Use of Analogue Exposures of Fractured Rock for Enhanced Geothermal Systems
Round 1
Reviewer 1 Report
REVIEW of the manuscript entitled: “Use of Analogue Exposures of Fractured Rock for Enhanced Geothermal Systems” by D.C. Peacock et al. submitted to the Special Issue of Geosciences “Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project”
(Manuscript ID: geosciences-1820048)
Dear Geosciences Assistant Editor, Dr. Anna Mozer
This paper provides a new and original knowledge and experience sharing about the use of outcrop analogues in support to enhanced geothermal systems (EGS) applications. In this interesting paper the Authors elegantly share with the community their experience and learnings gained during the study of analogue exposures of fractured and deformed Devonian and Carboniferous meta-sedimentary rocks in the Harz Mountains used to constrain flow models and make predictions about enhanced geothermal systems and thermal stimulation in a proposed enhanced geothermal reservoir at Göttingen.
Important questions are raised on the selection and use of outcrop analogues for subsurface characterization and modelling including definition of the key problems being addressed and the aims of the work. The proper use of the fracture terminology is also discussed with the specific case study presented by the Authors.
The topic is of great interest to both geoscientists facing the challenges in the use of outcrop analogues for subsurface reservoir characterization and modelling as well as for specialists interested in the local geology and specific geothermal application. The paper is very well suited for this special issue.
The reference list is wide, and high credit is given to previous work, both on the local geology and on the methodological background.
Because of all the aforementioned reasons, I found this manuscript thought-provoking and very much interesting for a broad, international audience. It is my opinion it represents an innovative and important rationale in the use of outcrop analogues. Therefore, I enthusiastically recommend its publication on Geosciences without absolute reservation, pretty much as it stands.
I would only suggest to add some details about the lithological units in the map represented in Figure 1
I require no anonymity, wish to be identified by the Authors and remain available for eventual questions and/or clarification. Thank you very much for having provided me with this opportunity to act as referee for Geosciences. I remain available and enthusiastic to act as a reviewer again for the Journal, upon request.
Yours sincerely,
Reviewer 2
Author Response
Please see attached file
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Dear Authors,
I revised the manuscript "Use of analogue exposures of fractured rock for enhanced geothermal systems" by Peacock et al., submitted to Geosciences.
Authors describe the methodological approaches used in a specific case study and applicable to study analogue EGS systems, suggesting a standard procedure as a guide which should be useful to approaching EGS investigation. English is well written. This paper could be interesting for a large audience and a detailed methodological approach could be useful for scientist which study EGS.
Despite that, overall, the manuscript seem to be approximative, methodologies are not sufficiently described and the general approach is not scientifically convincing. Some method's steps seem obvious. Neither parts is really deepen.
Author Response
Please see attached file
Author Response File: Author Response.pdf
Reviewer 3 Report
This paper is interesting in that it not only shows the favorable aspects but also the problems encountered while working on exposed analogues, which, sometimes, are the only way to have information on a potential EGS geothermal reservoir.
According to the title of the paper “ Use of Analogue Exposures of Fractured Rock for Enhanced Geothermal Systems”, it is only about EGS. Hence, either it should be enlarged to geothermal reservoirs in general if the authors want to include sedimentary systems, or they would rather avoid all that concerns sedimentary systems in the paper. In table 2, the authors speak about sedimentary rocks and diagenesis. The authors have to be consistent all along their paper. Is it only dedicated to EGS and hence to basement or is it wider and then can include reservoirs in sedimentary units? In that second case, the title of this paper has to be enlarged.
In the introduction, in addition to Morley (1995), reference has to be made to others papers in this special issue that indeed also are about the use of analogue to better know sub-surface geothermal reservoirs in basements. These papers are : Klee et al. (2 papers, 2021 a and b) : https://doi.org/10.3390/geosciences11080325,https://doi.org/10.3390/geosciences11110433; Chabani et al. (2 papers, 2021 a and b),https://doi.org/10.3390/ geosciences11070280., https://doi.org/10.3390/geosciences11120520; Hermann et al., 2022, https://doi.org/10.3390/geosciences12050195; Cheng et al., 2021, https://doi.org/10.3390/geosciences11110443; Cabidoche et al., 2021: https://doi.org/10.3390/geosciences11110437. Some of these papers are about fractures, others are broader.
