The Role of Fluid Overpressure on the Fracture Slip Mechanism Based on Laboratory Tests That Stimulating Reservoir-Induced Seismicity

Round 1

Reviewer 1 Report

The paper should be rewritten in good English. The paper should be founded on the series of experiments: authors present results of a sole experiment.  

Author Response

Responds to the reviewer 1:

Reply: Sincerely thank you for your careful reading and revision of the manuscript. Regarding the comments and suggestions proposed in the review, we made the following changes.

1. The paper should be rewritten in good English. The paper should be founded on the series of experiments: authors present results of a sole experiment.

Response:

Thanks for your advice. Because you would not like to sign your review report, we only can reply your advice according to the overall evaluation. Thanks for your suggestion.

Author Response File: Author Response.docx

Reviewer 2 Report

This work is focusing on the mechanisms of seismic activity induced by reservoirs by conducting indoor experiments. The work is meaningful and the analysis is sufficient to the conclusion. I would recommend acceptance of this article after authors answering the following questions:

1.       Why is the inlet pressure changing? Is this set for dams?

2.       Add more detailed comments of fig 5 in the content.

3.       Explain all the parameters’ definition in equation(5), especially the a and b.

4.       Explain the definition of deformation energy (Ed) in equation(6), and is it calculated or measured in this experiment?

5.       Please check line 275-277 if there is any unclear description of parameters.

Author Response

Responds to the reviewer 2:

Reply: Sincerely thank you for your careful reading and revision of the manuscript. Regarding the comments and suggestions proposed in the review, we made the following changes.

1. Why is the inlet pressure changing? Is this set for dams?

Response:

We change the inlet pressure to simulate the process of impoundment of water reservoir. The rate of water pressurization is set for dams.

2. Add more detailed comments of fig 5 in the content.

Response:

Thanks. The detailed comments of Figure 5 are added. The content is listed in the following:

Before the shearing test, the fracture surface of each sample are scanned to indicate the evolution of fracture after the process. Before the experiments, we polish the fracture surface and scan them to record the root mean square (RMS) of the fractures. The RMS can represent the roughness of fracture surface. When the RMS gets larger, the roughness of the surface will become larger. Figure 5(a) shows that the fault surface is quite smooth after polishing. Only the up surface is showing in this article due to the up surface and down surface are mirror images. Before the shearing test, the root mean square is 0.6575. After the shearing test, unstable slip happens that will damage the fracture surface. Figure 5(b) shows that the RMS of fracture surface, which changes to 0.7175 after shearing. The roughness increases. And broken minerals are discovered inside the fracture. Figure 5(c) is the shear surface calculated by Figure 5(a) and Figure 5(b). The shear surface can show the sheared thickness of the fracture. This phenomenon indicates that the fracture surface gets damage during the process while the pore pressure increases triggering the slip.

 

3. Explain all the parameters’ definition in equation(5), especially the a and b.

Response:

Thanks. The explanation of equation (5) is added in the content.

 Fault stiffness k_c can be calculate using equation(5). D_c is the critical slip distance for the friction of fracture surface changing from static friction to dynamical friction. Parameter a is the fracture friction at velocity V₁. Parameter b is the fracture friction at velocity V₂. σ_neff is the effective normal stress on the fracture. All the parameters can be measured in rate and state law friction test.

4. Explain the definition of deformation energy (Ed) in equation(6), and is it calculated or measured in this experiment?

Response:

Thanks. The deformation energy E_d in equation(6) the total deformation energy. It is calculated using following equation for the increase in aperture of the fracture. The A the is the area of the fault surface, P is the water pressure applied on the fault plane, and h is the width of aperture ranging from the initial state to the final state.

5. Please check line 275-277 if there is any unclear description of parameters.

Response:

Thanks. The description has been modified in the content.

Line 275-277: Results show that fracture begins to slip when both inlet and outlet pressure exceeds the theoretical water pressure and continue to stick slip when both inlet and outlet pressure increase. Slip distance, slip rate and stress drop indicate that stick slips happen periodically.

