Research into the Mechanism and Application of Liquid CO₂ Phase-Transition Fracturing in a Coal Seam to Enhance Permeability

Round 1

Reviewer 1 Report (Previous Reviewer 3)

Authors have put minimal effort to address my concerns and comments. Apart from some grammatical changes, the manuscript still looks pretty much the same. Most of my original comments still stand. Further, I provided several minor comments in the pdf file of the manuscript, which were also ignored.

Author Response

Dear Editors and Reviewers:

Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled “Mechanism and Application Research of Liquid CO2 Phase Transition Fracturing Coal Seam to Enhance Permeability” (ID：2078851). The comments are both valuable and very helpful for revising and improving our paper. We have studied comments carefully and have made correction which we hope meet with approval. The main corrections in the paper and the responds to the reviewers’ comments are as flowing:

Responds to the reviewer s’ comments:

Reviewer 1

1. English and Grammar usage in the manuscript is poor which made it hard to read. I would recommend getting the paper proofread by an English expert. I stopped correcting the grammar/English after the first few sections of the manuscript as there were too many errors.

Response:

We have revised the paper proofread by “Language Editing Services” of MDPI.

2. Different sections in the manuscript are not well organized. The manuscript should be divided into 4 major sections: 1) Introduction 2) Methods 3) Results and Discussion 4) Conclusions. There are several sections in the paper that should be combined in the introduction section. There is no methods section or results and discussion section in the current version of the manuscript. Please see my detailed comments in the pdf file on how to divide the information into 4 sections.

We have divided the manuscript into 4 major sections: 1) Introduction 2) Methods 3) Results and Discussion 4) Conclusions.

3. There are several previous studies performed using liquid CO₂ and understanding their effect on rock permeability. However, the manuscript fails to mention/acknowledge them. Please expand the introduction section to reflect the previous contributions.

We have added the related content as follows:

The liquid carbon dioxide phase change fracturing technology was first proposed by the Cardox International, UK, called the Cardox Tube System [16]. Singh introduced the main structure and application method of the device and pointed out that the de-vice can be used for large-scale mining and excavation of a quarry. Due to its high safe-ty, the device can be used for underwater operation and for fast and safe blasting near reservoirs and dams [17]. In Turkey, coal mines use the Cardox device in the working face to split the coal mass by the high-pressure carbon dioxide gas generated instantly, thus improving the lump coal rate [18]. Lekontsev compared several explosion-proof rock fracture technologies and suggested that the Cardox device does not belong to the scope of explosion but only a high-pressure gas generator [19]. Therefore, it is not lim-ited by the control of explosives, which have a limited scope of use. With its safety and stability, the liquid carbon dioxide phase change cracking can be applied to the clean-ing of large storage tank walls. As carbon dioxide gas is an inert gas, the device can be applied to the treatment of flammable and combustible materials. Lisienko studied the carbon dioxide cannon by simulating coal blasting on the ground and stated that the liquid carbon dioxide blasting was a slow, expansive, diffusing, and shearing process, which caused the released carbon dioxide gas to cut along the natural cracks of coal or explosives and was most suitable for the blasting of porous brittle materials [20]. Xiang Cheng tested carbon dioxide blasting in the working face of Luling Coal Mine. After blasting, the effect of the coal briquetting was good, the amount of coal thrown was large, and the ratio of fine coal was significantly reduced [21].

The phase change fracturing technology of liquid carbon dioxide is a new technol-ogy for the coal industry, which uses the huge energy released by liquid carbon dioxide in the process of the liquid-gas two-phase transformation in a confined space to act on the original fractures, and the fracturing effect of the high-energy gas expands the orig-inal fractures [22]. At the same time, the impact force generated during fracturing breaks the hole wall, creating new cracks, which increases the crack development of materials, improves the permeability of materials, and even breaks materials.

4. There is a disconnect between the numerical modeling section and the field-testing section. When I started reading the manuscript, my expectation was that the field testing would be used to validate the numerical modeling results. However, that was not the case. Is there a way to validate the numerical modeling (COMSOL) results using the field data?

