Experimental Investigation of Pore Structure and Movable Fluid Traits in Tight Sandstone

Round 1

Reviewer 1 Report

General comments 

The manuscript entitled "Experimental investigation of pore structure and  movable fluid traits in tight sandstone" presented an integrated methodology to evaluated the pore structure in sandstones. 

The paper is in general well-written and well-motivated. The methodology is quite interesting and presents a good solution. 

I found minor problems regarding the methodology specially for the micro computed X-ray tomography. Also the state of the art is not well-investigated for this method, currently the Synchrotron-based X-ray microCT can reach good spatial resolution able  to resolved pores of diameters near to the micron. Also, there is plenty of this work made in porous carbonates and sandstone. In addition, X-ray microCT images have been used for performing direct fluid flow simulations.  I recommend to add some literature to fill this gap in the state of the art. Also, some literature could be useful to strengthen the argument of the authors in the discussion. 

The processing of the images and the segmentation is not described in the text. In the results, a couple of images for both reconstructed and segmented slices/volumes could help the reader to have a better idea of the quality of the images. 

Some figures are too small, difficult to understand the meaning, or simply are not described. 

Please be sure that a mother tongue in English check your manuscript.

Good luck 

Comments for author File: Comments.pdf

Author Response

Response to Reviewer 1 Comments

Point 1: The manuscript entitled "Experimental investigation of pore structure and movable fluid traits in tight sandstone" presented an integrated methodology to evaluated the pore structure in sandstones. The paper is in general well-written and well-motivated. The methodology is quite interesting and presents a good solution.

Response 1: T    he authors appreciate the comments from the reviewer.

Point 2: I found minor problems regarding the methodology specially for the micro computed X-ray tomography. Also the state of the art is not well-investigated for this method, currently the Synchrotron-based X-ray microCT can reach good spatial resolution able to resolved pores of diameters near to the micron. Also, there is plenty of this work made in porous carbonates and sandstone. In addition, X-ray microCT images have been used for performing direct fluid flow simulations.  I recommend to add some literature to fill this gap in the state of the art. Also, some literature could be useful to strengthen the argument of the authors in the discussion.

Response 2: Thank you. The minor problems regarding the methodology have revised according to the reviews from the attached PDF file, and the detailed responses were presented below. Besides, we have added some up-to-date literatures to fill the gap in the state of the art for the micro CT method and strengthen the argument in the discussion.

Point 3: The processing of the images and the segmentation is not described in the text. In the results, a couple of images for both reconstructed and segmented slices/volumes could help the reader to have a better idea of the quality of the images.

Response 3: The authors appreciate the good questions from the reviewer. The processing of the images and the segmentation are very important, and sometimes it is necessary to provide these details. However, the purposes of the research were describing the characteristics of pore structures and movable fluid traits accurately, and micro CT was only one of the tests, hence, we could not provide too much information about the reconstruction and segmentation. Nevertheless, we do believe that reconstructed slice is helpful for the better understanding of the quality of the images, thus, we replaced Fig. 8 (a) and (d) to the reconstructed 3D CT images.

Point 4: Some figures are too small, difficult to understand the meaning, or simply are not described.

Response 4: Thank you. We have enlarged Fig. 6 and Fig. 7 to make them clear. We have provided the detailed description of each image in Fig. 2.

Point 5: Please be sure that a mother tongue in English check your manuscript.

Response 5: Thank you. We have asked some native speakers to carefully check the grammar and language and revised accordingly.

Point 6: Line 3, traits?

Response 6: It’s equal to movable fluid characteristics. We believe we use the appropriate word.

Point 7: Line 15, “aroused” or “caused”?

Response 7: We have changed it to “caused”.

Point 8: Also in other kind of reservoir rocks like porous sandstones and deformed porous carbonates.

Response 8: Thank you, we have added the literatures

Point 9: Line 65. Synchrotron-based CT can be useful technique for pore sizes near to the micron. For instance the Elettra Synchrotron (Italy)-SYRMEP beamline- previous works focuses in sandstones and porous carbonates have been done with voxel sizes in the range of 9.0 to 1.25. It is important to mention that the spatial resolution is very close to the voxel size with this technique.

Actually, this synchrotron and others facilities that can reach a sub-micron resolution. Of course, there is always a compromise between resolution and study volume.

