An Experimental Analysis of Microcrack Generation during Hydraulic Fracturing of Shale

Round 1

Reviewer 1 Report

The manuscript entitled "An experimental analysis of microcrack generation during hydraulic fracturing of shale" has sufficient innovation. The manuscript is not well explained and needs major revision. Following are some specific comments:

1. The abstract lacks quantitative information and seems very general.
2. The last paragraph of the introduction needs revision. It should clearly show the novelty and objectives of your study.
3. The authors did not mention the Methodology section.
4. The authors missed the result and discussion section.
5. Authors should highlight these challenges.
6. Why other material is not taken into account for comparative study?
7. Is authors study specific to shale only? If yes, why authors didn’t mention the characteristics of the material, other properties, etc.
8. The authors did not mention about the experimental set-up. How authors performed the quantitative analysis? explain it in the methodology section?
9. There should be space between units and numbers….line 100…. “25MPa” should be “25 MPa”.
10. Line 121, 122… “5mm*5mm” should be “5mm x 5mm”

 

Comments for author File: Comments.docx

Author Response

We would like to thank the editor and the reviewer for carefully reviewing our manuscript. The manuscript has been revised according to the comments and suggestions. Minor changes made in this version are listed in the following. In addition to correct the syntax and grammatical errors indicated by the reviewer, we also did a very careful proof reading of the manuscript. Moreover, to display the revisions conveniently, the updated revised section in the main text is highlighted in yellow color.

The manuscript entitled "An experimental analysis of microcrack generation during hydraulic fracturing of shale" has sufficient innovation. Following are some specific comments:

1. Comments:

The abstract lacks quantitative information and seems very general.

Reply:

It is very appreciated for the comment. The authors reorganize the structure of the abstract part and add quantitative information to perfect the statement of the paper. The new revised version of the abstract can be updated in the following:

“Hydraulic fracturing is commonly applied into shale gas exploitation industry. However mechanical mechanism of permeability under the fracturing has been unclear so far. In this study, an analysis of laboratory experiments on hydraulic fracture propagation and bedding plane reactivation in shale is presented. To investigate the microcrack under the fracturing, several small slices were collected from the fractures surface and scanned with the scanning electron microscope (SEM). It is demonstrated that the microscopic observed microcracks could not be produced by fluid pressure as the latter generated compressive stresses on the places of microcracks. On the contrary, the microcracks are produced by tensile stress concentration in front of the propagating fracture. This implies that the bedding plane reactivation is caused by fracture propagation along the plane. An analysis of microcrack lengths shows that shale exhibits anisotropy in fracture toughness with the resistance to fracture propagation parallel to bedding planes being as twice a small as compared with the resistance to fracture propagation in the directions normal to bedding planes.”

2. Comments:

Last paragraph of introduction needs revision. It should clearly show the novelty and objectives of your study.

Reply:

It is very thankful for the suggestions. The authors double check the last paragraph of introduction, and find out the problems of poor novelty. Following the suggestions, the part of the referred introduction has been modified as follows:

“In the present paper, the microcracking observed in laboratory experiments through scanning electron microscope (SEM) on hydraulic fracturing and bedding plane reactivation in shale are analysed in Section 2. Section 3 then presents the possible mechanisms of microcrack generation under the fracturing experiment. Following the above mentioned analyses, the notion of anisotropy of fracture toughness is derived based on the experiment of hydraulic fracturing in Section 4.”

3. Comments:

Authors did not mention the Methodology section.

Reply:

It is very appreciated for the kind reminder. The authors all agree with the reviewer. As suggested, the methodology of the experiment and analysis has focused on the section 2 and section 3. The method for the observation has been added into the abstract as “To investigate the microcrack under the fracturing, several small slices were collected from the fractures surface and scanned with the scanning electron microscope (SEM)”, and the analysical method is also presented in section 3.1.

4. Comments:

Authors missed the result and discussion section.