We will try our best to validate the numerical modeling (COMSOL) results using the field data in the future research work, and thank you very much. In this paper, the numerical modeling was validated the parameters of the “Test scheme”, and the field-testing was focused on monitoring the gas extraction volume to validated the perfect of liquid CO₂ phase transition fracturing coal seam to enhance permeability.

5. Several of the figures are missing axis labels, color coding labels, units etc. Most of them are of poor quality as well. Further, the captions of each figure need to be expanded to explain the figures in much more detail.

We have revised the Figures as follows:

Figure 1. Numerical model.

(a) Plastic deformation area

(b) Sectional view of plastic deformation area

Figure 2. Plastic deformation.

 

Figure 3. Division of the plastic deformation zone.

 

(a) The nephogram of volumetric strain            (b) The section value of volumetric strain

Figure 4. The volumetric strain.

Figure 5. The arrangement of boreholes.

Figure 6. The arrangement of boreholes.

Figure 7. Monitoring curves of gas drainage concentration.

Author Response File: Author Response.pdf

Reviewer 2 Report (Previous Reviewer 1)

The paper can be accepted after being modified according to the format specification of the journal.

Author Response

Dear Editors and Reviewers:

Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled “Mechanism and Application Research of Liquid CO2 Phase Transition Fracturing Coal Seam to Enhance Permeability” (ID：2078851). The comments are both valuable and very helpful for revising and improving our paper. We have studied comments carefully and have made correction which we hope meet with approval. The main corrections in the paper and the responds to the reviewers’ comments are as flowing:

Responds to the reviewer s’ comments:

Reviewer 2

1. The English of this paper is poor and needs to be optimized.

We have revised the paper.

2. The picture in this paper is not beautiful nor scientific enough.

We have revised the Figures as follows:

Figure 1. Numerical model.

(a) Plastic deformation area

(b) Sectional view of plastic deformation area

Figure 2. Plastic deformation.

Figure 3. Division of the plastic deformation zone.

 

(a) The nephogram of volumetric strain            (b) The section value of volumetric strain

Figure 4. The volumetric strain.

Figure 5. The arrangement of boreholes.

Figure 6. The arrangement of boreholes.

Figure 7. Monitoring curves of gas drainage concentration.

3. At present, many scholars had studied the liquid carbon dioxide fracturing technology of coal, compared with them, there is no obvious innovation in this paper.

I am sorry. The focus of this paper is to discuss the research contents of field scientific research projects. This article pays more attention to the guiding significance of the scene.

4. The section "1. Introduction" only briefly introduces the advantages and disadvantages of hydraulic punching technology, without introducing the research status of liquid carbon dioxide fracturing technology at all. It needs to be rewritten.

We have revised the “Introduction” as follows:

1. Introduction

For a long time, gas has seriously restricted the safe production of coal mine enterprises [1–3]. With the increase in the mining depth, the permeability of the coal seam becomes lower and lower. Enhancing the permeability of a coal seam is a prerequisite for gas extraction and the safe production of coal mines. At present, hydraulic fracturing technology is widely used in gas extraction [4–6]. However, the effect of the coal seam permeability improvement is poor for the downward layer through the borehole. The influence of the different thicknesses and hardness of a coal seam on the effect of permeability enhancement has been less studied. The understanding of the coal breaking mechanism of hydraulic punching, and the mechanism of increasing permeability and pressure relief are lacking. The process and technical parameters of hydraulic punching need to be further studied. The common punching process can easily cause problems such as holding holes, plugging holes, sticking, and running water. In particular, the coal water mixture flushed from the upward hole tends to accumulate at the upper opening of the casing, which directly affects the drilling and flushing process [7–9]. Therefore, there is an urgent need to find a penetration enhancement method with a good penetration effect and high safety. This paper analyzes and evaluates the technology of the liquid CO2 phase transition fracturing in a coal seam to enhance permeability.

