3D inversion and resolution analysis of land-based CSEM data from the Ketzin CO2 storage formation

Abstract

We evaluated 3D inversion of land controlled-source electromagnetic (CSEM) data collected across the Ketzin CO 2 storage formation. A newly developed, parallel and distributed 3D inversion code, which is based on a direct forward solver, has been used. This inversion scheme allowed us to calculate the Jacobian matrix explicitly within a reasonable time and use it to calculate regularization parameters, inspect survey coverage, and carry out resolution analysis. After demonstrating that the magnetic field components are sensitive to conductors only, whereas the electric field components are sensitive to all features of interest, we continued to work with electric field data only. Estimates of data uncertainty obtained from robust processing were used for automated data preselection and weighting during inversion. We tested different regularization techniques and a range of starting models to explore the model space and assess the influence of regularization on the inversion images. We further demonstrated an approach for handling numerical singularities due to sources located inside the inversion domain. We estimated survey coverage, horizontal and vertical resolution, and depth penetration using cumulative sensitivity, point spread functions, and depth-resolution plots. Based on data fit analysis, we determined a preferred subsurface conductivity model, which we compared to an independent regional structural geologic model, and we provided an interpretation for the structures resolved. The inversion approach we used provides robust results in good agreement with known geology, offers new possibilities for model assessment, and should be transferable to other CSEM data sets.

Quantitative evaluation of thin-layer thickness and CO2mass utilizing seismic complex decomposition at the Ketzin CO2storage site, Germany

“…A significant increase in the temporal thickness of the gaseous CO 2 plume is observed at Ktzi 203. This indicates a preferential migration of the gaseous CO 2 plume in the up-dip direction, which is in agreement with observations from the other seismic ) and electromagnetic methods (Bergmann et al 2012;Grayver et al 2014) employed at the Ketzin site. It is known that the velocity of the sandstone after CO 2 injection is influenced by the CO 2 saturation.…”

“…A combined approach using seismic and other geophysical methods may contribute to a decrease in uncertainties. Given that the electromagnetic method is also very sensitive to the presence of injected CO 2 due to the resistivity contrast (Grayver et al 2014;Bergmann et al 2015), 4-D joint seismic-electromagnetic monitoring for the injected CO 2 plume can potentially be used to improve coverage and provide useful complementary information on the CO 2 distribution.…”

A multi-domain decomposition-based Fourier finite element method for the simulation of 3D marine CSEM measurements

“…In this subsection, we first extend the DDFFE method [5] to multiple subdomains and then apply it to the secondary field formulation (16). We assume that the computational domain Ω = Ω(x, y, z) can be expressed as a tensor product Ω = R(y) × Ω 2D (x, z), and the cross-sectional area Ω 2D can be split into Ω 2D,1 and Ω 2D,2 .…”

“…Utilization of marine CSEM data for identification of oil and gas reservoirs and/or monitoring of fluid flows in a reservoir [7] is often performed through an inversion process [32,31,16]. Such an inversion process requires multiple solutions of the EM forward model, which is often the most time consuming task of any inversion algorithm.…”

Considerations about the Solution Space of a VTI Marine CSEM Inversion Problem Using Vertical Antennas

“…At these frequencies, the electromagnetic field propagates diffusively, which makes a direct interpretation using "events" like in seismics impossible. Therefore, the common processing flow contains an inversion for the subsurface conductivity distribution (e.g., Christensen and Dodds, 2007;Key, 2009;Ray et al, 2013;Grayver et al, 2014).…”