Reply:

It is very appreciated for the kind reminder. As suggested, the experiment results and discussion parts have been put into the last section of conclusions. The more details of the statements can be referred in the following:

“The laboratory experiment in this study demonstrate that the hydraulic fractures, showing two different types including newly produced fracture and reactivated fracture, are tensile fractures driven by the fluid pressure. The newly produced fracture grows along the maximum principal stress, while the reactivated fracture grows along the pre-existing bedding plane. These fractures are accompanied by the formation of microcracks with the orientation normal to the fracture faces. Further analysis in this study shows that the obvious and popular notion that the microcracks are generated by the pressure of the fracturing fluid after the fracture is produced is incorrect. The reason for that is the simple theoretical fact that the fluid pressure creates compressive stresses acting parallel to the fracture faces thus preventing the microcracks from being formed. Instead, the only place where tensile stresses are generated is the rock in front of the propagating fracture (outside of it). Therefore the microcracks are generated in the process of fracture propagation. Since the microcracks are also formed at the surfaces of the reactivated fracture the only conclusion is that the process of reactivation corresponds to a tensile fracture propagation as well. Of course the resistance to the fracture propagation of the weak plane is smaller than that of the rock where the main hydraulic fracture propagates, which translates into the difference between the values of fracture toughness. The smaller fracture toughness of the weak (bedding) plane means that the distances over which the tensile stress concentration is sufficient to generate microcracks are smaller than in the other parts of the rock. This leads to the traces of microcracks in the reactivated fractures being smaller than in a newly developed hydraulic fracture. This is precisely what is observed. The approximate four time difference in the mean microcrack lengths translates into two times difference in the values of fracture toughness. Given that fracture propagating parallel to the bedding planes will deviate to propagate along the weak planes the observed difference in the values of fracture toughness signals a twofold fracture toughness anisotropy. The results obtained will assist in the modelling of hydraulic fracturing and forecast of production capacity. The subsequent monitoring can be arranged necessarily for efficient hydrocarbon production and reduction of environmental impact of fracking operations.”

5. Comments:

Authors should highlight these challenges.

Reply:

It is very appreciated for the kind reminder. The authors all agree with the reviewer. Following the comment, to display the revisions conveniently, the updated revised section in the main text is highlighted in yellow color

6. Comments:

Why other material is not taken into account for comparative study?

Reply:

The authors thank for the kind suggestions. Since the present study focuses on hydraulic fracturing technology of the shale gas exploitation industry, the shale is taken into consideration for the experiment and mechanism analysis correspondingly.

7. Comments:

Is authors study specific to shale only? If yes, why authors didn’t mention characteristics of material, other properties etc.

Reply:

It is appreciated for the kind reminder. The authors study specific to the shale only as presented in the abstract part. Since the study focused on the observation of the microcracks of the hydraulic fracturing, the more observed experiment data has been taken into statistics analysis, and the characteristics of shale and other properties can be ignored to demonstrate the macro-mechanical behavior of the shale under fluid pressures.

8. Comments:

Authors did not mention about the experimental set-up. How authors performed the quantitative analysis? explain it in the methodology section?

Reply:

It is appreciated for the kind reminder. As the suggested, the authors double check the experiment part of the present paper and reorganize the structure of section 2.1. The new modified statements of the paper can be found in the following：

“In order to investigate the microcracks on the fracture face, small slices were collected from the fracture faces. Six slices with an area 5mm×5mm were obtained from the newly produced fracture and five slices with an area 3mm×3mm were obtained from the reactivated fracture along the bedding plane, as shown in Figure 3. All of the shale slices were polished by argon-ion to create a smooth surface for the improvement of SEM images.”

9. Comments:

There should be space between units and numbers….line 100…. “25MPa” should be “25 MPa”.

Reply:

It is appreciated for your suggestions. The authors agree with the reviewer and revise the previous statements of the referred point.

10. Comments:

Line 121, 122… “5mm*5mm” should be “5mm x 5mm”

Reply:

It is appreciated for your suggestions. The authors agree with the reviewer and correct the “5mm*5mm”to “5mm × 5mm”.

 

The last but not the least, the authors would like to thank the reviewer again for carefully reviewing our manuscript. The manuscript has been revised according to the comments and suggestions. In addition to correct the typo and grammatical errors indicated by the reviewer, we also did a very careful proof reading of the manuscript.

Author Response File: Author Response.docx

Reviewer 2 Report

Dear authors, 

The manuscript is very good, the experimental work and the data analysis are presented accordingly. There are some suggestions that need to be addressed. 

1. In the abstract session, please mention the main objective of this study.
2. In the introduction session, please provide the main contribution as well as the application of this study which makes it different from the previous study. 
3. The possible explanation of microcrack formation is best to be placed along with the experimental results.

