Thermally Assisted Dissociation of Methane Hydrates and the

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Thermally Assisted Dissociation of Methane Hydrates & the Impact of CO Injection 2

Swanand S Tupsakhare, Garrett C Fitzgerald, and Marco J. Castaldi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02509 • Publication Date (Web): 12 Sep 2016 Downloaded from http://pubs.acs.org on September 14, 2016

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Industrial & Engineering Chemistry Research

Thermally Assisted Dissociation of Methane Hydrates & the Impact of CO2 Injection Swanand S. Tupsakhare †, Garrett C. Fitzgerald ‡, Marco J. Castaldi *† †

Chemical Engineering Department, City College, City University of New York, New York, New York 10031,

United States ‡Rocky Mountain Institute, 22830 Two Rivers Road, Basalt, CO 81621, United States *E-mail: [email protected]. 1. Abstract: Largest amount of methane gas is trapped in less conventional natural gas resources such as methane hydrates. It is estimated that these reserves of methane gas in the form of hydrates are larger than all the conventional resources of methane gas combined. [U.S. Energy Information Administration - EIA - Independent Statistics and Analysis. Potential of Gas Hydrates Is Great, but Practical Development Is Far off]. Methane extraction from hydrates can be coupled with carbon dioxide sequestration to make this process carbon neutral. A largescale laboratory reactor is used to simulate the conditions existing in permafrost hydrate sediments to study the hydrate formation and dissociation processes. The dissociation process occurs via a cartridge heat source (to simulate the down-hole combustion) and carbon dioxide injection to study the CO2 sequestration behavior. The hydrate sediment studied was formed with 50 % saturation of hydrate by pore volume and the dissociation of this sediment was done using different combinations of high & low heating rates (100 W & 20 W) and high & low CO2 injection rates (1000 and 155 ml/min). Two baseline tests were conducted without any addition of heat at a CO2 injection rate of 155 ml/min and 1000 ml/min for comparison. The results indicate that at a constant heating rate, the number of moles of methane recovered decreases with an increasing flow rate of CO2 injection whereas the number of moles of CO2 sequestered increases with increase in the CO2 injection flow rate. At 50 % initial hydrate saturation (SH) and 100 W heating rate, the number of moles of methane recovered decreased from 96 to 58 when the CO2 injection rate was increased from 155 to 1000 ml/min respectively. Whereas, at 50 % initial saturation and 100 W heating rate the number of moles of CO2 sequestered increased from 13 to 40 when the CO2 injection rates was increased from 155 to 1000 ml/min. The recovery efficiency improved from 18 % to 22 % to 60 % when the heating rate was increased from 0 W to 20 W to 100 W respectively at 1000 ml/min CO2 injection.

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2. Introduction: Energy sources such as natural gas which have high hydrogen to carbon ratio are preferred these days over high carbon containing sources such as coal. Choice of energy sources is moving towards less carbon intense fuels such as natural gas. Currently the majority of the natural gas used is obtained from conventional reserves such as shale gas or oil wells. This has resulted in substantial investment and construction of chemical production facilities in the U.S. To ensure the continued utilization of natural gas as the feedstock of choice it will come from non-conventional sources such as gas hydrates. The proven conventional natural gas reserves in the U.S. increased from 305 Trillion Cubic Feet (TCF) in 2011 to 338 TCF in 2014 as reported in the U.S. Energy Information Administration (EIA) 1. Global estimates of methane gas in the form of hydrates are (1-5) x 1015 cubic foot 2 which are significantly larger than the conventional methane gas sources combined. Until renewable fuels develop to the point where significant replacement of fossil fuels occurs, combustion of cleaner fuels will contribute to sustainable energy developments. For example, in 2013 the U.S. had CO2 emissions that were approximately 10% below 2005 levels and nearly the same as 1996 levels. This will ensure continued reliance on natural gas for generations. As mentioned previously, extraction of methane gas from hydrate phase yields a fuel which is least carbon intense. In addition, this extraction process can be coupled with carbon dioxide sequestration which helps reduce carbon footprints. There have been successful field tests that demonstrated the feasibility of CO2 sequestration during the dissociation of methane hydrates and there are more such tests being planned 3. Equilibrium conditions of CH4 and CO2 hydrate formation indicate that when CH4 and CO2 coexist, CO2 is selectively converted to hydrate phase. CO2 sequestration tests make use of this concept to convert CO2 from gas phase to hydrate phase and dissociate methane hydrates at the same time. The process of replacement of CH4 by CO2 needs to be studied in detail to match the rate of dissociation of methane with rate of injection of CO2 and to understand the associated processes during CO2 injection. It is thermodynamically feasible to convert methane hydrates to CO2 hydrates at a temperature lower than 10 °C, also at a given temperature CO2 hydrates can form at lower pressures than methane hydrates 4. Therefore CH4-CO2 replacement can occur simultaneously 5 6. Hirohama et al. 7 used bulk methane hydrate sample to study the CH4-CO2 replacement process. They reported 15% replacement of CH4 hydrates by CO2 hydrate in a period of 800 hours. Lee et al. (2003) 8 used a relatively smaller setup with porous silica gel and liquid CO2. They were able to achieve a conversion of 50% CH4 hydrate. Baldwin et al. (2009)

