Experimental Study on the Forcible Imbibition Law of Water in Shale Gas Reservoirs
Round 1
Reviewer 1 Report
Dear Authors/Editor
The paper “ Experimental study on forcible imbibition law of water in shale gas reservoir” by ZHAO Zhihong，He Yanyan，GUO Jianchun，Zheng Xiaoqiang, TAO Liang, and DENG Xianan represents an interesting theoretical and experimental study on the imbibition phenomena occurring in shale formations during fracturing operations.
I suggest the authors to fix the following points for a potential publication of the paper:
1) The state of the art in excessively concentrate in the introduction. A dedicated paragraph is suggested together with and extension of the intro focusing on the problem statement
2) For sack of clarity, I warmly suggest to inset the explanation of each parameter/variable near each formula
3) In the “ analysis of experimental results “ very few comments concerned the effect of mineralogy
4) Numerous assumptions are adopted for the theoretical approach. Please clearly set them and highlight their simplifying effects concerning the real phenomena
5) A clear and straightforward experimental validation of the theoretical method is missed
6) Please explain source/methods for values in table 3
7) Be aware about Chinese letters in the 1
Best regards
Author Response
#Reviewer 1: The paper “ Experimental study on forcible imbibition law of water in shale gas reservoir” by ZHAO Zhihong，He Yanyan，GUO Jianchun，Zheng Xiaoqiang, TAO Liang, and DENG Xianan represents an interesting theoretical and experimental study on the imbibition phenomena occurring in shale formations during fracturing operations.
I suggest the authors to fix the following points for a potential publication of the paper:
1) The state of the art in excessively concentrate in the introduction. A dedicated paragraph is suggested together with and extension of the intro focusing on the problem statement
R: Thanks for your valuable suggestion, a dedicated paragraph has been added in Introduction. See L71-82 for details.
2) For sack of clarity, I warmly suggest to inset the explanation of each parameter/variable near each formula
R: As you suggest, the explanation of each parameter has been inserted near each formula.
3) In the “analysis of experimental results “very few comments concerned the effect of mineralogy
R: Thanks for your valuable suggestion. The results of mineral composition of rock samples indicate that the rock samples of Longmaxi Formation contain 10.2%-46.7% clay minerals, 21.2%-62.1% quartz, and 4.3%-37.2% carbonate minerals, and the rock samples have large differences in mineral composition content with strong non-homogeneity. The results of water imbibition experiments indicate that rock samples S4 and S5 with high clay content have strong imbibition capacity, S3 rock samples do not have poor imbibition capacity because of the high quartz content, while S1 rock samples with the lowest total clay mineral and quartz content show the worst imbibition capacity overall. The clay mineral content is the main control factor affecting the imbibition capacity characteristics of shale, while the quartz content also has a non-negligible influence on the imbibition capacity characteristics of shale, and the imbibition capacity of shale is a combination of clay mineral and quartz content, which is consistent with Zhao's study.
The comments concerned the effect of mineralogy have been added in L252-263.
4) Numerous assumptions are adopted for the theoretical approach. Please clearly set them and highlight their simplifying effects concerning the real phenomena
R: Thanks to your suggestion, we have included the underlying assumptions and an explanation of the simplifications in front of each model.
5) A clear and straightforward experimental validation of the theoretical method is missed
R: Thanks for your suggestion. The shale samples were subjected to imbibition experiments, and the imbibition depths of clay pore and non-clay pore of the samples were tested separately. The experimental results were compared with the calculated results of the theoretical model, which indicated that the multiple types of pores model established in this study was in good agreement with the experimental results and proved the usability of the theoretical model.
Figure 1. Comparison of experimental and theoretical model results..
6) Please explain source/methods for values in table 3
R: The viscosity, molar concentration and other parameters of the liquid of the Longmaxi group were tested using measuring instruments such as a fluid viscometer, the contact angles of advance and retreat were measured based on the seat-drop method, and the surface tension after equilibrium was measured by an optical tensiometer.
The explanation for values in table 3 has been added in L405-409.
7) Be aware about Chinese letters in the 1
R: We are very sorry for our carefulness, the Chinese letters have all been translated.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The review of the manuscript entitled: “Experimental study on forcible imbibition law of water in shale gas reservoirs”. The work is well-written and easy to follow it. By responding to the following comments and questions, the work can be ready for publication:
1. In the abstract, how the experimental results depicted 3 periods, imbibition diffusion, imbibition transition and imbibition balance periods. The reason or basis should be mentioned.
