Study on Oil Composition Variation and Its Influencing Factors during CO2 Huff-n-Puff in Tight Oil Reservoirs
, Hui Gao 1,3,4,*, Zhiwei Zhai 2,5,6,*, Xiaoyong Wen 7,8, Nan Zhang 9, Chen Wang 1,3,4, Zhilin Cheng 1,3,4, Teng Li 1,3,4 and Deqiang Wang 10,11
Round 1
Reviewer 1 Report
In this paper, the composition of oil and the change of oil properties, CO2 exhalation in tight oil reservoirs are studied. Experimental methods such as using numerical simulation methods to study different factors, such as original oil, reservoir temperature, reservoir pressure, and production time. Generally speaking, the idea of this paper is good. It provides some guidance for the implementation of CO2 huff-n-puff. However, there are still some weaknesses in the paper. The problems are summarized below. I think this paper can be published after solving the following questions.
1. There are some spelling mistakes in the paper. It is recommended to check the English writing carefully before publication.
2. Figure 1 shows a schematic diagram of the experimental setup. However, it is also recommended to include an experimental system in the paper.
3. In Section 2.1, the authors state that quartz sand and clay are mixed together and packaged into the model to achieve permeability in ultra-low tight reservoirs. The permeability of the sandbag model is 0.3×10-3μm2. Are quartz sand and clay mixed volume ratio or mass ratio? If so, what percentage? The authors should add more information about this content.
4. In Figure 2, the volatile components (C1-C5) appear to be producing oil. At the same time, the light component content (C6-C12) is less compared to the original oil. What are the reasons for this phenomenon?
5. In section 3.1, recommendations are on paper.
6. In the summary section, I strongly suggest that the author add a concluding sentence to emphasize the meaning of the text.
7. What is the abbreviation for GEM in section 3.1?
8. In Section 3.1, it should be stated that "the fluid PVT model used in reservoir numerical simulation is based on the equation of state established by Pen-Robinson".
9. In Section 3.1, what parameters are adjusted by the author to accommodate the properties of the actual crude oil?
Correct some spelling mistakes
Author Response
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Reviewer 2 Report
In this manuscript (processes-2538793), the authors investigate the CO2 huff-n-puff experiments with two different types of crude oil. It also uses numerical simulation to study how the production process affect the oil composition and property variation. However, some major revisions are needed before the manuscript can be considered further. The authors should address the following comments:
1. The introduction should include a review on the variation of oil composition and physical properties. This would support the statement that “the dynamic variation of oil composition and physical properties has not been fully investigated yet.”
2. The citation location is incorrect in some places. Moreover, some most recent work close to this study should be commented as background, e.g., “Experimental study of enhanced oil recovery by CO2 huff-n-puff in shales and tight sandstones with fractures. Pet. Sci. 18, 852–869 (2021).”, “An integrated model with stable numerical methods for fractured underground gas storage. Journal of Cleaner Production, 2023, 393, 136268”, etc. Please also clarify the difference of this paper with the first paper in Pet. Sci., since their titles are similar.
3. The section 2.2 Experiment procedures is too long and dense. It is suggested to split it into multiple paragraphs to enhance readability.
4. The terms “extraction” and “production” are used interchangeably in the text. Could the authors please clarify the difference between them, or standardize them to one term?
5. The connection between experiment and simulation is not clear enough. The authors are requested to explain how the experiment and simulation are related, and how they complement each other.
6. The values of Radius and Thickness in Table 1 are not clear. It is suggested to add model figure to illustrate them.
7. The dotted lines in Figure 5, 6, 8, and 10 are not in the right position. Please adjust them accordingly.
Author Response
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Reviewer 3 Report
The manuscript is presented well but the authors must address the following concerns:
1. Where was the 2nd heavy oil obtained?
2. The permeability of the sandpacks seems to be a few orders of magnitude higher than a typical tight oil reservoir (less than 0.1 mD)
3. How was the oil composition measured?
4. How was the sand contained inside the sandpack reactors?
5. How do you account for dead volume inside lines?
6. What was the injection pressure during the Huff stage? Was it above MMP of the crude oil?
Author Response
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Round 2
Reviewer 2 Report
All the review comments have been properly addressed. This paper can be considered to be accepted.
Author Response
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Reviewer 3 Report
Please add some data related to the MMP of the crude oil. Also, please include some sentences about the sand's permeability compared to a typical shale rock in the manuscript. This is important so that the reader has context about the disparity in this property.
Author Response
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