6

used MRI imaging to show the spontaneous conversion from CH4 to CO2

hydrate in porous media. They report that there was no hydrate dissociation during this exchange process. 2 ACS Paragon Plus Environment

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Schicks et al. (2001) 9 used XRD and Raman spectroscopy and reported a conversion of 20 to 50 % from CH4 to CO2 hydrate. Yongwon Seo et al (2013)

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measured the mixed CO2 and CH4 hydrate phase equilibrium using

13

C NMR spectroscopy. They found that 70 % of the methane was recovered after the dissociation in these tests.

Seungmin Lee et al (2013) 11 used micro-differential scanning calorimetry to quantify the methane replacement by carbon dioxide. They indicate that no significant CH4 hydrate dissociation takes place during this CO2 replacement. Xiao-Sen Li et al 12 studied the methane hydrate dissociation via depressurization in porous media using silica gel. They selected different ranges of initial formation pressure and the temperature. They observed that the initial formation pressure and temperature has an effect on the rate at which methane gas is released during the dissociation. Higher the initial formation pressure, higher is the recovery rate of methane. They also observed that the recovery rate decreases with the temperature. They also studied the effect of pore size and found that the rate of gas release increases with an increase in the pore size. Yong Liu et al

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developed a

mathematical model and carried out numerical simulations for dissociation of methane hydrates by using thermal stimulation as well as depressurization methods. They concluded that the major factors dominating the moving front are temperature and pressure of the reservoir. Wang et al

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studied dissociation of methane hydrates in a large scale reactor of volume 117.8 L by using

thermal stimulation, depressurization and thermal stimulation combined with depressurization. Authors also report an analytical model that studies heat and mass transfer during this process. The authors claim that the combination of depressurization with thermal stimulation enhances the methane recovery process. Methane gas recovery behavior observed in the experiments and that predicted by analytical method are in good agreement with each other in this work. In another independent study, Wang et al

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used the same large scale reactor to

study the hydrate dissociation behavior below the quadrupled point. They observed that the water being released in the hydrate dissociation process turns into ice immediately following the dissociation and also the pore water in the reservoir turns into ice form. Authors propose that lower production pressures used in the study help increase the dissociation rate of methane hydrates. It is possible because lower production pressures increase the driving force for the dissociation. Zhao et al

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reported the use of different forms of CO2 for the methane

replacement process; they report results using gaseous CO2, liquid CO2 and CO2 emulsions. They report the thermodynamic and kinetic feasibility of the CO2-CH4 replacement and provide experimental data to support the theoretical calculations. They found that CO2 emulsions may provide an effective, efficient way to replace methane in the hydrate phase. Results from simulation work show that diffusivity of CO2 into the hydrate phase is the key for efficient replacement. Deusner et al 17 reported the use of hot supercritical CO2 for the dissociation purpose. The high temperature (95 °C) CO2 was used to act as a heat source to provide the necessary heat of dissociation to CH4 hydrate. They used three different temperatures (2°C, 8 °C & 10°C) and two different 3 ACS Paragon Plus Environment