Author Response File: Author Response.docx

Reviewer 3 Report

Water injection induced seismicity is very important for unconventional resourses recovery (shale gas, shale oil, etc), this work conducts indoor experiments on fluid pressurization in fractured granite. The work is meaningful and the workload is large, and the data is also sufficient. I would recommend accept this manuscript after the authors carefully clarified the following comments:

1. the english needs to be further polished, and some grammar errors can be found. eg. "line 11: leading to induced seismicity", "line 225: is accordant with.." ...;

2. Fig.5 (a) and (b) cannot see any difference, and what's the meaning of fig 5(c)? need more comments in the content;

3. line156. "ratio τc is 98 %" while τc is defined as shear strength in line 148. please check and keep in consistency;

4. line 150, the citation style is not in formal, please keep it the same as other citations ;

5. Fig.7, how can you measure the seismic event? energy or stress fluctuation? please add more descriptions. 

6. Fig.9, the small figure inside figure 9 is blurred.

7. the subscript should not be in italic, eg. sigma neff, Dc...

Author Response

Responds to the reviewer 3:

Reply: Sincerely thank you for your careful reading and revision of the manuscript. Regarding the comments and suggestions proposed in the review, we made the following changes.

1. the English needs to be further polished, and some grammar errors can be found. eg. "line 11: leading to induced seismicity", "line 225: is accordant with.." ...;

Response:

Thanks for your advice. The grammar errors are checked and revised.

Line 11: The critical stressed fracture will slide as the response to the increase of fluid pressure inside fractures while impounding, triggering induced seismicity.

Line 225: This phenomenon is consistent with the experiment investigated by Scuderi.

 

2. Fig.5 (a) and (b) cannot see any difference, and what's the meaning of fig 5(c)? need more comments in the content.

Response:

Thanks. The figures have been modified. The contour has been adjusted to illustrate the difference between the fracture surface before and after the shearing experiment. The figure 5(c) is the sheared layer calculated by figure5(a) and figure5(b). More detailed description has been added in the content. The modified content is listed in the following:

 

Before the shearing test, the fracture surface of each sample are scanned to indicate the evolution of fracture after the process. Before the experiments, we polish the fracture surface and scan them to record the root mean square (RMS) of the fractures. The RMS can represent the roughness of fracture surface. When the RMS gets larger, the roughness of the surface will become larger. Figure 5(a) shows that the fault surface is quite smooth after polishing. Only the up surface is showing in this article due to the up surface and down surface are mirror images. Before the shearing test, the root mean square is 0.6575. After the shearing test, unstable slip happens that will damage the fracture surface. Figure 5(b) shows that the RMS of fracture surface, which changes to 0.7175 after shearing. The roughness increases. And broken minerals are discovered inside the fracture. Figure 5(c) is the shear surface calculated by Figure 5(a) and Figure 5(b). The shear surface can show the sheared thickness of the fracture. This phenomenon indicates that the fracture surface gets damage during the process while the pore pressure increases triggering the slip.

 

3. line156. "ratio τc is 98 %" while τc is defined as shear strength in line 148. please check and keep in consistency.

Response:

Thanks. The τ_c is a mistake. The correct description is r_τ, defined as the ratio between shear stress and shear strength of fracture.

 

4. line 150, the citation style is not in formal, please keep it the same as other citations.

Response:

Thanks for your advice. The citation style is revised to keep the same as other citations.

 

5. Fig.7, how can you measure the seismic event? energy or stress fluctuation? please add more descriptions.

Response:

Thanks for your advice. The seismic event is defined when stick slip happens. The seismic event can be investigated by several parameters, such as stress drop, sudden slip distance and rapid slip rate. The seismic event is a phenomenon when fracture sudden slides. The shear stress will drop suddenly because the machine cannot track the movement within millisecond. The fracture slides along the surface in a moment. The shear displacement will increase rapidly. Slip rate is the ratio of shear displacement and slip time. When the fracture slides, the slip rate will rise suddenly. Therefore, the stress drop, shear displacement (slip distance) and slip rate are the symbol judging a seismic event.

 

6. Fig.9, the small figure inside figure 9 is blurred.

Response:

Thanks. Figure 9 is revised.

 

7. the subscript should not be in italic, eg. sigma neff, Dc...

Response:

Thanks for your advice. The subscript is revised.

 

Reviewer 4 Report

Interesting work on evaluating the mechanisms of seismic activity induced by reservoirs. The research is very relevant and aims in providing an insight into a topic that still has a lot of unanswered questions.