The liquid carbon dioxide phase transition cracking device uses a heating tube to heat the liquid carbon dioxide in the fluid storage tube, changing it from a liquid state to a gas state, and the pressure increases sharply [10–12]. Then the high-pressure gas smashes the constant pressure energy sheet and generates stress waves that propagate. The generated gas passes through the release channel of the exhaust pipe and reaches the coal mass in a short time to form an explosive airflow. Stress waves and high-energy gas can not only generate new cracks in the coal mass but can also make the original cracks expand [13–15].

The liquid carbon dioxide phase change fracturing technology was first proposed by the Cardox International, UK, called the Cardox Tube System [16]. Singh introduced the main structure and application method of the device and pointed out that the device can be used for large-scale mining and excavation of a quarry. Due to its high safety, the device can be used for underwater operation and for fast and safe blasting near reservoirs and dams [17]. In Turkey, coal mines use the Cardox device in the working face to split the coal mass by the high-pressure carbon dioxide gas generated instantly, thus improving the lump coal rate [18]. Lekontsev compared several explosion-proof rock fracture technologies and suggested that the Cardox device does not belong to the scope of explosion but only a high-pressure gas generator [19]. Therefore, it is not limited by the control of explosives, which have a limited scope of use. With its safety and stability, the liquid carbon dioxide phase change cracking can be applied to the cleaning of large storage tank walls. As carbon dioxide gas is an inert gas, the device can be applied to the treatment of flammable and combustible materials. Lisienko studied the carbon dioxide cannon by simulating coal blasting on the ground and stated that the liquid carbon dioxide blasting was a slow, expansive, diffusing, and shearing process, which caused the released carbon dioxide gas to cut along the natural cracks of coal or explosives and was most suitable for the blasting of porous brittle materials [20]. Xiang Cheng tested carbon dioxide blasting in the working face of Luling Coal Mine. After blasting, the effect of the coal briquetting was good, the amount of coal thrown was large, and the ratio of fine coal was significantly reduced [21].

The phase change fracturing technology of liquid carbon dioxide is a new technology for the coal industry, which uses the huge energy released by liquid carbon dioxide in the process of the liquid-gas two-phase transformation in a confined space to act on the original fractures, and the fracturing effect of the high-energy gas expands the original fractures [22]. At the same time, the impact force generated during fracturing breaks the hole wall, creating new cracks, which increases the crack development of materials, improves the permeability of materials, and even breaks materials.

1.1. Effect of the stress wave on the coal mass

When high-pressure carbon dioxide gas is ejected from the fluid storage pipe, it is accompanied by the generation of stress waves. When the coal mass is regarded as an ideal elastomer, the motion equation of the coal-rock particle when the elastic wave propagates is as follows:

(λ+G)  ∂θ/(∂x_i )+G∇^2 u_i=ρ (∂^2 u_i)/(∂t^2 )  i=1,2,3…                                         (1)

where ui is the displacement of the particle in the coordinate direction; θ = εij = uij is the volumetric strain; ∇2 is the Laplace operator; λ, G is the lame elastic constant; ρ is the density of coal-rock mass; and t is time.

1.2. High-pressure gas gathering cutting by the phase change of liquid carbon dioxide

The unique design of the end of the release pipe in the liquid carbon dioxide phase transition equipment can solve the problem of the blasting energy dispersion and dilution. The exhaust hole of the release pipe allows the gas to be discharged in a fixed direction, so that the energy is gathered and fractures the coal mass. When the high-pressure liquid carbon dioxide gas impacts the coal in the form of jet, the high-pressure gas radiates from the center of the jet.

