Investigations and Prevention Strategies on the Fracturing Water Pollution Triggering Permeability and Production Decrease in Underground Gas Reservoirs
Round 1
Reviewer 1 Report
This work focus on the water pollution to the production of underground gas reservoir. The fracturing water pollution was studied experimentally and a model was derived to reflect the reservoir production considering fracturing water pollution. Preventions for fracturing water pollution was proposed as well. The abstract is concise and clear, and the introduction is logical and clearly clarify the innovation and significance of this work. I recommend minor revision before publication, and here are the comments:
1. The equations in section 2.1 “Fracturing water pollution research” are formed without complying with the standard format. Mathematical symbols should be used instead of using phrases directly in the equations. And don’t forget to interpret each symbol clearly.
2. The word “damage” is broadly used in figures without any definitions in the text, however. I suppose the authors mistakenly refer it to the word “pollution”. Check the manuscript carefully.
3. The effects of stress sensitivity and threshold pressure gradient are discussed as well, which seems less correlated to the paper topic, however. Clarify and interpret the significance of investigating the in stress sensitivity and threshold pressure gradient this work.
4. Authors claimed that reservoirs with an initial permeability over 0.20mD are preferably produced by natural energy rather than fracturing. Is it feasible for any type of gas reservoirs? If not, please clarify the conditions and limits of this water pollution prevention strategy.
Author Response
Dear reviewer,
We appreciate your comments and kind suggestions. Below is the point-by-point response to your comments.
(1) The equations in section 2.1 “Fracturing water pollution research” are formed without complying with the standard format. Mathematical symbols should be used instead of using phrases directly in the equations. And don’t forget to interpret each symbol clearly.
Responses:
Thanks for your comments. We have revised the equations in section 2.1 carefully according to your comments. We interpret each symbol carefully as well.
(2) The word “damage” is broadly used in figures without any definitions in the text, however. I suppose the authors mistakenly refer it to the word “pollution”. Check the manuscript carefully.
Responses:
Thanks for your comments. We have revised the whole text and all the figures carefully by correcting the word “damage” to “pollution”.
(3) The effects of stress sensitivity and threshold pressure gradient are discussed as well, which seems less correlated to the paper topic, however. Clarify and interpret the significance of investigating the in stress sensitivity and threshold pressure gradient this work.
Responses:
Thanks for your comments. We have clarified the significance of considering stress sensitivity and threshold pressure gradient in the Introduction section.
(4) Authors claimed that reservoirs with an initial permeability over 0.20mD are preferably produced by natural energy rather than fracturing. Is it feasible for any type of gas reservoirs? If not, please clarify the conditions and limits of this water pollution prevention strategy.
Responses:
Thanks for your comments. It is notable that the critical permeability, 0.2mD, is obtained by the established model according to the specific boundary and initial conditions we adopt in this work. We have clarified the adaptability of our model and analysis in terms of the critical permeability in section 5.
Best regards,
Ruifei Wang
Reviewer 2 Report
In this paper, the fracturing water pollution in the underground gas reservoir is quantitatively investigated and preventions for the fracturing water pollution are provided. The effects of fracturing water pollution on reservoir permeability are studied by core experiments. A mathematical physical model by integrating the reveal parameters from the core experiments is established to study the production of the underground gas reservoir considering fracturing water pollution. This work brings insights to water pollution prevention of gas development. The paper requires minor revisions before being accepted. Comments could be seen below:
1. The schematic of solid pollution of fracturing water to the underground gas reservoir is ambiguous. Please clarify each part of Figure 1. Only pore and reservoir are marked in the present version, even the marked pores are unclearly identified.
2. The abbreviations are confusing. For instance, the SD refers to Solid solute pollution from fracturing water, and the LD refers to Water solvent pollution from fracturing water. It makes no sense. Please revise it carefully.
3. The descriptions in Figure 2, 4 and 5 are inconsistent to that in the text. In particular, the word ‘damage’ used in these figures is confusing. Is that means equally the ‘pollution’ in the text? Please carefully revise it.
4. In Model validation section, the solution results from COMSOL are directly provided. I suggest the authors to enhance the meshing information, boundary and initial conditions and adopted time step of the simulation configuration.
5. Please carefully check the spelling and format of the text. There some writing errors such as superscript missing in section 3.3 Model validation; ‘reservoir Production under’ in line 301, page 10; ‘Figure 88’ in line 323, page 11; ‘Fracturing water water’ in line 332, page 12.
