Effect of Residual Water in Sediments on the CO2-CH4 Replacement Process
, Caili Huang 1,2,3,5, Dongliang Li 1,2,3,5,* and Deqing Liang 1,2,3,5Round 1
Reviewer 1 Report
Manuscript ID: energies-2270985
Type of manuscript: Article
Title: Effect of Residual Water in Sediments on the CO2–CH4 Replacement Process
The comments for authors are provided in the word file, attached below.
Comments for author File: 
Comments.docx
Author Response
The paper gives insight into the residual water effect in sediments on the CO2-CH4 replacement method, considering it is a promising method of gas hydrate recovery. However, some minor revisions should be done. These are given below.
- In the Abstract it is stated that “The replacement amount of CH4 increased with the increase in initial hydrate saturation, while the recovery efficiency decreased”. The recovery is decreased, so this is not the drawback of the study.
Response: Thank you for your comments. We agree that the decrease in recovery is not a drawback of the study. The abstract "CH4 replacement recovery increases with increasing initial hydrate saturation, while methane recovery efficiency decreases" means that CH4 replacement recovery increases with increasing initial hydrate saturation, while CH4 recovery efficiency decreases with increasing initial hydrate saturation. From commercial aspect, a high recovery efficiency means more CH4 can be produced which is useful to offset the cost for CO2 sequestration. If we only consider the CO2 storage, CH4 recovery efficiency is not the drawback as you noted. In this case, the word “while” has been replaced by “and” according to your suggestion.
- In the 2.4 section, some equations are provided for various processes like methane hydrate saturation, the Recovery efficiency of CH4, and CO2 storage efficiency etc, but no specific values are given and no calculations are done, this is not important to provide the exact value to get out output for recovery efficiency or storage efficiency?. Please comment on it.
Response: Thank you for your comments. The various computational procedures listed in section 2.4 are very important for the recovery efficiency and storage efficiency of the discussion section. The specific steps in the whole calculation process are tedious, for example, in the calculation of the methane hydrate saturation, the calculation about the compression factor involves iterative algorithm, and we use MATLAB mathematical software to assist the calculation, so we do not put the detailed calculation process into the manuscript, but only give the final results of the calculation. The recovery efficiency of CH4 and the sequestration efficiency of CO2 were calculated by combining the data measured from the Gas Chromatograph（GC）analysis with the equations in Section 2.4 to obtain the final results.
- The factors like the chemical properties of the sediment and the salinity of the sediment such as pH and mineral composition, can affect the amount of residual water in it. Sediments with high pH or high mineral content can retain more residual water, which can hinder the CO2-CH4 replacement process. Can you study these factors?
Response: Thank you for giving us valuable suggestions and ideas. There are many factors affecting the CO2-CH4 replacement process including high pH or high mineral content. The pH value in the pore water is closely related to the residual water content and sediment composition. In this case, the effects of pH and mineral composition on the CO2-CH4 replacement process is the important direction in our following research.
- The English language and spelling should be checked and try to avoid the repetition of a similar word.
Response: Thank you for your valuable suggestions. We have read the manuscript carefully, rechecked for linguistic and grammatical errors, and then made careful changes and optimizations
Reviewer 2 Report
Effect of Residual Water in Sediments on the CO2–CH4 replacement process
Lu et al. studied the CO2-CH4 replacement process in natural gas hydrate using simulated experimental conditions. The author has studied the effects of sediment particle size and initial hydrate saturation on CO2-CH4 displacement.
The experiments are carefully conducted, and the results are straightforward. The article could be accepted with minor revisions. My comments are mentioned below.
1. minor spell check and typo errors need to be addressed. For example, provide space in the initial12 h (Page 2, Line 48)
2. Check superscripts of CO2, and CH4 in all the figures.
3. Since CO2-CH4 displacement depends upon the sediment particle size, the trend of increase in pressure in Exp 1 (Figure 3), and its explanation seem too ambiguous. Please provide more details and references. The explanation with reference to surface area is not clear.
4. Recyclability on CO2 sequestration and CH4 replaced study need to be carried out for the best (say for 101 µm particle size)
5. Provide the error on the data generated during experiments.
Author Response
Lu et al. studied the CO2-CH4 replacement process in natural gas hydrate using simulated experimental conditions. The author has studied the effects of sediment particle size and initial hydrate saturation on CO2-CH4 displacement.