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values of pressure (8 MPa and 13 MPa). They found that the process is most efficient in terms of CH4 production and CO2 sequestration at 8 °C and 13 MPa. Based on tests and calculations the water in this work is saturated with CH4. The theoretical value of solubility of CH4 in water under the operating conditions is 0.0012 mole of CH4 per mole of water

18, 19

. Experiments

show that the value of solubility of CH4 obtained in this work is 0.0010 in 24 hours period. The experimental time scale for hydrate formation is 120 hours therefore the water is in contact with CH4 for a sufficient time under the operating conditions. This work makes use of the concept of down-hole combustion to study the effect of simultaneous addition of the heat and CO2 on the gas recovery. In addition, we make use of a relevant scale setup that closely approximates the aspect ratio of field operations. Heat and CO2 were simultaneously injected in the hydrate sediment to realistically achieve down-hole combustion that will allow us to study the CH4-CO2 hydrate exchange while sequestering CO2 in the sediment. The test matrix in this work has been setup to study: 1) the effect of change in CO2 injection rate on gas recovery and gas exchange process in the absence of any heat input; 2) the effect of addition of the heat, simultaneously with CO2 injection on gas recovery and gas exchange; 3) the enhancement in the recovery efficiency of methane by comparison of CO2 injection tests with pure heating tests. There is significant interest in gas hydrates research in countries with limited reserves of conventional hydrocarbons. Although there have been a few field tests for extraction of methane from hydrate phase, the process still needs to be optimized for large scale and economically favorable production. As a result, lab scale tests are needed to provide fundamental understanding of the hydrate dissociation process. We have developed a 59.3 liter vessel to explore the impacts of methane production and CO2 sequestration from simulated permafrost methane hydrates. The presence of a free gas zone has been confirmed during the field tests from the Messoyakha well in Siberia

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and the Kumano fore-arc basin

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. The apparatus used to in this work also

contained a free gas zone, which makes this setup close to the actual permafrost conditions. We have shown in our previously published

25

work that this apparatus correctly observes the phenomenon

predicted by Tsypkin et al 22 where hydrate dissociation can lead to an abrupt change in produced methane flow. In addition, this apparatus predicted the gas discharge values 25 that are within an order of magnitude of what is predicted by Moridis et al

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for field simulations. We have successfully demonstrated that this system can be

used to approximate anticipated field tests

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and the potential to greatly increase the efficiency

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and CO2

25

sequestration . This work attempts to understand the effect of changing the heating rate and CO2 injection rate on the CH4 gas recovery and the amount of CO2 sequestered in the sand matrix. 4 ACS Paragon Plus Environment

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3. Equipment and methods: The large-scale hydrate vessel (LSHV) was used for formation and dissociation studies of methane hydrates. The volume of LSHV used in this work is 59.3 liters and it is filled with quartz sand. We have provided an empty head-space of 3 cm which is free of any sand, to accommodate for volume expansion. This quartz sand is saturated with de-ionized water to 50 % by pore space volume. Figure 1 shows the schematic of LSHV indicating the gas lines and instrumentation.

Figure 1 Schematic of LSHV showing gas injection lines and instrumentation Figure 2 shows the picture of the LSHV that is used for studying the hydrate formation and dissociation. The material of construction of LSHV is 315 stainless steel. The diameter and height of the LSHV is 0.305 m and 0.813 m respectively, that provides the total internal volume of 59.3 liters. The two ends of this cylinder body are closed with two end plates of 5 cm thickness which are secured in place by using two threaded collars. The maximum value of pressure used in the hydrate formation study in this work is ~ 4.8 MPa. The reactor was designed taking into consideration the highest pressure required. Calculations using Clavarino equation indicate that this system can sustain a pressure of 15.8 MPa. We have tested this system hydraulically to 13.8 MPa.



