Reactive Transport Modeling and Sensitivity Analysis of CO₂–Rock–Brine Interactions at Ebeity Reservoir, West Kazakhstan

Round 1

Reviewer 1 Report

This paper illustrates the simulation work focus on CO2-rock-brine interactions at Ebeity reservoir, west Kazakhstan by using 1D PHREEQC software. The results show that the change in brine water promoted the CO2 mineralization process, forming dawsonite and ankerite from Fe, Ca, and Al elements. The simulation data are enriched and the analysis and discussion are in place. Therefore, I think the research results of this paper are very meaningful and should be able to accepted for publication.

 

Some comments as follow:

1.      It is suggested that the abstract and conclusion should be rewrite to focus on the simulation results and sensitivity analysis, to show new discovery of this work.

2.      The Figure 2 should include more information on the stratigraphic column chart, and so need to be revised.

3.      Need to explain why you chose this PHREEQS software to simulate over others such as tough2, CMG, etc.

4.      The chemical reaction formula needs to be modified.

Author Response

Reviewer #1:

This paper illustrates the simulation work focus on CO₂-rock-brine interactions at Ebeity reservoir, west Kazakhstan by using 1D PHREEQC software. The results show that the change in brine water promoted the CO2 mineralization process, forming dawsonite and ankerite from Fe, Ca, and Al elements. The simulation data are enriched and the analysis and discussion are in place. Therefore, I think the research results of this paper are very meaningful and should be able to accepted for publication.

Some comments as follow:

• It is suggested that the abstract and conclusion should be rewrite to focus on the simulation results and sensitivity analysis, to show new discovery of this work.

Thanks for the reviewer’s comment. We have rewritten the abstract and conclusion, including detailed sensitivity analysis results.

• The Figure 2 should include more information on the stratigraphic column chart, and so need to be revised.

We have revised Figure 2. Please check the updated figure. The figure was adapted from Duffy et al. [8], related to the geology of Ebeity reservoir, located in the Zharkamys West area.

• Need to explain why you chose this PHREEQS software to simulate over others such as tough2, CMG, etc.

PHREEQC has been known for its user-friendly interface, which makes it accessible to researchers with fundamental knowledge of geochemistry. Also, it is well-designed for geochemical speciation modeling, particularly in terms of CO₂ storage research. Finally, the results produced by PHREEQC serve as a foundation for subsequent academic research development and publications for Kazakhstan's local environments.

 

• The chemical reaction formula needs to be modified.

Equation 2 in the manuscript has been modified by following  https://doi.org/10.1016/j.ijggc.2013.01.027

   

 

Author Response File: Author Response.docx

Reviewer 2 Report

Review report on sustainability-2579928
A reactive transport modeling and sensitivity analysis of CO2–rock–brine interactions at Ebeity Reservoir, West Kazakhstan

This manuscript studied CO2-brine-rock interactions and conducted sensitivity analysis through a reactive transport modelling using PHREEQC, with an application in Ebeity reservoir, west Kazakhstan. The authors tried to identify the mineralogical and porosity changes as results of the geochemical reactions (mineral dissolution/precipitation, new mineral phase generation), and also tested the effect of surface area and gas purity on CO2 storage capability. The modelling results indicated that pH significantly decrease due to the presence of CO2, and consequently lead to the dissolution of carbonates. Sensitivity analysis showed it is important to characterize the surface area for an accurate simulation.

This paper could be a good supplement to the current understandings on CO2-brine-rock interactions and CO2 storage potential in Ebeity Reservoir, West Kazakhstan. From my point of view, this manuscript has the potential for publication after minor revisions. A number of suggestions are given, but not limited to the following:

1.     Page 6: contents of line 139-147 and line 147-156 are exactly the same.

2.     Section 3.1: ‘…complete filling of the pore space by 1 L of brine’. You assumed that the Swi is 100% and no gas phase in the initial state. That is a rare and strong assumption since usually it is more reasonable to have both water phase and gas phase (pre-existing CH4 and other gas in reservoirs) in the pore. Without strong evidence from the field, it is hard to convince me making that assumption.

3.     What is the mass of rock you considered in the initial model (compared to the mass of water is 1kg)?

4.     Section 3.2: what is the CO2 injection rate considered in the RTM?

5.     Figure 3 and Figure 5: it would be wonderful if the authors provide more details on the calculation of porosity. The initial porosity is 15%, and the change of porosity is due to the mineral dissolution/precipitation. But how did you quantitatively characterize the change?

6.     The main results and discussion make sense to me.

 

7.     Glad to see that the authors mentioned several assumptions of the modelling on Page 9. However, the limitations of the modelling or the consequence of these assumption on results were not well discussed.

Minor edits

Author Response

Reviewer #2:

This manuscript studied CO2-brine-rock interactions and conducted sensitivity analysis through a reactive transport modelling using PHREEQC, with an application in Ebeity reservoir, west Kazakhstan. The authors tried to identify the mineralogical and porosity changes as results of the geochemical reactions (mineral dissolution/precipitation, new mineral phase generation), and also tested the effect of surface area and gas purity on CO2 storage capability. The modelling results indicated that pH significantly decrease due to the presence of CO2, and consequently lead to the dissolution of carbonates. Sensitivity analysis showed it is important to characterize the surface area for an accurate simulation.

