SUMMARY Metallic well casings strongly impact electromagnetic fields due to their high electrical conductivity. We can take advantage of their presence and increase the sensitivity of EM signals to deeper subsurface structure by driving electric currents down through existing casings, or using the metal as a wave guide for EM telemetry. Interpretation of such well-casing-enhanced measurements requires accurate simulation of these setups. The Method of Moments (MoM) can be used for modelling well casings without having to discretize them finely as part of the subsurface model. Extending the MoM for EM sources directly connected to well casings is straightforward in principle. However, we find that the accuracy of MoM results for such configurations depends strongly on details of the computational model definition, such as the exact position of the connection point and the discretization of the source wire. We explain those important details and provide strategies for the most accurate MoM modelling of electric currents injected into well casings.

Controlled-source electromagnetic methods have the potential to be used in reservoir monitoring problems due to their sensitivity to subsurface resistivity distribution. For example, time-lapse electromagnetic (EM) measurements can help to determine reservoir changes during enhanced oil recovery processes, such as water/steam injection or CO2 sequestration. Although metal infrastructure, such as pipelines and casings, can strongly influence EM data and mask the underlying geologic response, these effects have not previously been quantified for time-lapse surveys. We have analyzed the effects of well casings on time-lapse surface-to-surface EM measurements using 1D and 3D modeling. First, using a synthetic example of an onshore 1D hydrocarbon reservoir, we quantified the effect of single and multiple casings at several source and receiver locations. We found that time-lapse responses are significantly distorted when a source or receiver is located near a casing. Next, we approximated a hydrocarbon reservoir as a thin bounded resistive sheet. We developed a method of moments algorithm to calculate the respective secondary currents and charges on a well casing and resistive sheet combination and validated the electric fields these secondary sources generate against finite-element modeling. Finally, we calculated and explicitly demonstrated time-lapse amplitude changes in the well casing-thinsheet interaction matrix, secondary currents, charges, and surface electric fields. Our 3D modeling results indicated that the conductive casing reduces the ability of the resistive sheet to impede current flow and distorts time-lapse responses. Therefore, one cannot fully eliminate casing effects by subtraction of time-lapse data and must fully incorporate such infrastructure into forward models for time-lapse EM inversion.

Modeling and inversion of electromagnetic (EM) data contaminated by steel infrastructure remains a numerically challenging task. In this study, we collected controlled-source EM (CSEM) field data at two test sites. First, we conducted a CSEM survey over a dry and abandoned well in eastern Colorado. Next, we collected CSEM data over a test well located at the Colorado School of Mines. The purpose of these experiments was to examine methods to invert EM data contaminated by infrastructure effects and recover undistorted subsurface conductivity models. We used a hybrid approach to model casing effects by considering the conductive metal as a distribution of secondary sources with magnitudes calculated using an approximate layered background. In particular, we applied the method of moments algorithm to calculate the magnitude of the secondary electric dipole sources along the casing. Subsequently, we included these secondary sources in a 3D finite-volume-based forward calculation and used the Gauss-Newton method to invert the contaminated field data. As expected, our preliminary inversion results show that by not considering casing effects, significant artifacts are introduced in the recovered models. Furthermore, we show that such artifacts are reduced significantly through the introduction of casing physics in the forward model, enabling the surrounding subsurface conductivity and corresponding geology to be characterized.