The impact of the high-pressure carbon dioxide gas to fracture the coal mass is divided into the following categories: the cavitation damage, the impact cutting effect of the high-pressure gas, the dynamic pressure effect of high-pressure gas, and the gas wedge effect formed by high pressure gas. Under the impact of high-pressure carbon dioxide gas, the internal stress distribution of the coal mass is similar to the half-space elastomer under the concentrated load. When the compressive stress generated by the impact of high-pressure gas reaches or exceeds the compressive strength of the coal body, the original coal body is damaged, and cracks are formed in the coal body. When the compressive stress generated by the impact of high-pressure gas exceeds the compressive strength of the coal mass, the original coal mass is damaged, and cracks are formed. The high-pressure gas entering the cracks causes the cracks to develop and expand rapidly, resulting in the fracture of the coal seam.

1.3. Coal seams fractured by the liquid carbon dioxide phase change expansion force

The liquid carbon dioxide in the liquid storage pipe gradually changes into gaseous carbon dioxide after heating, the volume expands continuously, and the pressure of the carbon dioxide gas increases continuously [23–25]. When the carbon dioxide gas breaks through the constant pressure valve and is ejected through the release pipe, the carbon dioxide gas still expands and increases, generating expansion work [26–28]. The high-pressure carbon dioxide gas exerts pressure on the wall of the fracturing boreholes, forcing new fractures in the coal seam. Otherwise, the high-pressure carbon dioxide gas enters the coal seam and expands the original fractures in the coal seam.

5. The experiment part of this paper is too simple and the simulation content has no depth.

We are very sorry. The experiment and the simulation content pay more attention to the guiding significance of the scene.

6. The conclusions of this paper are not innovative.

We have revised the conclusions as follows:

In this paper, the mechanism of the liquid carbon dioxide fracturing coal seam to enhance permeability was studied. The COMSOL software was used to numerically analyze the influence radius of the liquid carbon dioxide phase transition fracturing, which provided a basis for reasonable field distribution parameters and analyzing the effect of the onsite liquid carbon dioxide fracturing. The main conclusions were as fol-lows:

(1) The technology of liquid carbon dioxide fracturing coal seam enhanced the permeability through three aspects. First, the action of stress wave caused new cracks in the coal mass, and stress concentration occurred at the tip of the crack, which pro-moted the expansion and development of tiny cracks. Second, the intrusion of the high-pressure liquid carbon dioxide gas into the coal, resulted in cutting, impact, and a gas wedge. Third, the carbon dioxide gas broke through the constant pressure valve, and a large expansion thrust was generated, which acted on the coal mass, generating new cracks and disturbing the primary cracks in the coal seam.

(2) The COMSOL software was used to simulate the influence radius of the liquid carbon dioxide cracking, and it was determined that with the geological conditions of the #8 coal seam in the Baode Coal Mine, the influence radius of the carbon dioxide phase transition cracking was 13.4 m.

(3) The gas extraction volume, gas concentration, and coal seam permeability coef-ficient of the fracturing hole and the contrast hole were monitored onsite. The results showed that the fracturing effect was good.

Author Response File: Author Response.pdf

Reviewer 3 Report (New Reviewer)

The present study discusses the effect of CO2 gas fracturing on permeability gassy coal seams. The study further investigates numerically the mechanical effects (i.e., stress and strain) by the injected pressure on the coal matrix.

General Comments:

1.      CO2 gas fracturing technique and its impact (improvement) on permeability has already been discovered, so what is the novelty of the present study? What is the new here?

2.       There is no cohesion (consistency) of the present work: the authors provided on-site test data, while the numerical analysis on the stress/strain distribution during the CO2 transition. This does not validate the numerical model unless the authors must measure experimentally the stress/strain distribution in order to validate their numerical model.

3.      Radius influence is not clearly stated or not suffeiecntly studied. The use of moving mesh - Lagrangian mesh  -  (deforming the domain during the CO2 transition) would be a good start to analyse the  impact of different radii as well as CO2 on permeabilty.

Content Comments:

1.      In Section 2.1, reference is required for Eq.1, also where is Darcy's law? ui should be a velocity not a displacement?

2.      In Table 1, the authors need to show an equation where those parameters are applied?

3.      In line 105, What do you mean by "mesh size is differentiated"?

4.      In lines 105-106, “The mesh size near the fracturing boreholes is small and dense, and the mesh size away from the fracturing boreholes are large and sparse [22–24].” The authors need to elaborate more justify such context?

