Multi‐method virtual electromagnetic experiments for developing suitable monitoring designs: A fictitious CO₂ sequestration scenario in Northern Germany

Abstract

We present a numerical study for 3D time‐lapse electromagnetic monitoring of a fictitious CO2 sequestration using the geometry of a real geological site and a suite of suitable electromagnetic methods with different source/receiver configurations and different sensitivity patterns. All available geological information is processed and directly implemented into the computational domain, which is discretized by unstructured tetrahedral grids. We thus demonstrate the performance capability of our numerical simulation techniques. The scenario considers a CO2 injection in approximately 1100 m depth. The expected changes in conductivity were inferred from preceding laboratory measurements. A resistive anomaly is caused within the conductive brines of the undisturbed reservoir horizon. The resistive nature of the anomaly is enhanced by the CO2 dissolution regime, which prevails in the high‐salinity environment. Due to the physicochemical properties of CO2, the affected portion of the subsurface is laterally widespread but very thin. We combine controlled‐source electromagnetics, borehole transient electromagnetics, and the direct‐current resistivity method to perform a virtual experiment with the aim of scrutinizing a set of source/receiver configurations with respect to coverage, resolution, and detectability of the anomalous CO2 plume prior to the field survey. Our simulation studies are carried out using the 3D codes developed in our working group. They are all based on linear and higher order Lagrange and Nédélec finite‐element formulations on unstructured grids, providing the necessary flexibility with respect to the complex real‐world geometry. We provide different strategies for addressing the accuracy of numerical simulations in the case of arbitrary structures. The presented computations demonstrate the expected great advantage of positioning transmitters or receivers close to the target. For direct‐current geoelectrics, 50% change in electric potential may be detected even at the Earth's surface. Monitoring with inductive methods is also promising. For a well‐positioned surface transmitter, more than 10% difference in the vertical electric field is predicted for a receiver located 200 m above the target. Our borehole transient electromagnetics results demonstrate that traditional transient electromagnetics with a vertical magnetic dipole source is not well suited for monitoring a thin horizontal resistive target. This is due to the mainly horizontal current system, which is induced by a vertical magnetic dipole.

Three-Dimensional Modeling of the Casing Effect in Onshore Controlled-Source Electromagnetic Surveys

“…This study shows that the hollow casing can be adequately approximated with a rectangular prism, thus allowing to avoid a large number of tiny elements in the discretization and to reduce the computational cost. Finally,Börner et al (2015) conducted a set of modeling studies using finite-element formulations on unstructured grids for CSEM, borehole transient EM, and the direct current resistivity method to define a set of optimal source/receiver configurations with respect to coverage, resolution, and detectability of the anomalous CO2 plume.This article explores the sensitivity of a borehole-to-surface CSEM configuration with a deep electric dipole installed in the injection well and compares the modeling results with the baseline data set collected at the Hontomín CO2 storage site, Spain(Vilamajó et al 2015). We begin with the description of the geophysical setup at Hontomín and the acquired data set in Sect.…”

Monitoring Offshore CO2 Sequestration Using Marine CSEM Methods; Constraints Inferred from Field- and Laboratory-Based Gas Hydrate Studies

“…The thickness of 100 m is one commonly used in simulations and is conveniently one cell tall in our model. We assign the reservoir a conductivity of 0.01 S/m, a value used by [46] but on the low side of other studies. While a simple model, this is consistent with those used in other studies [40,44,46] and has the advantage of being identical to the canonical oilfield model [64], allowing comparisons to be made with many existing studies designed to examine the use of CSEM for oilfield exploration (e.g., [53,54]).…”

“…A critical component for safe and reliable CO 2 sequestration strategies involves monitoring the injection of CO 2 into the targeted region and verifying long-term stability and integrity of the storage reservoir, including possible leakage. Since the early 2000s, a variety of geophysical methods have been applied towards monitoring geological and marine storage of CO 2 (reviewed in [39,40]), including both seismic and nonseismic technologies, with increasing application of electromagnetic (EM) or electrical technologies that complement seismic or gravity measurement methods [41][42][43][44][45][46][47][48][49][50][51]. EM methods are considered particularly useful for monitoring CO 2 sequestration where displacement of pore fluid by CO 2 increases electrical resistivity.…”

Repeatability of land-based controlled-source electromagnetic measurements in industrialized areas and including vertical electric fields

“…A comprehensive review of CSEM methods for exploration and monitoring of hydrocarbon reservoirs is provided by Streich (2016). But targets under investigation also include CO 2 storage applications (Streich et al 2010;Vilamajó et al 2013;Zhdanov et al 2013;Börner et al 2015b) or geothermal reservoirs (Börner et al 2015a). Nearly all of the published work concludes that monitoring of the investigated subsurface process is feasible with CSEM techniques, though a few authors point out that the expected time-lapse responses are probably close to the detectability threshold (Tietze et al 2015; see also Streich 2016).…”