Experimental Study on CH₄ Hydrate Dissociation by the Injection of Hot Water, Brine, and Ionic Liquids

Round 1

Reviewer 1 Report

This work explores the effect of using hot water, brine, and ionic liquids on the efficiency of the methane recovery from hydrate reservoirs using the thermal stimulation method. The manuscript is within the aim and scope of JMSE and can be published after the following comments are addressed by the authors:

 

1.    I was wondering if “mining” is a suitable word to describe the process of methane hydrate exploitation? Can the authors confirm that this is the widely accepted term in the hydrate community?

Please see this work, where “mining” has NOT been used:

-       Yang et al., 2017. Flue gas injection into gas hydrate reservoirs for methane recovery and carbon dioxide sequestration. Energy conversion and management, 136, pp.431-438.

2.    Please declare BMIM-Cl and TMACl in the abstract and main body of the manuscript. Apart from that, I see some typos and wrong English grammar; please have your manuscript proofread. Please also make sure that the procedure is detailed in a narrative form not instructive. The equations must be numbered properly too.

3.    The authors provided interesting discussion on the exploitation methods for natural gas hydrates in introduction. However, this is inadequate as I see almost no discussion on the environmental impacts associated with the exploitation of hydrate reservoirs and dissociation of hydrates. I would suggest they expand their discussion to cover the above topic; the following works can be referred to:

-       Farahani et al., 2021. Effect of thermal formation/dissociation cycles on the kinetics of formation and pore-scale distribution of methane hydrates in porous media: a magnetic resonance imaging study. Sustainable Energy & Fuels, 5(5), pp.1567-1583.

-       Farahani et al., 2021. Insights into the climate-driven evolution of gas hydrate-bearing permafrost sediments: Implications for prediction of environmental impacts and security of energy in cold regions. RSC advances, 11(24), pp.14334-14346.

4.    “The hydrate-bearing sediments was 6.9 MPa, 9 C and 30% hydrate saturation.” Can the authors elaborate on why they chose to conduct their experiments at these very specific PT conditions?

5.    Please add more details on the experimental apparatus section. For example, the make, model, and accuracy of the temperature and pressure sensors are not provided. Figure 1 is also blurred and it’s difficult to read its content.

6.    “The sands were washed and dried to reduced the influence of purities on hydrate stability.” First, I think there are a couple of typos here, please check the sentence. Can I also ask what impurities are expected to be present in the natural sediments and how they may influence the hydrate stability? I believe the main reason for using the natural sediments is to simulate the natural conditions in the lab!

7.    “In hydrate preparation stage, 44 g natural sand and 6 g distilled water were mixed evenly and densely packed in the PTFE bushing.” How did the authors ensure the homogeneous packing and homogeneous initial water distribution? How did they ensure that the evacuation process did not have affect on the initial water distribution?

8.    Can the authors clarify why they kept the cell at 2 C during the vacuuming stage? Why not keeping it at the target temperature of 9 C?

9.    It’s unclear to me how the authors ensured that the hydrate saturation is 30%? Did they conduct any thermodynamic calculations? The details must be provided in the manuscript. I also see in the reported results that the hydrate saturations of 20 and 40% have been obtained, which is not consistent with what mentioned in the abstract.

10.  The results in Figure 3 are blurred and difficult to read. Please replace them with a high-quality figure.

11.  What is the porosity of the sediments? Can the authors discuss whether the gas production efficiency from the sediments can be also related to the pore-scale habit of gas hydrates in the sediments?

Author Response

Responses to the comments of Reviewer #1:

1. I was wondering if “mining” is a suitable word to describe the process of methane hydrate exploitation? Can the authors confirm that this is the widely accepted term in the hydrate community?

Please see this work, where “mining” has NOT been used:

-       Yang et al., 2017. Flue gas injection into gas hydrate reservoirs for methane recovery and carbon dioxide sequestration. Energy conversion and management, 136, pp.431-438.

