Feasibility Study of Controlled-Source Electromagnetic Method for Monitoring Low-Enthalpy Geothermal Reservoirs

Round 1

Reviewer 1 Report

My comments are:

1. English language should be revised in many places within the manuscript

2. Why the authors used 1.5 km separation distance?

3. Can the measurements of the present work be used for high enthalpy reservoirs?

4. What is the value of the vertical electric field?

5. What are the physical properties of the soil used in this study? such as thermal conductivity, thermal diffusivity, density, etc.

6. Why the composite casing has no influence on the electric field?

7. The literature review of this work should show the importance of geothermal energy, such information can be found from

Performance of geothermal power plants (single, dual, and binary) to compensate for LHC-CERN power consumption: comparative study. Geotherm Energy 5, 17 (2017). https://doi.org/10.1186/s40517-017-0074-z

6. 

Minor corrections required.

Author Response

• English language should be revised in many places within the manuscript.      

 Answer: The English has been corrected by MDPI English editing service

• Why the authors used 1.5 km separation distance?

Answer: The doublet was designed to have 1.5 km separation between the injection and the production wells. This design is provided in Vardon et al.,. "A Geothermal Well Doublet for Research and Heat Supply of the TU Delft Campus." arXiv preprint arXiv:2003.11826 (2020). However, this work has not been published.

• Can the measurements of the present work be used for high enthalpy reservoirs?

Answer: In the last sentence of the conclusions, we mention that our feasibility study can serve as a framework for future CSEM monitoring campaigns in general, which includes CSEM monitoring of the high-enthalpy geothermal reservoirs .

• What is the value of the vertical electric field?

Answer: We presume a vertical borehole is the best option and the vertical field propagates in horizontal directions, which is beneficial for our monitoring purposes. Lateral changes in the electric conductivity will lead to relative large changes in the vertical electric field. Hence that component is a good proxy for resistivity changes due to temperature changes. Its magnitude is expressed as The amplitude of the vertical electric field is shown in Fig. 2.

• What are the physical properties of the soil used in this study? such as thermal conductivity, thermal diffusivity, density, etc.

Answer: we assume a volumetric change in the cold volume rather than model it. We also assume a temperature change, which seems realistic but is not the result of modelling. As we already know the temperature change  and there is a direct relationship between temperature and electrical resistivity, the thermal and density properties of the soils are not relevant.

• Why the composite casing has no influence on the electric field?

Answer: It has negligible conductivity and magnetic permeability. The reason for the negligible of the composite casing on the electric field is now included in our study.

• The literature review of this work should show the importance of geothermal energy, such information can be found from

Performance of geothermal power plants (single, dual, and binary) to compensate for LHC-CERN power consumption: comparative study. Geotherm Energy 5, 17 (2017). https://doi.org/10.1186/s40517-017-0074-z

Answer: The mentioned paper is focused on the benefit of high-enthalpy geothermal reservoirs, which are beyond the scope of our paper. We mention the benefit of low-enthalpy geothermal reservoirs in lines 25-27.

Reviewer 2 Report

I reviewed this paper, which attempts to investigate the use of the CSEM method for monitoring geothermal reservoirs with low geothermal gradients, to experimentally determine the effective observation frequency and electrical stage device parameters and observation parameters used to distinguish temperature-dependent conductivity. After considering Recording noise, Survey repeatability errors, Near-surface temperature change, and Casing effect, it is found that CSEM can effectively monitor Low-Enthalpy Geothermal Reservoirs under the conditions affected by the observation environment.

For the readability and better comprehension of the paper, it is recommended to:

1. The author provides the results shown in Figure 2 and Figure 3 and the Relative difference related to temperature when doing Undesired Effects analysis.

2. It is recommended to analyze the influence of near-field and far-field sources separately and consider the shadow superposition effect of far-field.

Author Response

Reviewer's comments are agreed on.

Reviewer 3 Report

It is very interesting paper devoted to the actual topic of monitoring the underground movement of conducting fluids. The authors selected the appropriate research methodology, which is a ground-well version of electromagnetic sounding with an artificial source. Alternative methods of work are considered and the advantages of the used option are shown. Calculations of the sensitivity of the ground-well research variant to variations in the parameters of a fluid-saturated reservoir have been carried out. The influence of the conductive casing of wells on the propagation of an electromagnetic field is studied.

The article is well organized, contains clear and readable illustrations. The conclusions are justified and follow from the results of the conducted research. It seems that such type of work can be carried out on other objects with similar physical and geological models. For example, when studying the contours of flooding of hydrocarbon deposits and the like. The article is recommended for publication with minimal editorial edits.

Author Response

• Lines 36-37: What is the meaning of “cold plume”? For ease of understanding, please provide a detailed explanation in the paper.

Answer: agreed, and a definition of the cold plume has been provided in the manuscript, lines 28-29.

• Lines 52-62: The authors point out that CSEM has the most potential among all EM techniques to monitor the resistivity changes in the TU Delft campus geothermal reservoir. So, what are the shortcomings of CSEM compared to ERT and MT?

Answer: We purpose the CSEM method to monitor the subtle resistivity changes in low-enthalpy geothermal reservoirs.  Technical limitations of the CSEM which are associated with each survey configuration are mentioned in lines 64-77.

• 3D-MT imaging measurement has been widely used to describe the 3D resistivity structure of geothermal reservoirs. Can the CSEM method, which can only provide 2D resistivity information, accurately describe the temperature changes of reservoirs?

Answer: It is true that 3D-MT imaging has been used to map the 3D resistivity structure of geothermal reservoirs. The MT method is valuable in the exploration phase to detect large geothermal targets. When it comes to monitoring small and locally increasing resistivity values, the MT method is not expected to be sufficiently sensitive, as mentioned in lines 57-58. The CSEM has the potential to make 3D estimates of the changes in the resistivity structure with a 3D survey layout.

• Should “Rijland” and “Schiland” in Table 1 be replaced by “Rijnland” and “Schieland”?

Answer: This a very good point, the group names have been corrected.

• Can you provide a stratigraphic column marked with depth, group, temperature, and resistivity for TU Delft campus in this paper? This can make the content described in Table 1 more intuitive.

Answer: I agree that the stratigraphic column would  be more intuitive in general, and I added one

Reviewer 4 Report

Please refer to the Word attachment for detailed comments.

Comments for author File: Comments.pdf

Author Response

No reviewer.