Dynamic Evolution Law of Production Stress Field in Fractured Tight Sandstone Horizontal Wells Considering Stress Sensitivity of Multiple Media

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1.      How does the stress sensitivity coefficient influence the principal stress change amplitude in low-permeability reservoirs, and what are the underlying mechanisms behind the observed 0.27% change?

2.      What specific properties of main fracture parameters lead to a significant impact on the stress field, resulting in a variation amplitude of up to 2.85%?

3.      In what ways does the stress concentration area transition from an elliptical to a semi-circular distribution, and what factors contribute to this spatial evolution during constant pressure production?

4.      How do the density and angle of natural fractures affect the pore pressure changes within the range of 3.32%, and what implications does this have for the diffusion area of pore pressure and reservoir communication?

5.      What are the key factors that cause the stress to diffuse more effectively from the fracture tip to the surrounding areas, and how can this be quantified in terms of reservoir performance?

6.      How does the random variability in natural fracture parameters impact the overall stress field and pore pressure distribution in tight sandstone reservoirs?

7.      What role do matrix self-supporting fractures play in the stress and pore pressure evolution, and how does their interaction with hydraulic and natural fractures affect the overall reservoir behavior?

8.      How can the numerical model be validated against field data to ensure accurate predictions of stress field changes, especially considering the observed variations in principal stress and pore pressure?

9.      What are the implications of a 0.27% change in principal stress and a 3.32% change in pore pressure for the operational strategies of multi-stage hydraulic fracturing in tight sandstone formations?

 

10.  How do changes in the primary fracture parameters influence the stress field, and what are the potential strategies for optimizing these parameters to enhance production efficiency and minimize adverse effects?

Author Response

1. How does the stress sensitivity coefficient influence the principal stress change amplitude in low-permeability reservoirs, and what are the underlying mechanisms behind the observed 0.27% change?

Answer：

Thank you very much for your question. In response to your question, the stress sensitivity coefficient has been defined in Section 2.2.

The underlying mechanisms: With the increase of matrix porosity stress sensitivity coefficient, the minimum horizontal principal stress decreases along the whole fracture length direction, but the amplitude is small, only 0.27%. The maximum horizontal principal stress increases with the increase of matrix porosity stress sensitivity coefficient before 120m (fracture region), and decreases with the increase of mass porosity stress sensitivity coefficient after 120m. With the increase of matrix porosity stress sensitivity coefficient, the minimum horizontal principal stress decreases along the whole fracture length direction, but the amplitude is small, only 0.27%. The maximum horizontal principal stress increases with the increase of matrix porosity stress sensitivity coefficient before 120m (fracture region), and decreases with the increase of mass porosity stress sensitivity coefficient after 120m. With the increase of stress sensitivity coefficient of matrix permeability, the minimum horizontal principal stress has an increasing trend, and the maximum horizontal principal stress first decreases and then increases, and the influence trend is opposite to the stress sensitivity coefficient of matrix porosity

2. What specific properties of main fracture parameters lead to a significant impact on the stress field, resulting in a variation amplitude of up to 2.85%?

Answer:

Thank you very much for your question. As the number of principal fractures increases from 3 to 11, the minimum horizontal principal stress in the fracture area decreases by 1.9MPa and the maximum horizontal principal stress increases by 0.64MPa, and the variation is within 2.85%. The spacing and length of principal fractures also have a minor influence on the stress field.

3. In what ways does the stress concentration area transition from an elliptical to a semi-circular distribution, and what factors contribute to this spatial evolution during constant pressure production?

Answer:

Thank you very much for your question. This phenomenon is caused by the change of induced stress field around the main fracture. With the increase of the number of main fractures, the larger the transverse distribution of the main fracture zone, the wider the stress field generated at the fracture tip.

4. How do the density and angle of natural fractures affect the pore pressure changes within the range of 3.32%, and what implications does this have for the diffusion area of pore pressure and reservoir communication?