5.      In lines 108-109, What do the authors mean by saying “the boundary condition of the orifice is Darcy”. The orifice (the hole) is a porous zone, while surroundings are solid zones?

6.      In Fig.3, the caption is not informative and  all boundary conditions should be illustrated through the model (e.g., stress, porous, solid, inlet, etc).

7.      In Fig.4(a), in the legend, what does the negative numbers tell us (blue)? need to say?

8.      In line 123, you need to identify for the reader what does it mean by "state of destruction"?

9.      In line 124, do you mean (no plastic deformation) for stable state?

10.   In section 4.3, fracturing has already been studied, what is the new contribution of the present paper?

11.   In table 3, contrast boreholes in Fig.7, the authors only highlighted two, need to mention all?

12. Please go through those articles for equations involved in the numerical model during such a transport phenomenon.

https://doi.org/10.1177/0144598718785998

https://doi.org/10.1016/j.fuel.2022.124958

https://doi.org/10.3390/en15103828

 

 

 

Author Response

Dear Editors and Reviewers:

Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled “Mechanism and Application Research of Liquid CO2 Phase Transition Fracturing Coal Seam to Enhance Permeability” (ID：2078851). The comments are both valuable and very helpful for revising and improving our paper. We have studied comments carefully and have made correction which we hope meet with approval. The main corrections in the paper and the responds to the reviewers’ comments are as flowing:

Responds to the reviewer s’ comments:

Reviewer 3

1. CO₂ gas fracturing technique and its impact (improvement) on permeability has already been discovered, so what is the novelty of the present study? What is the new here?

I am sorry. The focus of this paper is to discuss the research contents of field scientific research projects. This article pays more attention to the guiding significance of the scene.

2. There is no cohesion (consistency) of the present work: the authors provided on-site test data, while the numerical analysis on the stress/strain distribution during the CO₂ transition. This does not validate the numerical model unless the authors must measure experimentally the stress/strain distribution in order to validate their numerical model.

We will try our best to validate the numerical modeling (COMSOL) results using the field data in the future research work, and thank you very much. In this paper, the numerical modeling was validated the parameters of the “Test scheme”, and the field-testing was focused on monitoring the gas extraction volume to validated the perfect of liquid CO₂ phase transition fracturing coal seam to enhance permeability.

3. Radius influence is not clearly stated or not suffeiecntly studied. The use of moving mesh - Lagrangian mesh - (deforming the domain during the CO₂ transition) would be a good start to analyse the  impact of different radii as well as CO₂ on permeabilty.

We have revised the figures in this paper. Thank you very much, and we will try our best in our future research work.

4. In Section 2.1, reference is required for Eq.1, also where is Darcy's law? ui should be a velocity not a displacement?

We have added the reference for Eq.1, and u_i is the displacement of the particle in the coordinate direction.

5. In Table 1, the authors need to show an equation where those parameters are applied?

The mechanical parameters in Table 1 are used to values the materials in numerical simulation tests.

6. In Fig.3, the caption is not informative and all boundary conditions should be illustrated through the model (e.g., stress, porous, solid, inlet, etc).

We have revised the information of the model as follows:

The model was constructed with the 81506 working face of Baode Coal Mine as the engineering background, and the model is shown in Figure 1. The size of the model was 150 m×100 m, and the fracturing boreholes were in the middle of the calculation model. To facilitate the calculation, the mesh size was differentiated [29-31]. The mesh size near the fracturing boreholes was small and dense, and the mesh size away from the fracturing boreholes was large and sparse [32–34]. The boundary of the model was fixed, and the diameter of the borehole was 113 mm. The boundary condition of the or-ifice was Darcy's law, and the pressure was 270 MPa to simulate the fracturing gas pressure generated during the phase change of liquid CO2. The upper boundary of the model was subject to a uniformly distributed original rock stress of 2.08 MPa.