 

This paper could be a good supplement to the current understandings on CO2-brine-rock interactions and CO2 storage potential in Ebeity Reservoir, West Kazakhstan. From my point of view, this manuscript has the potential for publication after minor revisions. A number of suggestions are given, but not limited to the following:

 

• Page 6: contents of line 139-147 and line 147-156 are exactly the same.

 

Thanks for the reviewer’s valuable feedback. The sentences have been revised and corrected.

• Section 3.1: ‘…complete filling of the pore space by 1 L of brine’. You assumed that the Swi is 100% and no gas phase in the initial state. That is a rare and strong assumption since usually it is more reasonable to have both water phase and gas phase (pre-existing CH4 and other gas in reservoirs) in the pore. Without strong evidence from the field, it is hard to convince me making that assumption.

 

We want to express our appreciation for the reviewer’s insightful comments. In this paper, our model has been developed to verify the implications of geological CO₂ injection and geochemistry changes investigation in the Kazakhstani reservoir. This is the main goal of the paper at this moment. The diverse assumptions and uncertainties raised by the reviewer will be the subjects of the upcoming research investigations, where detailed simulation input information about the reservoir geo-environmental conditions will be carefully obtained in the field and laboratory, and experimental verifications will be thoroughly applied. We expect the research will help to advance the exploration of CO₂ injection in Kazakhstan in the near future.

 

• What is the mass of rock you considered in the initial model (compared to the mass of water is 1kg)?

 

In the initial model, we have considered a rock mass of approximately 2.75 kg per 1 kg of water. This calculation incorporates the rock's density of 3.23 g/cm³ and accounts for the initial porosity of 15% within the cell. Therefore, for every 1 kg of water, there are approximately 2.75 kg of rock present in the modeled system.

 

• Section 3.2: what is the CO2 injection rate considered in the RTM?

 

In the RTM, Gas_PHASES 1 maintains a CO₂ injection rate by adjusting its dissolution/exsolution to maintain a specific partial pressure of 10^-1.75 within cell 1. This control mechanism ensures that the partial pressure of CO₂ is maintained even in the presence of CO₂-consuming reactions.

 

• Figure 3 and Figure 5: it would be wonderful if the authors provide more details on the calculation of porosity. The initial porosity is 15%, and the change of porosity is due to the mineral dissolution/precipitation. But how did you quantitatively characterize the change?

 

Thanks for the reviewer’s comment regarding porosity calculations. We have quantitatively characterized the change in porosity resulting from mineral dissolution/precipitation using the following approach. To calculate the change of the porosity based on the molar concentration changes in the system, we used the formula,

volume = molar volume x molarity

 

We also used the formula,

 

where Porosity represents the final porosity of the material as a fraction or percentage, V_voids represents the ratio of the volume of void spaces (pores, fractures, etc.) within the material to the total bulk volume of the material, and Porosity_initial is the initial porosity of the material. This calculation allows us to account for changes in porosity due to mineral reactions.

 

We have added more details to the supporting information.

 

• The main results and discussion make sense to me.

 

Thanks for the reviewer’s feedback. This helps us to maintain the importance of the subject in the local area.

 

• Glad to see that the authors mentioned several assumptions of the modelling on Page 9. However, the limitations of the modelling or the consequence of these assumption on results were not well discussed.

 

Thanks for the reviewer’s valuable feedback. The assumptions could have limitations, i.e., (1) Isothermal and Isobaric conditions may not fully capture the dynamic variations found in real geological formations; (2) The assumption of uniform distribution of brine species and minerals within the cell streamlines calculations but may not represent the natural heterogeneity often encountered in geological systems; (3)

The omission of gas phase migration should be noted when applying the model to scenarios where gas mobility plays a significant role; (4) As a result of the constant flow rate assumption, the model may not accurately represent the complexities of fluid transport in such systems. Consequently, the model's predictions should be understood in the context of these simplifications. The limitations and their discussions have been added to the manuscript.

 

 

Author Response File: Author Response.docx

Reviewer 3 Report

I have found the article interesting and of quality. The analyses carried out and showed in this manuscript are accurate and concrete; moreover, the storage of carbon dioxide within porous sediments is worthy of investigation and within the scope of the Journal.

Therefore, I suggest to consider this article for publication.

I have only some minor comments:

- Revise subscripts along the whole text.

- References are not correctly cited in the text, use numbers within square brackets.

The quality of the english language is suitable for publication.

Author Response

Reviewer #3:

I have found the article interesting and of quality. The analyses carried out and showed in this manuscript are accurate and concrete; moreover, the storage of carbon dioxide within porous sediments is worthy of investigation and within the scope of the Journal.

Therefore, I suggest to consider this article for publication.

I have only some minor comments:

• Revise subscripts along the whole text.

All the subscript issues have been corrected throughout the paper.

 

• References are not correctly cited in the text, use numbers within square brackets.

Thanks for the reviewer’s comment. The references have been re-styled accordingly.

 

Author Response File: Author Response.docx