Answer:

Thank you very much for your question. 3.32% means that the density of natural fractures increases from 0.1 to 0.3, pore pressure decreases by 1.59Mpa (3.32%), and the diffusion area of pore pressure expands significantly. When pore pressure is low, rocks may be more susceptible to the effect of stress concentration, resulting in the closure of fractures and affecting reservoir connectivity. On the contrary, higher pore pressure can resist external stress and maintain the stability of rock structure, which is conducive to the maintenance of reservoir connectivity.

5. What are the key factors that cause the stress to diffuse more effectively from the fracture tip to the surrounding areas, and how can this be quantified in terms of reservoir performance?

Answer:

Thank you very much for your question. According to the cloud map of the change of minimum horizontal principal stress under different number of principal fractures in FIG. 9, both the number of principal fractures and the distance between principal fractures will affect the diffusion of the crack tip to the surrounding area. With the increase of reservoir permeability, the fracture tip diffuses faster.

6. How does the random variability in natural fracture parameters impact the overall stress field and pore pressure distribution in tight sandstone reservoirs?

Answer:

Thank you very much for your question. As for the overall stress field of the reservoir, the influence of main fractures on it is mainly considered. With the increase of the Angle of natural fractures, hydraulic fractures are easier to communicate with natural fractures on both sides, and ultimately lead to the expansion of the diffusion area of pore pressure.

7. What role do matrix self-supporting fractures play in the stress and pore pressure evolution, and how does their interaction with hydraulic and natural fractures affect the overall reservoir behavior?

Answer:

Thank you very much for your question. The self-supported matrix fractures play a role of pressure conduction in the evolution of stress and pore pressure, and the hydraulic fractures communicate with the natural fractures, increase the reservoir production range, shorten the matrix seepage distance, and lead to a larger production spread range.

8. How can the numerical model be validated against field data to ensure accurate predictions of stress field changes, especially considering the observed variations in principal stress and pore pressure?

Answer:

Thank you very much for your question. In this paper, the existing model is verified, and the results obtained by Anusarn Sangnimnuan and Wu Kan in 2018 combined with the fluid flow/geomechanical coupling model of EDFM are compared and verified, and the model in this paper is basically consistent with the verified model.

9. What are the implications of a 0.27% change in principal stress and a 3.32% change in pore pressure for the operational strategies of multi-stage hydraulic fracturing in tight sandstone formations?

Answer:

Thank you very much for your question. The change of principal stress has a small impact on pressure channeling, while the change of pore pressure has a greater impact on hydraulic fracturing operations, which is more prone to pressure channeling. According to the construction parameters, the amount of fluid, displacement and cluster number are adjusted to alleviate pressure channeling.

10. How do changes in the primary fracture parameters influence the stress field, and what are the potential strategies for optimizing these parameters to enhance production efficiency and minimize adverse effects?

Answer:

Thank you very much for your question. It can be seen from Figure.8 (a) and (b) that as the number of principal fractures increases from 3 to 11, the minimum horizontal principal stress in the fracture region decreases by 1.9MPa and the maximum horizontal principal stress increases by 0.64MPa, with a change within the range of 2.85%

As can be seen from Figure. 10 (a) and Figure. 10 (b), with the increase of the principal fracture spacing from 8m to 16m, the minimum horizontal principal stress near the wellbore decreases by 0.92MPa and the maximum horizontal principal stress increases by 0.43MPa, and the variation range is within 1.36%. It can also be seen from Figure. 10 (c) that with the increase of principal fracture spacing, the decrease of principal stress and pore pressure increases.

As can be seen from Figure. 12, as the crack length increases from 80m to 160m, the maximum horizontal principal stress of the joint increases by 0.66MPa, the minimum horizontal principal stress decreases by 0.13MPa, and the stress change is within the range of 0.83%.

The longer the length of the main fracture, the easier it is for the child well to communicate with the mother well, and the longer the main fracture, the easier it is to communicate with the natural fracture, resulting in faster production of the well and faster pressure reduction. This adverse effect can be reduced by controlling the construction scale.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

1.     What software is used for simulation in this paper and what is the basis for the parameter settings please reflect in Part II.

2.     How are the natural fractures in the strata represented in the simulation? Why are rock bodies with natural and secondary fractures considered isotropic?

3.     The stress concentration at the tip of the cleavage is a classical rock mechanics theory and cannot be used as the main conclusion of the article.