7. In Fig.4(a), in the legend, what does the negative numbers tell us (blue)? need to say?

With the condition that the gas pressure of 270 MPa was applied at the orifice, fail = 0 meant it was in a critical state of destruction; fail < 0 meant that plastic deformation had occurred; and fail > 0 meant it was in a stable state.

As shown in Figure 3, the plastic damage of the broken circle (Region 1) was the most serious, the plastic damage of the fracture circle (Region 2) was smaller, and the plastic damage of the disturbance circle (Region 3) was the smallest, and it still had a promotion effect on the gas drainage. For the borehole to have a good gas drainage ef-fect, the borehole should be arranged within the range of the fissured circle. According to the numerical simulation, the influence radius of the fissured circle was 13.4 m.

8. In line 123, you need to identify for the reader what does it mean by "state of destruction"?

Increasing the pore pressure was equivalent to applying tensile stress to the coal matrix. Due to the tensile stress, cracks in the coal matrix began to develop and expand, and plastic deformation occurred in the coal mass. The state of destruction is that the coal mass produces plastic deformation with the action of tensile stress, cracks develop and expand, and permeability increases.

9. In line 124, do you mean (no plastic deformation) for stable state?

Yes, the table state was no plastic deformation.

10. In section 4.3, fracturing has already been studied, what is the new contribution of the present paper?

Through the research of this paper, the technology of liquid CO₂ phase transition fracturing coal seam to enhance permeability was applied in Baode Coal Mine successfully. The technology improves the effect of gas drainage, ensures the safe and efficient production of the coal mine, and has significant economic benefits.

11. In table 3, contrast boreholes in Fig.7, the authors only highlighted two, need to mention all?

We have revised the table 3 and table 4 as follows:

Table 3. Gas drainage volume of fracturing boreholes and contrast boreholes.

Location	Type	Gas drainage volume
		Maximum value	Mean value
Fracturing zone	Fracturing boreholes	0.0426	0.0276
#1 Contrast zone	#1 contrast boreholes	0.0052	0.0046
	#2 contrast boreholes	0.0129	0.0122
#2 Contrast zone	#3 contrast boreholes	0.0049	0.0049
	#4 contrast boreholes	0.0158	0.0148

Table 4. Gas drainage concentration of fracturing boreholes and contrast boreholes.

Location	Type	Gas drainage concentration/%
		Maximum value	Mean value
Fracturing zone	Fracturing boreholes	71	62.3
#1 Contrast zone	#1 contrast boreholes	11.2	9.9
	#2 contrast boreholes	56.4	55
#2 Contrast zone	#3 contrast boreholes	12.4	12
	#4 contrast boreholes	62.6	61.6

12. Please go through those articles for equations involved in the numerical model during such a transport phenomenon.

We have carefully studied these papers and benefited a lot. Thank you very much for your recommendation.

Fan C, Li S, Luo M, et al (2019). Numerical simulation of hydraulic fracturing in coal seam for enhancing underground gas drainage. ENERG EXPLOR EXPLOIT 37(1):166-193

Mohammadreza ZR, Mohammad S, Manouchehr H (2022). Stress and permeability modelling in depleted coal seams during CO2 storage. FUEL 325. https://doi.org/10.1016/j.fuel.2022.124958

Qu Q, Shi J, Wilkins A (2022). A Numerical Evaluation of Coal Seam Permeability Derived from Borehole Gas Flow Rate. ENERGIES. 15(10):3828. https://doi.org/10.3390/en15103828

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report (Previous Reviewer 3)

I am happy with the revised manuscript and author(s) responses. I recommend the manuscript for publication.

Author Response

 Thank you for your letter and for your comments concerning our manuscript
entitled “Research into the Mechanism and Application of Liquid CO2 Phase Transition Fracturing
in a Coal Seam to Enhance Permeability” (ID： 2078851). The comments are both valuable and
very helpful for revising and improving our paper.  