2. Is it possible to use the results for other reservoirs? For example, for Longmaxi formation, there are 3 periods. How will be these periods for other conditions and reservoirs? It should be explained.
3. In the introduction section (first paragraph), it is recommended to add a sentence about the production recovery techniques such as nanoparticle application. For this purpose, the next reference can be used in the revision stage: https://doi.org/10.1007/s13202-022-01583-1
4. The purposes of the manuscript should be presented in a separate paragraph at the end of the introduction section with more details.
5. Please add a reference for the next sentence: “As we know, in the initial stage of shut-in after fracturing of shale gas well, the fluid in hydraulic fractures would imbibe into shale matrix and natural fractures under the effect of fluid pressure and reservoir temperature and pressure, the imbibition in this stage is called forcible imbibition.”
6. What is the role of the reactor in the core flood apparatus (Fig. 1)? Is there any chemical reaction?
7. Please translate Chinese terms to English in Table 1.
8. The method of porosity and permeability measurement should be mentioned.
Author Response
#Reviewer 2: The review of the manuscript entitled: “Experimental study on forcible imbibition law of water in shale gas reservoirs”. The work is well-written and easy to follow it. By responding to the following comments and questions, the work can be ready for publication:
- In the abstract, how the experimental results depicted 3 periods, imbibition diffusion, imbibition transition and imbibition balance periods. The reason or basis should be mentioned.
R: According to imbibition saturation growth rate, the shale forcible imbibition can be divided into 3 periods, imbibition diffusion, imbibition transition and imbibition balance periods. Among them, the imbibition diffusion period is the main period for imbibition capacity rise. The reason for this phenomenon is that due to the fluid pressure difference effect, the shale fills its large pores and microfractures rapidly in the early stage, and in the percolation transition period, the percolation rate decreases continuously due to the gradual increase of fluid saturation. Due to the Jamin effect, it is difficult for the fluid to enter the small pores, and the fluid fills the pore roar channel, the seepage saturation tends to stabilize, and the seepage equilibrium period appears.
The reason and basis for 3 periods has been added in L18-23.
- Is it possible to use the results for other reservoirs? For example, for Longmaxi formation, there are 3 periods. How will be these periods for other conditions and reservoirs? It should be explained.
R: Three periods of shale forcible imbibition are also observed in other reservoirs. Such a pattern is also observed in the static percolation experiments of Ordos core, which is because the rate of water entering large pores and microfractures is relatively fast first, and then the rate tends to slow down gradually as the rock samples are gradually saturated. Most of the percolation experiments have a pattern of water ab-sorption rate from fast to slow and finally leveling off.
The explanation has been added in L211-215.
- In the introduction section (first paragraph), it is recommended to add a sentence about the production recovery techniques such as nanoparticle application. For this purpose, the next reference can be used in the revision stage: https://doi.org/10.1007/s13202-022-01583-1
R: Thanks for your suggestion, the reference have been used in the introduction section.
- The purposes of the manuscript should be presented in a separate paragraph at the end of the introduction section with more details.
R: Thanks for your suggestion, the purposes of the manuscript have been presented in a separate paragraph at the end of the introduction section.
- Please add a reference for the next sentence: “As we know, in the initial stage of shut-in after fracturing of shale gas well, the fluid in hydraulic fractures would imbibe into shale matrix and natural fractures under the effect of fluid pressure and reservoir temperature and pressure, the imbibition in this stage is called forcible imbibition.”
R: Thanks for your valuable suggestion, a reference has been added.
- What is the role of the reactor in the core flood apparatus (Fig. 1)? Is there any chemical reaction?
R: The role of the reactor in the device is to provide a reaction site for shale water imbibition, shale rock samples through the reactor for the water imbibition process, there is no chemical reaction.
- Please translate Chinese terms to English in Table 1.
R: Thanks for your careful reading, we have translated Chinese terms to English in Table 1.
- The method of porosity and permeability measurement should be mentioned.
