Influence of principal stress effect on deformation and permeability of coal containing beddings under true triaxial stress conditions

In situ stress is generally an anisotropic/true triaxial stress (σ1 > σ2 > σ3). Bedding weakens the continuity and integrity of coal. It is critical to understand the mechanical behaviour and gas migration of coal under true triaxial stress conditions. We performed experiments of cubic coal samples to investigate the permeability evolution and mechanical behaviour of coal under true triaxial stress conditions by using newly developed true triaxial geophysical apparatus. We analysed the effect of principal stresses on deformation and permeability characteristics of coal containing bedding planes. The results show that volumetric strain, stress states and bedding directions determine the permeability comprehensively. The variable quantity of strain was the largest in the direction normal to the bedding plane. The expansion or compression degree was characterized by the difference between the major and minor principal strain (ɛ1 − ɛ3). Essentially, this represents the difficulty degree with regard to coal being compressed at the initial stress state and the deformation degree in ɛ1 and ɛ3 direction. The variation of (ɛ1 − ɛ3) was consistent with that of permeability. Under an identical true triaxial stress condition, permeability was smaller when larger stress was applied in the direction normal to the bedding plane. Additionally, stress level in the direction parallel to the bedding planes and the directions between stresses in the direction parallel to the bedding planes and the flow direction also affect the permeability and strain. By solving lateral expansion coefficient, coal also exhibited anisotropic properties.

potential readers. Specific comments for revision are as follows, (1) In abstract, the language is confused and the logic is not clear, so it needs to be completely revised. (2) Why not consider the effective stress effect? (3) How did you make sure that flow regime was dominated by Darcy flow during experiment? (4) For the rigid loading, the end effect inevitably exist, which can affect the deformation measurement results of the specimen. What methods did you use to reduce the end friction effect? (5) How did you measure sample deformation in each direction, especially for volumetric strain? (6) Please provide in-situ stress conditions in the operation field where you took this specimen from? (7) Explain why the gas pressure in this paper is 3 MPa? (8) I recommended the author delete the figure about (ε1-ε2) and (ε2-ε3) in Fig. 12. (9) Please indicate the beddings in the coal specimen.

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Recommendation?
Major revision is needed (please make suggestions in comments)

Comments to the Author(s)
It is very confused regarding the primary objective of the study. Do the authors wanna figure out the role of bedding planes in fluid flow in coal reservoirs? The main problem with the study is that it lacks novelty. Some detailed comments are as follows: 1. Line 39: this is a completely incorrect statement. Lots of studies have proved that bedding planes of coal have little influence on coal permeability. 2. In Introduction, more than 90% of the content is about sandstone and shale. The goal of the study is to investigate the mechanical behavior and coal permeability, so the authors should pay their attention to coal. Considering the significant difference in reservoir structure between coal and other reservoir rocks, the studies on other rocks and the corresponding findings cannot be simply embezzled to coal. 3. How many specimens were prepared and tested? 4. Section 2.3, the experimental process is far from clear. What is the purpose for each step? More importantly, what kind of gas was used for testing? Different conclusions can be drawn when using different test gases. 5. Line 159-160: it is impossible to get the conclusion from Fig. 5. 6. In Conclusion: "the strain variation was largest, regardless of loading or unloading in the direction vertical to bedding". This conclusion is incorrect when it comes to coal if using nonsorptive gas as the test gas. 7. No new findings are reported in this study.
Decision letter (RSOS-181438.R0) 20-Nov-2018 Dear Dr Liu, The editors assigned to your paper ("Influence of Principal Stress Effect on Deformation and Permeability of Coal Containing beddings under True Triaxial Stress Conditions") have now received comments from reviewers. We would like you to revise your paper in accordance with the referee and Associate Editor suggestions which can be found below (not including confidential reports to the Editor). Please note this decision does not guarantee eventual acceptance.
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Comments to Author:
Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) 1. Why has the manuscript been uploaded twice? 2. Throughout the manuscript, no detailed information was provided regarding the experimental of strains and the permeability. Simply, they were mentioned rather than any proper information. Authors are advised to add detailed information. 3. No chemical analysis of the studied coal was included in the manuscript. 4. For the measurement of permeability, CO2 was utilized. Authors might be aware of the swelling nature of coal during the interaction with the carbon dioxide. So, while measuring the permeability was the swelling factor was considered. 5. What can be the possible cause for the increase in the permeability of the coal after a certain value of the intermediate stress? 6. Can the PR of the coal be up to 0.5? Please verify with the calculations.

