Experimental Study on the Effect of ASP Flooding on Improving Oil Recovery in Low Permeability Reservoirs Based on a Partial Quality Tool

Round 1

Reviewer 1 Report

The authors have reported that using partial quality tool, the high Mw ASP solutions can be sheared so that it injectivity can be improved in low permeable layer whereas in high permeable layer, recovery efficiency of high Mw ASP solutions remain unaffected. How can this be possible in the actual reservoir? In actual reservoir, a low permeable and high permeable layer coexists and how can we control the fluid-flow with that tool? Can we place the tool in the reservoir? From the figure 6, I understand that the tool is placed outside of the core. This is my major concern. The write up is not satisfactory.  Also I have other comments which are as follows.

1. Abstract: Without changing the oil displacement capability of high molecular weight ASP solution in high permeability oil layers, the ASP solution is actively sheared in low permeability oil layers by using a partial quality tool to increase the injection capability of the solution and improve the overall oil recovery

The sentence is not clear.

2. Page 1: better to say depleted oil fields
3. Page 1: “However, with the increase of exploitation time, the crude oil recovery ratio is decreasing year by year, and the remaining oil content in the formation is relatively large, so the traditional water flooding is not effective [1]”

Better explanation is needed on why water flooding is ineffective. 

4. Page 1: At present, the chemical flooding method is adopted for oil displacement, which can improve oil recovery to the greatest extent [2].

Where?

5. Page 2: ASP solution with a high molecular weight and high concentration can greatly improve oil recovery in high permeability oil layers

What is meant by high Mw in ASP slugs?

6. Page 2: However, most reservoirs in China belong to heterogeneous reservoirs

Weak statement

7. The injection effect of the displacement agent in high permeability reservoirs is better and the injection amount is greater [5,6]

Please give more details about the heterogeneity coefficient

8. Page 2: At present, CO2 injection technology is considered to be an effective technology to improve the recovery ratio of low permeability reservoirs [12]

What is meant by low permeability reservoirs – range? Also cite as per the EOR screening aspects

9. Page 2: Chen et.al [14] simulated CO2 displacement through long core displacement experiments and nuclear magnetic resonance experiments. The results showed that under the current formation pressure, the minimum miscible pressure of CO2 flooding was 32.6MPa, the CO2 solubility of crude oil was large, and after injecting CO2, the crude oil had a strong swelling capacity, which was conducive to improving the recovery

What is current formation pressure? Is miscibility or oil swelling- what is the driving mechanisms?

10. Page 3: Huang et al. [22] studied the changes in the molecular micro-morphology and physical parameters of the ASP solution before and after it flowed through the partial quality tool through micro-experiments and shear tests. The research results provided an experimental basis for the research on the effect of the separation tool and the improvement of the oil recovery of ASP solution in low permeability reservoirs. However, these studies only addressed the theoretical and microscopic mechanisms of the partial quality tools.

It is not clear to me what is the novelty of this work if Huang et al. (22) has already studied. The authors clearly highlight the research problems/gaps by providing more information. Other-wise this work will lack the merit.

11. Page 3: An experiment using the partial quality tool to improve the oil recovery of low permeability reservoirs in the oil displacement experiment has not been carried out, and the degree of influence on the oil recovery is still unknown.

Is this the novelty that authors have investigated using the experiment- which experiment it is? Huang have used micro-model. Is core flood used in this work? If so, why performing core flooding experiments will advances our technical knowledge on the usage of partial quality tool? This should be mentioned clearly.

12. Page 3: It is very confusing to read the para containing the objective section- English needs to be improved and it should convey the clear message.
13. The research results are of great significance for improving the recovery ratio of low permeability reservoirs

What is meant by recovery ratio?

14. Figure 1: Is this tool is going to shear the ASP solutions externally or going to be placed in reservoir?- If it is going to shear externally, then abstract needs to be written.
15. Page 7: what does maximum injection pressure indicate? Can authors plot the pressure profile?
16. Page 9: This shows that ASP solutions with molecular weights of 1900 × 104 and 2500 × 104 have a poor injection ability with a low and medium permeability and low matching degree to the oil layer

What is meant by oil layer?