Response 9: The authors appreciate the knowledge shared by the reviewer. We have added some literatures.

Point 10: Line 84. Please define. The definition is in lines 182, too far.

Response 10: Thank you, we have revised it.

Point 11: Line 175. Not sure if it is a correct term. Another problem is that the resolution of micro-CT facilitates enhance year after year. So, super-high resolution is meaningless.

It is important to distinguish conical beam and parallel beams like Synchrotron, where, additional to the distance, it is possible to exploit the phase contrast effects.

Response 11: The resolution of micro-CT facilitates enhance year after year, therefore, we have changed “super-high resolution” to “relatively high resolution” to make the content meaningful.

The apparatus produced conical beam, we have added the related content in the revised manuscript.

As far as the distance and phase contrast effects, we believe they may have little relationship with the content, however, the authors will go on to study in detail in the future.

Point 12: Line 177. Is this the spatial resolution or voxel size?

Response 12: It is spatial resolution.

Point 13: Line 180. It is not clear, how did you segment the images before analyses.

Response 13: There are multiple methods for image segmentation, we used the threshold value segmentation in this research. We have added the related information, and the details for how this is done are shown in Yang and Liu (2007) and Ma and Chen (2014).

Point 14: Line 234. The images (a-l) are not explained in the caption.

Response 14: We have added the detailed description of each image.

Point 15: Line 345. Too small, instead of 3x2 try 2x3 arrangement.

Response 15: We have enlarged the figures.

Point 16: Line 365. I prefer as a reader to see a reconstructed slide of your sample. Also, you should briefly describe the processing and the segmentation of the images. The segmentation is perhaps the most important step. So, if you do not show the reconstructed images, and the segmented images it is hard to validate your results.

Response 16: Thank you, we have replaced Fig. 8 (a) and (d) to the reconstructed 3D CT images. Besides, we used the threshold value segmentation in this research and have added the related information in the revised manuscript.

Point 17: Line 382. Skeletonized volume? Which method did you use? Probably you should make a zoom to better appreciate the skeleton structure.

Some authors previously investigating the pore-network of porous carbonates have obtain the connectivity density (from skeletonized pore networks) and also differentiated between connected and isolated porosity (using connected regions, like FIJI plugging) . Maybe these methods can help to better use this data.

Response 17: Thank you, we have replaced Fig. 8 (a) and (d) to the reconstructed 3D CT images. Therefore, the figures of skeletonized volume were abandoned.

Point 18: Figure 8. If the spatial resolution is 3 microns, how did you resolve pores radius lower than 1.5 microns?

Response 18: The micro CT has drawbacks in the calculation of the pores larger than 3 μm, suggesting that the pore radius that below to 3 μm were unreliable, but the data can still reflect the results.

Point 19: Line 431. Tiny is relative, please use a reference.

Response 19: Thank you, we have added the references.

Point 20: Line 431. Micro-CT images have been used for performing direct fluid flow simulations like lattice-Boltzmann and computed permeability values. The results have been very close (same order of magnitude) to laboratory based methods. This observation can strengthen your argument.   See some recent publication in Advances in Water Resources, also in Geosphere journal.

Response 20: The authors appreciate the share from the review. We have added some literatures.

Point 21: Line 434. In fact, isolated pores' contribution to fluid flow and permeability is negligible. But, why not showing this differences between isolated and connected pores?

Response 21: Thanks for your valuable advice. In fact, we believe if we want to show this difference, some tests need to be supplemented. For instance, in order to eliminate the effects of pore radius on fluid flow, we need to choose at least two homogenous materials with the same pore radius, one has isolated pores, the other has connected pores. Then compared their differences. We believe your suggestion is valuable, we could do it in the future.

Point 22: Line 508. “aroused” or “caused”?

Response 22: We have changed it to “caused”.

Point 23: Line 607. X-ray micro CT?

Response 23: Sorry, it should be X-ray diffraction.

Point 23: Line 629. It is difficult to get the message here.

Response 23: Sorry, we have rewritten the sentence to make it clear.

References:

Ma T, Chen P. Study of meso-damage characteristics of shale hydration based on CT scanning technology[J]. Petroleum Exploration and Development, 2014, 41(2): 249-256.

Yang G, Liu H. Study on the rock damage characteristics based on the technique of CT image processing [J]. Journal of China Coal Society, 2007, 32(5): 463-468.

