Coupled Numerical Method for Modeling Propped Fracture Behavior
Round 1
Reviewer 1 Report
Review comments: This paper presents a coupled numerical method incorporated DEM, FEM and CFD to investigate the effect of particle size and closure pressure on fracture behavior involved with proppant transport. The topic is interesting and the logical structure is well. The results could be useful for better understanding the proppant embedment mechanism and optimizing the hydraulic fracturing operation. Some comments I list below:
>> 1. In Figure 11, what is the color bar indicates and what is the discrepancy of input parameters between the undeformed and deformed assembly case? Does the fracture have an initial aperture or not? Why the undeformed case demonstrate the bigger aperture than the deformed case?
>> 2. Why the greater particle size can lead to a lower permeability? In terms of common sense, the greater particle size can form a higher porosity structure and the permeability increase.
>> 3. Why the effect of particle size on permeability is significant at low closure pressure condition but gradually weakening with increasing the closure pressure?
>> 4. In line 473, I can’t locate the position of chapter 2.12, probably the chapter number should be 2.1.2.
>> 5. The authors should examine the heading order carefully and rearrange the section number.
>> 6. As a review paper, it should be as complete as possible. A few topics on propped fracture behavior are missing, such as channel fracturing and layered fracturing. Sample publications are listed below.
Zhu, H., Shen, J., Zhang, F. (2019). “A fracture conductivity model for channel fracturing and its implementation with discrete element method”, Journal of Petroleum Science and Engineering, 172, 149-161. https://doi.org/10.1016/j.petrol.2018.09.054
Zhang, F. and Dontsov, E. (2018). “Modeling hydraulic fracture propagation and proppant transport in a two-layer formation with stress drop”, Engineering Fracture Mechanics, 199, 705-720. https://doi.org/10.1016/j.engfracmech.2018.07.008
Author Response
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Author Response.pdf
Reviewer 2 Report
In this work, a DEM-FEM-CFD coupled numerical tool is developed to model the propped fracture behavior. DEM is used to generate randomly distributed proppants in the fracture; FEM is utilized to determine the proppants be deformed and embedded into the formation; CFD is employed to investigate the permeability and conductivity of the propped fractures. The results is important and interesting. It is recommended to be published in our journal. Here are the detailed comments.
1) I am a little confused about the coupling between the FEM and CFD. At the coupling interface, the mesh sizes of FEM are consistent with that of the CFD, or NOT? If not, the Interpolation algorithm should be used to map the meshes between FEM and CFD. Please provide more discussions about the coupling between the FEM and CFD.
2) In this work, the one-way coupling method is used. While the two-way coupling methods, such as the Picard method and the Newton-Krylov method, have widely used in the coupling issues to achieve the high computational performance. Here are some successful examples, which may be useful. So I suggest to develop the two-way coupling methods in the DEM-FEM-CFD framework in the future.
[1] Numerical analysis of particulate flows with the finite element method, Ph.D. thesis, 2019.
[2] A Fully-coupled Newton-Krylov Solution Method for Parallel Unstructured Finite Element Fluid Flow, Heat and Mass Transfer Simulations, 2007.
[3] An assessment of coupling algorithms in HTR simulator TINTE, 2018.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
This paper was well revised and can be accepted with the current form.
Reviewer 2 Report
no further comment