Reviewer 3 Report (New Reviewer)

1. the proposed numerical analysis needs to be validated with the experimental data, or any existing rigorous works in the literature.

2. more focus on the numerical part is required, since the the experimental part has already been discussed in the literature.

Author Response

 Dear Editors and Reviewer:
Thank you for your letter and for the reviewer’s comments concerning our manuscript
entitled “Research into the Mechanism and Application of Liquid CO2 Phase Transition Fracturing
in a Coal Seam to Enhance Permeability” (ID： 2078851). The comments are both valuable and
very helpful for revising and improving our paper. We have studied comments carefully and have
made correction which we hope meet with approval. The main corrections in the paper and the
responds to the reviewer’s comments are as flowing:
Responds to the reviewer’s comments:
1. the proposed numerical analysis needs to be validated with the experimental data, or any existing
rigorous works in the literature.
Response:
We have added the related content as follows:
2.2. Theoretical calculation
According to the theory of elastic mechanics and fracture mechanics [35], the fracturing
radius of the coal mass of the gas phase transition can be calculated by the following formula:
??
= ?ℎ × (????? ??? )
1 ?
(2)
where rc is the fracturing radius of the coal mass of the gas phase transition in m; rh is the
radius of the borehole in mm; Pmax is the pressure peak of the gas phase transition in MPa; Kt is the
improving coefficient of tensile strength with dynamic load; St is the tensile strength of coal mass
with static load in MPa; α is the attenuation index.
The static stress of the coal mass around the borehole can be calculated by the following
formula:
??
= ? × (???)-? (3)
where P is the gas pressure in the initial crack area in MPa; Kb is the bulk modulus of the coal
mass in MPa; σr is the static stress of the coal mass around the borehole, MPa; r is the distance
from the borehole, m.
When the value of σr is equal to the extreme tensile strength of the coal mass, the value of r is
the crack radius of the gas phase transition. According to the actual conditions of the Baode coal
mine, the field parameters were chosen as follows: rh=113mm, Pmax=270MPa, Kt=6.6, St=5MPa,
α=1.5, P=150MPa, Kb=10.5×103MPa.
According to the aforementioned theoretical calculation, when the σr=0.9MPa, which is equal
to the extreme tensile strength of the coal mass, the crack radius of the gas phase transition is
13.8m.
2. more focus on the numerical part is required, since the experimental part has already been
discussed in the literature.
Response:
We have revised the numerical part as follows:
According to the numerical model and the stress applied to the model, as shown in Figure
2(a), the plastic deformation characteristics of the coal mass around the bore-hole after phase
transition cracking were simulated. To analyze the damage area by high pressure gas on the coal
mass, as shown in Figure 2(b), a section was made in the middle of the coal mass.
(a) Plastic deformation area
(b) Sectional view of plastic deformation area
Figure 2. Plastic deformation.
Increasing the pore pressure was equivalent to applying tensile stress to the coal matrix. Due
to the tensile stress, cracks in the coal matrix began to develop and expand, and plastic
Section line
Plastic deformation zone
deformation occurred in the coal mass. With the condition that the gas pressure of 270 MPa was
applied at the orifice, fail = 0 meant it was in a critical state of destruction; fail < 0 meant that
plastic deformation had occurred; and fail > 0 meant it was in a stable state. The numerical
simulation results showed that the radius of plastic deformation of the coal mass was 23 m.
According to the numerical simulation, the plastic deformation region of the coal mass was
divided as follows: the area of fail < 1.0×-108 Pa was the broken circle, the radius r1 was 4.6 m;
the area of 1.0×-108 Pa < fail < 0.5×-108 Pa was the fissured circle, the radius was 4.6 m< r2
<13.4 m; and the area of 0.5×-108 Pa < fail < 0 was the disturbance circle, the radius was 13.4 m<
r3 < 23.1 m. The area was divided as shown in Figure 3.
Figure 3. Division of the plastic deformation zone.
As shown in Figure 3, the plastic damage of the broken circle (Region 1) was the most
serious, the plastic damage of the fracture circle (Region 2) was smaller, and the plastic damage of
the disturbance circle (Region 3) was the smallest, and it still had a promotion effect on the gas
drainage. For the borehole to have a good gas drainage effect, the borehole should be arranged
within the range of the fissured circle. According to the numerical simulation, the influence radius
of the fissured circle was 13.4 m, which was basically consistent with the results of the
aforementioned theoretical calculation.
3.1. Features of the volumetric strain
To improve the accuracy of the division of different regions, the volumetric strain was
obtained during the simulation process by COMSOL, as shown in Figure 4(a). Ac-cording to the
position of the section line in Figure 3, the volumetric strain contour in Figure 4(a) was crosssectioned, and the result is shown in Figure 4(b).
(a) The nephogram of the volumetric strain (b) The section value of the volumetric strain
Figure 4. The volumetric strain.
As shown in Figure 4, a gas pressure of 270 MPa was applied at the orifice, and the
volumetric strain of the coal mass showed that the closer the distance to the orifice, the larger the
volumetric strain. The distribution of the volumetric strain was consistent with the distribution of
the plastic deformation of the coal mass. The volumetric strain in the broken circle was the largest,
followed by the fissured circle, and the disturbance circle was the smallest. 