Reviewer 2 Report

With a multidisciplinary approach, through the use of a combination of different tools and methodologies such as Thin Sections, Scanning Electron Microscope, X-ray, Pressure Controlled Mercury Intrusion, Rate Controlled Mercury Intrusion, Computer Tomography and Nuclear Magnetic Resonance, the paper presented by Liu et al.  discusses in detail how the variation of pore structures and movable fluid within Upper Triassic Yanchang tight sandstone could deteriorate the quality of a reservoir. The study, suggests in particular, that the deterioration of reservoir quality it is strongly affected by the reduction of larger pores, while the aqueous phases remain in the tiny pores as surface water. As results, the Authors suggest that pore structures control the lower limits of the movable pore radius affecting the reservoir quality. Indeed intergranular pores represents the lower limits of pore radius in higher permeability sandstones while, pore structure heterogeneity limits the improvement of the deposit permeability.

The paper is well organized and the English is fluent and smoothly. References are complete and up-to-date.

Despite this, few corrections must be made as follow:

Fig. 1-a) The location map of the study site has no coordinates. It is fundamental to add latitude and longitude coordinates. Furthermore, a North arrow must be added.

Fig. 3) The four graphs shown in the figure show an inverted abscissa axis. I honestly do not understand the reason since in a percentage scale the zero is always placed on the left and 100% to the right. To improve the interpretation and the comparison of the graphs I suggest to invert the abscissa axes.

Fig. 6) Same problem of figure three. The figure show an inverted abscissa axis with 0% placed to the right. The abscissa axes must be inverted.

Overall, this work represent a good contribution and deserves to be published after minor revisions.

Author Response

Response to Reviewer 2 Comments

Point 1: With a multidisciplinary approach, through the use of a combination of different tools and methodologies such as Thin Sections, Scanning Electron Microscope, X-ray, Pressure Controlled Mercury Intrusion, Rate Controlled Mercury Intrusion, Computer Tomography and Nuclear Magnetic Resonance, the paper presented by Liu et al.  discusses in detail how the variation of pore structures and movable fluid within Upper Triassic Yanchang tight sandstone could deteriorate the quality of a reservoir. The study, suggests in particular, that the deterioration of reservoir quality it is strongly affected by the reduction of larger pores, while the aqueous phases remain in the tiny pores as surface water. As results, the Authors suggest that pore structures control the lower limits of the movable pore radius affecting the reservoir quality. Indeed intergranular pores represents the lower limits of pore radius in higher permeability sandstones while, pore structure heterogeneity limits the improvement of the deposit permeability.

The paper is well organized and the English is fluent and smoothly. References are complete and up-to-date.

Response 1: The authors appreciate the comments from the reviewer.

Point 2: Despite this, few corrections must be made as follow:

Fig. 1-a) The location map of the study site has no coordinates. It is fundamental to add latitude and longitude coordinates. Furthermore, a North arrow must be added.

Response 2: Thank you, we have revised it.

Point 3: Fig. 3) The four graphs shown in the figure show an inverted abscissa axis. I honestly do not understand the reason since in a percentage scale the zero is always placed on the left and 100% to the right. To improve the interpretation and the comparison of the graphs I suggest to invert the abscissa axes.

Response 3: The authors appreciate the good questions from the reviewer. In fact, there is no strict requirement on the abscissa axis of capillary pressure curves, however, the scholars in the field have their routine. We always choose the inverted abscissa axis. Please refer to other publication (Sakhaee-Pour and Bryant, 2014; Wu et al., 2017).

Point 4: Fig. 6) Same problem of figure three. The figure show an inverted abscissa axis with 0% placed to the right. The abscissa axes must be inverted.

Response 4: Please refer to response 3.

Point 5: Overall, this work represent a good contribution and deserves to be published after minor revisions.

Response 5: The authors appreciate the comments from the reviewer.

References:

Sakhaee-Pour A, Bryant S L. Effect of pore structure on the producibility of tight-gas sandstonesEffect of Pore Structure on the Producibility of Tight-Gas Sandstones[J]. AAPG bulletin, 2014, 98(4): 663-694.

Wu H, Ji Y, Liu R, et al. Insight into the pore structure of tight gas sandstones: A case study in the Ordos Basin, NW China[J]. Energy & fuels, 2017, 31(12): 13159-13178.