The Mechanism of Casing Deformation before Hydraulic Fracturing and Mitigation Measures in Shale Gas Horizontal Wells

Round 1

Reviewer 1 Report

The paper addresses the major issue of casing buckling (CB) of the horizontal wells drilled to extract shale gas. The damaged casing could lower the efficiency of hydraulic fracturing or lead to the complete loss of the well. The paper approaches the issue through evaluation of the operational and geological factors, including both fault/fractures and horizontal slip, contributing to the CB. The analysis performed by this study shows that the role of operation factors seems to be minimal compared to the geological factors. The modeling performed by numerical analysis confirms the importance of geological factors. The conclusion of this paper accurately summarizes the results of the study. The study could definitely contribute to the science of the casing buckling after the following issues are resolved:

-        - The paper does not analyze the impact of hydraulic fracturing in the casing buckling. Therefore, it is basically a study on the casing buckling in horizontal wells. The introduction should be revised to reflect that.

-        - The finite element analysis demonstrates the importance of geological factors. However, no details of the numerical analysis and model set up, including the horizontal and vertical stresses, pore pressure, rock elastic properties and other geomechanical parameters are provided. A full section needs to be added for this missing information.

-        - The source of fracture and fault processing is not fully disclosed. It is suggested to explain the fault and fracture processing workflow to make sure that no fracture/fault is missed during analysis.

-       - The paper requires editing to improve readability before publication.

Author Response

• Comment: The paper addresses the major issue of casing buckling (CB) of the horizontal wells drilled to extract shale gas. The damaged casing could lower the efficiency of hydraulic fracturing or lead to the complete loss of the well. The paper approaches the issue through evaluation of the operational and geological factors, including both fault/fractures and horizontal slip, contributing to the CB. The analysis performed by this study shows that the role of operation factors seems to be minimal compared to the geological factors. The modeling performed by numerical analysis confirms the importance of geological factors. The conclusion of this paper accurately summarizes the results of the study. The study could definitely contribute to the science of the casing buckling after the following issues are resolved:

Response: Thanks for your high evaluation for our paper and recommendations for publication. We also appreciate your positive and constructive comments regarding our manuscript, and we have made careful modifications on the original manuscript based on major assessment with results from other authors.

• Comment: The paper does not analyze the impact of hydraulic fracturing in the casing buckling. Therefore, it is basically a study on the casing buckling in horizontal wells. The introduction should be revised to reflect that.

Response: The advice suggested by reviewer is very accurate and reasonable. Therefore, The abstract and introduction has been rearranged and shortened (see the revised paper).

• Comment: The finite element analysis demonstrates the importance of geological factors. However, no details of the numerical analysis and model set up, including the horizontal and vertical stresses, pore pressure, rock elastic properties and other geomechanical parameters are provided. A full section needs to be added for this missing information.

Response: The advice suggested by reviewer is very accurate and reasonable. Therefore, we have reorganized, and the content including rock elastic properties, pore pressure and geostress etc are added in the paper (see the paper).

• Comment: The source of fracture and fault processing is not fully disclosed. It is suggested to explain the fault and fracture processing workflow to make sure that no fracture/fault is missed during analysis.

Response: Thanks for reviewer’s reasonable advice. Therefore, we have described how fracture/ fault are tested in engineering and how they form naturally in revised paper.

• Comment: The paper requires editing to improve readability before publication.

Response: The advice suggested by reviewer is very accurate and reasonable. Based on these comments and suggestions, we have made careful modifications on the original manuscript and employed an English-language editing service to help polish our wording. All changes made to the text are in track type.

Special thanks to you for your good comments.

Reviewer 2 Report

This work should be of interest to researchers and practitioners alike. My few recommendations are as follows:

1. There are significant language issues in some sections of the manuscript. These include grammatical errors and spelling issues. Since I cannot point out all of them, it is best that the authors re-process the manuscript using professional English language editing service.

2. Provide some additional background around fracture/ fault map (Fig. 3) and how it was generated. Uncertainties, etc.

3. Fig. 4, it is not clear what the black curves labeled wellbore mean? since the x-axis range is ~ +-4 mm. Please clarify or modify the figure.

4. How exactly the downhole deformations were measured needs to be clarified in the discussion (i.e., what logging tools/ interpretation methods were used?).

5. Fig. 8 add axis values. Depth or relative depths are of interest to understand how significant the out of zone drilling is as an issue.

6. Since depths are used throughout the discussion, figures should also highlight relevant depths.

Author Response

• Comment: There are significant language issues in some sections of the manuscript. These include grammatical errors and spelling issues. Since I cannot point out all of them, it is best that the authors re-process the manuscript using professional English language editing service.

Response: The advice suggested by reviewer is very accurate and reasonable. Based on these comments and suggestions, we have made careful modifications on the original manuscript and employed an English-language editing service to help polish our wording. All changes made to the text are in track type.

• Comment: Provide some additional background around fracture/ fault map (Fig. 3) and how it was generated. Uncertainties, etc.

