the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Aug 2023
Evolution of fault reactivation potential in deep geothermal systems. Insights from the greater Ruhr region, Germany
Abstract. The success of deep geothermal systems depends on the presence of fault zones in the subsurface. Faults play a vital role in the Earth's plumbing system by facilitating fluid flow when they dilate, but are simultaneously known to enhance the hazard of the system once slipping in shear mode. As dilation of a fault enhances its permeability significantly, shear failure can lead to loss of boreholes or seismic events of economic concern. In this study, we present the evolution of reactivation potential of major faults during 25-year production period in deep generic geothermal systems in the greater Ruhr region in western Germany. To determine the pre-operational in situ stress state we use a recently published comprehensive dataset of stress magnitude data from the greater Ruhr region in an analytical-probabilistic model accounting for uncertainties of in situ stress, fault geometry, and frictional properties for a prospective reservoir in the Devonian Massenkalk formations. The resulting cumulative distribution functions of dilation and slip tendency of given fault sets suggests that more than half of the combined length of NW-SE-striking faults have a high reactivation probability, whereas the NE-SW-striking faults remain not optimally-oriented in the regional stress field. Using the relationship between dilation and slip tendency, we propose fault segments suitable for geothermal development that exhibit high hydraulic conductivity, i.e. high dilation tendency, and lower potential for shear failure, i.e. low slip tendency. In the second step, we employ generic thermo-hydro-mechanical models to quantify induced spatio-temporal stress changes on selected fault planes due to long-term geothermal production. We find that after 25 years thermal stress changes contribute significantly to the change of the reactivation potential which should be accounted for while planning deep geothermal systems.
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RC1: 'Comment on egusphere-2023-1889', Anonymous Referee #1, 27 Sep 2023
General comments
This manuscript intends to discuss the fault reactivation potential in deep geothermal systems that would be developed in the greater Ruhr region, Germany. To do so, two main approaches are considered. The first one is based on the well-known static slip and dilation tendencies complemented by the critical pore pressure. The second one is based on a thermo-hydro-mechanical numerical modelling of a synthetic geothermal doublet.
This manuscript presents approaches and results very similar to previous studies without relevant increase of information. The concepts and ideas are already well established. Along the manuscript, necessary details and quality controls are missing, especially regarding the numerical reservoir modelling, which prevents from being confident in the quantified results. Moreover, the deterministic THM modelling remains very generic and therefore delivers common knowledge and general results. The scientific significance is missing over the manuscript. Besides, the manuscript is wordy and extrapolates the results. Consequently, I recommend that this manuscript be rejected for publication in Solid Earth and any other journal.
Specific comments
The two main criticisms are associated with both approaches developed in the manuscript.
Slip tendency, dilation tendency and critical pore pressure
1. The methodology and the results presented have been discussed in a very similar way and for the same area by the same main author in:
- Kruszewski, M., G. Montegrossi, T. Backers, and E. H. Saenger, 2021, In Situ Stress State of the Ruhr Region (Germany) and Its Implications for Permeability Anisotropy: Rock Mechanics and Rock Engineering, 54, 6649–6663.
- Kruszewski, M., G. Klee, T. Niederhuber, and O. Heidbach, 2022a, In situ stress database of the greater Ruhr region (Germany) derived from hydrofracturing tests and borehole logs: Earth System Science Data, 14, 5367–5385.
- Kruszewski, M., G. Montegrossi, M. Balcewicz, G. de Los Angeles Gonzalez de Lucio, O. A. Igbokwe, T. Backers, and E. H. Saenger, 2022b, 3D in situ stress state modelling and fault reactivation risk exemplified in the Ruhr region (Germany): Geomechanics for Energy and the Environment, 32, 100386.
Despite the probabilistic approach was applied in this manuscript, the added-value is questionable.
2. Regarding the probabilistic approach, why the procedure presented by Seithel et al. (2019) (a paper your refer to) is not used although it was developed for the same purpose?
3. The slip tendency results (Fig. 4) highlight fault patches that have Ts higher than 1. How can it be?
4. With the presented results, many faults should be already critically stressed. How do you explain that no natural seismicity is observed in the area? A chapter discussing the natural seismicity of the area is missing.
