The Law of Acid Pressure Fracture Propagation in Maokou Formation Carbonate Reservoir in Central Sichuan

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This research article describes the techniques of acidizing in carbponates rocks. Despite the well-described methods to investigate and explore the law of acid pressure fracture growth in acid rock reservoirs under different acid systems, and carrying out four sets of actual triaxial physical simulation experiments of acid pressure, the fracture morphology, fracture growth law, pump pressure curve characteristics and acid rock reaction mechanism, I will recommend the following comments below: 

1. The results are not supported quantitatively in the abstract section. Some numerical results should be presented in the abstract and the conclusion sections. There are some mistakes in the English sentences. Therefore, the English of this paper should be improved. 
2. The introduction section does not answer why this article is significant, and the paper's novelty is hidden. The authors should follow the journal's instructions, as they have cited the introduction section wrongly. 
3. Lines 43 and 44 have wrong citations and should be fixed. Line 58, the m3, should have the three superscripted. Line no. 63: There should be a space between Mao and Formation.
4. Why is the title of Table 1 italic?
5. There should be no dot between figures and numbers (Figure.1.). All the figures should be fixed.
6. Figures 3 and 8 are not clear. 
7. There is no reasonable comparison between experimental and field application works. 
8. Colcusions is not supported qunatitaivley. 

Comments on the Quality of English Language

The English language of this paper should be improved.

Author Response

Please refer to the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Dear authors

Thank you very much for your submission entitled "The Law of Acid Pressure Fracture Propagation in Maokou Formation Carbonate Reservoir in Central Sichuan". Although it is a well-established paper, several parts need significant improvements. General comments are listed below:

1- The abstract section is too long to comprehend. Please prepare an abstract section, highlighting the most striking results obtained from your laboratory and field assessments.

2- At the end of the introduction section, you should clearly state your primary motivation and hypothesis of your manuscript. What is the main aim of this study? What is lacking in the existing literature on using acids on carbonate platforms?

3- Since you did not have a materials and methods section in the manuscript, the title of section 2 should be changed to Materials and Methods. You should prepare a flow chart indicating your sampling methodology. In this regard, Figure 1 should be improved. 

4- As far as I am concerned, the values listed in Table 2 don't represent your field conditions. Assuming that the density of dolomite is 2.8 g/cm3. For a depth of 6000 m, the primary vertical stress should be at least 6000 m  x 0.028 MN/m3 = 168 MPa. According to the fundamental rock mechanics theories, your corresponding horizontal stress should also be within 50 - 70 MPa.  In this regard, you should clearly state solid evidence why you selected 26 and 29 MPa for vertical and horizontal stresses, respectively. It is one of the most critical problems of your manuscript.

5- You should also present the initial physical and mechanical properties of the carbonate rocks. a) Density b) Porosity c) Tensile strength d) Uniaxial compressive strength e) Young modulus f) Poissons ratio. In addition to the intact rock properties, you should have theoretical knowledge about the rock masses at those depths. For example, the geological strength index (GSI) is vital to assess rock mass stability. 

6- In order to observe the effect of acids on stress-strain behaviour more clearly, you should present the stress-strain curves before and after acid treatment.  You should also calculate the crack initiation stress value as a function of the uniaxial compressive strength. What should your acid concentration be, depending on the crack initiation stress?

7- Figures 5, 6,9, and 10 need appropriate scale bars.

8- You should discuss your laboratory results and field applications together. They should complement each other. 

9- In your conclusion section, you should clearly state

a) The effects of acid concentration on the crack initiation and propagation of the carbonate rocks.

b) Although acids dissolve carbonate rocks, please indicate how they also cause cracks in rock materials and rock masses. 

c) The relationship between pump pressure, acid concentration, and crack initiation of carbonate rocks. 

d) The effects of rock porosity and dissolution rates of carbonate rocks.

The points mentioned above undoubtedly improve the quality of your work. I guess the comments raised would be beneficial when revising your manuscript.

Kind regards

 

 

Author Response

Please refer to the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This report presents acid fracturing experiments conducted on four groups of carbonate rocks at varying depths to investigate the morphology of acid-induced fractures and the acid-rock interactions under different acid systems.  The findings offer a novel approach to diminish cracking pressure and enhance acid fracture complexity in carbonate reservoirs, while also establishing a basis for future studies on acid pressure in these reservoirs.

The aim of this study is surely within the SI of Processes: Environmentally Friendly Production of Energy from Natural Gas Hydrates. I appreciate the low similarity level (only 17%), nicely prepared figures and illustrations. The style is also clear and concise. However, there are some aspects that should be revised and clarified.

Major comments:

Lines 36-37, what are their typical composition?

Line 40, please provide the values of T and p (range).

Line 118, the PXRD results must be described in more detailed way, starting from the description of the diffractometer used, analysis conditions, presenting the results (patterns), etc.

Line 135, what kind of test? How this QPA has been performed?

Lines 138-155, please provide the producers of those chemicals.

Line 150, was it Fe(II) or Fe(III)?

Line 194, have you obtained the permission to reuse this figure? If yes, it should be clearly stated in its caption.

Table 2, “Rock sample type” column is not needed as this information doesn’t change; the same for “output volume”, those kind of repetitive information should be included in the figure’s caption

Figures 5 and 6, please add a scale for comparison

Line 252, please provide equations for those reaction

Figure 8b, why there are two overlapping peaks?

Figure 11, this figure must be improved, there are some Chinese (?) letters (?) or symbols in this figure and I don’t understand them

Line 390, what is the location of this well?

