Simulation Analysis of CO2-EOR Process and Feasibility of CO2 Storage during EOR
Roman Berenblyum
Round 1
Reviewer 1 Report
The title imposes that CO2 storage is the key topic, however it receives minor attention compared to optimisation of EOR process as such.
Abstract do not give a clear picture on storage aspect. On 20-22 "In this paper, those two segments (oil production and retention) were observed and compared in 72 reservoir simulation cases and, after that for various CO2 and oil prices." - unclear
33-35 states "Enhanced oil recovery (EOR) with CO2 injection might be attractive because of the carbon dioxide retention in the reservoir [1], which provides a positive effect on emission reduction" - I do not believe that during EOR process more CO2 is retained than will be generated using oil and gas recovered. This aspect needs to be addressed.
On 39: CO2EOR is not the only commercial level - CO2 is used in beverage and agriculture. It is however the only process that can contribute to storing CO2 on industrial scale.
60-63: “There are no guidelines for the analysis or selection of WAG ratios, well distance, permeability and time of primary production parameter based on multi-case simulation study as an input. The main reason for the absence of such guidelines and in general the reason such analysis is not performed is the long run-time of a typical compositional reservoir model.” – that’s a very questionable statement. May be that’s because WAG ratios, well distance would be highly dependant upon difference in geological and fluid properties and general dependency is too complicated and potentially not needed?
182: “Earnings of a CO2 EOR storage project comes from oil production and avoided CO2” – avoided CO2 “earnings” would typically be given to emitters capturing their CO2. Therefore, EOR project can benefit from reduced CO2 price and other may be other contractual arrangements. States phrase is oversimplification of the real picture.
Introduction similarly to abstract barely touches on storage witch is stated as the topic of the paper in the title, optimisation criteria on page 5, after line 216 shows optimisation for pure EOR case. Co-optimisation of EOR and storage was studies and published around 10 years ago.
On materials and models: How well pattern was selected is not explained and its very special and uncommon. Is reservoir open or closed? That could affect all results. Is CO2 dissolution in water accounted for or not?
In results positive aspect of EUA is presented, however additional costs for permissions / monitoring / liability are not mentioned or discussed. EOR process do not automatically qualify as storage in many countries and additional costs are needed.
On 399: “It is not possible to achieve positive NPV if CO2 price is below 15 €/t.” CO2 is bought to produce oil. How lower price makes the whole process uneconomic???
In conclusions:
Numbering is wrong.
In conclusion 4: “an optimal case has permeability of 50 mD” Its fundamentally wrong to present reservoir and control parameters like this. WAG ratios are controlled, geology is given. Correct way to address it is to conclude how geology and fluid properties affect what optimal WAG ratio and other control parameters should be in order to achieve maximum NPV. And / Or what is the optimal range of field parameters for application of EOR.
Storage is only mentioned in conclusions under “Finally” as is given very small attention in the paper in general.
Author Response
In following answers, we marked part added to the manuscript in green and all deleted parts in red.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors presented the numerical simulation results of the CO2-EOR process. The topic is of interest to the CCUS industry. However, an extensive revision must be conducted for its publication due to the following reasons:
[Abstract]
- p1 line 19: In abstract, If it were CCS, I would be with "retention maximization." However, it is hard to agree to the phrase "retention maximization" for CO2-EOR; because it is expected the injected valuable CO2 to keep moving and contacting with more oil for the incremental oil production.
- line 21: 72 simulation cases; however, 54 simulation cases were mentioned in Conclusions (line 427 on p19). Check the consistency.
[Introduction]
- The current form is too long to read because detailed literature reviews were included. Please consider split the current Introduction into Introduction and Literature Review.
- There are many papers related to CO2-EOR and storage in oil reservoirs. Compared to those, it is unclear what are the novelty and originality of this paper. Please highlight them specifically.
[Results]
- The authors considered the following variables: permeability, depth, well distances, WAG ratios, interest rate, CO2 price, oil price, and electricity prices. The design level of variables is 2 to 4. However, the analysis was simple. Also, the results would be specific to the reservoir case described on p.6. Meanwhile, the authors pointed out there are no guidelines for CCS (see lines 60-61 on page 2). Although I do not agree with this claim, please strengthen the logic with some grounds that the results of this study can be utilized as a guideline.
[Discussion]
- It seems that the current form is part of Results.
- Consider rewriting the Discussion by leaving the limitations of this study.
Author Response
In following answers, we marked part added to the manuscript in green and all deleted parts in red.
Author Response File: Author Response.pdf
Reviewer 3 Report
In this manuscript, the authors evaluated oil production and retention by comparing different CO2-EOR reservoir simulation cases and economic analysis.
Comments
- The abstract needs to be rewritten to present a concise summary of the work. Sentences 1, 2 and 3 in the abstract section should be removed.
- The authors should clearly state the novelty of the work.
- Given that this is a numerical simulation work, one would expect numerical convergence test, numerical error analysis and benchmarking/validation to be carried. This is missing in this work.
- The relationship between storability, retention as defined by equation 5 and inverse of permeability is not clear. A mathematical equation with reference(s) should be presented for this purpose.
- Equations 4 and 5 should be properly typeset.
- A 3D simulation model from Eclipse should be included to demonstrate CO2 -oil displacement and retention process.
- The purpose of the economic analysis using NPV is not very clear. This appears to overshadow the main focus of the manuscript which is CO2-EOR.
- The title is confusing and should be rewritten to give a more succinct highlight of the project focus.
Author Response
In following answers, we marked part added to the manuscript in green and all deleted parts in red.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Overall, the revised manuscript was fairly amended for publication in its current form.
Correct some typos such as Where -> where on lines 132, 227, 233, and so on.
Author Response
Thank you for all your comments and suggestions.
Author Response File: Author Response.pdf
Reviewer 3 Report
The authors have responded to few of the comments raised. However, some critical comments are still outstanding and the manuscript is not publishable in its current form.
The abstract needs to be rewritten to present a concise summary of the work; not a cosmetic change. The first sentence in the abstract section should be removed.
Given that this is a numerical simulation work, one would expect numerical convergence test, numerical error analysis and benchmarking/validation to be carried. This is missing in this work. Not providing such simple evaluation of the methodology would make this manuscript weak and not publishable in Energies journal. Specific references where such evaluation has been carried out should also be provided.
The relationship between storability, retention as defined by equation 5 and inverse of permeability is not clear. Proving the relationship between these variables without adequate mathematical basis will make it difficult for readers to comprehend the manuscript. Therefore, the authors should provide a mathematical equation with reference(s) for this purpose.
A 3D simulation model from Eclipse should be included to demonstrate CO2-oil displacement and retention process. Displaying the 3D models and outcomes should not be difficult if numerical simulations have been carried out. Not providing such evidence and other numerical analysis mentioned earlier would diminish the quality of the manuscript.
The purpose of the economic analysis using NPV is not very clear. This needs to be clarified.
Author Response
Thank you for all your comments and suggestions.
Author Response File: Author Response.pdf
Round 3
Reviewer 3 Report
The authors have addressed the concerns raised previously.
Minor comments:
- In Fig. 10, the legend and texts on the axes are not readable.
- Equation 8 must be properly typeset by removing the words and replacing with mathematical symbols.
Author Response
Thank you!
Author Response File: Author Response.pdf