Author Response

We would like to thank the editor and the reviewer for carefully reviewing our manuscript. The manuscript has been revised according to the comments and suggestions. Minor changes made in this version are listed in the following. In addition to correct the syntax and grammatical errors indicated by the reviewer, we also did a very careful proof reading of the manuscript. Moreover, to display the revisions conveniently, the updated revised section in the main text is highlighted in yellow color.

 

The manuscript is very good, the experimental work and the data analysis are presented accordingly. There are some suggestions that need to be addressed.

1. Comments:

In the abstract session, please mention the main objective of this study.

Reply:

It is appreciated for your suggestions. The authors agree with the reviewer and reorganize the structure of the abstract. More details of the modifications can be found in the following:

“Hydraulic fracturing is commonly applied into shale gas exploitation industry. However mechanical mechanism of permeability under the fracturing has been unclear so far. In this study, an analysis of laboratory experiments on hydraulic fracture propagation and bedding plane reactivation in shale is presented. To investigate the microcrack under the fracturing, several small slices were collected from the fractures surface and scanned with the scanning electron microscope (SEM). It is demonstrated that the microscopic observed microcracks could not be produced by fluid pressure as the latter generated compressive stresses on the places of microcracks. On the contrary, the microcracks are produced by tensile stress concentration in front of the propagating fracture. This implies that the bedding plane reactivation is caused by fracture propagation along the plane. An analysis of microcrack lengths shows that shale exhibits anisotropy in fracture toughness with the resistance to fracture propagation parallel to bedding planes being as twice a small as compared with the resistance to fracture propagation in the directions normal to bedding planes.”

2. Comments:

In the introduction session, please provide the main contribution as well as the application of this study which makes it different from the previous study.

Reply:

It is appreciated for the kind reminder. The authors would like to thank the reviewer again. The updated version of the manuscript has considered the reviewer suggestions and reorganized the introduction as following:

“The fracture network generated by hydraulic fracturing is aimed to form a dense hydraulically conductive pathway connecting as many isolated hydrocarbon-rich pores as possible to increase the permeability. Hydraulic fracture tends to be a dominating fracture in the network with random multiple branches (secondary fractures).”

Furthermore, the present study focuses on the micro-scale research to solve the mechanism of permeability increase, especially the cracks at micro-scale, which is the different from the previous study.

3. Comments:

The possible explanation of microcrack formation is best to be placed along with the experimental results.

Reply:

It is appreciated for the kind suggestions. The authors agree with the reviewer. As suggested, to explain the microcrack and experiment results clearly, the conclusions part of the paper has been reorganized and revised in the new updated version. More details can be found in the following:

The laboratory experiment in this study demonstrate that the hydraulic fractures, showing two different types including newly produced fracture and reactivated fracture, are tensile fractures driven by the fluid pressure. The newly produced fracture grows along the maximum principal stress, while the reactivated fracture grows along the pre-existing bedding plane. These fractures are accompanied by the formation of microcracks with the orientation normal to the fracture faces.

Further analysis in this study shows that the obvious and popular notion that the microcracks are generated by the pressure of the fracturing fluid after the fracture is produced is incorrect. The reason for that is the simple theoretical fact that the fluid pressure creates compressive stresses acting parallel to the fracture faces thus preventing the microcracks from being formed. Instead, the only place where tensile stresses are generated is the rock in front of the propagating fracture (outside of it). Therefore the microcracks are generated in the process of fracture propagation. Since the microcracks are also formed at the surfaces of the reactivated fracture the only conclusion is that the process of reactivation corresponds to a tensile fracture propagation as well. Of course the resistance to the fracture propagation of the weak plane is smaller than that of the rock where the main hydraulic fracture propagates, which translates into the difference between the values of fracture toughness. The smaller fracture toughness of the weak (bedding) plane means that the distances over which the tensile stress concentration is sufficient to generate microcracks are smaller than in the other parts of the rock. This leads to the traces of microcracks in the reactivated fractures being smaller than in a newly developed hydraulic fracture. This is precisely what is observed. The approximate four time difference in the mean microcrack lengths translates into two times difference in the values of fracture toughness. Given that fracture propagating parallel to the bedding planes will deviate to propagate along the weak planes the observed difference in the values of fracture toughness signals a twofold fracture toughness anisotropy.

The results obtained will assist in the modelling of hydraulic fracturing and forecast of production capacity. The subsequent monitoring can be arranged necessarily for efficient hydrocarbon production and reduction of environmental impact of fracking operations.