17. Page 11: According to the research on the matching relationship between the molecular weight of the ASP solution and core permeability, it

Please reference

18. “According to the influence of the partial quality tool on the molecular weight of the ASP solution, in order to compare the influence of the high molecular ASP solution after shearing with that of the low molecular ASP solution without shearing on the injection”

Please give the rationale for this comparison at the beginning of each para. Similarly, the rationale should be given for each para.

19. Conclusion: For heterogeneous oil layers, the injection of a high molecular weight ASP solution can improve the oil recovery of high permeability oil layers, and the use of a partial quality tool can improve the oil displacement effect of low permeability oil layers and enhance the overall oil

How can we achieve this if the tool is externally degrading the polymers?

Author Response

Response to Reviewer 1 Comments

First of all, thank you very much for your comments on this paper. We tried our best to improve the manuscript and made some changes in the manuscript. And the replies to your comments are as follows. We appreciate for your warm work earnestly, and hope that the correction will meet with approval. Once again, thank you very much for your comments and suggestions.

Point 1: The authors have reported that using partial quality tool, the high Mw ASP solutions can be sheared so that it injectivity can be improved in low permeable layer whereas in high permeable layer, recovery efficiency of high Mw ASP solutions remain unaffected. How can this be possible in the actual reservoir? In actual reservoir, a low permeable and high permeable layer coexists and how can we control the fluid-flow with that tool? Can we place the tool in the reservoir? From the figure 6, I understand that the tool is placed outside of the core. This is my major concern. The write up is not satisfactory.

Response 1: Thank you very much for your comments on the content of the paper, but we sincerely hope that you can consider our paper for publication, and we have made relevant explanations.

In order to improve the oil displacement efficiency, high concentration and high molecular weight ASP solution is used for oil displacement and oil recovery. However, due to the heterogeneity of the reservoir, the high concentration and high molecular weight ASP solution has high compatibility with the high permeability oil layer and strong injection capacity, most of the ASP solution flows into the high permeability oil layer, and a very small amount of polymer solution is injected into the low permeability oil layer, but these high molecular weight polymers have poor compatibility with the low permeability oil layer. Therefore, the injection capacity in low permeability reservoirs is poor, and the overall oil recovery is low. Therefore, this paper proposes for the first time to use a partial quality tool in low permeability reservoirs to improve this phenomenon in the process of ASP flooding. However, the partial quality tool is a tool for active shearing of ASP solution, which breaks the high molecular weight polymer chain, produces mechanical degradation, reduces the molecular weight of the polymer injected into the low permeability oil layer, and improves the injection capacity, thus improving the oil recovery of the low permeability oil layer without affecting the oil recovery of the high permeability oil layer, so it is only used for the low permeability oil layer, thus conforming to the actual situation. However, it is very effective to improve the overall oil recovery without changing the underlying structure and polluting the formation by using the partial quality tool. The precondition for the application of the partial quality tool is the layered injection allocation technology. Its process flow is shown in the following figure 1. This technology is mature enough, and the partial quality tool is applied in this kind of technology, which corresponds to whether the partial quality tool is installed or not in different formations.

Figure 1

Point 2: Abstract: Without changing the oil displacement capability of high molecular weight ASP solution in high permeability oil layers, the ASP solution is actively sheared in low permeability oil layers by using a partial quality tool to increase the injection capability of the solution and improve the overall oil recovery

The sentence is not clear.

Response 2: Thank you very much for your amendment to the abstract. According to your amendment, we have clarified the mechanism of the quality-dividing tool and briefly described it. The specific amendments are as follows:” The partial quality tool degrades the polymer through active shearing action, reduces the molecular weight of the polymer, to improve the matching degree to the low permeability oil layer and the oil recovery.”

The specific revision content is on the page 1 of the revised paper, lines 12-28.

Point 3: Page 1: better to say depleted oil fields

Response 3: Thank you very much for your revision of the relevant language in the paper. According to your revision comments, we have revised the relevant words.

The specific revision content is on the page 1 of the revised paper.

Point 4: Page 1: “However, with the increase of exploitation time, the crude oil recovery ratio is decreasing year by year, and the remaining oil content in the formation is relatively large, so the traditional water flooding is not effective [1]”

Better explanation is needed on why water flooding is ineffective.