Response: Thank you for your valuable comments. In the field of hydrate production, the term “Mining” has appeared in some journals and patents, such as Li et. Al, 2016, Optimization of the Production Pressure for Hydrate Dissociation by Depressurization, 34, pp. 4296. Yao et al, 2020, Numerical simulation of hydrate slurry flow characteristics in vertical pipes based on population balance theory. And the patent by Hamamatsu P et. al. Methane hydrate mining method used for exploiting methane hydrate from methane hydrate stratum under sea-bottom, involves turning and outputting laser beam from laser light source to methane hydrate stratum (JP2009046882). However, in general, the word “mining” appears infrequently, and it is generally believed that mining is related to “coal mining”, to avoid ambiguity, the word “mining” is modified to “gas production” or “exploitation” in this paper.

2. Please declare BMIM-Cl and TMACl in the abstract and main body of the manuscript. Apart from that, I see some typos and wrong English grammar; please have your manuscript proofread. Please also make sure that the procedure is detailed in a narrative form not instructive. The equations must be numbered properly too.

Response: We are very sorry for our negligence. We have added the abbreviated name of the ionic liquid. And we have corrected the manuscript by fixing incorrect grammar and typos. The description of the experimental procedure has been supplemented with the description of many experimental details, such as the preparation before heat injection, and the relevant contents have been added (pages 4-5). In addition, we have numbered the equations in the manuscript.

3. The authors provided interesting discussion on the exploitation methods for natural gas hydrates in introduction. However, this is inadequate as I see almost no discussion on the environmental impacts associated with the exploitation of hydrate reservoirs and dissociation of hydrates. I would suggest they expand their discussion to cover the above topic; the following works can be referred to:

-       Farahani et al., 2021. Effect of thermal formation/dissociation cycles on the kinetics of formation and pore-scale distribution of methane hydrates in porous media: a magnetic resonance imaging study. Sustainable Energy & Fuels, 5(5), pp.1567-1583.

-       Farahani et al., 2021. Insights into the climate-driven evolution of gas hydrate-bearing permafrost sediments: Implications for prediction of environmental impacts and security of energy in cold regions. RSC advances, 11(24), pp.14334-14346.

Response: We sincerely appreciate the valuable comments. We have carefully read the references you listed and added a discussion of the environmental impact of hydrate exploitation in the introduction. Because of the high energy density of natural gas hydrates and a large amount of carbon trapped in natural gas hydrates, the decomposition of natural gas hydrates is considered a potential threat associated with global warming. The amount of CH4 released by gas hydrate decomposition is typically in the range of 5Tg yr-1(5 million tons per year), and methane is 20–30 times more potent as a greenhouse gas than CO2, thereby increasing global warming. Even if methane is released during hydrate dissociation, there may be very few places where gas hydrate dissociation releases significant amounts of methane into the atmosphere, given many factors, such as the depth of gas hydrate in the sediment, the strong sediment and water column sinks, and the inability of bubbles emitted at the seafloor to deliver methane to the sea-air interface in most cases. And there is no conclusive proof that hydrate-derived methane is reaching the atmosphere now. On the current level of technology, gas hydrate extraction is slow and therefore does not have a huge impact on the marine ecosystem or the atmosphere. Problems such as submarine landslides may occur, but these can be avoided as much as possible when designing the production plan. Overall, natural gas hydrate development is safe. The reference you noted has been added in the manuscript to support the environmental impact of hydrate exploitation.

4. “The hydrate-bearing sediments was 6.9 MPa, 9℃ and 30% hydrate saturation.” Can the authors elaborate on why they chose to conduct their experiments at these very specific PT conditions?