Author Response File: Author Response.pdf

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

In this paper, the coal permeability enhancement technology based on liquid carbon dioxide was studied. But this paper has not yet met the requirements for publication, and it has the following problems:

1. The English of this paper is poor and needs to be optimized.

2. The picture in this paper is not beautiful nor scientific enough.

3. At present, many scholars had studied the liquid carbon dioxide fracturing technology of coal, compared with them, there is no obvious innovation in this paper.

4. The section "1. Introduction" only briefly introduces the advantages and disadvantages of hydraulic punching technology, without introducing the research status of liquid carbon dioxide fracturing technology at all. It needs to be rewritten.

5. The experiment part of this paper is too simple and the simulation content has no depth.

6. The conclusions of this paper are not innovative.

 

Reviewer 2 Report

1. Introduction : you should put more details about your study and explain your objective clearly

2.  Keywords : should be keyword not the phase

3. Some equation, you don’t give explanation

4. your figure is not clear, for example figure 1,2 and 7

5. Your topic is very interesting but The content is not smooth. It hard to understand your work. 

 

Reviewer 3 Report

Study on the permeability improved technology of fracturing coal seam based on liquid carbon dioxide

The manuscript aims to understand the impact of using this carbon dioxide in enhancing permeability of deep coal seams using CO₂. The results of the manuscript are scientifically sound but have several major limitations. The major comments in the manuscript are as follows:

1)     English and Grammar usage in the manuscript is poor which made it hard to read. I would recommend getting the paper proofread by an English expert. I stopped correcting the grammar/English after the first few sections of the manuscript as there were too many errors.

2)     Different sections in the manuscript are not well organized. The manuscript should be divided into 4 major sections: 1) Introduction 2) Methods 3) Results and Discussion 4) Conclusions. There are several sections in the paper that should be combined in the introduction section. There is no methods section or results and discussion section in the current version of the manuscript. Please see my detailed comments in the pdf file on how to divide the information into 4 sections.

3)     There are several previous studies performed using liquid CO2 and understanding their effect on rock permeability. However, the manuscript fails to mention/acknowledge them. Please expand the introduction section to reflect the previous contributions.

4)     There is a disconnect between the numerical modeling section and the field-testing section. When I started reading the manuscript, my expectation was that the field testing would be used to validate the numerical modeling results. However, that was not the case. Is there a way to validate the numerical modeling (COMSOL) results using the field data?

5)     Several of the figures are missing axis labels, color coding labels, units etc. Most of them are of poor quality as well.  Further, the captions of each figure need to be expanded to explain the figures in much more detail.

 

Please find more detailed comments in the pdf file attached. 

Comments for author File: Comments.pdf