L 56-57: “This was aimed at developing cost-effective techniques for Enhanced Geothermal Systems (EGS) in a variety of geological settings across Europe”. In this sentence, the authors should probably indicate “This project” instead of only “This” in order that there is no confusion with “this paper”.
L 64 : Is the phrasing “to use of… “ correct in the sentence “… we attempted to use of analogue exposures …”?
L 124 to 127 : The authors indicate: “Exhumation means that many of the post-Variscan structures observed in the Harz Mountains, especially the joints, may not occur at reservoir depths at Göttingen. We suggest, however, that the joints observed in the Harz Mountains give us strong indications of the patterns of induced fractures that would be created by stimulation of Variscan rocks beneath Göttingen.” The joints created thanks to exhumation might not at all have the same mechanical origin as those occurring diring stimulation. How can they be compared? The authors have to explain this point.
L 275: 3.4. Avoid distracting topic. Is this paragraph really relevant ? What does it bring to the “analogue” topic? The second part, from L 284 to L 290 is however useful and should be kept, but the title of the subsection should be changed if the 1st part about “Avoid distracting topic” is deleted.
Table 2 caption : “…how exposed analogues can be used for study these elements.” Is it correctly phrased or would it rather be “for studying these elements” or “to study these elements”?
Table 2 might be amended. For example, is an exposed analogue able to provide data about a modern geothermal site (1st line of Table 2: What type of system is being studied: palaeo hydrothermal and modern geothermal)? I don’t think so, except if hot fluids come to the surface. In that case, what is the relationship between this hot surface fluid and the deep geothermal reservoir (see Cabidoche, 2021). However, an exposed analogue can indeed provide information about a palaeo-hydrothermal system. Hence, is the box “Modern geothermal” really useful here? If the authors think it is necessary to keep it, then in addition to “porosity” as indicated here, permeability is a key parameter to indicate for modern geothermal sites which can be provided by flow logs in boreholes. In the box “what aspect of the system is of interest?”, Fluid phase should be replaced by Liquid as fluids comprise gas (Gazeous phase is mentioned as well as Fluid phase). In the box “what aspects of the rocks are of interest?”, diagenesis only applies to sedimentary rocks, while EGS are mostly developed in basement rocks. This diagenesis term is hence not the best one maybe. Prefer another one such as “alteration” which could include diagenesis (e.g: alteration: hydrothermal, diagenesis)? In the box “What information is needed?”, rather than “exposed analogues”, prefer “geological data”, to be consistent with geophysical and geochemical data topics. “exposed analogues” can be indicated into brackets.
L 436 : CO2 sequestration (e.g., Ogata et …) : please replace by CO2
L 464 : “… they must be properly provide useful insights into the geothermal reservoir. “. The phrasing seems wrong : “be properly provide”
L 472 to 474: Conclusions, section 4: “Science is generally about solving problems, and exposed analogues for EGS must answer particular questions about the behaviour of reservoir rocks. It is therefore important to avoid letting interesting new topics cause too much distraction from the main aims of the fieldwork. “. Like in the discussion part, I wonder if this mention is really important since those interesting topics might be studied sideway to that of the analogue.
L 476 to 478: Conclusions, section 5, the authors state: “It is unhelpful to use the term “fracture” as a field description, and the types of “fractures” must be defined during fieldwork. While veins give useful information about palaeo fluids, they are less likely to contribute to fluid flow in the sub-surface than are such open fractures as joints.” In fact, depending on the type of minerals they contain, veins can be useful for present-day fluid flow after dissolution through chemical stimulation, example given: calcite or clay minerals. Hence, the thorough study of veins can be as important as that of other fracture sets (maybe except joints as already discussed by the authors).
L 484: “…it is important to understanding what those people need and to gain a common understanding …”. Should probably be rephrased (twice understanding).
L 504 : “… an appropriate exposed analogue should be studied in appropriate way …” ought to be rephrased to avoid redundance.
Literature references have to be numbered following the rules of the journal Geosciences, unless it might be made by the people in charge of final layout just before paper release.
Author Response
Please see attached file
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Best Regards
Author Response
I have added the following (although it breaks up the flow of the paragraph): “Note that certain minerals can be utilised for chemical stimulation (e.g., Na et al., 2016)”.
I still don't know the name of the reviewer who wanted to be named
Author Response File: 
Author Response.docx
