the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Oct 2024
Modeling Commercial-Scale CO2 Storage in the Gas Hydrate Stability Zone with PFLOTRAN v6.0
Abstract. Safe and secure carbon dioxide (CO2) storage is likely to be critical for mitigating some of the most dangerous effects of climate change. In the last decade, there has been a significant increase in activity associated with reservoir characterization and site selection for large-scale CO2 storage projects across the globe. These prospective storage sites tend to be terrestrial sites selected for their optimal structural, petrophysical, and geochemical trapping potential. However, it has also been suggested that storing CO2 in reservoirs within the gas hydrate stability zone (GHSZ), characterized by high pressures and low temperatures (e.g., Arctic or marine environments), could provide natural thermodynamic and solubility barriers to gas leakage. Evaluating the prospect of commercial-scale, long-term storage of CO2 in the GHSZ requires reservoir-scale modelling capabilities designed to account for the unique physics and thermodynamics associated with these systems. We have developed the HYDRATE flow mode and accompanying fully implicit parallel well model in the massively parallel subsurface flow and reactive transport simulator PFLOTRAN to model CO2 injection into the marine GHSZ. We have applied these capabilities to a set of CO2 injection scenarios designed to reveal the challenges and opportunities for commercial-scale CO2 storage in the GHSZ.
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RC1: 'Comment on gmd-2024-162', Anonymous Referee #1, 30 Oct 2024
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This paper presents the development of the HYDRATE flow model within the PFLOTRAN framework, designed to simulate CO₂ storage in the form of CO₂-hydrate. The model is applied to assess two distinct scenarios of CO₂ injection into the hydrate stability zone. This modeling approach provides valuable insights into the potential challenges and opportunities associated with commercial-scale CO₂ storage as hydrate. The manuscript is well-constructed and includes high-quality figures. I recommend the following minor revisions to enhance its clarity:
- The description about well model is not quite clear. How is the well pressure calculated? I suggest a separate section describing the well model.
- Please briefly explain “Neighbor cell ghosting” in the text.
- Please add the depth of BHSZ in section 3.1
- I suggest adding a phase diagram used in this study.
Citation: https://doi.org/10.5194/gmd-2024-162-RC1 -
RC2: 'Comment on gmd-2024-162', Ewa Burwicz-Galerne, 11 Nov 2024
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The authors provided well-written and concise modeling study on two CO2 injection scenarios into marine sediments within the gas hydrate stability zone. Please find below my suggestions on how to further improve the content of this paper related mostly to the modeling scenarios description and presentation of the results.
Detailed comments:
- line 10: 'prospective storage sites tend to be terrestrial' - this is probably true from the US perspective, however, the entire sentence on structural, petrophysical etc. trapping is absolutely right also for marine settings. I would therefore recommend to remove the terrestrial part.
- line 14: I would say upfront whether you refer to the CO2 GHSZ or CH4 GHSZ which might be much more common to the readers
- line 23: 'terrestrial' CO2 storage - again, perhaps the 'terrestrial' part could be removed as it applied to all types of sites
- line 39: if you use CO2 throughout the text, then also use CH4 instead of methane
- line 41: no need to have both terms- carbon dioxide and CO2 here, it was stated already in the abstract
- line 42: 'CO2 hydrate that forms in similar pressures and temperatures...' - I would say that these stability differences between CH4 and CO2 are rather crucial? To help the readers, you could give some examples on GHSZ thickness for both components at identical conditions.
- line 46: please stick to CH4 instead of methane for consistency
- line 48: 'to kick out methane' is a bit of a jargon
- line 49: what about released CH4 during this substitution process? Could you model this process as well?
- line 56: 'Relative to deeper sediment, this point usually represents a minimum temperature and pressure' - please rephrase or remove this sentence
- lines 66-67: repetition of 'demonstrated'
- line 134-135: 'This means that for one well, only one extra row and one extra column are used in the fully implicit flow Jacobian, not an extra row and extra column for each reservoir cell associated with a well' - please rephrase the sentence for clarity or add some additional description
- line 156: 'salt is tracked in the aqueous phase' - since there is salt precipitation in the system, you could explain the salt mass balance in a bit more detail here
- line 218 and 220: '...and very far into the future.' - please explain a bit more precisely the time frame
- line 286: 'seafloor temperature' - do you mean bottom water temperature? is it realistic to assume a constant bottom water temperature for 150 years?
- line 291: how realistic is the injection rate of 5,000 kg/year for 150 years? Why did you use these exact numbers?
- line 292: what is 'BHP'?
- line 305: please indicate the base of GHSZ in fig. 1
- line 320: 'CO2 trapping mechanisms insufficient due to thermal buffering effect' - when would the buffering effect work? what would be the critical conditions e.g. injection rates or temperature of CO2? How long would it take to equilibrate the system? Some discussion needed
- lines 336-337: why in scenario 2 dTdz and Ttop are different than in scenario 1 (20/30 deg.C/km, 3/5 deg.C)?
- line 351: what is 'DOE CarbonSAFE storage complex'?
- line 362: 'well model adjusts the gas flow rate' - what do you mean? is your simulations accounting for that or is it physically happening during injection experiments? please explain.
- lines 374-375: how does salt precipitation affect your modeled migration fields for both gas and pore fluids? I would suggest to include the velocity fields either in one of the existing figures or as a separate material. Your discussion also refers a lot to the permeability reduction effect (which is nice) which could be more highlighted in the paper as one of your key results.
- line 442: ' we demonstrate these new (model) capabilities on a series of test problems' - in the current paper, there are two scenarios which might seem not sufficient for this term. I would rather explicitly say '(...) on two test problems', just for clarity.
Figures:
- in general, GHSZ should be indicated on all figures. Fig. 1 Sh and Ss could be presented in a zoom since non-zero concentrations are only in the uppermost part.
- perhaps it would be a good idea to compile a movie from each injection scenario as a supplementary material
- Fig.2 - please mark intervals from fig. 3
- Fig. 3: what is alpha (Pa-1) in the last panel?
A suggestion for enhanced content: I would be glad to see a discussion on the implications of multi-phase migration within sediment overburden above the well/injection points which might lead to a potential leakage, and your predictions on challenges related to the injection inside a mixed CO2-CH4 GHSZ with pre-existing CH4-GH deposits.
Citation: https://doi.org/10.5194/gmd-2024-162-RC2
Data sets
Modeling Commercial-Scale CO2 Storage in the Gas Hydrate Stability Zone: Input Data Michael Nole https://zenodo.org/records/13619874
Model code and software
PFLOTRAN v6.0 Glenn Hammond et al. https://pflotran.org
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