Response 4: Thank you very much for your comments on the water flooding part of the introduction. According to your comments, we have improved the relevant contents and specifically revised them as follows:” However, with the increase of exploitation time, the oil recovery is decreasing year by year, and the remaining oil content in the formation is relatively large, however, the remaining oil mainly exists in microscopic remaining parts of pores that cannot be swept by water flooding, so the traditional water flooding is not effective [1].”

The specific revision content is on the page 1 of the revised paper, lines 33-39.

Point 5: Page 1: At present, the chemical flooding method is adopted for oil displacement, which can improve oil recovery to the greatest extent [2].

Where?

Response 5: Thank you very much for your suggestions on this part of the paper. As described in reference 2, core from outcrop Berea sandstone were used, relevant displacement experiments were designed, and the experimental results show that the effect of polymer injection was found to be more positive when low salinity was initialized from the start of water injection (secondary mode). In this case, the final recovery factor increased to about 90% OOIP. From this, we found that under the same circumstances, only changing water flooding into chemical flooding can greatly improve the oil recovery, and from the reference, chemical flooding has a wider application range and is generally applicable to other environments. Therefore, from the perspective of chemical flooding, this part of the content is only to highlight the advantages of chemical flooding compared with water flooding, thus leading to the following contents and making a general description.

Point 6: Page 2: ASP solution with a high molecular weight and high concentration can greatly improve oil recovery in high permeability oil layers

What is meant by high Mw in ASP slugs?

Response 6: Thank you very much for your comments on this part of the paper. For ASP solution with high molecular weight, different experimental studies have different definition ranges for high molecular weight. We only summarize this in general description. In the introduction part, we choose high molecular weight to describe in a unified way in order not to target a specific situation. For this paper, according to the actual application of ASP flooding agent in Daqing Oilfield, when the molecular weight is higher than 1900 × 10⁴, it is high molecular weight polymer solution. For oil layers with different permeability, the matching degree of ASP solutions with different molecular weights to oil layers is different. This is because the molecular weight of polymer in ASP solution increases, the effective volume of its molecules in the solution increases, the longer the macromolecular chain is, the easier the molecular chain is tangled, the conformation of the molecular chain is relatively stable, and the formed reticular tangled points are relatively firm, so its viscosity is also larger, resulting in the increase of the flow resistance of the solution. Moreover, the high molecular weight will promote the mechanical trapping behavior of polymer molecules on the pore surface of the core, which increases the retention of polymer in the porous medium, resulting in a decrease in the injection capacity of the polymer solution and a decrease in the oil displacement effect. Therefore, it is not that the higher the molecular weight, the better the oil displacement effect of the polymer solution.

Point 7: Page 2: However, most reservoirs in China belong to heterogeneous reservoirs

Weak statement

Response 7: Thank you very much for your comments on this part of the article. I'm sorry that the situation described in this part is not accurate. According to your comments, we have revised this part of the content as follows:” However, for heterogeneous reservoirs, the injection effect of the displacement agent in high permeability reservoirs is better and the injection amount is greater [5,6], which results in less displacement agent flowing into low permeability reservoirs”

The specific revision content is on the page 2 of the revised paper, lines 40-50.

Point 8: The injection effect of the displacement agent in high permeability reservoirs is better and the injection amount is greater [5,6]

Please give more details about the heterogeneity coefficient

Response 8: Thank you very much for your comments on references 5 and 6. According to your suggestions, it is necessary to put forward specific heterogeneity coefficient in the process of description, but most documents define indexes such as high and low permeability and injection capacity differently. References use 2.2, 1.8 and 0.7mD as indexes for low permeability reservoirs, and the other uses permeability range as indexes, so there is no specific method to define these classification methods. Therefore, we have made a general description, and we have explained the experimental fact that the injection capacity of polymer solution in the high permeability oil layer is stronger than that in the low permeability oil layer according to the contents of the literature, leading to the following contents. Therefore, we have cited references 5 and 6 for factual explanation, which meets the description requirements.

