Coupling Mechanism of Coal Body Stress–Seepage around a Water Injection Borehole
Round 1
Reviewer 1 Report
Review comments:
Based on the self-developed radial seepage experimental system, this paper tests the gas permeability of large-size raw coal samples, and the experimental method is relatively novel. At the same time, the COMSOL Multiphysics 5.4 simulation software is used to invert and summarize the water injection drill under different stress environments. Stress-seepage dynamic evolution law of coal mass around pores. Overall, this article deserves publication in the journal Sustainability, but there are still several issues that need to be addressed before the paper can be published.
(1) In Chapter 2, the author tested the gas permeability of large-sized raw coal samples based on the self-developed radial seepage experimental system, which is very innovative. Please explain the application of the cover during the experiment. After compression loading, it is how to simulate the real stress of the formation.
(2) In Chapter 2, what is the relationship between the radial seepage experiment and the simulation research content that follows?
(3) In Section 2.3, the data in Figure 4 is exponentially fitted to obtain the corresponding fitting relation and fitting coefficient. What is the specific meaning of the fitting relation obtained, and the author is advised to use this part based on The size of the fitting coefficient is used to supplement the fitting effect.
(4) In Chapter 3.3.2, with the increase of the radial distance of the borehole, the stress shows a change trend of decrease increase decrease. Why this change trend is presented is not explained in detail.
(5) In Section 3.4.2, the pore seepage velocity around the borehole gradually decreases in steps from the vicinity of the borehole to the surrounding, showing a turbulent non-Darcy seepage state. This phenomenon is a bit confusing, you can explain it, and The reason for this phenomenon is explained in the text.
(6) In Chapter 4, the author summarizes the dynamic evolution law of stress seepage in the whole process of water injection, and analyzes the change law of each stage in detail. It is suggested that the author appropriately introduce relevant literature in this chapter to make the analysis conclusion more convincing.
Author Response
Response to Reviewer 1 Comments
Based on the self-developed radial seepage experimental system, this paper tests the gas permeability of large-size raw coal samples, and the experimental method is relatively novel. At the same time, the COMSOL Multiphysics 5.4 simulation software is used to invert and summarize the water injection drill under different stress environments. Stress-seepage dynamic evolution law of coal mass around pores. Overall, this article deserves publication in the journal Sustainability, but there are still several issues that need to be addressed before the paper can be published.
Point 1: In Chapter 2, the author tested the gas permeability of large-sized raw coal samples based on the self-developed radial seepage experimental system, which is very innovative. Please explain the application of the cover during the experiment. After compression loading, it is how to simulate the real stress of the formation.
Response 1: Thank you very much for your comments and questions. The raw coal sample used by the experimental equipment relied on in the manuscript is large in size. The simulation of the real stress of the formation is mainly through the overburden loading unit, using hydraulic oil as the medium, using the oil pump to boost the pressure inside the cylinder block, and the oil pressure pushes the piston downward to apply different sizes of axial pressure to the coal sample to simulate the overburden pressure. The cylinder block is surrounded by rigid constraints, so as to provide lateral stress to the coal sample.
Point 2: In Chapter 2, what is the relationship between the radial seepage experiment and the simulation research content that follows?
Response2: Thank you very much for your questions. Because the radial seepage experiment has made it clear that stress is the direct cause of coal deformation, but as a typical gray system, the coal seam cannot directly obtain the actual change of stress in the process of water injection, and the radial seepage experiment system cannot completely reproduce the evolution characteristics of stress. Therefore, based on the coal seam permeability obtained from the radial seepage experiment, combined with the mechanical characteristic parameters of the coal sample, the stress seepage field coupling relationship in the water injection process is inverted by using COMSOL multiphysics5.4 numerical simulation software, and the stress distribution characteristics around the borehole and the dynamic evolution law of Darcy seepage in the water injection process are further analyzed.
