Comparative Laboratory Study of the Geochemical Reactivity of the Marcellus Shale: Rock–Fluid Interaction of Drilled Core Samples vs. Outcrop Specimens

Round 1

Reviewer 1 Report

Figure numbers referred in the in the text do not match those in the figure captions from Figure 10 onward.; therefore I cannot proceed with the review until this is fixed. 

Author Response

Figure numbers were corrected and the manuscript was updated.

 

Reviewer 2 Report

This study focuses on characterizing the mineral content of flowback waters and evaluating potential pollution impacts on drinking water, surface water, and soil quality. The experimental scheme design is systematic and complete, and has certain innovation.

(1)   Whether the quality of the sample is 8g is justified and can meet the test requirements (line 113).

(2)   Is there any difference between A and H in Figure 5? It is suggested to indicate the difference between pictures in the picture.

(3)   What is the basis for selecting outcrop samples in the experiment?

(4)   This paper is directly related to your research. Please read it, hoping to inspire you.( Bai J, Kang Y, Chen M, et al. Dual effects of retained fracturing fluid on methane diffusion in shale containing adsorbed methane[J]. Gas Science and Engineering. 2023, 110: 204872.)

Author Response

Response 1: An 8g sample is used because it provides sufficient material for imaging and analysis while minimizing the potential for sample loss or damage during preparation. According to BTech protocols, the minimum weight required is 10mg. https://www.iitg.ac.in/biotech/BTechProtocols/Demo_SEM.pdf  

Response 2: Yes, there is. Figure 5 A is the outcrop and 5 H is the core sample 7.

Response 3: According to the availability and the characteristics of the area of interest (lithology, mineralogy and texture). Equally, obtaining a representative sample.

Response 4: Thank you so much for the paper, it was valuable.

 

Reviewer 3 Report

The research paper (Comparative laboratory study of geochemical reactivity of the Marcellus shale: Rock-fluid interaction of drilled core samples vs outcrop specimens) is well written. The objective of this study is to identify the chemical content of flowback waters generated when hydraulic fracturing fluid additives and DI-water interact with the Marcellus shale using SEM-EDS and ICP-OES. However, there are still some issues that need to be explained and elaborated in the paper.

1. Explain the reason for Fracture frequency increases for deeper samples.

2. Were samples subjected to argon ion polishing before SEM?

3. The image in Figure 6d is too blurred, so it is recommended to replace it with a high-resolution image.

4. Why not choose seven samples for the corresponding experiment at the same time, but choose two depths to do it. How representative is this sample selection? The samples could be changed a lot while varies depths. For example, the study doi: 10.1016/j.coal.2022.104079

5. It seems more practical to replace point scanning with surface scanning (Figure 7). For example: 10.1306/08031817416.

6. The composition of Marcellus shales is mainly formed by the neoformation and transformation of clays, including illite, chlorite, kaolinite, and mixed-layer clays. Provide test data to support this conclusion. Doi: 10.2138/am-2016-5263

7. The high concentration of dissolved Si in shale samples might be attributed to the presence of more soluble silica polymorphs (possibly biogenic). Is there any biogenic silicon found in SEM? For example, Doi: 10.1016/j.marpetgeo.2020.104567

8. When conducting mechanism analysis in the discussion section, some substantive evidence should be supplemented, such as SEM images of shale before and after fracturing fluid action.

9. It is recommended to rewrite the conclusions and express the main points without describing them.

Author Response

Response 1: Due to increased pressure and temperature, the frequency of fractures in rock samples tends to rise with depth. This causes the rocks to distort and disintegrate brittlely, resulting in fracture formation. Furthermore, the presence of fluids within the rock can increase pressure and facilitate fracturing, increasing the fracture frequency with depth.

Response 2: No, Argon polishing was not used. Silicon Carbide sandpaper and polycrystalline diamond in suspension were used for polishing.

Response 3: The figure is focusing on a pyrite framboid.

Response 4: The coring didn’t follow this procedure. The sample selections are representative in the sense that the pay zone covers all the depths of the core samples. We are, therefore, analysing the variations at the different depths of the same pay zone.

Response 5: Thank you for the suggestion, surface scanning is not available.

Response 6: This statement was the introductory phrase for the discussion on the chemical composition of Marcellus shale. It is supported by previous research as indicated in the references.

Response 7: Yes, there is biogenic silicon found.

Response 8: We do not have images from after the fracturing fluid was applied, because the samples were crushed for the experiment.

Response 9: Changes have been done accordingly.

 

Round 2

Reviewer 1 Report

The paper presents analytical results on an important topic, and can be of interest to both geological and environmental scientists. I recommend the authors include data on elements strontium and lithium in the revision as these two elements have been reported at elevated concentrations in many other similar geological samples. Please refer to the annotated MS for comments and suggested changes.

Comments for author File: Comments.pdf

Author Response

Please, find the answers in the file uploaded below.

 

 

Author Response File: Author Response.docx

Round 3

Reviewer 1 Report

Thanks for the comments that lithium and strontium were not detected in the effluents. Please make a statement in the paper to make it clear to the readers.

Author Response

Thank you for your response and comments. The statement has been added as a conclusive statement of the results.