Point 9: Page 2: At present, CO2 injection technology is considered to be an effective technology to improve the recovery ratio of low permeability reservoirs [12]

What is meant by low permeability reservoirs – range? Also cite as per the EOR screening aspects

Response 9: Thank you very much for your comments on reference 12 in the paper. We have consulted the relevant methods for improving and improving the oil recovery of low permeability oil layers, and found that CO₂ flooding can effectively improve the recovery rate of low permeability oil layers, while reference 12 summarizes the effectiveness and practicability of CO₂ flooding method from a general perspective. Therefore, we have summarized here that CO₂ can improve the oil recovery of low permeability oil layers, and only described the introduction from the perspective of improving the oil recovery of low permeability oil layers, thus leading to the use of partial quality tool to improve the oil recovery of low permeability oil layers. For low permeability reservoirs, many literatures define low permeability reservoirs in different ranges, some of which are defined as ultra-low permeability, low permeability, medium permeability, high permeability and ultra-high permeability, but this paper divides them into high permeability and low permeability by taking 200 × 10^-3 μm² as the approximate boundary.

Point 10: Page 2: Chen et.al [14] simulated CO2 displacement through long core displacement experiments and nuclear magnetic resonance experiments. The results showed that under the current formation pressure, the minimum miscible pressure of CO2 flooding was 32.6MPa, the CO2 solubility of crude oil was large, and after injecting CO2, the crude oil had a strong swelling capacity, which was conducive to improving the recovery.

Response 10: Thank you very much for your suggestion to the reference 14 in paper. We have revised it according to your suggestion, in which the current formation pressure is 32MPa. Chen et al. simulated CO₂ displacement and CO₂- water alternate displacement, and evaluated the displacement efficiency of different types of pores. the mechanism is the same as that of the whole CO₂ displacement and is miscible. However, the overall displacement mechanism has nothing to do with the overall content of the paper in the foreword, so it is not reflected in the foreword, but highlights CO₂ flooding to solve the problem of low oil recovery in low permeability reservoirs, so the overall method and results are of concern to us. We have revised this part of the content as follows:” Chen et.al [14] simulated CO₂ displacement through long core displacement experiments and nuclear magnetic resonance experiments. The results showed that under the current formation pressure 32 MPa, the minimum miscible pressure of CO₂ flooding was 32.6MPa, the CO₂ solubility of crude oil was large, and after injecting CO₂, the crude oil had a strong swelling capacity, which was conducive to improving the oil recovery.”

The specific revision content is on the page 2 of the revised paper, lines 72-76.

Point 11: Page 3: Huang et al. [22] studied the changes in the molecular micro-morphology and physical parameters of the ASP solution before and after it flowed through the partial quality tool through micro-experiments and shear tests. The research results provided an experimental basis for the research on the effect of the separation tool and the improvement of the oil recovery of ASP solution in low permeability reservoirs. However, these studies only addressed the theoretical and microscopic mechanisms of the partial quality tools.

It is not clear to me what is the novelty of this work if Huang et al. (22) has already studied. The authors clearly highlight the research problems/gaps by providing more information. Other-wise this work will lack the merit.

Response 11: Thank you very much for your comments on the reference of Huang. First Huang studied the changes of molecular micromorphology and viscoelasticity and other properties before and after ASP solution was treated by a partial quality tool. He did not apply it to actual oil displacement. He only conducted superficial research from a theoretical and microscopic point of view. In this paper, we studied the effect of partial quality tool on improving the oil recovery of heterogeneous oil layers through laboratory oil displacement tests. It is completely different from Huang's research content. This paper studies the influence of quality tool on enhancing oil recovery from a practical point of view. Therefore, this paper has practical research value on enhancing oil recovery in heterogeneous reservoirs, and has certain innovative application value considering the influence factors such as different polymer solutions and reservoirs with different permeability.

Point 12: Page 3: An experiment using the partial quality tool to improve the oil recovery of low permeability reservoirs in the oil displacement experiment has not been carried out, and the degree of influence on the oil recovery is still unknown.

Is this the novelty that authors have investigated using the experiment- which experiment it is? Huang have used micro-model. Is core flood used in this work? If so, why performing core flooding experiments will advances our technical knowledge on the usage of partial quality tool? This should be mentioned clearly.