Reviewer: 2
Comments to the Author(s) The manuscript presented an interesting investigation on permeability and deformation properties of coals containing bedding planes under true triaxial stress conditions. The manuscript seems organized well. The approach taken and result are interesting and useful for potential readers. Specific comments for revision are as follows, (1) In abstract, the language is confused and the logic is not clear, so it needs to be completely revised.
(2) Why not consider the effective stress effect? (3) How did you make sure that flow regime was dominated by Darcy flow during experiment? (4) For the rigid loading, the end effect inevitably exist, which can affect the deformation measurement results of the specimen. What methods did you use to reduce the end friction effect? (5) How did you measure sample deformation in each direction, especially for volumetric strain? (6) Please provide in-situ stress conditions in the operation field where you took this specimen from? (7) Explain why the gas pressure in this paper is 3 MPa? (8) I recommended the author delete the figure about (ε1-ε2) and (ε2-ε3) in Fig. 12. (9) Please indicate the beddings in the coal specimen.
Reviewer: 3 Comments to the Author(s) It is very confused regarding the primary objective of the study. Do the authors wanna figure out the role of bedding planes in fluid flow in coal reservoirs? The main problem with the study is that it lacks novelty. Some detailed comments are as follows: 1. Line 39: this is a completely incorrect statement. Lots of studies have proved that bedding planes of coal have little influence on coal permeability. 2. In Introduction, more than 90% of the content is about sandstone and shale. The goal of the study is to investigate the mechanical behavior and coal permeability, so the authors should pay their attention to coal. Considering the significant difference in reservoir structure between coal and other reservoir rocks, the studies on other rocks and the corresponding findings cannot be simply embezzled to coal. 3. How many specimens were prepared and tested? 4. Section 2.3, the experimental process is far from clear. What is the purpose for each step? More importantly, what kind of gas was used for testing? Different conclusions can be drawn when using different test gases. 5. Line 159-160: it is impossible to get the conclusion from Fig. 5. 6. In Conclusion: "the strain variation was largest, regardless of loading or unloading in the direction vertical to bedding". This conclusion is incorrect when it comes to coal if using nonsorptive gas as the test gas. 7. No new findings are reported in this study.

Comments to the Author(s)
Formatting error: page [190][191][192][193][194] In all the figures, some more information can be added to the caption mentioning what does in illustrates.

Comments to the Author(s)
The responds of authors are reasonable and the revised version of the paper can be also acceptable.

Recommendation?
Accept as is

Comments to the Author(s)
The authors have addressed all my concerns, and the manuscript is currently suggested for publication.

15-Jan-2019
Dear Dr Liu: On behalf of the Editors, I am pleased to inform you that your Manuscript RSOS-181438.R1 entitled "Influence of Principal Stress Effect on Deformation and Permeability of Coal Containing beddings under True Triaxial Stress Conditions" has been accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referee suggestions. Please find the referees' comments at the end of this email.
The reviewers and Subject Editor have recommended publication, but also suggest some minor revisions to your manuscript. Therefore, I invite you to respond to the comments and revise your manuscript.
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• Data accessibility It is a condition of publication that all supporting data are made available either as supplementary information or preferably in a suitable permanent repository. The data accessibility section should state where the article's supporting data can be accessed. This section should also include details, where possible of where to access other relevant research materials such as statistical tools, protocols, software etc can be accessed. If the data has been deposited in an external repository this section should list the database, accession number and link to the DOI for all data from the article that has been made publicly available. Data sets that have been deposited in an external repository and have a DOI should also be appropriately cited in the manuscript and included in the reference list.
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All contributors who do not meet all of these criteria should be included in the acknowledgements.
We suggest the following format: AB carried out the molecular lab work, participated in data analysis, carried out sequence alignments, participated in the design of the study and drafted the manuscript; CD carried out the statistical analyses; EF collected field data; GH conceived of the study, designed the study, coordinated the study and helped draft the manuscript. All authors gave final approval for publication.
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Point-by-point Responses to the reviewers' comments
Reviewer #1: Comment 1: Why has the manuscript been uploaded twice? Response: Thanks for the comment. We are very sorry for it. Since we are the first to the Journal: Royal Society Open Science, we are not familiar with the submission system. It's also related to our carelessness. If we have the opportunity to submit new manuscripts to the Appendix A journal, we will strictly comply with the requirements of the submission system and avoid such mistakes.