4.     4.1 Whether the results of the analysis of some of the different parameters are significantly different.

5.     Lines 202-203, why the compression coefficients of the primary and secondary fractures have less effect on pore pressure?

6.     Please list the theoretical formulas if they are involved in the article.

7.     The reason for the change in principal stresses due to the difference in compression coefficients between the primary and secondary cleavage is not explained.

8.     Where in the text is the effect of hydraulic fracturing on the fracture and stress evolution of a rock mass demonstrated?

9.     Many scholars have studied the stress-seepage coupling law of pore media and fracture media in low permeability bodies, in which aspect is the author's innovation reflected?

Comments on the Quality of English Language

Moderate editing of English language required

Author Response

1. What software is used for simulation in this paper and what is the basis for the parameter settings please reflect in Part II.

Answer:

Thank you very much for your question. Simulation was performed using comsol multiphysics software.

2. How are the natural fractures in the strata represented in the simulation? Why are rock bodies with natural and secondary fractures considered isotropic?

Answer:

Thank you very much for your question. The natural fractures in the formation are randomly distributed in the simulation. The isotropy of rock mass with natural and secondary fractures is considered to simplify the model.

3. The stress concentration at the tip of the cleavage is a classical rock mechanics theory and cannot be used as the main conclusion of the article.

Thank you very much for your proposal, which has been deleted in the relevant position of the paper.

4. 4.1 Whether the results of the analysis of some of the different parameters are significantly different.

Answer:

Thank you very much for your question. According to the simulation results, changing the stress sensitivity coefficient of matrix porosity has little effect on the principal stress.

5. Lines 202-203, why the compression coefficients of the primary and secondary fractures have less effect on pore pressure?

Answer:

Thank you very much for your question. From the pore pressure curves of the monitoring points as shown in Figure 6 (e) and 7 (e) over time, it can be seen that the compression coefficient of the primary crack and the secondary crack have little influence on the pore pressure, and the stress curves almost coincide.

6. Please list the theoretical formulas if they are involved in the article.

The theoretical formulas mentioned in this article have been supplemented in Section 2.2.

7. The reason for the change in principal stresses due to the difference in compression coefficients between the primary and secondary cleavage is not explained.

Thank you very much for your question. It can be seen from figure.6 and figure.7  that the compression coefficient of the primary crack and secondary crack has a small influence on the principal stress and pore pressure, and the stress curves almost coincide, and the change is only within 0.001MPa. The reason may be that the presence of cracks causes stress to be localized at and near the crack tip. Even if the compression coefficients of the primary and secondary cracks change, this localization effect may make the change of the overall stress distribution not significant. The compressive coefficient of primary and secondary fractures changes but the stress does not change much, which may also be the result of the interaction between fractures, healing effect, stress relaxation, nonlinear response of rock, threshold effect of fracture closure and anisotropy of rock.

8. Where in the text is the effect of hydraulic fracturing on the fracture and stress evolution of a rock mass demonstrated?

Answer:

Thank you very much for your question. The compression coefficient of primary and secondary fractures, number of primary fractures, spacing of primary fractures, length of primary fractures, density of natural fractures and Angle of natural fractures mentioned in sections 4.3 to 4.8 are all parameters related to hydraulic fracturing. The first five parameters are directly related to fractures artificially formed by hydraulic fracturing, while the last two parameters are related to the nature of natural fractures communicated in the fracturing process.

9. Many scholars have studied the stress-seepage coupling law of pore media and fracture media in low permeability bodies, in which aspect is the author's innovation reflected?

Answer:

Thank you very much for your question. Most studies on stress fields focus solely on factors such as fracture parameters and fluid properties. However, in real geological formations, there are often multiple seepage media such as hydraulic fracturing fractures, secondary fractures, and natural fractures. The acquisition of stress fields requires coupling of seepage stress, and in this process, factors such as permeability are stress sensitive. The changes in stress fields when these three factors are considered simultaneously are not yet clear. Therefore, this article considers multi-stage stress sensitivity and based on the parameters of fractured tight sandstone reservoirs, establishes a numerical model for the dynamic evolution of the stress field coupled with matrix self-supporting fractures supporting fractures seepage stress. The influence of various factors on the production stress field is analyzed, and the research results are helpful for analyzing the changes in the stress field around the production well.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors present simulation studies of different impacts, including matrix, HF, and NF properties, on stress change. The work could attract attention from readers with research interests of hydraulic fracturing. However, the authors should include more discussions on the reasons behind each sensitivity result. Here are the detailed comments.