Response 12: Thank you very much for your comments on the introduction of the paper. First, The research of Huang et al is only the derivation of the mathematical formula for the polymer solution flowing through the partial quality tool, the establishment of the relevant mathematical model, and the study of the changes of molecular micromorphology and viscoelasticity before and after the polymer solution flowing through the partial quality tool, and the study of the changes of the polymer solution before and after flowing through the partial quality tool from the perspective of theory and microscopic experiments. However, what we really want to know is whether the use of partial quality tool can improve the oil recovery in actual application, that is, in actual oil displacement experiments, which has a great promotion effect on improving the oil recovery in heterogeneous oil reservoirs. However, previous studies have not studied the actual effect of partial quality tool from the perspective of actual oil displacement experiments. Therefore, this is the first time that the actual oil displacement experiment has been used in this paper. On the basis of the experiment, this paper has studied the research of different factors on improving the oil recovery of heterogeneous oil layer and the matching degree between oil layer and polymer solution under the condition of using the partial quality tool. Therefore, this paper has carried out the research for the first time, which has certain innovation points and also provides a thought and technical means for future research on using physical methods to improve the oil recovery of heterogeneous oil layer.

Point 13: Page 3: It is very confusing to read the para containing the objective section- English needs to be improved and it should convey the clear message.

Response 13: Thank you very much for your comments on the language part of the paper. According to your comments, we modified the English usage in the paper in MDPI language polishing organization. We have uploaded the specific modification certificate to the supplementary document.

Point 14: The research results are of great significance for improving the recovery ratio of low permeability reservoirs

What is meant by recovery ratio?

Response 14: Thank you very much for your comments on the expression of relevant words in the paper. In order to express the contents of the paper more clearly, we have revised the relevant words, which actually mean “oil recovery”.

Point 15: Figure 1: Is this tool is going to shear the ASP solutions externally or going to be placed in reservoir?- If it is going to shear externally, then abstract needs to be written.

Response 15: Thank you very much for your suggestions on the modification of the operation mechanism of the partial quality tool in the paper. According to your suggestions, we have made some preliminary modifications. The main operation mechanism of the partial quality tool has been explained in the introduction of the partial quality tool in the paper and in the mentioned above Figure 1. However, the overall layered injection technology is very mature, and the research is based on laboratory tests. The layered injection technology in the formation is too complicated. Therefore, in order to highlight the effect of the partial quality tool, this aspect is not highlighted in this paper.

The specific revision content is on the page 1 of the revised paper, lines 12-28.

Point 16: Page 7: what does maximum injection pressure indicate? Can authors plot the pressure profile?

Response 16: Thank you very much for your comments on the part of maximum injection pressure in this paper. We used the parameter of maximum injection pressure in the oil displacement experiment to determine the injection capacity of polymer solution to heterogeneous cores before and after the action of the partial quality tool. The greater the maximum injection pressure in the oil displacement process, the poorer the injection capacity of polymer solution to heterogeneous cores, the lower the matching to cores, which indirectly leads to the poorer oil displacement effect of polymer solution. However, the overall pressure change is not our concern, but only the maximum injection pressure in the oil displacement process is needed to reflect the injection capacity of polymer solution, which is also reflected in previous documents and widely accepted, and is explained in the results and discussion part of the paper. Therefore, in order to highlight the research content, we only recorded the maximum injection pressure in the oil displacement process and conducted research.

Point 17: Page 9: This shows that ASP solutions with molecular weights of 1900 × 104 and 2500 × 104 have a poor injection ability with a low and medium permeability and low matching degree to the oil layer

What is meant by oil layer?

Response 17: Thank you very much for your comments on this part of the paper. As we can see from figure 6, when the molecular weight reaches 1900 × 10⁴ and 2500 × 10⁴, the maximum injection pressure reaches 6.12 and 6.63 MPa, respectively and the minimum is 4.18 and 4.61 MPa, respectively, with the change of concentration, which are both very high pressure values, and with the increase of permeability, the maximum injection pressure decreases to a lesser extent. From this, we can see that when the molecular weight reaches 1900 × 10⁴ and 2500 × 10⁴, the maximum injection pressure reaches the maximum. Before using the partial quality tool, its injection ability to the low permeability oil layer is also the worst, and the matching degree to the oil layer is low. Therefore, we take this experiment as the premise. The ASP solutions with molecular weights of 1900 × 10⁴ and 2500 × 10⁴ are compared by using the partial quality tool to verify whether the partial quality tool can reduce the maximum injection pressure of the high molecular weight ASP solution, increase the injection capacity of the solution in low permeability reservoirs and improve the oil recovery. Therefore, this experiment is the premise of the whole verification study.

Point 18: Page 11: According to the research on the matching relationship between the molecular weight of the ASP solution and core permeability, it

Please reference.

