Temporal Stability of the Transient Electromagnetic Response with Respect to the Development of Monitoring Techniques
Final Report Abstract
The purpose of this project is to prepare, investigate, and methodologically develop a borehole transient electromagnetic monitoring technique for observing CO2 plumes in the subsurface. Both electrical and electromagnetic methods are particularly sensitive to changes in the conductivity of the pore fluid in porous rock media, which generally qualifies them for monitoring high-resistivity CO2 plumes in conductive saline aquifers. With fundamental and extensive preliminary petrophysical work in the laboratory, we have shown what changes in rock electrical conductivity can be expected in the presence of CO2 in the aquifer. Based on this, our virtual electromagnetic experiments in the time domain showed that the magnitude of the measurement effect for a CO2 sequestration at the pilot site Brooks, Alberta, Canada, is expected to be about 10 to 20% if 600 t CO2 are injected into an approximately 300 m deep saline aquifer in the course of a scientific experiment planned by Carbon Management Canada and our transient electromagnetic surface-to-borehole equipment is used. Our preparations for the electromagnetic field survey had been underway since 2015 and were then carried out in two measurement campaigns in 2016 and 2018. While technical difficulties in 2016 only allowed the acquisition of a small data set, a comprehensive, high-quality data set was obtained in the second measurement campaign in 2018 to determine the baseline before CO2 injection. For this purpose, eight transient electromagnetic sources with edge lengths of up to 400 m and dipole moments of up to 6.4*10^6 Am2 were positioned at the Earth's surface. The measurements of the electromagnetic transient could be performed along the entire accessible observation well in the range of 60 to 320 m depth with high data quality. Since the expected measurement signal is small, the main focus was put on the quantification of the error and the quality of the repeatability of the measurements. Therefore, statistical methods were developed to assess the similarity or dissimilarity of electromagnetic transients. These procedures are mathematically demanding for several reasons. First, because transient electromagnetic signals represent non-stationary time series, and second, because these time series exhibit extremely large dynamics in both the time domain and amplitude. With the help of a suitable normalization and transformation of the data as well as various distance metrics such as dynamic time warping and autoregressive time series models (ARIMA), a reliable workflow could be developed that separates similar and dissimilar time series. Thus, all methodological prerequisites have been created to prove the detectability of a CO2 plume in a practical field experiment in the next step.
Publications
- 2020. Classification and repeatability studies of transient electromagnetic measurements with respect to the development of CO2 monitoring techniques, Dissertation, Faculty of Geosciences, Geoengineering and Mining of the Technische Universität Bergakademie Freiberg
Bär, Matthias