Point 3: In Section 2.3, the data in Figure 4 is exponentially fitted to obtain the corresponding fitting relation and fitting coefficient. What is the specific meaning of the fitting relation obtained, and the author is advised to use this part based on The size of the fitting coefficient is used to supplement the fitting effect.
Response 3: Thank you very much for your questions and suggestions. There are mainly linear, exponential, univariate quadratic and power-law relationships between coal permeability and external overburden pressure load. In Figure 4, when the exponential relationship is used to fit the variation law of coal seam permeability and gas flow with overburden pressure, the fitting relationship satisfies , where a and b are the fitting coefficients, and p is the overburden pressure in the experiment (that is, the in-situ stress in the real formation). After fitting, it is found that there is a good matching relationship between the two. The fitting coefficients of coal seam permeability and gas flow with overburden pressure are greater than 0.99, indicating that the fitting effect is good. As for the fitting effect of the experimental curve, we have supplemented and annotated it in Chapter 2.3, please you review.
Point 4: In Chapter 3.3.2, with the increase of the radial distance of the borehole, the stress shows a change trend of decrease increase decrease. Why this change trend is presented is not explained in detail.
Response 4: Thank you very much for your comments. We have explained and analyzed in detail that the stress shows a decreasing increasing decreasing trend with the increase of the radial distance of the borehole. The main reason is that in the process of water injection, due to the existence of water pressure around the water injection borehole, the axial pressure and water pressure superimposed to form a high stress area, while the coal body in the distance has the phenomenon of stress concentration due to the stress around the borehole. At the same time, because the coal body in the distance from the water injection borehole is not affected by the intrusion and pressure relief of dynamic water, the original stress area appears. Therefore, along the direction of drilling hole diameter, the stress shows a decreasing increasing decreasing trend. The above explanation has been supplemented and marked in the corresponding position of chapter 3.3.2 in the manuscript, please you review.
Point 5: In Section 3.4.2, the pore seepage velocity around the borehole gradually decreases in steps from the vicinity of the borehole to the surrounding, showing a turbulent non-Darcy seepage state. This phenomenon is a bit confusing, you can explain it, and The reason for this phenomenon is explained in the text.
Response 5: Thank you very much for your comments and suggestions. I'm very sorry that due to our negligence, the pore seepage velocity around the borehole gradually decreases step by step from the vicinity of the borehole to the surroundings, presenting a turbulent non Darcy seepage state, which has not been clearly stated, causing your confusion. According to your comments, we explain the reasons for this phenomenon as follows: In the process of water injection, the pressure of injected fluid gradually increases, causing stress accumulation. When the pressure reaches the opening pressure of coal fractures, the fractures in the coal expand or hydraulic fracturing, resulting in the gradual decline of seepage velocity in the fractures from near the borehole to the surrounding, presenting turbulent unsteady seepage phenomenon. The above explanation has been supplemented and marked in the corresponding position of chapter 3.4.2 in the manuscript, please you review.
Point 6: In Chapter 4, the author summarizes the dynamic evolution law of stress seepage in the whole process of water injection, and analyzes the change law of each stage in detail. It is suggested that the author appropriately introduce relevant literature in this chapter to make the analysis conclusion more convincing.
Response 6: Thank you very much for your comments and suggestions. According to your opinion, we have introduced the following references in Chapter 4 of the manuscript to support the contents of the manuscript, please you review.
[1] Shi Y, Wang C, Hui C, et al. Analysis of Coal Permeability Revolution Laws Around Drillings Along Coal Seam [J]. Safety in Coal Mines, 2015,46(12).
[2] Liu Z, Yang H, Wang W, et al. Experimental Study on the Pore Structure Fractals and Seepage Characteristics of a Coal Sample Around a Borehole in Coal Seam Water Infusion[J]. Transport in Porous Media,2018,125(2):
[3] Zhang C, Liu Z, Wang B, et al. Numerical Simulation and Test Study on Mechanical Properties Evolution of Hing- Pressure Water Injection Coal Seam [J]. Chinese Journal of Rock Mechanics and Engineering, 2009, 28(A02):5.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript is well written. I particularly appreciate the meticulous design of the research in which the experiment and simulation were both considered and the results were elaborately discussed. The findings provide extra insight and can add value to the body of this research field.