Response 18: Thank you very much for your comments on section 3.2 of this paper. According to your comments. This part of “According to the research on the matching relationship between the molecular weight of the ASP solution and core permeability’’ is the research conclusion in section 3.1. We use this part of conclusion as the basis for further research. We are sorry for any confusion in expression, so according to your opinion, we have improved the expression. We have revised this part of the content as follows:” In order to improve the oil displacement ability of the polymer ASP solution in cores with medium and low permeability, the shearing action of the polymer ASP solution (>1600 × 104) was carried out by using a partial quality tool to break the polymer chain, reduce the molecular weight, and improve the injection ability. According to the research in the above section on the matching relationship between the molecular weight of the ASP solution and core permeability, it can be seen that the concentration change has no obvious effect on the injection capacity and oil displacement effect of high molecular weight ASP solution in medium and low permeability cores.

”

The specific revision content is on the page 11 of the revised paper, lines 289-292.

Point 19: “According to the influence of the partial quality tool on the molecular weight of the ASP solution, in order to compare the influence of the high molecular ASP solution after shearing with that of the low molecular ASP solution without shearing on the injection”

Please give the rationale for this comparison at the beginning of each para. Similarly, the rationale should be given for each para.

Response 19: Thank you very much for your comments on the explanation of the first paragraph in the paper. We have supplemented the contents of the relevant paragraphs according to your comments and explained the purpose of the relevant operations. The specific amendments are as follows:” In order to study the improvement of core injection ability of ASP solutions with different molecular weights after the action of the partial quality tool, the injection pressure was selected during oil displacement as the verification parameter.” and “In order to study the ability of ASP solutions with different molecular weights to enhance the oil recovery of heterogeneous cores after being acted on by the partial quality tool, relevant experiments were carried out to verify”

The specific revision content is on the page 11 of the revised paper, lines 286-292, and the page 12 of the revised paper, lines 325-330

Point 20: Conclusion: For heterogeneous oil layers, the injection of a high molecular weight ASP solution can improve the oil recovery of high permeability oil layers, and the use of a partial quality tool can improve the oil displacement effect of low permeability oil layers and enhance the overall oil

How can we achieve this if the tool is externally degrading the polymers?

Response 20: Thank you very much for your comments on the conclusion part of the paper. In order to avoid misunderstanding, we have changed the conclusion part to read as follows:” For heterogeneous oil layers, the use of a partial quality tool can improve the oil displacement effect of low permeability oil layers and enhance the overall oil recovery”

The main function of the partial quality tool is based on layered injection technology (as shown in figure. 1). When injecting high molecular weight polymer solution for oil displacement, most of the high molecular weight polymer solution will enter the high permeability oil layer, so the oil recovery of the high permeability oil layer is higher, while a few of the high molecular weight polymer solution will enter the low permeability oil layer, but due to the excessive molecular weight, the matching degree to the low permeability oil layer is lower. Therefore, the poor injection ability of the high molecular weight polymer solution in the low permeability oil layer indirectly leads to the low oil displacement efficiency and the decrease of the overall oil recovery. However, when the high molecular weight polymer solution flows through the low permeability oil layer, the use of the partial quality tool enables the high molecular weight polymer to undergo active shear degradation, breaking the molecular chain and reducing the molecular weight. Thus, the matching degree of the limited polymer solution to the low permeability oil layer increases, and the injection ability is stronger. Compared with not using the partial quality tool, the oil recovery of the low permeability oil layer is greatly improved.

The specific revision content is on the page 21 of the revised paper, lines 536-537

Author Response File: Author Response.docx

Reviewer 2 Report

This study presents how to improve oil recovery by reducing the molecular weight of ASP using Partial Quality Tool.

Let me hold the decision until the followings are improved.

1. Need more explanation of "partial quality tool" and how it reduces the molecular weight. Of course the authors put the citations however necessary background directly related to study must be still described. How does it reduce the molecular weight? by breaking down polymer molecules? 
2. Figure 6 and 7 have errors in the legend or descriptions in each figure. Molecular weights are all the same or mostly likely written in the reverse order. Explanation and discussion do not match the figure results.
3. In figure 7, the recovery vs permeability trends show change as molecular weight increases (from log-style increasing trend through linear-increasing trend to exponential-type increasing trend). Are there any explanations? 
4. Is it the reduced molecular weight itself or the relative reduction of the molecular weight (new molecular weight/original molecular weight) which affects the oil recovery?
5. Table 3 and 5...isn't it better to present those results using graphs?
6. English is not bad. However, some sentences and paragraphs show redundancy. Maybe one more revision by a native English speaker can improve the quality.