The experiments are carefully conducted, and the results are straightforward. The article could be accepted with minor revisions. My comments are mentioned below.
- minor spell check and typo errors need to be addressed. For example, provide space in the initial12 h (Page 2, Line 48)
Response: Thank you for your valuable suggestions. We have read the manuscript carefully, rechecked for linguistic and grammatical errors, and then made careful changes and optimizations
- Check superscripts of CO2, and CH4 in all the figures.
Response: Thank you for your valuable suggestions. We have carefully completed the revision of the manuscript under your suggestion. We modified the superscripts of CO2 and CH4 in Figures 5, 6, 8, 10, 11, and 12.
- Since CO2-CH4 displacement depends upon the sediment particle size, the trend of increase in pressure in Exp 1 (Figure 3), and its explanation seem too ambiguous. Please provide more details and references. The explanation with reference to surface area is not clear.
Response: Thank you for your comments. We are very sorry for not explaining the reason clearly and we have revised the manuscript with your suggestion. The pressure variation trend of experiment 1 in Figure 3 shows a gradual increase in pressure during the first 10 h of the reaction and a gradual decrease in pressure starting after 10 h of the reaction. The pressure trend of Experiment 1 in Figure 3 shows that the pressure gradually increases during the first 10 h of the reaction, and then the pressure starts to gradually decrease after 10 h of the reaction. We believe that the amount of CH4 gas replaced during the first 10 h of the reaction is greater than the sum of CO2 gas consumed in the replacement reaction and CO2 gas consumed in the formation of CO2 hydrate with water, so the total pressure in the kettle finally tends to rise. The reason is that the specific surface area of the porous medium is larger when the particle size is smaller, and the area of the CO2 replacement reaction surface is larger, which means the effective area of the replacement reaction is larger, so the amount of CH4 replaced will be larger. We refer to the following literature: Kawasaki, T. , Ukita, T. , Fujii, T. , Noguchi, S. , & Ripmeester, J. A. . (2011). Particle size effect on the saturation of methane hydrate in sediments - constrained from experimental results. Marine and Petroleum Geology, 28(10), 1801-1805. The literature mentions that the smaller the particle size of porous media, the larger the specific surface area and the larger the reaction area. We are sorry to confuse surface area and specific surface area, what we want to express is specific surface area. Specific surface area is the total area per unit mass of material in m2/g. It usually refers to the specific surface area of solid materials, such as powder, fiber, granule, flake, block, etc.
- Recyclability on CO2 sequestration and CH4 replaced study need to be carried out for the best (say for 101µm particle size)
Response: We sincerely thank you for your suggestion and we strongly agree that more experiments or data are needed to support the conclusions of this work. We are currently conducting additional experimental studies on CO2 sequestration and replacement to verify the reproducibility of the experiments.
- Provide the error on the data generated during experiments.
Response: Thank you for your valuable suggestions. During this experiment, the following errors existed:
The first is that the pressure before the replacement reaction will fluctuate up and down at 3 MPa, and it is difficult to ensure that the replacement pressure of each reaction starts at 3 MPa. The initial replacement pressures of Experiments 1-8 are: 3.069, 3.023, 3.014, 3.024, 3.032, 3.024, 3.039, and 3.021 MPa, respectively, and their relative errors for the set 3 MPa produced were 2.3%, 0.8%, 0.5%, 0.8%, 1.1%, 0.8%, 1.3%, and 0.7%, respectively.
The second is that the temperature before the replacement reaction will fluctuate up and down at 277.15 K. It is difficult to ensure that the replacement temperature of each reaction starts at 277.15 K. The initial replacement temperatures of Experiments 1-8 are: 276.97, 276.55, 277.12, 276.50, 277.2, 276.50, 277.19, 276.59 K, which are relative to the set of 277.15K produced relative errors of 0.06%, 0.22%, 0.01%, 0.23%, 0.01%, 0.02%, 0.01%, and 0.02%.
The third is the error of gas chromatograph in analyzing CO2 and CH4 gas components, the error of analysis of CO2 and CH4 are: 0.12% and 0.03% respectively.