5. For the range of pressure found in the Ruhr area at that depth (<2 kbar), Byerlee (1978) observed friction coefficients of 0.85. However, the limit assumed in the manuscript is 0.6 (L132, L187, Table 2) and not 0.85, using the same reference, why? L210, however, refereeing to Byerlee (1978) again, the 0.85 friction is written to be a possible value!
6. For Sf, in Eq. 3, Co is accounted for, but it does not appear for the slip tendency although both parameters (Ts and Sf) have the same theoretical background (Mohr-Coulomb failure criterion). Why is it so?
7. In section 5.2, L378-379: “Scalar values used for fault stability evaluation based on the contribution of fluid pressure only, such as Sf, will not provide a full picture of the fault stability in situ”. This is also true for the slip tendency, so mention it as well.
THM modeling
8. The last sentence of the conclusion is not surprising and does not need any result of the numerical simulation that was described in the manuscript. Below is a (non-exhaustive) list of papers that are already discussing the importance of thermally induced stress changes on a long term basis in geothermal contexts:
- De Simone, S., V. Vilarrasa, J. Carrera, A. Alcolea, and P. Meier, 2013, Thermal coupling may control mechanical stability of geothermal reservoirs during cold water injection: Physics and Chemistry of the Earth, Parts A/B/C, 64, 117–126.
- Egert, R., Gaucher, E., Savvatis, A., Goblirsch, P., Kohl, T., 2022. Numerical determination of long-term alterations of THM characteristics of a Malm geothermal reservoir during continuous exploitation. Presented at the European Geothermal Congress 2022, Berlin, Germany.
- Jeanne, P., J. Rutqvist, and P. F. Dobson, 2017, Influence of injection-induced cooling on deviatoric stress and shear reactivation of preexisting fractures in Enhanced Geothermal Systems: Geothermics, 70, 367–375.
- Jeanne, P., J. Rutqvist, P. F. Dobson, J. Garcia, M. Walters, C. Hartline, and A. Borgia, 2015, Geomechanical simulation of the stress tensor rotation caused by injection of cold water in a deep geothermal reservoir: Journal of Geophysical Research: Solid Earth, 120, 8422–8438.
- Kivi, I. R., E. Pujades, J. Rutqvist, and V. Vilarrasa, 2022, Cooling-induced reactivation of distant faults during long-term geothermal energy production in hot sedimentary aquifers: Scientific Reports, 12, 2065.
- Koh, J., H. Roshan, and S. S. Rahman, 2011, A numerical study on the long term thermo-poroelastic effects of cold water injection into naturally fractured geothermal reservoirs: Computers and Geotechnics, 38, 669–682.
- Wassing, B. B. T., T. Candela, S. Osinga, E. Peters, L. Buijze, P. A. Fokker, and J. D. Van Wees, 2021, Time-dependent Seismic Footprint of Thermal Loading for Geothermal Activities in Fractured Carbonate Reservoirs: Frontiers in Earth Science, 9.
Many references regarding THM modelling in similar contexts should be given but they are missing. They could have been used as inspiration source.
9. When presenting THM numerical modelling, it is necessary to develop much more what is actually done to give confidence in the results. So far, it is not the case, and a lot of information is missing, e.g. what are the physical processes activated (equations)? The above-mentioned papers could help to do so.
10. Was a mesh sensitivity study carried out? This is questionable when looking at the discontinuous curves of Fig. 7a and b.
11. In the THM results, it would be most important to see space and time distribution of, at least, the pore-pressure field and the temperature field before jumping directly to the shortest distance between wells and fault.
12. Section 3.2, L165-168: “Effects such as fault permeability enhancement due to the dilation, change of rock properties due to Pp or temperature, T, the influence of fluid chemistry on rock mass and fault properties, mechanisms of earthquake interactions, and the Kaiser effect are not considered in the simulation” This looks like COMSOL could account for all of these. I am not aware that COMSOL can simulate earthquakes.
Additional comments
13. First sentence of abstract: This is wrong as underlined e.g. by the deep geothermal exploitation in the Paris basin for many decades.
