Numerical Simulation of Hydrogen Diffusion in Cement Sheath of Wells Used for Underground Hydrogen Storage

Round 1

Reviewer 1 Report

Numerical Simulation of Hydrogen Diffusion in Cement 2 Sheath of Wells Used for Underground Hydrogen Storage

       i.          Introduction section is poor. There are many studies which are presented in lumped form without any explanation. Contribution of each study should be acknowledged properly.

     ii.          Assign proper reference to governing equation.

   iii.          Font size is not clear, especially in tables and figures. It should be uniform.

   iv.          Justify following statement in abstract

“Hydrogen should take 13 about 7.5 days to fully penetrate a 35cm cement sheath under expected well conditions.”

     v.          How diffusion phenomenon is uniform and controlled for Carbon emissions and hydrogen. ?

   vi.          There are some old fashioned references are presented to justify the literature survey. Replace such studies with recent contributions on current topic.

   vii.          Proper commas/full stop needed after all equations.

  viii.          The source of Fick's second law needed. The suggested source is   “https://doi.org/10.1080/17455030.2022.2121443”

     ix.          Improve the novelty of work. The motivation of work should be clear and convincing. Justify how current work is different from already performed studies. Authors fails to present goal of work clearly.

      x.           

     xi.          Explanation of numerical scheme is completely missing. The steps provided in appendix cannot be justified. How computational simulations are performed? At which accuracy, the results are prepared?

   xii.          Conclusion section is not convincing. Provide some more quantities observations.

  xiii.          More discussion on results needed. Include some physical explanation of parameters.

Some typo errors should be corrected. 

Author Response

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Reviewer 2 Report

This paper is about numerical simulations of hydrogen diffusion through the cement sheath of oil and gas wells used in underground hydrogen storage (UHS). This numerical modelling was based on simulating Fick's second law of diffusion for fixed concentration boundary conditions. Finite Difference Method (FDM) was employed in this research to solve the equation numerically. There are minor comments requiring explaining:

1) Finite Difference Method (FDM) requested to extend for readers.

2) Figure 1: It needs to explain 2.89 days (see legend text). The hydrogen takes about 2.89 days to penetrate the cement sheath fully.

3) It needs one or more examples from other methods/models to do comparisons with the current study.

Author Response

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Reviewer 3 Report

The manuscript is well-written and clearly structured. It aims to evaluate the hydrogen diffusion in cement sheath. However, the overall quality of the research work is low. It is a basic simulation based on a simple model. Although the manuscript claimed that simulation models were validated, it also admitted that it was validated using “a simplified homogeneous system”, and the reviewer was unable to see the efficacy of such validation.

Meanwhile, some of the statements were controversial and confusing, like the statement “Although the solution was verified analytically, the input dataset used for the numerical studies was not matched with experimental data because of the absence of existing experimental literature that measures the rate and depth of hydrogen degradation in class G or H well cement under UHS conditions.” It is unclear to the reviewer with mismatched dataset etc how validation can be carried out?

Furthermore, the simulation results presented in Figures 2 and 5, it is not clear to the reviewer how those parameters were selected, there is no justification in the manuscript.

In all, this is an interesting manuscript and may be suitable as an undergraduate project and investigation, but it lacks the depth to be a scientific publication.

Generally good, minor editing is required.

Author Response

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Round 2

Reviewer 1 Report

The contests of work are now improved. 

Fine. 

Author Response

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Reviewer 3 Report

Comments addressed.

Author Response

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