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Figure 2 Large Scale Hydrate Vessel made with stainless steel body The LSHV has a total of 5 ports at the top end plate. The middle 3/8 inch port is used to insert a custom-made cartridge heater (Hotset CU375) that is used for simulating combustion (shown in red in Figure 1). Thermocouples labeled as T set (T1-T4) and C set (C1-C4) are inserted through the two of the remaining four ports on the top end plate. B set (B1-B4) of thermocouples is inserted through the wall of the cylinder body as shown in Figure 1. The relative locations of all the thermocouples are also highlighted in Figure 1. All the thermocouples used in this study are ungrounded, 1/16 inch, K type thermocouples supplied by Omega Engineering. A pressure relief valve (Swagelok, SSR3D) and a pressure transducer (Trans metrics P100/200 series) (P1) are connected to the remaining two ports on the top end plate. The pressure relief valve is provided to allow for automatic release of pressure in the event pressure exceeds the set point of 10 MPa. Pressure transducer (P1) measures the pressure in the empty head-space of the LSHV. Two additional pressure transducers, P2 and P3 are located in the gas sampling port and in the CO2 injection line, respectively. P2, which is located in the gas sampling line is used for measuring the pressure in the sand matrix and for hydrostatic pressure. The bottom end plate of the LSHV is equipped with four gas injection ports used for injection of CH4 into the system. The CO2 injection line allows injection of CO2 right on the surface of the heating element as shown in Figure 1.


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As mentioned previously, LSHV is filled with quartz sand of ~ 500 micrometer mean particle size diameter such that 3 cm portion at the top remains unoccupied to allow for gas accumulation and volume expansion during hydrate formation. The sand is saturated with de-ionized water to 50 % by pore volume. The CH4 and CO2 gases used in this work are Ultra High Purity (UHP) grade supplied by Praxair. The gas sample from top of the reactor (effluent) and gas sampling port are analyzed by using a binary gas thermal conductivity detector (TCD) from GOWMAC (model 20-150). The TCD is calibrated by using an Agilent micro gas chromatography (Agilent 3000). The top sample (effluent) is taken from the empty head space of the reactor whereas the pore space sample is taken using a sample probe that is inserted into the sand matrix at the center of the reactor near the cartridge heater. The entire setup is kept in an automatically controlled cold room from KOLPAC (Model PR195MPD208) to maintain the LSHV at desired temperatures and also to maintain the constant far field temperature. This refrigerator is capable of maintaining the temperature between -4 to 20 °C. The rate of gas injection and the gas released during the dissociation is measured by using flow meters. Omega FMA-873A-V flow meter (0-50 standard liter per minute), calibrated for CH4 is used for injecting methane into the reactor. Aalborg 17 series flow meter (0-5 standard liter per minute) is used for measuring the gas released during dissociation. Two Aalborg 17 series mass flow controllers are used for injection of CO2 (0-200 ml/min and 0-1000 ml/min). All the thermocouples, pressure transducer, flow meters, flow controllers and TCD are connected to the Omega USB data acquisition system which makes used of Personal DaqViewTM software to record the data on a computer. The hydrate equilibrium pressure for CH4 at 2.5 °C is 3.3 MPa and the system pressure during the dissociation is set just above this value. Pressure in the reactor is maintained constant by using a back pressure regulator. The exit gas stream is a mixture of CH4 and CO2 which is then allowed to flow through the TCD and micro gas chromatography unit. Injected CO2 partially flushes the CH4 existing in gas phase in the reactor and the composition of the gas exiting the reactor is measured using TCD. This work includes results from a total of 8 tests that were carried out at a constant initial hydrate saturation of 50 % by volume of pore space. The heating rate and the injection flow rate of CO2 were varied in each test. All saturation conditions reported correspond to initial water saturation for simplicity. It is recognized that hydrate saturation will be slightly different but less than 10%. This 50 % saturation is achieved in 5 stages of pressurization each contributing 10 % toward the total saturation. In each step, the reactor is pressurized with methane and the hydrate saturation is calculated based on the difference between initial and final pressure taking into account the specific density of hydrates and the compressibility factor of methane at the operating conditions. Each hydrate formation cycle takes approximately 24 hours to reach the equilibrium. Hydrates 7 ACS Paragon Plus Environment