Response: The hydrate saturation in Shenhu Area of South China Sea ranges from 20% to 48%, and the average hydrate saturation is 30%, so we choose the reservoir with 30% saturation in the control experiment; In 2007, China Geological Survey executed GMGS1 hydrate drilling voyage, and obtained hydrate solid samples in three boreholes SH2, SH3 and SH7 in the Shenhu Area. At site SH2 the sea floor is at an elevation of Z=−1235 m, and hydrate is endowed at 185 m below the seafloor surface. From the past logging data, the initial temperature at the seafloor level (1000 m below sea level) is 276.9 K, and the temperature gradient is 0.03 k/m, so the hydrate reservoir temperature at SH2 station is 276.9+185*0.03=282.45 K. Therefore, the water bath temperature is set to 9℃ in the experiment to simulate the temperature of the Shenhu Area. When the water bath temperature is set to 9℃, the reactor is about 10℃, and the corresponding phase equilibrium pressure is 6.8MPa. before heat injection, the backpressure valve is adjusted to 6.9MPa, which is 0.1MPa higher than the phase equilibrium pressure, and the hydrate will not decompose, so the subsequent gas production from hydrate decomposition is due to the heat injection decomposition.

5. Please add more details on the experimental apparatus section. For example, the make, model, and accuracy of the temperature and pressure sensors are not provided. Figure 1 is also blurred and it’s difficult to read its content.

Response: As Reviewer suggested that details of the experimental apparatus have been added to the manuscript, including water baths, advection pumps, temperature and pressure sensors, etc. The gas production measurements were carried out in a high pressure cylinder 50 mm in diameter and 50 mm in length. A PTFE bushing with a diameter of 49 mm, a height of 43.5 mm and a thickness of 4 mm was used to hold the hydrate-bearing sediments. Two thermal resistances (Beijing West AVIC Co., Ltd, PT100, 223.15K-473.15K, ± 0.1K) were inserted from the top of the cylinder, which were used to measure temperature in gas and sediments respectively. There is a pressure sensor(Beijing West AVIC Co., Ltd, CYB-20, 0-20MPa, ± 1MPa ) on the top of the reactor to monitor the pressure change in the reactor. A stainless-steel pipe that 3 mm in diameter was inserted along the axis of the cylinder which was to simulate the production well. The cylinder was equipped with an injection system that composed of a water bottle, an advection pump(Shanghai Wufeng Instrument Co., Ltd, LC-P100, 0-21MPa, 0-9.999ml/min, accuracy: ±0.001ml/min) and a piston water jacket as seen in Fig. 1. The cooling jacket was connected with the high temperature thermostatic bath(Ningbo Tianheng Co., Ltd, THGD-1020, 262.15K-374.15K, accuracy: ±0.1 ℃ ) for heat exchange, to maintain the constant temperature of the injected hot water. The water in the water bottle was injected into the bottom of the water jacket through the advection pump, pushing the piston inside the water jacket to move upward, and the hot water was pushed into the reactor. The gas collection system is composed of back pressure valve(OSK, Beijing, 15-2500psig, 250.15-373.15K, accuracy: ±0.001MPa), wet gas flow meter(Changchun Alpha Instrument Co., Ltd, LMF-1, accuracy: ±1%). The data acquisition unit records pressure and temperature varying with time. The temperature of the cylinder was maintained by low temperature thermostatic bath(Ningbo Tianheng Co., Ltd, THGD-2015, 252.15K-374.15K, accuracy: ±0.1 ℃.  Figure 1 has been replaced with a high resolution image.

6. “The sands were washed and dried to reduced the influence of purities on hydrate stability.” First, I think there are a couple of typos here, please check the sentence. Can I also ask what impurities are expected to be present in the natural sediments and how they may influence the hydrate stability? I believe the main reason for using the natural sediments is to simulate the natural conditions in the lab!