14. Nothing in the manuscript supports the simulation of seismicity or aseismic slip or seismic hazard. Consequently, these aspects should be mentioned with care.
Technical corrections
15. Second sentence of abstract: what is the Earth’s “plumbing” system? I have never read such wording in a geothermal context. Do you mean “circulation”?
16. L18: […] a complex “web” of faults […]? I have never read such wording in a geothermal context. Do you mean “network”? It is found L401 as well.
17. Avoid using “the distance to failure” (e.g. L31), you mean in meters (?), prefer the “reactivation potential”.
18. The Appendix does not correspond at all to what is announced in the main part of the manuscript.
There would be a lot more to say, but that was the gist.
Citation: https://doi.org/10.5194/egusphere-2023-1889-RC1 -
RC2: 'Comment on egusphere-2023-1889', Anonymous Referee #2, 18 Oct 2023
General comments
The Kruszewski et al. manuscript discusses the potential to reactivate faults associated with potential geothermal energy system development in the greater Ruhr region, Germany. The authors employ two approaches. The first, using deterministic faults, is probabilistic analysis of static fault slip potential and propensity for opening-mode behavior under assumed in situ stress and pore fluid pressure conditions. The second type of analysis is 3D mechanical simulation with the consideration of fluid pressure and thermoelastic coupling of a pair of wells straddling an idealized fault.
This reviewer must assume that there is high interest in the choice of the region analyzed because the approaches used are certainly not novel and no new geologic, geophysical, or geomechanical data is presented. The concepts and ideas are already well-established. The numerical analysis is highly idealized and generic in its applicability.
The rationale for having the two types of analyses, static and numerical continuum, is not clear to me as insights from the first do not strongly pertain to the second. The analyses do agree with each other and support the findings and the discussion so I propose that the linkage between the two be strengthened.
The manuscript is written fairly clearly but suffers significantly from being highly repetitive (e.g., the text in the appendix is also presented in the main text). It is repetitive in many other aspects.
The illustrations are are of good quality.
I recommend that the manuscript be thoroughly rewritten as it is considered for publication. Unless a figure/table maximum prohibits, the figures in the appendix can be moved to the main text.
Specific comments
The manuscript deals with static analysis of faults and coupled simulation of a faulted continuum with the main application being the likelihood of faults reactivation. There is no treatment of seismicity (e.g., moments, maximum magnitude, slow vs fast rupture, rate and state, etc). Therefore, mention of application to seismicity should be kept to a minimum with the focus kept on reactivation.
As specifically written, equation 3 is not in Streit and Hills (2004) therefore some additional description is required.
Cohesion appears in some of the analysis but not consistently. Either don’t use it and discuss why, or use it throughout.
"distance to failure" is colloquial wording. Replace with the technical equivalent pertaining to stress condition relative to failure condition.
A marked pdf with many suggestions for rewording and clarification has been provided.
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RC3: 'Comment on egusphere-2023-1889', Anonymous Referee #3, 24 Nov 2023
Summary :
The manuscript proposed by Kruszewski et al. details the results of a methodology intended to provide an understanding of the evolution of fault zone reactivation potential associated with deep geothermal energy projects. The document, presented in a clear and detailed manner, have scientific and/or industrial impact for geothermal energy. The detailed methodology begins by processing a set of stress magnitude data from the greater Ruhr region. The results of this first approach allow targeting fault orientations with higher or lower reactivation potentials. These results are complemented by 3D numerical modeling, with THM coupling, to quantify the spatiotemporal variations of stress change generated by geothermal production. However, before publication in EGUsphere, a few minor modifications and clarifications should be made in the manuscript. I would like to emphasize that the boundary conditions and physical properties of the numerical models need to be more thoroughly explained, and the consequences for the results presented more thoroughly discussed. Overall, considering the quality of the results, the quality of the writing, and the clarity of the figures, I propose minor corrections to the manuscript. These comments are placed point by point below.
Minor Comment
Abstract :
-Ln 1 : The initial sentence should be tempered. The success of deep geothermal systems significantly relies on the integration of various geological and physical processes, which could be defined in numerical modelling by a complete THMC coupling (Chester and Logan 1986; Byerlee, 1994; Barton et al., 1995; Scholz, 2002; Violay et al., 2017, ...). Furthermore, many other parameters such as cost drilling and feed in tariffs supporting the development of geothermal energy must be taken into account to evaluate the feasibility and viability of the project.