Author Response
Dear reviewer,
We appreciate your comments and kind suggestions. Below is the point-by-point response to your comments.
(1) The schematic of solid pollution of fracturing water to the underground gas reservoir is ambiguous. Please clarify each part of Figure 1. Only pore and reservoir are marked in the present version, even the marked pores are unclearly identified.
Responses:
Thanks for your comments. We have revised the Figure 1 carefully and clearly marked each part in the figure.
(2) The abbreviations are confusing. For instance, the SD refers to Solid solute pollution from fracturing water, and the LD refers to Water solvent pollution from fracturing water. It makes no sense. Please revise it carefully.
Responses:
Thanks for your comments. We have revised the abbreviations carefully, particularly corrected the “SD” to “SSP” which indicates the solid solute pollution and the “LD” to “WSP” which indicates the water solvent pollution.
(3) The descriptions in Figure 2, 4 and 5 are inconsistent to that in the text. In particular, the word ‘damage’ used in these figures is confusing. Is that means equally the ‘pollution’ in the text? Please carefully revise it.
Responses:
Thanks for your comments. We have revised the whole text and all the figures carefully by correcting the word “damage” to “pollution”.
(4) In Model validation section, the solution results from COMSOL are directly provided. I suggest the authors to enhance the meshing information, boundary and initial conditions and adopted time step of the simulation configuration.
Responses:
Thanks for your comments. We have enhanced the meshing information and initial and boundary conditions in the manuscript.
(5) Please carefully check the spelling and format of the text. There some writing errors such as superscript missing in section 3.3 Model validation; ‘reservoir Production under’ in line 301, page 10; ‘Figure 88’ in line 323, page 11; ‘Fracturing water water’ in line 332, page 12.
Responses:
Thanks for your comments. We have revised the whole text carefully.
Best regards,
Ruifei Wang
Reviewer 3 Report
1. This paper is interesting. My major concern is the general adaptivity. The authors should at least comment on the possible effect of fluid species and chemical equilibrium. Or, the authors can figure out the representative rules and conditions that can be referred in general scenarios.
2. The authors should enhance the background introduction by referring to more advanced researches on unconventional reservoir development, to demonstrate more on the significance of the topic, for example, "https://doi.org/10.1016/j.fuel.2019.02.117".
3. Units are needed in the axis in Figure 12.
4. The authors may also comment on the extension from 2D to 3D. The effect of 3D heterogeneity may be interesting in their next paper.
5. Did the authors consider the possible recovery of the fluidity? For example, injecting CO2 to help precipitation dissolution.
Author Response
Dear reviewer,
We appreciate your comments and kind suggestions. Below is the point-by-point response to your comments.
(1) This paper is interesting. My major concern is the general adaptivity. The authors should at least comment on the possible effect of fluid species and chemical equilibrium. Or, the authors can figure out the representative rules and conditions that can be referred in general scenarios.
Responses:
Thanks for your comments. Our model and analysis are adaptable for precisely forecast of gas production and re-liable prevention of fracturing water pollution of tight rock gas reservoirs developed by water fracturing. In terms of other effects such as fluid species and chemical equilibrium, we discuss them in the added Discussion section.
(2) The authors should enhance the background introduction by referring to more advanced researches on unconventional reservoir development, to demonstrate more on the significance of the topic, for example, "https://doi.org/10.1016/j.fuel.2019.02.117".
Responses:
Thanks for your comments. The recommended literature is inspiring. We prefer to cite this paper in the added Discussion section where we enhanced the comments on further investigations based on this work.
(3) Units are needed in the axis in Figure 12.
Responses:
Thanks for your comments. We have revised the Figure 12 carefully.
(4) The authors may also comment on the extension from 2D to 3D. The effect of 3D heterogeneity may be interesting in their next paper.
Responses:
Thanks for your comments. We have discussed the extension from 2D to 3D in the added Discussion section.
(5) Did the authors consider the possible recovery of the fluidity? For example, injecting CO2 to help precipitation dissolution.
Responses:
Thanks for your comments. It is literally a great idea. We have discussed the coupling researches on gas production and fracturing water pollution considering CO2 underground sequestration in the added Discussion section.
Best regards,
Ruifei Wang