 

The last but not the least, the authors would like to thank the reviewer again for carefully reviewing our manuscript. The manuscript has been revised according to the comments and suggestions. In addition to correct the typo and grammatical errors indicated by the reviewer, we also did a very careful proof reading of the manuscript.

Author Response File: Author Response.docx

Reviewer 3 Report

The conclusion on nature of the micro cracks formation, lines 349 to 357 is rather questionable. Especially since the last sentence (lines 355-357) seems to contradict the first portion of this conclusion: "Further analysis in this study shows that the obvious and popular notion that the since the micro cracks are also formed at the surfaces of the reactivated fracture the only conclusion is that the process of reactivation corresponds to a tensile fracture propagation as well". This is just a speculation. Please elaborate.

 

Some "technical" remarks:

- Page 3 – lines 111-117 – Figure 1 – the letters are too small and practically illegible.

- Page 5 – lines 146 to 148 – Figure 4 – white lettering is almost invisible – try to use another more prominent color.

- Page 5 – line 156 – Should it be "presence of minerals" instead of "presentation"?

- Page 11 – line 228 – Reference "Kachanov et al. 2003" is not listed in the "Reference" section. Please, provide the adequate reference to this article.

The scanned pages of the manuscript with marked errors and suggested correction are enclosed.

Comments for author File: Comments.PDF

Author Response

We would like to thank the editor and the reviewer for carefully reviewing our manuscript. The manuscript has been revised according to the comments and suggestions. Minor changes made in this version are listed in the following. In addition to correct the syntax and grammatical errors indicated by the reviewer, we also did a very careful proof reading of the manuscript. Moreover, to display the revisions conveniently, the updated revised section in the main text is highlighted in yellow color.

 

• Comments:

The conclusion on nature of the microcracks formation, lines 349 to 357 is rather questionable. Especially since the last sentence (lines 355-357) seems to contradict the first portion of this conclusion: "Further analysis in this study shows that the obvious and popular notion that the since the micro cracks are also formed at the surfaces of the reactivated fracture the only conclusion is that the process of reactivation corresponds to a tensile fracture propagation as well". This is just a speculation. Please elaborate.

Reply

It is appreciated for the kind suggestions. The authors agree with the reviewer. As suggested, the conclusion part of the submitted version has been revised and modified to clarify the study. Furthermore, the updated version of the manuscript is attached in the following:

The laboratory experiment in the present study demonstrates that the hydraulic fractures of the shale, including two different types of newly produced fracture (PF) and reactivated fracture (RF), are tensile fractures driven by the fluid pressure, respectively. The newly PF grows along the maximum principal stress, while the RF propagates along the pre-existing bedding plane. These fractures are accompanied by the microcracks generation with the orientation normal to the fracture surface. The further analysis following the experiment shows that the microcracks are generated in the process of fracture propagation, not after the hydraulic fracturing. For the fluid pressure creates compressive stresses acting parallel to the fracture faces, the microcracks have been prevented, instead of that, the only place where tensile stresses are generated in front of the propagating fracture creates the microcracks. Since the microcracks are also formed at the surfaces of the RF, it is concluded that the process of reactivation corresponds to a tensile fracture propagation as well.

Of course, the resistance to the fracture propagation of the bedding plane is smaller than that of the rock where the main hydraulic fracture propagates, which translates into the difference between the values of fracture toughness. The smaller fracture toughness of the bedding plane means that the distances over which the tensile stress concentration generating microcracks are smaller than in the other parts of the shale. This leads to the traces of microcracks in the RFs being smaller than in a newly developed PF, which are the observed experiment results. It is also found the the approximate four times difference in the mean microcrack lengths translates into two times difference in the values of fracture toughness. Given that fracture propagating parallel to the bedding planes will deviate to propagate along the weak planes, the observed difference in the values of fracture toughness signals a twofold fracture toughness anisotropy.

It can be forecasted that the proposed results obtained will assist in the modelling of hydraulic fracturing and forecast of production capacity. The subsequent monitoring can be arranged necessarily for efficient hydrocarbon production and reduction of environmental impact of fracking operations.

• Comments:

Some "technical" remarks:

- Page 3 – lines 111-117 – Figure 1 – the letters are too small and practically illegible.

- Page 5 – lines 146 to 148 – Figure 4 – white lettering is almost invisible – try to use another more prominent color.