Response: Our experiment studied the production of hydrates from 40-60 mesh (0.3-0.45 mm) reservoirs of medium-coarse sand. In the process of screening 40-60 mesh sand by sieve, some fine particles of sand as well as dust will be interspersed. In addition, natural sand comes from the sea, and the Cl^-, Na⁺, Mg²⁺, SO₄^2-, etc. will be attached to the surface of the sand, which will have an impact on hydrate generation. We cannot guarantee that every grain of sand contains the same impurities, and if we do not flush the sand, we cannot guarantee that the reservoir conditions are consistent for each experiment. Besides we are not sure whether these ions will be involved in promoting/inhibiting hydrate decomposition during the experiment, so we have to flush the sand. In order to simulate the water environment of the seafloor, we will choose artificially prepared brine for hydrate generation experiments in the future. “reduced the influence of purities on hydrate stability.” Perhaps the expression is not accurate enough, we have changed it to “Rinsing and drying of sand to reduce the effect of dust and impurities on the experiment” (line 147 of page 4).

7. “In hydrate preparation stage, 44 g natural sand and 6 g distilled water were mixed evenly and densely packed in the PTFE bushing.” How did the authors ensure the homogeneous packing and homogeneous initial water distribution? How did they ensure that the evacuation process did not have affect on the initial water distribution?

Response: In hydrate preparation stage, we stirred 44g of natural sand and 6g of distilled water well in a container, because the sand was small in volume and the stirring time was sufficient, we considered that the initial water was evenly distributed in the reservoir. After sufficient mixing, a certain amount of sand and water mixture was weighed and filled into the PTFE bushing, and compaction for every two scoops of mixture. The same mass of the mixture was taken for each experiment and the height of the reservoir was the same after the sample was filled, i.e. the volume of the mixture was the same. Since the reservoir was compacted layer by layer, we believe that the sand and water were uniformly distributed in the bushing. To prevent the effect of evacuation on the reservoir, the evacuation time before each experiment does not exceed 60 s. After the evacuation is finished, open the gas cylinder and the inlet and outlet valves of the reactor and slowly deflate the air, repeating two to three times to exhaust the residual air in the kettle.

8. Can the authors clarify why they kept the cell at 2℃ during the vacuuming stage? Why not keeping it at the target temperature of 9℃?

Response: Because the reactor volume of this experiment is small, after feeding methane gas, the gas can only make contact with the upper part of the reservoir, which is dense, so it is more difficult for the gas to penetrate into the reservoir. Keeping the cell at 2°C, hydrate generation has higher subcooling, which accelerates the rate of hydrate generation. However, even if the cell keeping at 2°C, it takes about 5 days to generation 30% saturated hydrate. Explanations has been added in the manuscript.

9. It’s unclear to me how the authors ensured that the hydrate saturation is 30%? Did they conduct any thermodynamic calculations? The details must be provided in the manuscript. I also see in the reported results that the hydrate saturations of 20 and 40% have been obtained, which is not consistent with what mentioned in the abstract.

Response: During hydrate generation, we have performed thermodynamic calculations of hydrate saturation, which have been added in the section 2.2.1 (pages 5-6). Only in the experiments of hydrate dissociation with hot water injection, we have performed experiments with 20% and 40% saturation, the main purpose of which is to test the feasibility of experimental methods and devices from different perspectives, because in addition to the thermal injection parameters (thermal injection temperature and volume) that affect hydrate dissociation, geological factors (such as hydrate saturation and permeability) also have an influence on gas production. To verify the reliability of the data, we analyzed the effect of saturation on gas production in the experiments with hot water injection, and the results were consistent with those in previous literature (Ref. 42-44). Since the lack of explanation can easily lead to misunderstanding, relevant explanations have been added in the manuscript (page7, 9).

10. The results in Figure 3 are blurred and difficult to read. Please replace them with a high-quality figure.

Response: We are very sorry for this. Blurred images in the manuscript have been replaced with high-quality images, including Figure 1, Figure 3-11.

11. What is the porosity of the sediments? Can the authors discuss whether the gas production efficiency from the sediments can be also related to the pore-scale habit of gas hydrates in the sediments?

Response: The porosity of particle size measured by the volume method is 0.40, which has been added in line 149-150 of page 4 of this paper. The results of previous studies show that the gas production efficiency of sediments is related to the porosity, but in this paper, we only studied the gas production of hydrate in 40-60 mesh particle size, so we could not explore the relationship between porosity and gas production efficiency. In the future, we will choose different particle sizes to explore the hydrate gas production.