1.       Lines 52-53: The sentence "81% of the gas production from frac-hit wells is more affected than 80%" is confusing.

2.       Line 59: it should be "The presence of inter well...."

3.       Lines 142-143: despite the modeling methodology was described somewhere else, it's recommended that the authors summarize the methodology in a short paragraph, so that the readers won't need to immediately check other papers.

4.       Line 148: "as shown in Figure 2" should be added after "a geological numerical model of the target block".

5.       Figure 2: it needs more information to describe this figure. Which are hydraulic fractures? Which are natural fractures? The authors may use different colors to differentiate them.

6.       Line 166: What's the meaning of "the stress sensitivity coefficient"? Please provide equations or some descriptions for it.

7.       Lines 178-181: the authors described what was found in Figure 4. However, what's the meaning of this result? Can this result be verified against other works? Similar comments also go to the other figures.

8.       Figure 3: please add descriptions of Cp, sx, sy, and p in the caption of Figure 3. Please split Figure 3 (c) to 3 subfigures with different stress range in y axis to clearly show the variation of stresses sx, sy, and p with changing Cp.

9.       Figure 4: please provide the "matrix porosity stress sensitivity coefficient" of each subfigures.

10.   Lines 191-192: please explain the reason for the opposite trend.

11.   Figure 5: Describe Ca. Please split Figure 5 (c) to 3 subfigures with different stress range in y axis to clearly show the variation of stresses sx, sy, and p with changing Ca.

12.   Figure 8: please change Chinese to English.

13.   Figure 9: add captions of number of main fractures.

14.   Lines 271-272: the bottom two subfigures of Figure 15 show different shapes in blue to green areas. Thus we could not say that "the diffusion range did not increase or decrease". The authors should come up with methods to quantify the area of the blue/green area in the figure to justify their statement regarding diffusion range.

15.   Figure 14: what's the unit "l/m"?

 

Comments on the Quality of English Language

Generally readable.

Author Response

The authors present simulation studies of different impacts, including matrix, HF, and NF properties, on stress change. The work could attract attention from readers with research interests of hydraulic fracturing. However, the authors should include more discussions on the reasons behind each sensitivity result. Here are the detailed comments.

1. Lines 52-53: The sentence "81% of the gas production from frac-hit wells is more affected than 80%" is confusing.

Answer:

Thank you very much for your question. 81% refers to 81% of the Wells in the block that have undergone pressure channeling, and 80% refers to the Wells that have undergone pressure channeling resulting in an 80% drop in gas well production.

2. Line 59: it should be "The presence of inter well...."

Answer:

Thank you very much for your question. It has been modified in the corresponding position

3. Lines 142-143: despite the modeling methodology was described somewhere else, it's recommended that the authors summarize the methodology in a short paragraph, so that the readers won't need to immediately check other papers.

Answer:

Thank you very much for your question. The modeling approach has been described in Section 2.2.

4. Line 148: "as shown in Figure 2" should be added after "a geological numerical model of the target block".

Answer:

Thank you very much for your question. It has been modified in the corresponding position.

5. Figure 2: it needs more information to describe this figure. Which are hydraulic fractures? Which are natural fractures? The authors may use different colors to differentiate them.

Answer:

Thank you very much for your question. It has been modified in the corresponding position.

6. Line 166: What's the meaning of "the stress sensitivity coefficient"? Please provide equations or some descriptions for it.

Answer:

Thank you very much for your question. The explanation and equation expression have been given in Section 2.2.

7. Lines 178-181: the authors described what was found in Figure 4. However, what's the meaning of this result? Can this result be verified against other works? Similar comments also go to the other figures.