Comment 2:
Throughout the manuscript, no detailed information was provided regarding the experimental of strains and the permeability. Simply, they were mentioned rather than any proper information. Authors are advised to add detailed information. Response: Thanks for the comment. The coals extracted from the well were processed to get the cube specimens, and then we started the experiment. The experimental steps are as follows: (1) Place the specimen on the loading plate, and then put on a heat-shrinkable tube, which should cover the sealing washer of the loading plate (Fig. R1). The heat-shrinkable tube is tightly attached to the surface of the specimen with a hot air gun. Then the heatshrinkable tube is tightly fastened at the sealing washer to prevent air leakage during the experiment.
(2) The specimen is fed into the loading cell, and the LVDT is installed and connected.
The force of the true triaxial control system is cleared and the loading plate is moved slowly towards the specimen by displacement control. Meanwhile, the indicator changes of the LVDT displayed on the system are observed. When the system shows that the force of the six loading plates is 0. 5 kN to 2. 5 kN (indicating that the loading plate has been in contact with the specimen) and the LVDT is within -1 mm ~ +1mm (the range of LVDT is -5 mm ~ +5mm), the installation of the LVDT is feasible. Otherwise, the LVDT will be readjusted (Measurements of the specimen's deformation are made independently along each principal stress axis using the linear variable displacement transducers as shown in Fig. R2).
(3) Seal the loading cell and fill it with hydraulic oil. When it is full, close the intake valve. The hydraulic oil was pressurized to 5 MPa by the servo valve (according to the principle of force and reaction force, the stress designed by the experiment is correspondingly reduced by 5 MPa) and the oil pressure should be greater than the gas pressure in the experiment to prevent the heat-shrinkable tube from bursting. Then enter the gas and adjust the pressure valve to 3 MPa.   In the experiment, what we are most concerned about is the change of LVDT. The strain can be obtained by the displacement data recorded by the data acquisition system, which is the basis of the mechanical analysis.
We applied steady state method to measure permeability. For steady state gas flow measurements, the inlet pressure was kept constant while the gas on the outlet side was at atmospheric pressure (Pout=0.1 MPa). The inlet and outlet pressure data were recorded in short intervals (1.2 s for the control and data acquisition system of the multifunctional true triaxial geophysical apparatus) using the pressure transducers. The gas flow rate at the outlet side was monitored using a mass flow meter with the precision of 1mL/min. To start a steady state measurement, the inlet pressure was adjusted to a desired pressure (it is 3 MPa in the paper) using the pressure regulator connected to the gas cylinder. Once the steady state flow conditions were reached, the flow rates were recorded and a higher stress impact on the potential for fracture shear failure, the evolution of stress-dependent aperture and the corresponding evolution of permeability [1]. Therefore, in the paper, the purpose of the research is to investigate the deformation mechanism of the coal and gas migration in various directions under the effect of bedding. For studied coal, we only conducted proximate analysis as shown in Table R1.
The Poisson's ratio in the full text is changed to the lateral expansion coefficient.

Reviewer #2:
Comment 1: In abstract, the language is confused and the logic is not clear, so it needs to be completely revised.

Response:
Thanks for the comment. According to the reviewer's suggestion, the abstract is modified as follows: In-situ stress is generally an anisotropic/true triaxial stress (σ1﹥σ2﹥ Essentially, this represents the difficulty degree with regard to coal being compressed at the initial stress state, and the deformation degree in ε1 and ε3 direction. The variation of (ε1-ε3) was consistent to that of permeability. Under an identical true triaxial stress condition, permeability was smaller when larger stress was applied in the direction normal to the bedding plane. Additionally, stress level in the direction parallel to the bedding planes and the directions between stresses in the direction paralleling to the bedding planes and the flow direction also affect the permeability and strain. By solving lateral expansion coefficient, coal also exhibited anisotropic properties.