Reviewer 2 Report

This paper studied the thermal stimulation of methane hydrate by injecting the ionic liquids, hot water and also hot brine. It is quite fine with me in general; however, this paper is not suitable for publication in its current form. The following comments should be addressed in order to enhance the quality of the paper:     

-       What is the accuracy and operational range of the temperature and pressure sensors in the cell? It would be better to mention this information in the experimental section.

-       The authors have only used the abbreviated name of the ionic liquid. Please mention their extended names on first use.

-       In Tables 2-5, it would be useful if the authors differentiate between the input and output of the tests and also arrange the input columns according to their explanation in the text.

-       Based on the results given in Table 2, there is a maximum in the gas production and thermal efficiency as the injected hot water increased from 30 to 35 mL and then they decreased for 40 mL of the hot water. Therefore, it seems that 35 mL of hot water is an optimum amount to get higher efficiency, is it right? Please explain a bit more about this.

-       In Page 7, they wrote “Since the specific heat of brine is lower than pure water, less heat was injected into the hydrate-bearing sediments when the injection volume of brine was the same as hot water”, What does "less heat" mean?

-       Something is missing in the text. For example, on line 257 of Page 9, it was written "(Exp. 8, 9 and 10)" and the name of the table was not mentioned. Please check the entire text carefully for any typos.

-       Were the experimental data repeated and reproducible?

Author Response

Responses to the comments of Reviewer #2:

1. What is the accuracy and operational range of the temperature and pressure sensors in the cell? It would be better to mention this information in the experimental section.

Response: Details of the experimental apparatus have been added to the manuscript, including the brand, model, operating range, and accuracy (page 3).

2. The authors have only used the abbreviated name of the ionic liquid. Please mention their extended names on first use.

Response: Considering the reviewer’s suggestion, we have added the abbreviated name of the ionic liquid (line 16-17 of page1 and line 105-106 of page 3).

3. In Tables 2-5, it would be useful if the authors differentiate between the input and output of the tests and also arrange the input columns according to their explanation in the text.

Response: Considering the reviewer’s suggestion, we have rearranged the tables in the article. The first row of each table shows the controlled experiments, which are explained in text for the reader's understanding (pages 10-11). The tables are also placed at the beginning of each section, followed by a description of the graphs of the temperature and pressure changes during the experiment (description text at the top, graphs at the bottom)

 

4. Based on the results given in Table 2, there is a maximum in the gas production and thermal efficiency as the injected hot water increased from 30 to 35 mL and then they decreased for 40 mL of the hot water. Therefore, it seems that 35 mL of hot water is an optimum amount to get higher efficiency, is it right? Please explain a bit more about this.

Response: This is indeed the case in our experiment, where the thermal efficiency is maximum when the injection volume is 35 ml. When the injection time was increased from 35ml to 40ml, the volume of water in the cell increased, and the output water could not be discharged during the experiment due to the limitation of the experimental setup, which meant that the water layer in the reactor was thicker in the later stage of injection. When the injection volume was increased, the increased 5ml of injection heat was not fully utilized, and most of it was used to heat the decomposition gas and free water in the reactor. Besides, after filling the sand, the free space in the reactor is 49ml, and the volume available for water injection is even less (there is a 5mm between the PTFE and the reactor). When the injection volume is 40ml, the water layer has reached the top of the reactor, and the gas production well is surrounded by water, so it is more difficult for the gas to penetrate the thick water layer to reach the gas production well, resulting in the collected gas production volume being low. If this is the case, the experimental results will not change even if the experiment is repeated. In order to avoid that the gas could not be discharged due to the thick water layer, the injection volume was 30 ml in all subsequent experiments. Relevant explanations have been added on page 7.