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formed in this study are expected to be a combination of pore filling and grain cementing type. Methane saturated water leads to pore filling type hydrates whereas a gas rich environment leads to grain cementing type hydrates 26. Supporting Information includes the details about the reproducibility of the tests. Complete details of hydrate formation in this system can be found in previous publications 24 25 26. Table 1 below summarizes the experimental parameters during the tests. Table 1 Summary of the test parameters Heating rate & CO2 flow rate at 50 % SH Test #

Heating rate

CO2 Flow Rate

Test 1

0W

155 ml/min

Test 2

0W

1000 ml/min

Test 3

100 W

155 ml/min

Test 4

100 W

1000 ml/min

Test 5

20 W

155 ml/min

Test 6

20 W

1000 ml/min

Test 7

100 W

0 ml/min

Test 8

20 W

0 ml/min

Test 1 and 2, both were conducted without heating with a CO2 injection rate of 155 ml/min and 1000 ml/min respectively. Tests 1 and 2 were carried out without any heating so that they could be used as a base to compare results from other tests. Tests 3 and 4, both made use of a high heating rate of 100 W and a CO2 injection rate of 155 ml/min and 1000 ml/min respectively. Tests 5 and 6 both were conducted at a low heating rate of 20 W and a CO2 injection rate of 155 ml/min and 1000 ml/min respectively. Tests 7 and 8, were done to study the effect of only heating without any CO2 injection. Tests 7 and 8, were conducted at the high heating rate of 100 W and low heating rate of 20 W respectively without CO2 injection, enabling comparisons related to heat rate alone. Table 1 summarizes the parameters for all the 8 tests. The lower bound of the two CO2 flow rates was 155 mL/min which was chosen based on the concept of using some of the produced methane as a fuel for down-hole heating via combustion. This is a conceptual way to determine if the amount of carbon produced via methane can be sequestered via CO2. In our concept we propose to draw some of the produced methane, mix it with high pressure oxygen or air and initiate the combustion in the hydrate reservoir contained in a down-hole combustor. This concept has been detailed in a 8 ACS Paragon Plus Environment

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previous publication

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and there have been attempts by various companies to deploy such a scheme at field

tests 26. The calculated production rate of 155 mL/min of CO2 is based on the heating rate of 100 Wand 100% conversion of released methane to CO2. The upper bound of CO2 flow rate experiment was 1000 mL/min. This indicates that this process has a potential to sequester an order of magnitude more CO2 than that released in combustion. This provides room for injection of additional CO2 from outside making this process further carbon neutral. 4. Results and Discussion: Equation 1 and Equation 2 define the gas recovery efficiency and gas sequestration factor respectively. Gas recovery efficiency is a measure of the amount of gas recovered with respect the amount of gas initially present in the reactor. Gas sequestration factor is a measure of CO2 sequestration efficiency. A sequestration factor of less than 1 indicates that the number of moles of CO2 sequestered is less than the number of moles of methane recovered. Similarly if the sequestration factor is greater than 1, it indicates that more number of moles of CO2 are sequestered than the number of moles of methane recovered.

=

Equation 1

=

Equation 2

Here, the number of moles of CH4 and CO2 exiting the system are calculated using the data from mass flow meter and TCD. The number of moles of CO2 sequestered in the system is calculated by mass balance of CO2 entering and leaving the system. 4.1 Study of CH4 gas recovery and CO2 sequestered at constant heating rate and different CO2 injection rates 4.1.1 Test 1 and 2; Baseline CO2 injection without thermal stimulation Figure 3 shows the data from TCD for the gas composition of the samples taken from the top sample port of the reactor (effluent stream) and from the center sample port of the reactor (pore space stream). Composition of CH4 is plotted on Y axis with balance being CO2. This data is for test 1 which was carried out at 155 ml/min CO2 injection. No heat was supplied to the sediment during test 1.