Comment 2:
Why not consider the effective stress effect? Response: Thank you for this comment. In our experiments, the gas pressure was constant (3 MPa). During the whole permeability measurements, the inlet pressure kept at 3 MPa and the outlet pressure kept at 0.1 MPa. As the gas pressure kept constant, the variations of total stress and effective stress were the same. The overall trends of the curves plotting in terms of total stress and effective stress are the same. Therefore, we used total stress in the manuscript.

Comment 3:
How did you make sure that flow regime was dominated by Darcy flow during experiment? Response: Thanks for the comment. Fracture permeability is generally governed by Darcy's law [1,2,3]. Zhang and Wang also believed that the Darcy flow works within macropores (pore width＞50 nm) and fractures [4]. Wu et al. concluded that the flow rate, q, of 8.33×10-7 m 3 /s is sufficiently low to achieve the steady-state flow [5]. The magnitude of flow rate under true triaxial conditions we measured is also between 10 -6 and 10 -7 , which indicates that the gas flow through the coal body is in steady-state. The Darcy's flow equation has been widely used in the field to estimate fluid permeability through porous media under steady-state flow conditions [6].
In addition, we have measured the coal flow rate, q, under different stresses and gas pressures. As shown in Fig. R3, the flow rate and gas pressure are linearly fitted with a correlation coefficient, R 2 of 0.9981 and 0.9987. It also proves that we use the rationality of Darcy's law equation. Based on the rigid loading mode, this situation is inevitable, and we call it the end friction effect [7]. Lubrication is an effective method to decrease the friction between metal loadingplatens and specimen faces by applying anti-friction agent. In this paper, the first step is to wrap the coal specimen with a heat-shrinkable tube, and then use a heat air gun to tighten the heat-shrinkable tube to the coal specimen. Finally, daub lithium grease evenly on four horizontal loading platens. Due to ventilation for permeability experiment, no lubricant is applied to the vertical surface.
Reference: Response: Thanks for the comment. The coals extracted from the well were processed to get the cube specimens, and then we started the experiment. The experimental steps are as follows: (1) Place the specimen on the loading plate, and then put on a heat-shrinkable tube, which should cover the sealing washer of the loading plate (Fig. R1). The heat-shrinkable tube is tightly attached to the surface of the specimen with a hot air gun. Then the heatshrinkable tube is tightly fastened at the sealing washer to prevent air leakage during the experiment.
(2) The specimen is fed into the loading cell, and the LVDT is installed and connected.
The force of the true triaxial control system is cleared and the loading plate is moved slowly towards the specimen by displacement control. Meanwhile, the indicator changes of the LVDT displayed on the system are observed. When the system shows that the force of the six loading plates is 0. 5 kN to 2. 5 kN (indicating that the loading plate has been in contact with the specimen) and the LVDT is within -1 mm ~ +1mm (the range of LVDT is -5 mm ~ +5mm), the installation of the LVDT is feasible. Otherwise, the LVDT will be readjusted (Measurements of the specimen's deformation are made independently along each principal stress axis using the linear variable displacement transducers as shown in Thanks for the comment. The principal stress of the fully mechanized working face  Response: Thanks for the comment. The bedding in the coal specimen used in the experiment is shown Fig. R5. Thanks for the comment. Coal cleats are of two types: face cleats and butt cleat, which are often normal to the bedding plane and may be perpendicular to each other [1,2,3].
Fordsham and Gayer concluded that bedding plane slipping fractures are also tectonically induced and are widespread in coal seams, commonly associated with cleat [4]. It is well known that the coal permeability decrease significantly with cleat closure [5]. They pointed out that the bedding and joint structure had an important influence on the permeability and deformation [7,9]. Pan et al. took coal with different bedding directions as the research object, and studied the law of the permeability evolution during the loading process. It was concluded that drilling across bedding planes for gas extraction achieved better effects, at last reduce gas release and gas disasters [10]. Liang et al. conducted uniaxial compression tests on salt rock and coal specimens to obtain relevant basic mechanical parameters and strength characteristics of salt rock and coal in different bedding directions [11]. Therefore, the understanding of the contribution of bedding on seepage and deformation properties of coals containing beddings is both important and necessary.