Controlled-Source Electromagnetic (CSEM) methods have the potential to be powerful geophysical tools for imaging and monitoring the distribution of electrically resistive fluids, such as freshwater aquifers, CO2 injected into the subsurface or hydrocarbons during oil and gas production. However, the presence of metallic infrastructure (steel well casings, pipelines etc.) presents an enormous challenge, because the highly conductive metal masks the electromagnetic response of subsurface geology and distorts any associated time-lapse changes. Therefore, numerical techniques to predict and mitigate the contamination caused by pipelines and casings on CSEM surveys are critical for real world imaging and 4D applications near any such metal objects. In a collaborative project between the Colorado School of Mines and Shell, we have developed CSEM modeling and inversion tools that can handle realistic scenarios with multiple vertical as well as deviated casings and complex pipeline networks, as will be encountered in mature oil field environments. First, we implemented a forward modeling code based on the Method of Moments technique, which effectively turns the casings into extra sources, such that we do not need to discretize them into excessive numbers of very small model cells. We used this modeling tool to demonstrate quantitatively how steel casings impact synthetic and real time-lapse EM data. The forward modeling code was then combined with a newly developed Gauss-Newton inversion engine, which by itself has been demonstrated to provide images of superior resolution, depth penetration and data fit with less dependency on initial conditions compared to previous quasi-Newton inversion engines. In this contribution, we first demonstrate on synthetic data that the combination of these two algorithms provides high-quality electrical resistivity images in the immediate vicinity of well casings. Then, we show encouraging results of applying the new tools to field trial data acquired over known casings under semi-controlled conditions. The images obtained are nearly free of casing imprint and subsurface geology could be recovered. These results suggest that this technology may enable us to explain severely distorted field data that were previously uninterpretable.

The inversion of electromagnetic (EM) data contaminated by casing effects remains a numerically challenging task. In this study, we collected EM field data over a dry and abandoned well in eastern Colorado to examine methods to invert such infrastructure-contaminated measurements. We use a hybrid approach to model casing effects by considering the casing as a distribution of secondary sources with magnitudes calculated using an approximate 1D model. Subsequently, we include these secondary sources into a 3D finite volume-based forward calculation. We then use a Gauss-Newton method to invert the contaminated field data. The inversion results show that we are able to remove the casing artifacts from the recovered model and adequately characterize the surrounding geology.

Summary We have applied a new Gauss-Newton (GN) inversion code to re-image data that previously could not be interpreted with confidence. The data were acquired over an area where seismic data indicate a basement high about 3 km below an exploration target. Previous extensive field data imaging and scenario tests using a quasi-Newton (BFGS) inversion algorithm could not rule out the possibility that a slightly resistive feature at the depth of interest was a misplaced expression of the deeper basement high. GN inversion provides much-improved, more stable images that show a laterally extended, moderately resistive layer in the target depth range. When explicitly including the basement in the inversion starting model, that resistor becomes only marginally weaker, and remains clearly separated from the basement. Unlike BFGS inversion, the GN inversion results leave no room for misinterpreting the basement high as a hydrocarbon reservoir.

Controlled-source electromagnetics (CSEM) can be used to image subsurface resistivity and add value in petroleum exploration. However, the application of CSEM methods can be particularly challenging in mature oil and gas fields, where the presence of steel casings and complex seabed infrastructure may influence electromagnetic (EM) fields. The effect of this metal infrastructure can be modeled using the method of moments (MoM), which has previously been demonstrated to be effective in simple situations. We have now developed a methodology for modeling the EM response of complex pipeline geometry using MoM, and we validate our approach numerically and experimentally; the difference between finite-element and MoM modeling is less than 3%, whereas the difference between experimental data and MoM results is less than 30%. We further develop a fast and efficient approach to model the EM response of horizontal pipeline infrastructure. First, we create individual pipe sections of custom shape; when assembled, these sections are used to construct the entire pipeline system. We then identify “precise” and “approximate” zones based on how accurately we need to perform the calculations. We find that the electric field values in the approximate zone can be calculated accurately and quickly using previously saved table values, and therefore achieve a considerable reduction in computational requirements. Finally, we apply our methodology to a real situation of a 60 km horizontal seabed pipeline and find that the electric field distortion can be calculated rapidly and efficiently with our custom-built algorithm.

