Capacitively coupled EM sensors integrated in non-corrosive casings for longterm CSEM monitoring
Guy G. Drijkoningen (TU Delft - Applied Geophysics and Petrophysics)
M. Ravilov (TU Delft - Applied Geophysics and Petrophysics)
Karel Heller (TU Delft - Lab Geoscience and Engineering)
C. van Beek (TU Delft - EMSD CE&G)
More Info
expand_more
Other than for strictly personal use, it is not permitted to download, forward or distribute the text or part of it, without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license such as Creative Commons.
Abstract
We present a development of capacitively coupled EM sensors integrated in non-corrosive casings for permanent CSEM monitoring in boreholes. Capacitive sensors are required to detect low-frequency (diffusive-field) signals where voltage measurements fail and ammeters need to be used. The permanent installation in boreholes necessitates surface placement of the electronic components to ensure their longevity and accessibility. An issue is that small current signals need to be transferred over a large distance via cables whose capacitances are larger than the ones from the sensors, so a circuit of a Zero-Resistance Ammeter with Integrator (ZRA-I) was developed for annihilating the cable-capacitance effect. Via modelling, lab and small-scale field testing, we were able to show that capacitive sensors with ZRA-I electronics worked well: although the desired signal is slightly decreased compared to the one from galvanically coupled sensors, the signal-to-noise ratios are comparable, for the frequencies used. So we show that capacitive sensors can successfully be integrated in composite casings and, with the proper sensor electronics, can well be used for permanent CSEM monitoring in boreholes.