R: Thanks for your suggestion. We have added the method of porosity and permeability measurement: Using ELK-2 ultra-low permeability meter with pulse decay method as a prototype, for the decay characteristics and pressure changes during the ultra-low permeability test, calculating the permeability before and after the experiment and HXK-â…¢ helium porosity meter by Boyle's law, under the same temperature, the product of pressure and volume of the gas as a constant, to measure the porosity before and after the experiment.
Author Response File: Author Response.pdf
Reviewer 3 Report
The manuscript reports the study on forcible imbibition law of water in shale gas reservoirs in Longmaxi Formation located in southern Sichuan area. The results reported represent a notable advance in the shale gas development technologies. The research is interesting and my main concerns are the structure of the sections and the lack of justifications as follows:
1. Line 4: Formate the names of the authors according to journals' instructions.
2. Corrections needed throughout the text: An abbreviation, the ”etc“ must be followed by a dot (”etc.“). Also the reference citations „et al.“ must be corrected and consistent in the text. Sometimes you write ”et al“ or ”et. al.“
3. Line 89: Figure 1: Appart from diagram please provide the photographs of used experimental equipment.
4. Section 2.3. Why is the Longmaxi Formation chosen for this study? Please provide the appropriate description of this geological formation with its lithology followed by a map and borehole locations.
5. Table 1: Translate the name of the fourth column in English.
6. Please provide the photographs of the experimental samples and their exact position in the core sample.
7. Lines 117-118: ”It can be seen from this figure that the 7 shale samples are mainly composed of quartz, carbonate rocks, and clay minerals, and higher in content of clay minerals...”
This sentence is not clear. Shale is a sedimentary rock composed of very fine clay particles. Clay forms from the decomposition of the mineral feldspar. As shale is a rock it can’t be composed of carbonate rock! Instead of carbonate rock write the mineral calcite. Figure 2 must be explained in the text. Which minerals from the group of feldspars are present in studied samples?
8. Figures 9 and 10: Which minerals are considered as fragile? or “brittle minerals” (line 254)? This must be explained.
9. Figures 5-8, 12,13,15: Font type and formation are not unique!
10. Section 5: ”Fig. 13 shows the physical parameters of some of the shale samples before and after forcible imbibition under the formation temperature and pressure.” Which shale samples from the seven used in this study are presented in the figure 13 and how their mineralogical content affect the results obtained? Correct the spelling of figure 13.
11. Section 6: The Conclusion needs to be rewritten. Start with the brief summary of your work followed by the main findings. Explain why the clay minerals are the main controlling factors of microfactures induced by shale hydration. What about the other minerals presented in shale samples? The novelty explanation is desirable, what is new in this work? Authors should critically point out their weak parts and discussed them to support some future research in this subject.
Author Response
#Reviewer 3: The manuscript reports the study on forcible imbibition law of water in shale gas reservoirs in Longmaxi Formation located in southern Sichuan area. The results reported represent a notable advance in the shale gas development technologies. The research is interesting and my main concerns are the structure of the sections and the lack of justifications as follows:
- Line 4: Formate the names of the authors according to journals' instructions.
R: Thanks for your careful reading, we have formatted the names of the authors according to journals’ instructions.
- Corrections needed throughout the text: An abbreviation, the ”etc“ must be followed by a dot (”etc.“). Also the reference citations „et al.“ must be corrected and consistent in the text. Sometimes you write ”et al“ or ”et. al.“
R: Thanks for your careful reading, we have corrected to make it consistent in the text.
- Line 89: Figure 1: Appart from diagram please provide the photographs of used experimental equipment.
R: Thanks for your suggestion, the photograph of used experimental equipment has been provided in L103.
Figure 2. Used experimental equipment for water forced imbibition of water in shale.
- Section 2.3. Why is the Longmaxi Formation chosen for this study? Please provide the appropriate description of this geological formation with its lithology followed by a map and borehole locations.
R: Thanks for your careful reading, Longmaxi Formation. When fracturing the Longmaxi Formation reservoir, the shale absorbs water and leads to low fracturing fluid rejection rate. It is important to select shale rock samples from the Longmaxi Formation and analyze its forcible imbibition characteristics. The experimental samples were taken from Longmaxi Formation cores collected from southern Sichuan area. The reservoir, the major pay in Longmaxi Formation from which the cores were collected has burial depths of 2400-2450m, The black carbona-ceous shale, black carbonaceous shale interbedded with dark gray laminated siltstone, gray to gray-green mud siltstone and gray siltstone are developed from the bottom to the top, and the organic carbon and silica content gradually decreases and the color gradually becomes lighter. As the map is still in a confidential stage, we are sorry that we cannot provide a map and borehole locations.