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Figure 3 Effluent and pore space gas composition for Test 1 at 0 W heating and 155 ml/min CO2 injection The effluent gas composition recovered is initially pure methane followed by the onset of CO2 detection. The time that it takes to detect CO2 in the sample stream was 40 hours for test 1 (at 155 ml/min) and 4 hours for test 2 (at 1000 ml/min). At the beginning of the tests the reactor contains 83 moles of methane in the form of hydrate and 13.4 moles of methane in gas phase. Additionally there is 2.1 liter of free water present in the reactor. For further explanation and details regarding the 2.1 liters of free water please see 26 27. At the operating conditions of the process (2.5 °C and 3.3 MPa) the values of compressibility factors for CH4 and CO2 are 0.904 and 0.714 respectively. Calculations show that injection of 21 moles of CO2 is required to completely displace 13.4 moles of methane initially present in the reactor. These calculations are based on the difference in the compressibility of CH4 and CO2. These calculations also assume that there is no gas phase mixing. If the system was a plug flow reactor with no gas exchange occurring, in such case the break through time of the experiment for test 1 would be 53 hours to displace 13.4 moles of methane at a CO2 injection rate of 155 ml/min. However it is clear from Figure 3 that it takes less than 53 hours for the CO2 to appear in the sample gas stream. This indicates that the process do not resemble a Plug Flow Reactor (PFR), there is some extent of gas phase mixing as well as gas exchange. PFR comparison is only used to find a bound for highest expected time for displacing all the gas phase CH4 in the reactor. Figure 4 shows the amount of CH4 recovered and CO2 sequestered in case of tests 1 and 2. Triangles correspond to the high CO2 injection rate test and circles to the low CO2 injection rate test. The CO2 and CH4 composition profile of the effluent gas are similar to Figure 3. The tests were terminated as soon as the pore


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space gas sample becomes 5% in methane and 95% in CO2. Note that this sample is taken from the center of the reactor. Same criteria was used to end all the CO2 injection tests in this work.

Figure 4 Cumulative moles of CH4 recovered and CO2 sequestered for tests 1 and 2. The recovery efficiency for test 1 and 2 was 24% and 18% and the sequestration factors were 0.97 and 1.8 respectively. Test at 155 ml/min operated for longer time which allowed for relatively higher gas sequestration and as a result, higher methane recovery. The sequestration factor for the test at 155 ml/min indicated that more amount of methane was recovered than the amount of CO2 sequestered, however in case of 1000 ml/min more amount of CO2 was sequestered than the amount of CH4 recovered. Results from baseline tests indicate that in the absence of heating, higher CO2 injection rate results in higher CO2 sequestration and lower CH4 production 4.1.2 Test 3 and 4; High heating rate CO2 exchange tests: Addition of heat with CO2 causes an increase in the rate of CH4 hydrate dissociation and also generates more free water that can further combine with gas phase CO2 which leads to CO2 hydrate formation. This is demonstrated in tests 3 and 4 which were carried out with the addition of 100 W heat at the center of the reactor. The CO2 injection rates in tests 3 and 4 were 155 and 1000 ml/min respectively. Figure 5 shows the progression of the cumulative effluent gas quantities for tests 3 and 4 with the solid markers corresponding to methane released and the open markers for CO2 sequestered.

The recovery efficiency

increased from 60 % to 99 % when the CO2 injection rate was decreased from 1000 to 155 ml/min respectively. The sequestration factors were 0.14 for the low injection rate (155 ml/min), 0.68 for the high injection rate (1000 ml/min).



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Figure 5 Cumulative moles of CH4 recovered and CO2 sequestered for tests 3 and 4 The value of recovery efficiencies were 24% and 18% respectively for tests 1 and 2. In the case of tests 3 and 4 the recovery efficiency values are 99% and 60%. This indicates that addition of heat at 100 W increased the values of recovery efficiency at both, low and high CO2 injection rates. On the other side the sequestration factors for both the tests 3 and 4 decreased from their corresponding value in tests 1 and 2. Increasing the CO2 injection rate from 155 to 1000 ml/min at a constant heating rate (100 W) caused an increase in the CO2 sequestration factor at the expense of decreasing the CH4 recovery. 4.1.3 Test 5 and 6; Low heating rate CO2 exchange: Test 5 and 6 were conducted at 20 W heating rate and 155 and 1000 ml/min CO2 injection rates. The gas recovery and CO2 sequestration of the exchange tests are summarized in Figure 6 (inset) with solid markers showing methane recovered and open markers showing CO2 sequestered. The values of recovery efficiency for test 5 and 6 were 31% and 22% respectively. Similarly, sequestration factors were 1.4 and 1.34 for test 5 and 6 respectively. As stated previously, a sequestration factor of more than 1 indicates that the number of moles of CO2 sequestered is more than the number of moles of methane recovered.


