are visible in the measured data and after inversion by assuming good knowledge of the background resistivity distribution. The reservoir resistivity and size, however, have to be interpreted with care considering the intrinsically low resolution of electromagnetic (EM) which is further reduced by manmade EM noise.

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Geophysical methods are widely used for hydrocarbon exploration and time-lapse measurements. One method that can be applied in place of or in addition to the routinely used seismic method, is the Controlled-Source ElectroMagnetic (CSEM) method. The work described in this thesis explores various aspects to improve the land CSEM method for exploration and monitoring purposes. Therefore, three separate active land CSEM field experiments, a baseline and two repeat surveys, were carried out over a period of nearly two years at the Dutch Schoonebeek oil field, where heavy oil is produced by injecting steam to reduce the viscosity of the oil. Steam injection and oil production have to be closely monitored to prevent breakthrough of steam to production wells and to detect possible unwanted leakage or contamination of groundwater such that further measures can be taken.@en