Author Response
Response to Reviewer 2 Comments:
Point 1: The manuscript is well written. I particularly appreciate the meticulous design of the research in which the experiment and simulation were both considered and the results were elaborately discussed. The findings provide extra insight and can add value to the body of this research field.
Response 1: Thank you very much for your comments and affirmation of the manuscript. Your comments are of great help to our future research.
Author Response File: Author Response.pdf
Reviewer 3 Report
Through the combination of experiment and numerical simulation, this paper studies the dynamic evolution law of stress seepage around the water injection borehole, which can provide theoretical guidance for the process of coal seam water injection. The following problems still need to be solved.
(1) What are the characteristics of the large-scale radial seepage experimental system of raw coal in this paper, and what are the differences or advantages compared with other equipment? When the experiment is carried out, the axial pressure is mainly used. How is the side wall of the coal sample constrained by the force?
(2) The content of Section 3.2 describes the model, and selects the combination of mechanical equilibrium equation and seepage and continuity equation. It is suggested that the author supplement the reasons for choosing this combined model.
(3) Fig. 6 shows the stress distribution before water injection, which is not compared with the existing relevant research results in this paper. In order to prove the reliability of the model, the author can cite references to testify the stress evolution law around the borehole before water injection in Fig. 6.
(4) Section 3.4.2 does not introduce whether Darcy speed is obtained by calculation or by software measurement points?
(5) The Darcy velocity gradually decreases from the vicinity of the borehole to the surrounding steps. Why does this phenomenon appear? The author can explain and supplement.
(6) In the last paragraph of section 3.4.2 of the article, it is described that the flow in the open crack seepage channel is in a turbulent state, and whether the flow in the slit channel has turbulent characteristics at the same time?
(7) In Section 4, combined with the experimental and numerical simulation results, the dynamic evolution law of stress seepage in the whole process of water injection is discussed. In order to be more convincing, references can be appropriately introduced to testify in this part.
Author Response
Response to Reviewer 3 Comments
Through the combination of experiment and numerical simulation, this paper studies the dynamic evolution law of stress seepage around the water injection borehole, which can provide theoretical guidance for the process of coal seam water injection. In general, the paper is worth publishing in the journal Sustainability, but the following problems still need to be solved before the paper is published.
Point 1: What are the characteristics of the large-scale radial seepage experimental system of raw coal in this paper, and what are the differences or advantages compared with other equipment? When the experiment is carried out, the axial pressure is mainly used. How is the side wall of the coal sample constrained by the force?
Response 1: Thank you very much for your comments and questions. At present, for the study of coal seam permeability, a large number of scholars mainly conduct laboratory research on small-size briquette pressed coal samples or small-size raw coal samples. The large-scale radial seepage experimental system of raw coal independently developed in the manuscript further expands the size of the coal sample on the basis of similar research, striving to restore the original fracture structure of the coal seam, reduce the influence of the boundary effect of the coal sample on the measurement parameters, and improve the accuracy of the measurement results. During the experiment, hydraulic oil is used as the medium through the overburden loading unit, and the oil pump is used to boost the pressure inside the cylinder block. The oil pressure pushes the piston downward to apply axial pressure of different sizes to the coal sample to simulate the overburden pressure. There are rigid constraints around the cylinder block, so as to provide lateral stress to the coal sample.
Point 2: The content of Section 3.2 describes the model, and selects the combination of mechanical equilibrium equation and seepage and continuity equation. It is suggested that the author supplement the reasons for choosing this combined model.
Response 2: Thank you very much for your comments and suggestions. According to your opinion, we have added the reasons for selecting the mechanical equilibrium equation and the seepage and continuity equations in Section 3.1 of the manuscript, please you review.