Response: Thanks for reviewer’s reasonable advice. Therefore, we have described how fracture/ fault are tested in engineering and how they form naturally in revised paper.

• Comment: 4, it is not clear what the black curves labeled wellbore mean? since the x-axis range is ~ +-4 mm. Please clarify or modify the figure.

Response: Thanks for reviewer’s reasonable advice. As reviewer suggested, we have revised the describe of Fig. 4 and modified the Fig. 4 in the paper (see the revised paper).

• Comment: How exactly the downhole deformations were measured needs to be clarified in the discussion (i.e., what logging tools/ interpretation methods were used?).

Response: We are very sorry for our negligence of the problem. The advice suggested by reviewer is very accurate and reasonable, therefore, we have revised related contents and added the introduction of the logging tools in the Section 4.2(see the revised paper).

• Comment: 8 add axis values. Depth or relative depths are of interest to understand how significant the out of zone drilling is as an issue. Since depths are used throughout the discussion, figures should also highlight relevant depths.

Response: Thanks for reviewer’s reasonable advice, we have revised the describe in the paper and add axis values in Fig.8. And we have revised the figures associated with depths in the paper.

Special thanks to you for your good comments.

Reviewer 3 Report

The manuscript details in-field results and numerical study of casing buckling in horizontal shale gas wells. This reviewer finds this work of interest for researchers and practitioners dealing with avoinding this common type of failures in the nonconventional hydrocarbon upstream industry.

The article is overall well organized and written. Some grammar and style errors, and choice of words although not difficulting understanding, should be corrected.  For instance, introductory words like Moreover and Eventually in the abstract, foue (four) in caption fig. 4, and use of articles and plural-singular throughout the paper.

Title: clearly defines purpose

Abstract: it correctly defines purpose, and discusses procedure and main findings. However, in order to capture readers’ attention, it could be shortened and focused on what is of interest to practitioners in the industry.

Keywords: clear and complete

Introduction: complete, both generally in the industry and the case study.

Experimental: geological and numerical work is sound

Discussion: focused on the particular case under study. This reviewer suspects there might be broader discussions that could be of use for a larger audience.

Conclusions: for its most part is the conclusion of a technical report for the company operating that particular oil field.

 References are sufficient, well suited and up to date, note however that Chinese authors represent more than half of the references, in a field originally led by other countries.

Author Response

• Comment: The manuscript details in-field results and numerical study of casing buckling in horizontal shale gas wells. This reviewer finds this work of interest for researchers and practitioners dealing with avoinding this common type of failures in the nonconventional hydrocarbon upstream industry. The article is overall well organized and written.

Response: Thanks for your high evaluation for our paper and recommendations for publication. We also appreciate your positive and constructive comments regarding our manuscript, and we have made careful modifications on the original manuscript based on major assessment with results from other authors.

• Comment: Some grammar and style errors, and choice of words although not difficulting understanding, should be corrected. For instance, introductory words like Moreover and Eventually in the abstract, foue (four) in caption fig. 4, and use of articles and plural-singular throughout the paper.

Response: The advice suggested by reviewer is very accurate and reasonable. We are very sorry for our negligence of the problem. Therefore, we have revised the related sections (see the revised paper).

• Comment: Title: clearly defines purpose; Abstract: it correctly defines purpose, and discusses procedure and main findings. However, in order to capture readers’ attention, it could be shortened and focused on what is of interest to practitioners in the industry.

Response: The advice suggested by reviewer is very accurate and reasonable. The abstract has been rearranged and shortened (see the revised paper).

• Comment: Keywords: clear and complete Introduction: complete, both generally in the industry and the case study. Experimental: geological and numerical work is sound Discussion: focused on the particular case under study. This reviewer suspects there might be broader discussions that could be of use for a larger audience. Conclusions: for its most part is the conclusion of a technical report for the company operating that particular oil field. References are sufficient, well suited and up to date, note however that Chinese authors represent more than half of the references, in a field originally led by other countries.

Response: Thanks for your high evaluation for our paper and recommendations. We are very sorry for our negligence of the problem. Considering the reviewer’s suggestion, we have reorganized the described, and the references from other countries are added in the paper (see the paper).

Special thanks to you for your good comments.

Round 2

Reviewer 1 Report

The following issues have been resolved in the edited version:

-        - Readability

-         -The model setup and the initial stress field around the well

-        - The presence of faults and their intensity for different wells

-        - Including pertaining references in the Introduction

The latest version is ready for publication. However, the referencing format needs to be corrected to the standard referencing format before publication.

Author Response

• Comment: The following issues have been resolved in the edited version:

-- Readability

-- The model setup and the initial stress field around the well

-- The presence of faults and their intensity for different wells

-- Including pertaining references in the Introduction

The latest version is ready for publication. However, the referencing format needs to be corrected to the standard referencing format before publication.

Response: Thanks for your high evaluation for our paper and recommendations for publication. We also appreciate your positive and constructive comments regarding our manuscript, and we have made careful modifications on the original manuscript based on major assessment with results from other authors. The referencing format has been rearranged (see the revised paper).