5. In Page 7, they wrote “Since the specific heat of brine is lower than pure water, less heat was injected into the hydrate-bearing sediments when the injection volume of brine was the same as hot water”, What does "less heat" mean?

Response: The specific heat of brine is lower than that of water, i.e., the heat released from brine is less than that of water when the temperature is reduced by 1°C. The reservoir temperature is the same before heat injection, and when the same temperature and volume of brine and hot water are injected, the heat released from the brine is less and the heat absorbed by the sediment is less when the temperature of both decreases to the ambient temperature (water bath temperature) as gas production proceeds. Since the expression was not clear enough and caused misunderstanding, we changed it to "the heat used to heat the sediment becomes less", which is more appropriate (line 301-302 of page 10).

6. Something is missing in the text. For example, on line 257 of Page 9, it was written "(Exp. 8, 9 and 10)" and the name of the table was not mentioned. Please check the entire text carefully for any typos.

Response: The name of the table in which the experiment is located has been indicated after the experiment serial number to facilitate the reader's access to the data. Typos and grammatical corrections have been made in the text.

7. Were the experimental data repeated and reproducible?

Response: Some of the experiments were repeated. For the test results in line with the cognitive range of data did not repeat (such as the higher the temperature, the greater the gas production, the lower the thermal efficiency), for the data does not meet the linear law will repeat the experiment, such as the above-mentioned injection volume of 40ml, the thermal efficiency instead lower than 35ml, we repeat the experiment found that, although each experiment gas production is slightly different, but are lower than 35ml of. In the future, we will repeat the experiment for the data that do not conform to linearity and try to be rigorous.

Special thanks to you for your good comments. We tried our best to improve the manuscript and made some changes in the manuscript. These changes will not influence the content and framework of the paper. And here we did not list the all the changes but marked with red colors in revised paper. We appreciate for Editors/Reviewers’ warm work earnestly, and hope that the correction will meet with approval. Once again, thank you very much for your comments and suggestions.

Round 2

Reviewer 1 Report

I carefully evaluated the authors' response to my comments and their modifications in the manuscript. I believe the manuscript can be published after the following minor comments are addressed by the authors:

WRT Q1: I still cannot get convinced about using "mining" as it's not a common terminology in the hydrate scientific community. Please replace it with a suitable word such a "exploitation". 

WRT Q2: I still see some English grammatical errors and hopefully these are resolved during the proofreading stage.

WRT Q6: I strongly invite the authors to examine the effect of washing the natural sands on the hydrate formation in their future studies.

Author Response

I carefully evaluated the authors' response to my comments and their modifications in the manuscript. I believe the manuscript can be published after the following minor comments are addressed by the authors:

WRT Q1: I still cannot get convinced about using "mining" as it's not a common terminology in the hydrate scientific community. Please replace it with a suitable word such an "exploitation". 

Response: Thank you for your valuable comment. The word “mining” has been corrected We replaced it with the word “exploitation” according to the reviewer’s suggestion. Exploitation is more professional in describing the potential applications of the thermal stimulation.

WRT Q2: I still see some English grammatical errors and hopefully these are resolved during the proofreading stage.

Response: Thank you for your valuable comment. The English gramma has been checked again according to your advice. The improper expression in the abstract and the experimental section has also been corrected.

WRT Q6: I strongly invite the authors to examine the effect of washing the natural sands on the hydrate formation in their future studies.

Response: Thank you for your valuable comment. Washing the natural sands is to remove the impurities such as adsorbed salts, mud or organics on the sand surface. The impurities noted was assumed to influence the thermodynamic conditions of the CH4 hydrate and some other effects known or unknown to the hydrate formation or dissociation, and finally led to the significant variations of the experimental results. In this case, we should wash the natural sand before the preparation of hydrate bearing sediments. As you noted, the effect of impurities on hydrate exploitation is quite complicated which is an important area in hydrate research. In this work, we have discussed about the salt concentration on hydrate exploitation. In our future work, we will focus on the effect of mud on hydrate-bearing sediment.