Answer:

Thank you very much for your question. Figure 4 analyzes the influence of matrix porosity on stress field. With the increase of porosity sensitivity coefficient, the stability of cracks in rocks decreases, and cracks are more likely to expand under lower stress conditions. The spread of the crack will lead to the redistribution of the stress around it, but this redistribution will not change the overall shape of the stress field, only change the distribution of the stress.

8. Figure 3: please add descriptions of Cp, sx, sy, and p in the caption of Figure 3. Please split Figure 3 (c) to 3 subfigures with different stress range in y axis to clearly show the variation of stresses sx, sy, and p with changing Cp.

Answer:

Thank you very much for your question. It has been modified in the corresponding position.

9. Figure 4: please provide the "matrix porosity stress sensitivity coefficient" of each subfigures.

Answer:

Thank you very much for your question. It has been modified in the corresponding position.

10. Lines 191-192: please explain the reason for the opposite trend.

Answer:

Thank you very much for your question. The stress-sensitive coefficients of matrix porosity and matrix permeability have been briefly introduced in Section 2.2. The opposite reason is that with the increase of stress sensitivity coefficient of matrix porosity, the matrix porosity will decrease, and the amount of oil and gas resources per unit space will be less. With the production progress, the stress sweep range will increase, resulting in faster stress drop. : With the increase of matrix permeability stress sensitivity coefficient, matrix permeability will decrease, the slower the fluid flow, and with the progress of production, the stress sweep range will decrease, resulting in a slower and lower stress drop.

11. Figure 5: Describe Ca. Please split Figure 5 (c) to 3 subfigures with different stress range in y axis to clearly show the variation of stresses sx, sy, and p with changing Ca.

Answer:

Thank you very much for your question. It has been modified in the corresponding position.

12. Figure 8: please change Chinese to English.

Answer:

Thank you very much for your question. It has been modified in the corresponding position.

13. Figure 9: add captions of number of main fractures.

Answer:

Thank you very much for your question. It has been modified in the corresponding position.

14. Lines 271-272: the bottom two subfigures of Figure 15 show different shapes in blue to green areas. Thus we could not say that "the diffusion range did not increase or decrease". The authors should come up with methods to quantify the area of the blue/green area in the figure to justify their statement regarding diffusion range.

Thank you very much for your suggestion, which has been modified in the relevant part of the article. The quantitative analysis of the blue area by matlab software shows that the difference of the diffusion range between the two is only 0.5%, and there is no obvious change

15. Figure 14: what's the unit "l/m"?

"1/m" is the unit of natural fracture density, which is incorrectly expressed in the paper and has been modified in Figure 15 after searching the literature.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The comments have been incorporated in the revised version.

Author Response

I would like to express my sincere gratitude for your excellent guidance and valuable suggestions during my dissertation. Your expertise and critical insights have greatly enhanced the quality and depth of my work.

Reviewer 2 Report

Comments and Suggestions for Authors

(1) Please refer to the 'processes-template' format to modify the image name in the text, specifically: 'If there are multiple panels, they should be listed as: (a) Description of what is contained in the first panel; (b) Description of what is contained in the second panel. Figures should be placed in the main text near to the first time they are cited'.

(2) The formula layout in this paper needs to follow the journal format, and the author needs to check and modify it.

(3) The reference format cited in the article needs to be checked by the author.

Comments on the Quality of English Language

Minor editing of English language required

Author Response

comments 1：Please refer to the 'processes-template' format to modify the image name in the text, specifically: 'If there are multiple panels, they should be listed as: (a) Description of what is contained in the first panel; (b) Description of what is contained in the second panel. Figures should be placed in the main text near to the first time they are cited'.

Response 1 ：Thank you very much for your suggestions, which have been revised in relevant places in the article, and marked in yellow on the manuscript for your convenience.

comments 2：The formula layout in this paper needs to follow the journal format, and the author needs to check and modify it.

Response 2: Thank you very much for your suggestions. The formulas in the article have been revised in relevant places and marked in yellow in the manuscript for your convenience.

comments 3：The reference format cited in the article needs to be checked by the author.

Response 3: Thank you very much for your suggestion. The format of the reference literature has been revised in the relevant places.