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.

Transcript

1. Exploring the physics of electromagnetics with steel-cased wells using open-source tools Lindsey Heagy & Doug Oldenburg IUGG 2019
2. A motivating example Subsurface injections (hydraulic fracturing) 2
3. A motivating example 3 Maxwell’s equations (quasi-static) time frequency constitutive relations Subsurface injections (hydraulic fracturing)
4. Outline • Numerical modelling • DC Resistivity • Electromagnetics ◦ Conductivity ◦ Permeability 4
5. 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
6. DC resistivity 6 Kaufman, 1990 Validating the physics • Kaufman (1990): Charges, currents, electric fields • Augustin (1989): magnetic permeability
7. 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
8. DC Resistivity: casing integrity 8
9. DC Resistivity: data at the surface Intact well 9 primary Secondary (flawed - intact)
10. 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
11. EM: conductive well 11 Current density (A/m2)
12. EM: permeable well 12 t = 5ms t = 10ms t = 1ms conductive permeable
13. EM: permeable well 13 conductive permeable
14. EM: permeable well 14
15. EM: permeable well 15 conductive permeable difference
16. Summary DC 16 EM: conductive t = 10ms permeable conductive EM: permeability currents: galvanic + image + channelled
17. thank you! simpeg.xyz slack.simpeg.xyz github.com/lheagy/phd-thesis you!?