PreviousNext No AccessSEG Technical Program Expanded Abstracts 2020Can metal infrastructure effects be subtracted in time-lapse electromagnetic measurements?Authors: Gurban OrujovAndrei SwidinskyRita StreichGurban OrujovColorado School of MinesSearch for more papers by this author, Andrei SwidinskyColorado School of MinesSearch for more papers by this author, and Rita StreichShell Global Solutions International BVSearch for more papers by this authorhttps://doi.org/10.1190/segam2020-3427309.1 SectionsSupplemental MaterialAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail AbstractControlled-source electromagnetic (CSEM) methods have the potential to be used in reservoir monitoring problems. Due to their sensitivity to subsurface resistivity distribution, time-lapse electromagnetic (EM) measurements can help to determine reservoir changes during enhanced oil recovery (EOR) processes such as water/steam injection or CO2 sequestration. Although metal infrastructure such as pipelines and casings may influence EM data and mask the underlying geological response, one might expect that these effects could be subtracted during time-lapse surveys. Using a synthetic example of an onshore hydrocarbon reservoir, we analyze the effect of well casings on the time-lapse surfaceto-surface EM measurements at several source and receiver locations and quantify the effect of a single and multiple wells. Results show that we can not fully eliminate casing effects by subtraction and therefore need to incorporate them into forward modeling for time-lapse EM inversion.Presentation Date: Tuesday, October 13, 2020Session Start Time: 1:50 PMPresentation Time: 3:55 PMLocation: Poster Station 7Presentation Type: PosterKeywords: time-lapse, frequency-domain, electromagnetics, CSEM, wellsPermalink: https://doi.org/10.1190/segam2020-3427309.1FiguresReferencesRelatedDetailsCited by3-D Numerical Study on Controlled Source Electromagnetic Monitoring of Hydraulic Fracturing Fluid With the Effect of Steel-Cased WellsIEEE Transactions on Geoscience and Remote Sensing, Vol. 60VTI, HTI, TTI and combination of symmetries: Electrical anisotropy and its impact in unconventional reservoirsAna Curcio1 September 2021Symmetries and configurations of hydraulic fracturing electromagnetic monitoring: a 2D anisotropic approach7 January 2021 | Geomechanics and Geophysics for Geo-Energy and Geo-Resources, Vol. 7, No. 1 SEG Technical Program Expanded Abstracts 2020ISSN (print):1052-3812 ISSN (online):1949-4645Copyright: 2020 Pages: 3887 publication data© 2020 Published in electronic format with permission by the Society of Exploration GeophysicistsPublisher:Society of Exploration Geophysicists HistoryPublished Online: 30 Sep 2020 CITATION INFORMATION Gurban Orujov, Andrei Swidinsky, and Rita Streich, (2020), "Can metal infrastructure effects be subtracted in time-lapse electromagnetic measurements?," SEG Technical Program Expanded Abstracts : 606-610. https://doi.org/10.1190/segam2020-3427309.1 Plain-Language Summary Keywordstime-lapsefrequency-domainelectromagneticsCSEMwellsPDF DownloadLoading ...

Controlled-source electromagnetics (CSEM) can be used to image subsurface resistivity and add value in petroleum exploration. However, the application of CSEM methods can be particularly challenging in mature oil & gas fields, where the presence of steel casings and complex seabed infrastructure may influence electromagnetic (EM) fields. The effect of this metal infrastructure can be modeled using the Method of Moments (MoM), which has been demonstrated to be effective in simple situations. In this study we present a methodology for modeling the EM response of complex pipeline geometry using the MoM. We then confirm the validity of our approach both numerically and experimentally; the difference between finite element and MoM modelling is below 3%, while the difference between experimental data and MoM results is below 30%. Presentation Date: Tuesday, September 17, 2019 Session Start Time: 1:50 PM Presentation Time: 3:30 PM Location: 225C Presentation Type: Oral

Summary We will present results from a field trial of onshore time-lapse EM in a production environment, with special focus on the influence of well casings and pipelines on the acquired data, and lessons learned for future application of 4D EM.

The Method of Moments was originally developed in the 1960s for electrical engineering applications, as described in Roger Harrington’s famous book “Field Computation by Moment Methods”. The concept was quickly adopted by the mineral exploration community to model the electromagnetic response of volcanogenic massive sulfide deposits as thin conductive sheets, and the technique still forms the basis of many airborne electromagnetic interpretation packages. Targets are parameterized as specific geometric shapes and represented as a distribution of secondary sources whose moments are determined through the solution to a dense matrix equation describing field interactions within the system. The approach is closely related to integral equation modelling but the dimensionality of most problems is reduced by considering objects and shapes of infinitesimal thickness (like thin conductive sheets, plates or tubes). One advantage of the Method of Moments is its ability to describe highly conductive targets as is the situation for steel infrastructure problems, and algorithms can be custom built for rapid calculations. In this workshop we describe our experiences with this technique in the context of steel well casings and applied electromagnetic methods. Our presentation will outline the basic theory of the approach, describe the steps we have taken to validate the method against quasi-analytical and finite-element solutions and finally, give several illustrative examples showing the behavior of single casings, interactions between two casings and demonstrating the capability of modeling more realistic multi-casing scenarios. Presentation Date: Start Time: Location: Presentation Type:

In oil and gas production environments, controlled-source electromagnetics can be used to aid brownfield exploration, development, and reservoir monitoring efforts. However, such environments typically have many highly conductive steel-cased wells in the area of interest. We have developed a modeling algorithm using a method of moments (MoM) approach to calculate the electromagnetic response of multiple 3D steel-cased wells of arbitrary geometry in a layered earth conductivity model. This approach involves dividing each casing into a collection of segments, each carrying a uniform current density. A matrix is computed that describes how the casing segments interact with each other electromagnetically. Then, we solve a linear system for the current within each casing segment, given a transmitter of arbitrary frequency and location. From these currents we are able to solve for the secondary electromagnetic fields produced solely by the casings at any point in our layered model, and we add these to the primary fields produced by the transmitter. To validate the algorithm, we compared results with a pseudoanalytic MoM algorithm for a single vertical casing in a half-space. We also compare results with a finite-element solution using Comsol Multiphysics for vertical and single tilted wells buried in various layered earth models. Our results indicate a good match between these different approaches, with tilted casings in a layered model requiring further study. Finally, we applied our algorithm to a realistic synthetic model with three casings (one vertical and two deviated) extending into a layered earth model containing the classic thin resistive layer. This example illustrates how the algorithm can be used to compute the electromagnetic response of multiple steel casings. The example also illustrates how the electromagnetic field changes due to the presence of the casings and how the casings may be used to inject the signal at depth.

Controlled-source electromagnetic (CSEM) data are sensitive to the subsurface resistivity distribution, but 3D inversion results are ambiguous, and in-depth interpretation is challenging. Resolution and sensitivity analysis as well as the influence of noise on resolution have been used to quantify 3D inversion performance. Based on these numerical studies, a land-based CSEM survey was designed and carried out at the Schoonebeek oil field, the Netherlands. The acquired data were processed and subsequently inverted for the resistivity distribution. The 1D and 3D inversion of horizontal electric-field data show the reservoir at the right depth, matching well-log data without using a priori knowledge about the actual reservoir depth. We used a 1D model with fine layering as a starting model for 3D inversion. Synthetic data inversions and sensitivity tests demonstrate that resistive or conductive bodies inside the reservoir zone may be well-detectable with our limited acquisition geometry. Spatial variations in the reservoir resistivity are visible in the measured data and after inversion by assuming good knowledge of the background resistivity distribution. The reservoir resistivity and size, however, have to be interpreted with care considering the intrinsically low resolution of electromagnetic (EM) which is further reduced by manmade EM noise.

We investigate the potential of magnetotelluric (MT) imaging to aid exploration activities in fold-and-thrust belt settings. While MT is known to be sensitive to electrically conductive structures, here we examine resolution capabilities for strongly resistive thrust sheets. Images of a real data set clearly show the resistors, consistent with previous geological knowledge. The shape of a deeper resistive body, which is proven to exist based on exploration drilling, appears to be discernible in parts of the image, and blurred into a single body with a shallower resistor in other parts. Using synthetic studies to analyze the image reliability further, we find that such a resistive body is indeed detectable by the MT data, but would not be resolvable, even if the MT data set was amended in areal coverage and frequency range. This analysis prevents premature, biased geological interpretation. Presentation Date: Thursday, October 18, 2018 Start Time: 8:30:00 AM Location: 213A (Anaheim Convention Center) Presentation Type: Oral