-Ln 2 : The terms "fault" and "fault zone" are used successively between the first and second sentences. It might be clearer for the reader to use a single term, which could be defined in the introduction.
Introduction :
-Ln 18 : I would suggest adding references here. It might also be interesting for the reader to see, at this point, the definition of "fault" that as used throughout the remainder of the study.
-Ln 21 : Might use a less global term than “Anthropogenic”, perhaps geothermal activities? This would require a revision of the sentence structure.
-Ln 24 : I suggest revising the sentence structure, perhaps changing “On the other hand” to “Moreover”?
-Ln 26 : May I suggest some recent studies on this topic : Guillou-Frottier et al., 2013, Moeck, 2014, Duwiquet et al., 2019.
-Ln 55 & 60 : Though comprehensive and clear thus far, your introduction might benefit from incorporating a review of the current state of the art concerning the application of these two geomechanical criteria (Ts, Td) in analogous contexts. Consider referencing studies like Moeck et al. (2009), among potentially others. The same consideration applies to THM numerical modeling. Providing a logical justification for their utilization, accompanied by a review of prior studies on this subject, would enhance the overall contextual understanding. I have in mind Armandine Les Landes et al., 2019, and/or Duwiquet et al., 2021, but undoubtedly, there are other relevant studies as well.
Methodology :
Subsection 3.2
Here, you are directed towards the use of commercial software. At this stage, it might be prudent to conduct calibration tests for the employed THM coupling in comparison to the open-source codes that have been used and published.
It would be advisable to directly specify the boundary conditions of the employed numerical models on Figure 3. The clarity of the figure would be enhanced by clearly displaying the dimensions of the considered system. In the figure description, you provide information about the number of cells used, but the cell sizes vary between the fault and other lithologies. What are the minimum and maximum sizes, and how were these sizes chosen? Have convergence tests been conducted to ensure that the numerical results are no longer dependent on the cell size? Additionally, how are the meshes considered in the modeled wells? Is it a radial mesh? What impact does this have on the final result ?
Ln 181 : It may be necessary to reconsider the dimensions of the fault (a width of 9 km?) and provide information on the fault thickness.
Table 2 : The permeability of the fault (10-11 m²) is not referenced. Such a high permeability value could lead to fluid flow velocities that exceed the limits of the applicability of Darcy's law. It is essential to verify this by examining whether fluid flow velocities are consistent. For example, in comparison, other numerical models (but same software) impose fault permeability values close to 10-14 m² and find corresponding field data for this value (Roche et al., 2018; Taillefer et al., 2017). It seems important here to provide further explanation of your approach. Additionally, the imposed permeability value for the reservoir also lacks a reference."
Ln 200 : You fixed a temperature at the bottom of the model. In order to limit boundary conditions at the model's base, wouldn't it be preferable to use a heat flux instead? These aspects could be either modified or discussed in the relevant section.
Citation: https://doi.org/10.5194/egusphere-2023-1889-RC3 -
EC1: 'Comment on egusphere-2023-1889', Federico Rossetti, 27 Nov 2023
Dear Authors,
I apologize for the time it took to review your manuscript, but finding appropriate reviewers was not an easy task (more than twenty denies in this case).
During the public discussion stage, your manuscript received three independent reviews. Rev#1 criticizes the scientific rationale of the study and the novelty of the presented results. Comments from Rev#2 in part overlap with those of Rev#1, still commenting on the scientific approach and application to the case study. Rev#3 asks for further info on the numerical modeling technique and to implement discussion on the modelling results.
Despite relevance of the presented topics, which deal with quantifying the reactivation potential of faults in structurally-controlled geothermal reservoirs, the manuscript requires a major revision work to make the scientific message original and convincing, and to present a robust reconstruction. If a revised version is submitted, it will undergo a new revision round and re-evaluation.
Sincerely,
Federico Rossetti
Citation: https://doi.org/10.5194/egusphere-2023-1889-EC1
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