Author Response

Response to Reviewer 2 Comments

First of all, thank you very much for your comments on this paper. We tried our best to improve the manuscript and made some changes in the manuscript. And the replies to your comments are as follows. We appreciate for your warm work earnestly, and hope that the correction will meet with approval. Once again, thank you very much for your comments and suggestions.

Point 1: Need more explanation of "partial quality tool" and how it reduces the molecular weight. Of course the authors put the citations however necessary background directly related to study must be still described. How does it reduce the molecular weight? by breaking down polymer molecules?

Response 1: Thank you very much for your comments on the partial quality tool part of the paper. In the introduction part, we put forward the concept of partial quality tool and briefly introduced that partial quality tool is an active shearing tool that breaks the polymer molecular chain and is used to reduce the molecular weight of polymer solution. However, for partial quality tool, this tool is also proposed for the first time by Daqing Oilfield. There is little introduction about this tool. And there is no relevant research applied to the polymer solution flooding process, so this aspect is not explained in detail in the introduction, and the purpose of this paper is to introduce the effect of the partial quality tool in detail, so the partial quality tool and its mechanism are described in section 2.1.5 of this paper, so only a brief explanation is given in the abstract. We have explained for the problems of abstract as follows:” In the formation, the partial quality tool degrades the polymer through active shearing action, reduces the molecular weight of the polymer, to improve the matching degree to the low permeability oil layer and the oil recovery.”

The specific revision content is on the page 1 of the revised article, lines 14-17.

Point 2: Figure 6 and 7 have errors in the legend or descriptions in each figure. Molecular weights are all the same or mostly likely written in the reverse order. Explanation and discussion do not match the figure results.

Response 2: Thank you very much for your comments on figures 6 and 7 in the paper. We are very sorry that we didn't choose the correct picture due to negligence. According to your suggestion, we have modified the relevant information and parameters in the figures.

The specific revision content is on the pages 8-10 of the revised paper.

Point 3: In figure 7, the recovery vs permeability trends show change as molecular weight increases (from log-style increasing trend through linear-increasing trend to exponential-type increasing trend). Are there any explanations?

Response 3: Thank you very much for the relation between oil recovery and permeability and molecular weight in Figure 7 of the paper. This part mainly focuses on the general change trend and change rule between oil recovery and formation permeability and different polymer solutions. When oil displacement is carried out in formations with different permeability and using different polymer solutions, the oil recovery achieved is also different. The change rule does not conform to the linear, logarithmic and exponential changes. What the paper wants to show is the overall change trend. Under normal circumstances, the polymer solution with high molecular weight (> 1900 × 10⁴) has poor oil displacement efficiency and poor injection capacity at a lower permeability (< 200 × 10⁴μm²) oil layer. based on this phenomenon, the improvement degree of polymer solution to oil layer displacement after the action of the partial quality tool is determined in this paper. Therefore, in the description of figure 7, we will only describe it from the perspective of the overall change trend. However, considering the changes of straight line, logarithm and index, it may only conform to the phenomenon under this condition, and cannot summarize all the oil displacement conditions. When other parameters change, the change rule of this detail does not conform.

Point 4: Is it the reduced molecular weight itself or the relative reduction of the molecular weight (new molecular weight/original molecular weight) which affects the oil recovery?

Response 4: Thank you very much for your comments on the reduction of molecular weight in this paper. The polymer solution is injected into low permeability oil layers with less amount and poor matching to oil layers, so most of the polymer solution is injected into high permeability oil layers, while the high molecular weight polymer solution has poor matching to low permeability oil layers and cannot be used for efficient oil displacement. Therefore, in low permeability oil layers, we use partial quality tool to actively shear the high molecular weight polymer solution. The molecular chain of the high molecular weight polymer solution is broken, the molecular weight is reduced, and the matching property to the low permeability oil layer is increased, so that the polymer solution can effectively enter the low permeability oil layer with low permeability for oil displacement, and the overall oil recovery is improved. Therefore, the shear degradation of the polymer solution is carried out on the basis of the original injected polymer solution to reduce the molecular weight and improve the injection capacity.