The related description has been added in L124-132.
- Table 1: Translate the name of the fourth column in English.
R: We are very sorry for our carelessness, the Chinese in the fourth column has been translated into English
- Please provide the photographs of the experimental samples and their exact position in the core sample.
R: Thanks for your suggestion, the photographs of experimental samples are provided in L141.
Figure 3. Experimental samples (a) and their exact position in the core sample (b).
- Lines 117-118: “It can be seen from this figure that the 7 shale samples are mainly composed of quartz, carbonate rocks, and clay minerals, and higher in content of clay minerals...”
This sentence is not clear. Shale is a sedimentary rock composed of very fine clay particles. Clay forms from the decomposition of the mineral feldspar. As shale is a rock it can’t be composed of carbonate rock! Instead of carbonate rock write the mineral calcite. Figure 2 must be explained in the text. Which minerals from the group of feldspars are present in studied samples?
R: Thanks for your careful reading, we have written calcite instead of carbonate rock. According to the test results, the feldspar in the rock samples mainly consisted of sodium feldspar and potassium feldspar.
- Figures 9 and 10: Which minerals are considered as fragile? or “brittle minerals” (line 254)? This must be explained.
R: Thanks for your suggestion. The brittle minerals are mainly composed of quartz, feldspar and carbonate.
- Figures 5-8, 12,13,15: Font type and formation are not unique!
R: Thank you for your careful reading, we have standardized the type and formation of all the figures.
- Section 5: “Fig. 13 shows the physical parameters of some of the shale samples before and after forcible imbibition under the formation temperature and pressure.” Which shale samples from the seven used in this study are presented in the figure 13 and how their mineralogical content affect the results obtained? Correct the spelling of figure 13.
R: Fig. 13 shows the results of S4-6 shale samples. S4 has the most clay minerals content and S6 has the least clay minerals content. The porosity and permeability of the three groups of rock samples did not differ much before water imbibition, and with water imbibition in the shale, the porosity and permeability increased to different degrees, and the increase had a positive correlation with the clay minerals content.
- Section 6: The Conclusion needs to be rewritten. Start with the brief summary of your work followed by the main findings. Explain why the clay minerals are the main controlling factors of microfactures induced by shale hydration. What about the other minerals presented in shale samples? The novelty explanation is desirable, what is new in this work? Authors should critically point out their weak parts and discussed them to support some future research in this subject.
R: In this study, a new experimental device for spontaneous imbibition of fracturing fluid under the conditions of formation temperature and confining pressure was de-signed. A set of experimental apparatus and corresponding test and evaluation methods were developed to analyze laws of forcible imbibition of water in shale reservoirs. Clay minerals are the main controlling factors of shale hydration. We reach the following conclusions through the study.
- According to imbibition saturation, the shale forcible imbibition of Longmaxi Formation shale reservoirs in Southern Sichuan can be divided into 3 periods, imbibition diffusion, imbibition transition and imbibition balance periods.
- The contribution degree of different pore water imbibition depth of shale is clay pore, brittle mineral pore and organic pore in turn. The mineral composition of the induced microfracture areas was tested. The results shows that clay minerals dominate, with an average content of 50.1%, which indicates that clay minerals are the main controlling factors of microfractures induced by shale hydration.
- The reservoir confining pressure has prohibition on shale imbibition, but even under reservoir confining pressure, imbibition can still improve the fracturing effect of reservoir, resulting in the increase of porosity of 0.42-1.63 time and increase of permeability of 17.6-67.3 times. Under the experimental conditions, the initiation time of induced microfractures is 98.5h on average, and is in negative correlation with imbibition capacity. On this basis, the optimized shortest shut-in time of shale gas well is 5days.
- In this study, the pore network development characteristics and connectivity were not considered, and the pore network characteristics can be characterized by CT scan subsequently to study the effect of pore network characteristics on the water imbibition effect of shale rock samples.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The paper was revised based on my comments. Thus it is ready for publication.
Reviewer 3 Report
The authors have addressed all the comments and concerns raised. The revision has significantly improved the manuscript. I have no additional comments. Overall, the manuscript is now well written and the reported results are of interests to the readers.
