Optimization of Well Placement in Carbon Capture and Storage (CCS): Bayesian Optimization Framework under Permutation Invariance
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis is a good technical paper discussing the application of Bayesian optimization in well placement study in a CCS setting. Interesting results are presented showing the optimal sequestration of CO2 in a saline aquifer.
Here are some comments to improve the work:
- I don't think "applied sciences" is one of the keywords.
- Line 40: what is "CO2-Saline"?
- CO2 storage in saline aquifers is proven in some projects, but depleted fields are only emerging storage sites (not much proven with focus on storage and not EOR). I don't think these are the best references to deliver the message.
- Line 60: if you decide to name the analytical solutions (which are not much in the context of what is being discussed), you should provide more details and discuss how these citations fit in the paragraph.
- Line 79: what is "Petrophysical fluid properties"?!
- Line 83: why are you mostly talking about hydrocarbons? since the topic is CCS, I suggest to write more general, and e.g., discuss why phase behavior is important for CCS.
- Line 88 onward: reference(s) required when you discuss the CCS plan.
- Intro section by end of page 2 (before well placement section): there is no good flow of topics and a clear path to develop the discussion towards well placement. Please re-write for a better read.
- Line 100 & 101: again, focus is on production performance and oil production, while other topics relevant to paper can be discussed.
- Intro section should discuss more (and better) published studies about Bayesian optimization for well placement. You should also discuss well placement studies in CCS setting.
- Line 162: this is not the best assumption that the reader is familiar with Multivariate Gaussian distributions.
- section 3.1: more details and description of your reservoir system is required since this is not either discussed later in case study.
- Line 291: the goal should be to avoid not postpone reaching fracing pressure.
- Table 1: is your average pressure 5 psi?!
- Line 298: what is "several decades"?
- section 3.3: Decision variables & constraints?
- Line 396: what observations?
- section 4: intro to simulation case study is needed
- how realistic is it to have 8 producers in a CCS project? what is the basis for these numbers? more details is needed.
- Table 3&4: explain the basis for selecting the numbers in tables 3 and 4
- Line 609: explain more about the generalization. How?
- Discussion: the first page reads more as a summary rather than discussing the results presented in the previous section.
- Page 2 of discussion is much better, but can you support these explanations by showing more evidence, i.e., what happens if a different configuration is used?
- edit the caption of fig. 13. what does it show?
Line 640: what "two primary insights"?
Comments on the Quality of English LanguageI suggest double-checking of the language. I found these two issues:
- Line 65: "combined to" is not right.
- Line 396: principal?
Author Response
see attached file
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThis is a well-executed study that proposes a novel and mathematically sound approach for efficiently optimizing well placement in CCS.
Here are some specific suggestions:
Methods: Include some quick background on the reservoir simulation (governing equations, discretization scheme, key assumptions) for completeness.
Figure 2: Clarify if this is the permeability field or another property shown. Label colorbar and specify units.
Algorithm 2: Define variables the first time they are used (e.g. f^-1).
Line 460: Expand discussion of how the modified kernel satisfies Mercer's conditions.
Line 548: Quantify the speedup from the kernel modifications.
Conclusion: Discuss potential future work, e.g. extension to different well types, inclusion of geological uncertainty, etc.
Author Response
see attached file
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThank you for revising your manuscript and acting on most of the comments. I have still some comments/concerns to be addressed.
General comment: not easy to follow the changes. Sometimes, the whole section is highlighted in red but not everything is changed like in conclusions section. Please only highlight your changes. Also, since line numbers change, give proper address instead of just saying the manuscript is revised.
- The added text (line 62-75) is not helpful and is really not necessary.
I suggest to delete this section, but in Eq. 1: What is “mobility ratio of the phase”? What is influx/outflux?
- Line 79: if you name TPFA, you should provide a description.
- Line 163-176: Please refer to my comment 10 from the last review. I meant you provide more studies on Bayesian Optimization for well placement (not necessarily CCS), and you will find this topic covered in several good papers.
The comment also asked for including well placement studies in CCS setting (not necessarily BO) for which you provided 2 references but should discuss more-
- Line 200: I don’t see “The off-diagonal elements σi,j” in any of your equations.
- In the discussion section about your optimal case, also show the pressure development in the field to emphasize your choice of wells.
- Figure 14 and corresponding text: you should describe your sub-optimal case and how it deviates from the optimal case. Add more details.
- Line 705: either state explicitly what the “two primary insights” are, or move this to line 697.
- Line 714: I like this paragraph and the reflection on the limitations of the work.
Regarding the potential for aggregation into one or two total producers during the project’s design phase: Include an optimized case where you use fewer (or a single) producers and discuss if/why you see things differently. This is needed for an enriched discussion and solid conclusion.
- Back to my comment nr. 19 in last review: what is the basis for selecting 8 producers? Regarding your reply, legacy wells are potential leakage pathways, so it is really not like all the existing wells in a field are to be repurposed. Many should be plugged and monitored for this purpose.
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Round 3
Reviewer 1 Report
Comments and Suggestions for AuthorsThank you for revising and clarification. Please consider the following minor adjustments:
Line 62: "The most widely used flow model in reservoir simulation is the black-oil one." What is the basis for this? any reference?
Line 69: mobility ratio is defined as the ratio of mobility (M_a=k_a/mu_a) of two phases, but eq. 1 is written for one phase. Rewrite this part with correct terms in the formula and definitions.
Line 74: q is volumetric flow rate.
Line 734: "In the latter, injectors are not efficiently spaced to the injectors". Rephrase the sentence.
Line 747: Please describe "such aggregations", how many wells? 1 and 2?
Line 748: What is "the total sequestration rate"?
Author Response
please see the attachment
Author Response File: Author Response.pdf