- Page 5 – line 156 – Should it be "presence of minerals" instead of "presentation"?

- Page 11 – line 228 – Reference "Kachanov et al. 2003" is not listed in the "Reference" section. Please, provide the adequate reference to this article.

The scanned pages of the manuscript with marked errors and suggested correction are enclosed.

Reply

It is appreciated for the kind suggestions. The authors agree with the reviewer and thanks for the reminder again. Following the suggestions, the authors reorganize the manuscript and correct the referred mistakes of some "technical" remarks. The corrections and modifications can be found in the updated version of the manuscript. More details of the revised works are presented and listed as follows:

• Page 3 – lines 111-117 – Figure 1 – the letters have been enlarged;
• Page 5 – lines 146 to 148 – Figure 4 – white lettering is changed into green color;
• Page 5 – line 156 – Should it be "presence of minerals" instead of "presentation";
• Page 11 – line 228 – Reference "Kachanov et al. 2003" is listed in theupdated "Reference" section.

Kachanov M, Shafiro B, Tsurkov I (2003) Handbook of Elasticity Solutions. Kluwer Academic Publishers, Dordrecht/Boston/London, pp. 324.

 

The last but not the least, the authors would like to thank the reviewer again for carefully reviewing our manuscript. The manuscript has been revised according to the comments and suggestions. In addition to correct the typo and grammatical errors indicated by the reviewer, we also did a very careful proof reading of the manuscript.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

I appreciate the author's efforts in the revised version of the manuscript. However, there is not enough discussion of the experimental results, and they are not well organized. Authors must try to add separate results section and present them in a logical way.

In the “Conclusion” section some text must be added to discuss the future work or research opportunities.

Minor revision is recommended.

Author Response

  Reply to Reviewer 

We would like to thank the editor and the reviewer for carefully reviewing our manuscript. The manuscript has been revised according to the comments and suggestions. Minor changes made in this version are listed in the following. In addition to correct the syntax and grammatical errors indicated by the reviewer, we also did a very careful proof reading of the manuscript. Moreover, to display the revisions conveniently, the updated revised section in the main text is highlighted in yellow color.

 

Reply

It is appreciated for the kind suggestions. The authors agree with the reviewer. As suggested, the conclusion part of the submitted version has been revised and modified to clarify the study. Furthermore, the updated version of the manuscript is attached in the following:

The laboratory experiment in the present study demonstrates that the hydraulic fractures of the shale, including two different types of newly produced fracture (PF) and reactivated fracture (RF), are tensile fractures driven by the fluid pressure, respectively. The newly PF grows along the maximum principal stress, while the RF propagates along the pre-existing bedding plane. These fractures are accompanied by the microcracks generation with the orientation normal to the fracture surface. The further analysis following the experiment shows that the microcracks are generated in the process of fracture propagation, not after the hydraulic fracturing. For the fluid pressure creates compressive stresses acting parallel to the fracture faces, the microcracks have been prevented, instead of that, the only place where tensile stresses are generated in front of the propagating fracture creates the microcracks. Since the microcracks are also formed at the surfaces of the RF, it is concluded that the process of reactivation corresponds to a tensile fracture propagation as well.

Of course, the resistance to the fracture propagation of the bedding plane is smaller than that of the rock where the main hydraulic fracture propagates, which translates into the difference between the values of fracture toughness. The smaller fracture toughness of the bedding plane means that the distances over which the tensile stress concentration generating microcracks are smaller than in the other parts of the shale. This leads to the traces of microcracks in the RFs being smaller than in a newly developed PF, which are the observed experiment results. It is also found the the approximate four times difference in the mean microcrack lengths translates into two times difference in the values of fracture toughness. Given that fracture propagating parallel to the bedding planes will deviate to propagate along the weak planes, the observed difference in the values of fracture toughness signals a twofold fracture toughness anisotropy.

It can be forecasted that the proposed results obtained will assist in the modelling of hydraulic fracturing and forecast of production capacity. The subsequent monitoring can be arranged necessarily for efficient hydrocarbon production and reduction of environmental impact of fracking operations.

 

 

The last but not the least, the authors would like to thank the reviewer again for carefully reviewing our manuscript. The manuscript has been revised according to the comments and suggestions. In addition to correct the typo and grammatical errors indicated by the reviewer, we also did a very careful proof reading of the manuscript.

Author Response File: Author Response.docx