Comment 2:
In Introduction, more than 90% of the content is about sandstone and shale. The goal of the study is to investigate the mechanical behavior and coal permeability, so the authors should pay their attention to coal. Considering the significant difference in reservoir structure between coal and other reservoir rocks, the studies on other rocks and the corresponding findings cannot be simply embezzled to coal.

Response:
Thanks for the comment. We have added some literatures about the influence of bedding in coals on deformation and permeability evolutions, and deleted some literatures which are not relevant to the subject. The modified introduction are written in revised manuscript by red words. Thanks for the comment. The experimental process was as following The coals extracted from the well were processed to get the cube specimens, and then we started the experiment. The experimental steps are as follows: (1) Place the specimen on the loading plate, and then put on a heat-shrinkable tube, which should cover the sealing washer of the loading plate (Fig. R1). The heat-shrinkable tube is tightly attached to the surface of the specimen with a hot air gun. Then the heatshrinkable tube is tightly fastened at the sealing washer to prevent air leakage during the experiment.
(2) The specimen is fed into the loading cell, and the LVDT is installed and connected.
The force of the true triaxial control system is cleared and the loading plate is moved slowly towards the specimen by displacement control. Meanwhile, the indicator changes of the LVDT displayed on the system are observed. When the system shows that the force of the six loading plates is 0. 5 kN to 2. 5 kN (indicating that the loading plate has been in contact with the specimen) and the LVDT is within -1 mm ~ +1mm (the range of LVDT is -5 mm ~ +5mm), the installation of the LVDT is feasible. Otherwise, the LVDT will be readjusted (Measurements of the specimen's deformation are made independently along each principal stress axis using the linear variable displacement transducers as shown in (3) Seal the loading cell and fill it with hydraulic oil. When it is full, close the intake valve. The hydraulic oil was pressurized to 5 MPa by the servo valve (according to the principle of force and reaction force, the stress designed by the experiment is correspondingly reduced by 5 MPa) and the oil pressure should be greater than the gas pressure in the experiment to prevent the heat-shrinkable tube from bursting. Then enter the gas and adjust the pressure valve to 3 MPa.

Response:
Thanks for the comment. According to the counting rule of the multi-functional true triaxial geophysical apparatus system, the strain increases when the specimen is compressed in one direction, and decreases when it expands. Comment 6: In Conclusion: "the strain variation was largest, regardless of loading or unloading in the direction vertical to bedding". This conclusion is incorrect when it comes to coal if using non-sorptive gas as the test gas.

Response:
Thanks for the comment. We have revised this conclusion to 'The conclusion 'the strain variation was largest of coal saturating CO2 under applied stress and keeping gas pressure unchanged , regardless of loading or unloading in the direction vertical to bedding'.

Comment 7:
No new findings are reported in this study.

Response:
Thanks for the comment. Coal is a kind of sedimentary rock with obvious beddings [13], especially for normal coals [14]. The existence of beddings weaken the continuity and integrity of coal and makes it heterogeneity [15,16,17]. With respect to underground reservoirs, the in situ stress is generally anisotropic (σ1＞σ2＞σ3), and the hydrostatic stress and triaxial stress (i.e. varying confining stress) do not replicate the true in situ stress [18,19,20,21]. Therefore, we performed experiments of cubic coal samples to investigate the permeability evolution and mechanical behavior of coal under true triaxial stress conditions by using newly developed true triaxial geophysical (TTG) apparatus.
The results show that volumetric strain, stress states and bedding directions determine the permeability comprehensively. The variable quantity of strain was the largest in the direction normal to the bedding plane. The expansion or compression degree were characterized by the difference between the major and minor principal strain (ε1-ε3).
Essentially, this represents the difficulty degree with regard to coal being compressed at the initial stress state and the deformation degree in ε1 and ε3 direction. Under an identical true triaxial stress condition, permeability was smaller when larger stress was applied in the direction normal to the bedding plane. Additionally, stress level in the direction parallel to the bedding planes and the directions between stresses in the direction paralleling to the bedding planes and the flow direction also affect the permeability and strain.
This study can more realistically reflect the permeability and deformation evolutions of coals containing bedding under true triaxial conditions.