Summary We present a thoroughly tested modeling algorithm using a Method of Moments approach to calculate the controlled-source electromagnetic response of multiple 3D pipelines or steel-cased boreholes of arbitrary geometry in a layered earth host. To validate the algorithm, we compare results with a simpler Method of Moments approach for a single vertical casing in a halfspace. We also compare results with a finite-element solution using Comsol Multiphysics for both a single vertical and single tilted well crossing a subsurface boundary. Our tests show a very good match between these different approaches. We then apply our algorithm to a realistic synthetic marine model with three casings (one vertical and two deviated) extending into a layered earth model containing the classic thin resistive layer. This example illustrates how the algorithm can be used to compute the electromagnetic response of multiple steel casings, as well as how signal can be enhanced at depth through a casing with suitable transmitter geometry.

Controlled-source electromagnetic data is sensitive to the subsurface resistivity distribution, but 3D inversion results are ambiguous and in-depth interpretation is challenging. Resolution and sensitivity analysis as well as the influence of noise on resolution have been used to quantify 3D inversion performance. Based on these numerical studies, a land-based controlled-source electromagnetic survey was designed and carried out at the Schoonebeek oil field, the Netherlands. The acquired data were processed and subsequently inverted for the resistivity distribution. Both 1D and 3D inversion of horizontal electric-field data show the reservoir at the right depth, matching well-log data without using a-priori knowledge about the actual reservoir depth. We used a 1D model with a fine layering as starting model for 3D inversion. Synthetic data inversions and sensitivity tests demonstrate that resistive or conductive bodies inside the reservoir zone may be well detectable with our proposed limited acquisition ...

Electromagnetic methods are sensitive to conductivity contrasts in the subsurface. This makes the methods useful for monitoring oil and gas reservoirs, which will experience changes in electrical conductivity over their lifetime. In a modern production environment, such a survey would be completed in the presence of multiple deviated cased wells. Common well casing materials, such as steel, are very good conductors and can create a strong secondary electromagnetic field that contaminates the data at the receivers with unwanted signal. To resolve this problem, it is necessary to be able to accurately model the effect of the well casings on collected data. In this paper, we make use of earlier casing modeling work using the Method of Moments and extend the approach to model the electromagnetic response of a group of deviated wells in a layered geology subject to a marine CSEM survey and produce a tool fit for practical applications. An illustrative example is used to show the electric fields calculated by a group of deviated wells in a layered marine geologic setting. Results show that most of the electromagnetic fields channeled by the horizontal wells into a thin resistor are unable to escape into the surrounding media.

Electromagnetic methods that utilize controlled sources have been applied for natural resource exploration for more than a century. Nevertheless, concomitant with the recent adoption of marine controlled-source electromagnetics (CSEM) by the hydrocarbon industry, the overall usefulness of CSEM methods on land has been questioned within the industry. Truly, there are few published examples of land CSEM surveys carried out completely analogously to the current marine CSEM standard approach of towing a bipole source across an array of stationary receivers, continuously transmitting a low-frequency signal and interpreting the data in the frequency domain. Rather, different sensitivity properties of different exploration targets in diverse geological settings, gradual advances in theoretical understanding, acquisition and computer technology, and different schools in different parts of the world have resulted in a sometimes confusing multitude of land-based controlled-source EM surveying approaches. Here, I aim to review previous and present-day approaches, and provide reasoning for their diversity. I focus on surface-based techniques while excluding airborne EM and well logging and on applications for hydrocarbon exploration. Attempts at the very demanding task of using onshore controlled-source EM for reservoir monitoring are shown, and the possible future potential of EM monitoring is discussed.

We evaluated 3D inversion of land controlled-source electromagnetic (CSEM) data collected across the Ketzin [Formula: see text] 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.

Summary We investigate potential benefits of measuring the vertical electric field component in addition to the routinely measured horizontal electric field components in onshore time-lapse controlled-source electromagnetics. Synthetic electromagnetic data based on a model of the Schoonebeek onshore oil field are used. We confirm that the vertical electric field component is more sensitive to small changes in the reservoir than the horizontal components, yet its amplitudes are small. Accordingly, optimal source-receiver geometry and precise knowledge of the verticality of the receiver dipole will be required for successful utilization of the vertical electric field.