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Investigating the physics of electromagnetics o...

Investigating the physics of electromagnetics over steel cased wells

Presented at the Lawrence Berkeley National Lab Geophysics Seminar.

Economic and social motivations to understand the impact of activities such as hydraulic fracturing and carbon capture and storage continue to increase. Electromagnetic (EM) imaging techniques have the potential to be a valuable method for monitoring these activities. One particular challenge is the presence of steel-cased wells. Steel has both a high electrical conductivity (~106 S/m) and a significant magnetic permeability (~100𝜇0), thus, it can considerably (and non-intuitively) impact the behaviour of the currents, fields, and fluxes in an EM survey. Several authors have demonstrated that the presence of the casing can be beneficial because the casing acts as an “extended electrode” and can channel current to depth. However, there are still many open questions, particularly with respect to the role of magnetic permeability, about the finer details of the EM responses. To make progress on these questions we need to be able to simulate the responses from realistic wells. The geometry of steel-cased wells is a complicating factor as the wells are often a centimeter thick and may extend for several kilometers. We have developed software for simulating Maxwell’s equations using a 3D cylindrical discretization. The software is included within the open-source SimPEG ecosystem, which supports forward simulations and inversions across a range of geophysical methods including magnetics, gravity, direct current resistivity, induced polarization, electromagnetics and fluid flow. In this presentation, we will explore aspects of the physical responses in a time-domain EM experiment with a steel-cased well and I will provide context for how these developments fit into the wider open-source ecosystem of tools for geophysics.

Lindsey Heagy

March 27, 2019
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  1. Investigating the physics of EM over steel cased wells Lindsey

    J. Heagy UC Berkeley LBL Geophysics Seminar March 27, 2019
  2. Modeling electromagnetics on cylindrical meshes • Finite volume discretization ◦

    cylindrically symmetric ◦ 3D cylindrical meshes • DC, FDEM, TDEM • Open source • Implemented in SimPEG 5 Heagy & Oldenburg, 2018
  3. Modeling electromagnetics on cylindrical meshes 6 Kaufman, 1990 Validating the

    physics • Kaufman (1990): Charges, currents, electric fields • Augustin (1989): magnetic permeability
  4. DC Resistivity: fundamentals • Kaufman (1990), Kaufman & Whitman (1993),

    Schenkel & Morrison (1994) • Cross-sectional conductance (S · m) • Implications ◦ Survey design ◦ Approximating wells (to reduce computation) ◦ Sensitivity in an inversion Modeling electromagnetics on cylindrical meshes 7 currents charges short well long well
  5. DC resistivity with steel-cased wells • Casing integrity ◦ Fundamental

    physics ◦ Feasibility ▪ Full break, partial flaw ◦ Factors influencing detectability ▪ Conductivity (casing, background) ▪ Depth of flaw 8 (Heagy & Oldenburg, 2019)
  6. DC resistivity with steel-cased wells • Casing integrity • Survey

    design considerations • Detecting a target ◦ Conductor, resistor ◦ Electrical connection vs. not 11 Signal due to target resistor conductor
  7. EM with steel cased wells: impact points, conductivity 18 Survey

    design: currents = galvanic + image + channeled
  8. EM with steel cased wells: impact points, permeability 19 t

    = 10ms permeable Forward simulation: • interplay between conductivity and permeability • poloidal currents in casing Survey design: • Important to include permeability • longer response through time conductive
  9. Open questions 20 • Horizontal and deviated wells • Infrastructure

    • Scalability of numerical simulations • Inverse problem • ...
  10. • Modular: building blocks ◦ organized in a framework ◦

    pieces available to manipulation • Declarative: express intent ◦ write what you mean ◦ looks like the math • Extensible: new research ◦ quantitative communication ◦ built in feedback loops • Open: for the future ◦ reproducible ◦ opportunities for collaboration building for researchers
  11. building for researchers • Modular: building blocks ◦ organized in

    a framework ◦ pieces available to manipulation • Declarative: express intent ◦ write what you mean ◦ looks like the math • Extensible: new research ◦ quantitative communication ◦ built in feedback loops • Open: for the future ◦ reproducible ◦ opportunities for collaboration
  12. Modular, multi-physics 33 • Magnetics • Gravity • DC Resistivity

    • Induced Polarization (IP) • Electromagnetics ◦ Time ◦ Frequency ◦ With IP effects • Fluid Flow