Point 5: Table 3 and 5...isn't it better to present those results using graphs?

Response 5: Thank you very much for your comments on the expression form of the data in the document table. When we integrated the data, we found that when we drew the line chart and histogram, due to the large difference between the data, the data was extremely poor, which made the expression of the picture not obvious or beautiful. Therefore, considering the readers' senses and expressiveness, we chose the form of the table to explain.

Point 6: English is not bad. However, some sentences and paragraphs show redundancy. Maybe one more revision by a native English speaker can improve the quality.

Response 6: Thank you very much for your comments on the English description of the paper. According to your comments, we have revised the English usage of the paper in MDPI language polishing organization. We have uploaded the specific revision certificate to the supplementary document.

Round 2

Reviewer 1 Report

Dear Authors.

Thanks for the changes.

I still have two concerns. 

1. please incorporate the response 11 and 12 into draft accordingly with clear details
2. Partial quality tool- please include a clear para whether it was used in the field before atleast on a pilot scale- If not list the potential features of it so that it can be used in the field without any issues. Relevant references needs to be cited. This is very important. 

Thanks. 

Author Response

Response to Reviewer 1 Comments

First of all, thank you very much for your comments on this paper. We tried our best to improve the manuscript and made some changes in the manuscript. And the replies to your comments are as follows. We appreciate for your warm work earnestly, and hope that the correction will meet with approval. Once again, thank you very much for your comments and suggestions.

Point 1: please incorporate the response 11 and 12 into draft accordingly with clear details

Response 1: Thank you very much for your comments on the introduction. According to your comments, we have added the original responses 11 and 12 to the paper and specifically revised them as follows：“However, these studies only addressed the theoretical and microscopic mechanisms of the partial quality tools. An experiment using the partial quality tool to improve the oil recovery of low permeability reservoirs in the oil displacement experiment has not been carried out, and the degree of influence on the oil recovery is still unknown. Therefore, it is necessary to conduct oil displacement experiments from the point of view of actual production and consider the influence of different polymer solutions and oil layers with different permeability on the oil displacement process of partial quality tools, which provides a thought and technical methods for later research on using physical methods to improve the oil recovery of heterogeneous reservoirs.”

The specific revision content is on the page 3 of the revised paper, lines 102-110.

Point 2: Partial quality tool- please include a clear para whether it was used in the field before atleast on a pilot scale- If not list the potential features of it so that it can be used in the field without any issues. Relevant references needs to be cited. This is very important.

Response 2: Thank you very much for your comments on the effective application of the partial quality tool. First of all, the past research on the partial quality tool only studied the changes of polymer solution before and after flowing through the partial quality tool from the perspective of theory and micro-experiments. The introduction in this respect has been introduced in the introduction part of the paper（References 21-22）. What we really want to know is whether the use of the partial quality tool can improve the oil recovery in actual application, that is, in actual oil displacement experiments. However, in order to verify the effect of the partial quality tool, during the field experiment, due to factors such as production environment and time, we chose the indoor experiment to carry out relevant application, and the laboratory experiment can also verify the effect of the partial quality tool. This is the purpose of this study, and on the basis of this study, relevant field experiments can be carried out. However, due to the slightly complicated process and the use of the partial quality tool on the basis of layered injection, the overall oil displacement feasibility is not affected, so for the sake of brief explanation, this aspect is ignored in this paper. In other words, this paper is to verify the effectiveness of the partial quality tool from the perspective of laboratory experiments, providing theoretical and experimental basis for future field experiments.

Author Response File: Author Response.docx

Reviewer 2 Report

Answered my questions properly.

Recommend for publication.

About the English writing, please consult the editorial office.

Author Response

Response to Reviewer 2 Comments

First of all, thank you very much for your comments on this paper. We tried our best to improve the manuscript and made some changes in the manuscript. And the replies to your comments are as follows. We appreciate for your warm work earnestly, and hope that the correction will meet with approval. Once again, thank you very much for your comments and suggestions.

Point 1: About the English writing, please consult the editorial office.

Response 1: Thank you very much for your comments on the language part of the paper. According to your comments, we have already revised it through MDPI's polishing mechanism. We have uploaded the specific polishing documents to the supporting documents.

Author Response File: Author Response.docx