## 6 Undesired Effects
### Casing Effect  
As mentioned in Section 2, the design of the TU Delft geothermal doublet includes two deviated steel-cased wells. The impact of the very low electrical resistivity of the steel casing on the EM field response is significant [47; 48; 49; 50]. In [51], it was shown that it is also important to take into account the high magnetic permeability of the steel casing when conducting CSEM forward modelling.  
Thus, we incorporated the physical properties of a steel-cased vertical well into the TU Delft campus sub-surface model. The thickness of the casing wall is 2.5 cm and its  
Figure 6: Time-lapse monitoring of the TU Delft campus reservoir with errors in the source and receiver parameters. The (**left plot**) shows the relative difference due to the repeatability error, while the (**right plot**) shows the difference due to the repeatability error and resistivity change.  
Figure 7: Time-lapse monitoring of the TU Delft campus reservoir in the presence of sub-surface changes. The (**left plot**) shows the relative difference due to the 9 \({}^{\circ}\)C seasonal variation in soil temperature, while the (**right plot**) shows the difference due to the seasonal variation and resistivity changes.  
diameter is 15 cm. The considered steel has an electrical resistivity and a relative magnetic permeability equal to \(10^{-6}\)\(\Omega\cdot\)m and 100, respectively. The hollow of the casing was assumed to be filled with a brine of resistivity equal to \(5.68\times 10^{-2}\)\(\Omega\cdot\)m. The physical properties of the casing do not change over time, corresponding to a non-corroded casing. The model was discretized on a cylindrical mesh. Thus, only the impact of one vertical-cased well could be modelled.  
As shown in Figure 8, the modelled steel-cased well severely disturbed the electric field. The steel casing changed the field response by 30% or more within 10 m of its vicinity. The footprint of the steel casing reduced with distance, ranging between 10 and 30% within 100 m and less than 10% further away. Close to the source location, the footprint of the casing was less than 1%, except around the zero-crossing. This significant distortion in the electric field was caused by electric fields induced in the modelled steel medium. These fields propagate into the surrounding environment to distort the electric fields from the source. Figure 8 also shows that the detectability of the resistivity change decreased within 10 m around the steel casing, as well as some other areas within 100 m around the casing. Away from the casing, the resistivity change still created a sufficient difference.  
We changed the properties of the casing from steel to fibreglass-reinforced epoxy, which has resistivity of around \(10^{16}\)\(\Omega\cdot\)m and unit magnetic permeability. The effect of the composite casing on the electric field was limited to the borehole, and is not observable in Figure 9. Based on the right-hand plots in Figures 8 and 9, it seems that the composite casing had no influence on the electric field outside the casing, allowing for the CSEM monitoring of the geothermal reservoir close to the borehole. The negligible impact of the composite medium can be attributed to its very high electrical resistivity and no magnetization, therefore not producing induction fields.  
Figure 8: Time-lapse CSEM monitoring of the TU Delft campus reservoir in the presence of a steel casing. The (**left plot**) shows the effect of the steel casing on the electric field response, while the (**right plot**) shows the effect of the steel casing on the difference between time-lapse electric fields.  
Figure 9: Time-lapse CSEM monitoring of the TU Delft campus reservoir in the presence of a composite casing. The (**left plot**) shows the effect of the composite casing on the electric field response, while the (**right plot**) shows the effect of the composite casing on the difference between time-lapse electric fields.