I would recommend an English language check on the article to eliminate numerous awkward sentence constructions that make it a little hard to follow. I'm pointing out a few of them below:

a) Change basic to basis (lines 55 and 242).

b) The title is a bit unclear. Perhaps consider changing to: "The role of fluid overpressure on fracture slip mechanism based on laboratory tests simulating reservoir-induced seismicity". Just a suggestion. Feel free to change as you see fit. 

c) Line 26: avoid the phrase "confusing researchers"

d) Line 117: change "cyclic" to cycle. 

d) Change sentence constructions in the following lines to make them more clear: 42-43, 77-79, 269-271. 

Here are a few other comments/questions about the research:

1. Line 119: Increase and decrease of water pressure at 0.005 MPa/s has been used. Why did you use this rate? Please include in the text. 

2. What was the reasoning for using the different inlet pressures in FIg. 4. Are these typical inlet pressures for dams? Please clarify in text. 

3. Line 163: please define "permeability".

4. Line 203: Reference to Fig. 5 is not clear. Please double check figure and figure number.

5. In Fig. 7, explain if there is any relationship between slip distance, max slip rate and stress drop. Would a change in one parameter change the others? If so, can this change be reasonably estimated? 

6. In equation (5), what is b and a and Dc. Use proper subscripts for Dc. Define all paramaters in this equation. 

7. In Fig. 8, define delta_P. Is this the same that is used in Line 210? If so, please be consistent with capitalization of the variable. 

8. What is the physical significance of Mw (in equation 4.)? Is this empirical relationship dependent on any factors? Also, will the relationship between ER (radiated energy) and Mw be governed by site conditions such as type of dam, material, soil conditions? 

9. In equation (6), Please explain deformation energy (Ed) and how it is computed. 

10. Section 4.2 : it is unclear how seismic energy is predicted from hydraulic energy. Please explain Fig. 9 and how it was obtained. Elaborate this discussion. 

11. Rephrase the sentence in lines 275-277 clearly indicating the relationship (if established) between different parameters. 

12. Lines 282 - 283. Please reiterate how the two parameters determine sliding rate. 

Author Response

Responds to the reviewer 4:

Reply: Sincerely thank you for your careful reading and revision of the manuscript. Regarding the comments and suggestions proposed in the review, we made the following changes.

1. Change basic to basis (lines 55 and 242).

Response:

Thanks for your advice. The mistakes have been revised.

Line 55: Experiments analyzes that pore pressure can be predicted on the basis of elastic parameters.

Line 242:  The input and output energy can be described on the basis of conservation of energy.

2. The title is a bit unclear. Perhaps consider changing to: "The role of fluid overpressure on fracture slip mechanism based on laboratory tests simulating reservoir-induced seismicity". Just a suggestion. Feel free to change as you see fit.

Response:

Thanks for your advice. We have changed the title to make it clear.  

3. Line 26: avoid the phrase "confusing researchers".

Response:

Thanks. The sentence has been revised.

Line 26: Reservoir-induced seismicity is a tough task that there are still many challenges in figuring out how the seismicity triggered and how long it last.

4. Line 117: change "cyclic" to cycle.

Response:

Thanks.  The “cyclic” is changed to cycle.

5. Change sentence constructions in the following lines to make them more clear: 42-43, 77-79, 269-271.

Response:

Thanks for your advice.

Line 42-43: Evidences show that fluid overpressure is a significant factor in the nucleation and propagation of ruptures.

Line 77-79: This paper aims to discuss the fluid overpressure in inducing seismic when the fracture is critically stressed. HM coupling experiment shows that the unstable slip of fracture is driven by the inlet pressure and outlet pressure.

Line 269-271: The granite sample with a smooth fracture is adopted in this research. The injected water pressure increased cyclically to simulate the water reservoir impoundment. During the test, stick slip happens correspondingly to the inlet and outlet pressure.

 6. Line 119: Increase and decrease of water pressure at 0.005 MPa/s has been used. Why did you use this rate? Please include in the text.

Response:

Thanks. This rate is adopted because it is slow enough not to cause excessive pressure concentration around the inlet hole. And this rate is fast enough to quickly simulate the process of reservoir impoundment. This content is included in the text.