Point 3: Fig. 6 shows the stress distribution before water injection, which is not compared with the existing relevant research results in this paper. In order to prove the reliability of the model, the author can cite references to testify the stress evolution law around the borehole before water injection in Fig. 6.
Response 3: Thank you very much for your comments and suggestions. According to your opinion, we have supplemented relevant references in the manuscript to support the stress evolution law around the borehole before water injection, please you review.
[1] L. Zhen, W. Wendi, C. Dandan, Numerical Calculation and Analysis of Hole Sealing Parameters of Gas Drainage in Bedding, Mining Research and Development 40(12) (2020) 6.
[2] Shi Y, Wang C, Hui C, et al. Analysis of Coal Permeability Revolution Laws Around Drillings Along Coal Seam [J]. Safety in Coal Mines, 2015,46(12).
Point 4: Section 3.4.2 does not introduce whether Darcy speed is obtained by calculation or by software measurement points?
Response 4: Thank you very much for your comments. We are very sorry that due to our negligence, the acquisition of Darcy speed is not introduced in section 3.4.2. The Darcy velocity distribution curve in the manuscript is obtained by extracting the radial measurement points of the borehole through the numerical simulation software. We have added and marked, please you review.
Point 5: The Darcy velocity gradually decreases from the vicinity of the borehole to the surrounding steps. Why does this phenomenon appear? The author can explain and supplement.
Response 5: Thank you very much for your comments and suggestions. I'm very sorry that due to our negligence, the pore seepage velocity around the borehole gradually decreases step by step from the vicinity of the borehole to the surroundings, presenting a turbulent non Darcy seepage state, which has not been clearly stated, causing your confusion. According to your comments, we explain the reasons for this phenomenon as follows: In the process of water injection, the pressure of injected fluid gradually increases, causing stress accumulation. When the pressure reaches the opening pressure of coal fractures, the fractures in the coal expand or hydraulic fracturing, resulting in the gradual decline of seepage velocity in the fractures from near the borehole to the surrounding, presenting turbulent unsteady seepage phenomenon. The above explanation has been supplemented and marked in the corresponding position of chapter 3.4.2 in the manuscript, please you review.
Point 6: In the last paragraph of section 3.4.2 of the article, it is described that the flow in the open crack seepage channel is in a turbulent state, and whether the flow in the slit channel has turbulent characteristics at the same time?
Response 6: Thank you very much for your comments. Based on the change law of Darcy velocity distribution curve, it is found that the accumulation of water injection pressure makes the cracks in the coal body expand or hydraulic fracturing, and the seepage process in the open fracture seepage channel appears turbulent state. In the capillary seepage channel of the slit, under the action of mechanics, the water seepage is mainly capillary beam seepage. During the study of this manuscript, we cannot directly judge whether the flow in the slit channel has turbulent characteristics at the same time. For this reason, we are very sorry and hope to get your understanding.
Point 7: In Section 4, combined with the experimental and numerical simulation results, the dynamic evolution law of stress seepage in the whole process of water injection is discussed. In order to be more convincing, references can be appropriately introduced to testify in this part.
Response 7: Thank you very much for your comments and suggestions. According to your opinion, we have introduced the following references in Chapter 4 of the manuscript to support the contents of the manuscript, please you review.
[1] Shi Y, Wang C, Hui C, et al. Analysis of Coal Permeability Revolution Laws Around Drillings Along Coal Seam [J]. Safety in Coal Mines, 2015,46(12).
[2] Liu Z, Yang H, Wang W, et al. Experimental Study on the Pore Structure Fractals and Seepage Characteristics of a Coal Sample Around a Borehole in Coal Seam Water Infusion[J]. Transport in Porous Media,2018,125(2):
[3] Zhang C, Liu Z, Wang B, et al. Numerical Simulation and Test Study on Mechanical Properties Evolution of Hing- Pressure Water Injection Coal Seam [J]. Chinese Journal of Rock Mechanics and Engineering, 2009, 28(A02):5.
Author Response File: Author Response.pdf
