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Exploring the Physics of Electromagnetics with ...

Exploring the Physics of Electromagnetics with Steel-Cased Wells Using Open-Source Tools

Presented at the IUGG conference in Montreal (http://iugg2019montreal.com/).

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 large 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 ~1cm 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; we use this software to explore the physical responses in a time-domain EM experiment with a steel-cased well. In particular, we discuss how magnetic permeability alters the responses. All examples shown will be made openly available as Jupyter notebooks.

Lindsey Heagy

July 13, 2019
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  1. Modelling EM on cylindrical meshes • Finite volume discretization ◦

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

    (1990): Charges, currents, electric fields • Augustin (1989): magnetic permeability
  3. 7 currents charges short well long well DC Resistivity 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
  4. DC resistivity with steel-cased wells • Casing integrity • Survey

    design considerations • Detecting a target ◦ Conductor, resistor ◦ Electrical connection vs. not 10 Signal due to target resistor conductor
  5. EM: permeable well 12 t = 5ms t = 10ms

    t = 1ms conductive permeable
  6. Summary DC 16 EM: conductive t = 10ms permeable conductive

    EM: permeability currents: galvanic + image + channelled