7. What was the reasoning for using the different inlet pressures in FIg. 4. Are these typical inlet pressures for dams? Please clarify in text.

Response:

Thanks. Figure 4 shows the alteration of inlet pressures because the inlet pressure is a typical curve for water reservoir, such as Xiluodu Reservoir and Three Gorge Reservoir. This content is included in text. 

8. Line 163: please define "permeability".

Response:

Thanks for your advice. The permeability here is referring to the fracture permeability.

9. Line 203: Reference to Fig. 5 is not clear. Please double check figure and figure number.

Response:

Thanks. The figure and figure number have been checked. And we carefully add more detailed explanation the figure in the content.

10. In Fig. 7, explain if there is any relationship between slip distance, max slip rate and stress drop. Would a change in one parameter change the others? If so, can this change be reasonably estimated?

Response:

Thanks. Slip distance is the shear displacement when stick slip occurs. Max slip rate is the ratio of slip distance and slip time. Therefore, the max slip rate is calculated by the slip distance. Stress drop cannot be calculated by slip distance or max slip rate.

11. In equation (5), what is b and a and Dc. Use proper subscripts for Dc. Define all paramaters in this equation.

Response:

Thanks. Fault stiffness k_c can be calculate using equation(5). D_c is the critical slip distance for the friction of fracture surface changing from static friction to dynamical friction. Parameter a is the fracture friction at velocity V₁. Parameter b is the fracture friction at velocity V₂. σ_neff is the effective normal stress on the fracture. All the parameters can be measured in rate and state law friction test.

12. In Fig. 8, define delta_P. Is this the same that is used in Line 210? If so, please be consistent with capitalization of the variable.

Response:

Thanks for your advice. The Δp is the same that used in Line 210. The capitalization of the variable is adjusted as the same. 

13. What is the physical significance of M_w (in equation 4.)? Is this empirical relationship dependent on any factors? Also, will the relationship between E_r (radiated energy) and Mw be governed by site conditions such as type of dam, material, soil conditions?

Response:

Thanks. M_w is the moment magnitude of earthquakes. This is an empirical formula. This might be changed when the statistics belongs to a different area. The site conditions might have influence in determining the relationship between E_r and M_w. But this has not be verified yet.

14. In equation (6), Please explain deformation energy (E_d) and how it is computed.

Response:

Thanks for your advice. The deformation energy E_d in equation(6) the total deformation energy. It is calculated using following equation for the increase in aperture of the fracture. The A the is the area of the fault surface, P is the water pressure applied on the fault plane, and h is the width of aperture ranging from the initial state to the final state. 

15. Section 4.2 : it is unclear how seismic energy is predicted from hydraulic energy. Please explain Fig. 9 and how it was obtained. Elaborate this discussion.

Response:

Thanks.  According to equation(6), each seismic event has a certain hydraulic energy E_h and radiated energy E_r. Momenet magnitude (M_w) can be calculated according to equation(8). Therefore, the seismic energy(M₀) can be calculated according to equation(4). Above this, the relationship between seismic energy and hydraulic energy is established. In figure 9, the black line is the ratio between seismic energy and hydraulic energy. The 11 black lines represents the ratio at 100% to 0.00000001%, correspondingly. The triangles and circles in figure 9 are data collected from several field experiments and laboratory experiments.

16. Rephrase the sentence in lines 275-277 clearly indicating the relationship (if established) between different parameters.

Response:

Thanks for your advice. The sentence is modified. The slip rate is related to the slip distance. But the stress drop cannot establish any relationship to the slip rate and slip disatance.

Line 275-277: Results show that fracture begins to slip when both inlet and outlet pressure exceeds the theoretical water pressure and continue to stick slip when both inlet and outlet pressure increase. Slip distance, slip rate and stress drop indicate that stick slips happen periodically. 

17. Lines 282 - 283. Please reiterate how the two parameters determine sliding rate

Response:

Thanks. The explanation is added in the text.

Line 282-283: The value of Δp/σ_n and 1- r_c determines the sliding state.When Δp/σn < 1- rc, the increased pressure and shear stress cannot induce slip. When Δp/σn > 1- rc, the fault is transiting from stable state to unstable state.

Author Response File: Author Response.docx