Resistivity and reservoir monitoring in the lower Cretaceous Bockstedt oilfield using a borehole CSEM setup

Summary Reservoir monitoring is essential for the development of hydrocarbon accumulations. Resistivity changes in the subsurface may help to achieve this task. CSEM has already proven its usefulness for exploration projects. In this study an onshore field has been chosen to optimise the CSEM workflow for production purposes (monitoring fluid movements and drainage pattern). The unconventional CSEM setup involves the use of the metal casing of an abandoned production well as input electrode. A timelapse approach with several acquisition campaigns has been adopted. Repeatability of the EM signals over a 4-year period is good. The reservoir is a 15m sandstone interval situated at a shallow depth of around 1200m. Vertical as well as horizontal electric components have been measured and recorded. An inversion step is always needed to interpret the EM data. A finite difference method is used to solve the Maxwell equations for computing the electric time response. Simulation modelling demonstrated the need to incorporate induction effects of metal objects in the inversion scheme and showed that a 30m thick HC-filled reservoir at 1200m depth can be detected. Alternative timelapse strategies have been examined. The constrained inversion (seismic horizons as input) leaves scope for further improvement.