 

 

 

Minor issues:

Line 58, it should be “10¹²m³”

Table 2, it should be “stress (MPa)”

Lines 313, 314, it should be Fe³⁺

Author Response

Please refer to the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The revised manuscript does not fully address some of the comments. The authors must ensure that all reviewer comments are carefully and thoroughly addressed.
For each comment, a detailed and point-by-point response is expected. Authors should provide specific explanations and, where applicable, indicate the exact changes made in the manuscript.

Author Response

Please see the attachment. The parts that have been changed and added have been marked in red in the article for your review.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Dear authors

Most of my previous comments (e.g., comment 1, comment 4, comment 5, comment 6) have not been addressed properly. 

Kindly remember my previous comments as follows:

Comment 1: The abstract section is too long to comprehend. Please prepare an abstract
section, highlighting the most striking results obtained from your laboratory and field
assessments

Comment 4: As far as I am concerned, the values listed in Table 2 don't represent your field
conditions. Assuming that the density of dolomite is 2.8 g/cm3. For a depth of 6000 m, the primary vertical stress should be at least 6000 m x 0.028 MN/m3 = 168 MPa. According to the fundamental rock mechanics theories, your corresponding horizontal stress should also be within 50 - 70 MPa. In this regard, you should clearly state solid evidence why you selected 26 and 29 MPa for vertical and horizontal stresses, respectively. It is one of the most critical problems of your manuscript.

Comment 5: You should also present the initial physical and mechanical properties of the carbonate rocks. a) Density b) Porosity c) Tensile strength d) Uniaxial compressive strength e) Young modulus f) Poissons ratio. In addition to the intact rock properties, you should have theoretical knowledge about the rock masses at those depths. For example, the geological strength index (GSI) is vital to assess rock mass stability.

Comment 6: In order to observe the effect of acids on stress-strain behaviour more clearly, you should present the stress-strain curves before and after acid treatment. You should also calculate the crack initiation stress value as a function of the uniaxial compressive strength. What should your acid concentration be, depending on the crack initiation stress? 

These comments have not been addressed in your revised document (processes-3593279-peer-review-v2.pdf). Therefore, from my perspective, the manuscript can not be accepted for publication in Processes.

Kind regards

Author Response

Please see the attachment. In response to your uncorrected question，the parts that have been changed and added have been marked in red in the article for your review. 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The Authors have revised their works and answered my comments. However, in the future, please indicate the changes done in the manuscript using different color of the font or "track changes" mode.

Author Response

感谢您的建议，我会在文章中标记修改后的部分。

Round 3

Reviewer 1 Report

Comments and Suggestions for Authors

Despite improving the quality of the manuscript, the figures are not clear and are not suitable for publication in their current form.

Author Response

Thank you for pointing out the problem of figures clarity in the manuscript. All figures in the manuscript have been adjusted to maximum clarity. Please review the latest version of the manuscript later. You are welcome to criticize and correct.

Reviewer 2 Report

Comments and Suggestions for Authors

Dear authors

Thank you very much for the revisions. The revised manuscript can be accepted for publication in Processes.

Author Response

Thank you very much for your valuable feedback, I will continue to revise my manuscript.

Round 4

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript has been improved. However, the authors must ensure all comments are carefully and thoroughly addressed. The abstract and conclusion should include the qualitative and quantitative results. In addition, Figure 13 is unclear, and its quality should be improved.

Author Response

Comments :

The results are not supported quantitatively in the abstract section. Some numerical results should be presented in the abstract and the conclusion sections.

Response : Thank you very much for your valuable comments. Regarding the problem of numerical results, the summary has been corrected again, please check it:

Dolomite reservoir of Maokou Formation is rich in gas resources in central Sichuan Basin. Acid fracturing is an important technical means to improve reservoir permeability and productivity. The interaction mode of dolomite and limestone, acid system will affect the effect of reservoir reconstruction. In order to clarify the influence of complex structure on fracture morphology, explore the fracturing effect of different acid systems. Physical simulation experiments of true triaxial acid fracturing were carried out with two acid systems and downhole full-diameter cores. The experimental results show: (1) After the carbonate rock is subjected to acid fracturing using a "self-generated acid + gel acid" system, the fracture pressure drops significantly by up to 60%. The morphology of the acid-eroded fractures becomes more complex, with an increase in geometric complexity of about 28% compared to a single acid solution system. It is prone to form three-dimensional "spoon" shaped fractures, and the surface of the acid-eroded fractures shows light yellow acid erosion marks. Analysis of the acid erosion marks indicates that the erosion depth on the fracture surface reaches 0.8-1.2mm, which is deeper than the 0.2mm erosion depth achieved with a single system. (2) Acid solution is difficult to penetrate randomly distributed calcite veins with low porosity and permeability structure. When the fracture meets the calcite vein, the penetration rate of acid solution drops sharply to 15%-20% of the initial value, resulting in a reduction of about 62% of the acid erosion area in the limestone section behind. And the acid erosion traces in the limestone behind the calcite vein are significantly reduced. The acid erosion cracks are easy to open the weak surface between dolomite and limestone, inducing the fracture to turn. (3) The results of field engineering and experiment are consistent, injecting authigenic acid first in the process of reservoir reconstruction is helpful to remove pollution. The recovery rate of near-well permeability is more than 85% with pre-generated acid. Reinjection of gelled acid can effectively communicate the natural weak surface and increase the complexity of cracks. The average daily oil production of the completed well was increased from 7.8m³ to 22.5m³, and the increase factor reached 2.88.

In addition, the clarity of figure 13 has been improved again, please check the corresponding position in the revised manuscript.