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Figure 6 Cumulative moles of methane recovered and CO2 sequestered for tests 5 and 6 - magnified portion of inset from 0 to 50 hour & Cumulative moles of methane recovered and CO2 sequestered for tests 5 and 6 (inset). From Figure 6 inset, although at the end of test the sequestration factor is 1.4 in both the tests, the scenario in the first 50 hours of the tests is different. Inset in Figure 6 shows the data for the entire duration of the tests whereas the Figure 6 shows the magnified view of the first 50 hours. The number of moles of CO2 sequestered is higher than the number of moles of methane recovered during the entire duration of test 6. However, the number of moles of CO2 sequestered is less than CH4 recovered for approximately first 50 hours of the test 5. Comparing the slopes of the recovered CH4 curves in tests 5 and 6 in Figure 6, the slope of the CO2 sequestered curve starts to decrease simultaneously as the slope of CH4 recovered starts to decrease. This would mean the rate of CO2 sequestration decreases as soon as the rate of dissociation of CH4 hydrate decreases. This gives a message that CH4-CO2 replacement may be taking place in the hydrate phase. 4.2 Study of methane recovery and CO2 sequestration at constant CO2 injection rate and different heating rates 4.2.1 No heating (0 W) and low heating (20 W) tests at 155 ml/min and 1000 ml/min Figure 7a shows the comparison of the results for 0 and 20 W heating tests at155 ml/min CO2 injection rates.



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Figure 7a Cumulative moles of CH4 recovered and CO2 sequestered for tests 1 and 5 The major finding from Figure 7a is that the rate of production of methane for the 20 W test is higher than the unheated test at the low (155 ml/min) injection test. Referring to Figures 7a for the 20 W heating test the CO2 sequestered is higher than that for the unheated test. This can be explained on the basis of the availability of free water due to dissociation of methane hydrates after the addition of the 20 W heat. Here the 20 W heating rate keeps the sediment temperature within the CO2 hydrate stability zone and also generates more free water, giving more potential for CO2 sequestration. To understand the data in Figure 7a in a better way, a magnified view of Figure 7a between 0 to 85 hours is plotted in Figure 7b below.

Figure 7b Cumulative moles of CH4 recovered and CO2 sequestered for tests 1 and 5, magnified portion of Figure 7a from 0 to 85 hours 14 ACS Paragon Plus Environment

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It is observed form Figure 7b that, the data points (solid and open circles) for test 1 for CH4 recovered and CO2 sequestered track together until the end of the test, indicating that the rate of methane production was nearly equal to the rate of CO2 injection. On the other side for test 5, the rate of methane production was higher than the rate of CO2 injection until nearly 60 hours of test 5. After 60 hours, the CO2 sequestration rate becomes higher than methane production rate and stays at a higher value until the end of the test. This is due to the addition of heat at 20 W. Addition of heat at 20 W in test 5 (as opposed to 0 W heat addition in test 1) increases the rate of methane recovery in the initial stages. In the later stages (after ~ 60 hours), the free water that becomes available from CH4 hydrate dissociation; increases the rate of CO2 sequestration by converting the gas phase CO2 into a hydrate phase. Figure 8 below shows the comparison of results for 0 and 20 W heating tests at 1000 ml/min CO2 injection rate.

Figure 8 Cumulative moles of CH4 recovered and CO2 sequestered for tests 2 and 6 The common features in Figure 7a and Figure 8 is that the CO2 sequestration rate was nearly the same for both heated and unheated tests until approximately 60 hours and 20 hours respectively. After a careful look at Figure 7a and Figure 8, we observe that methane recovery rates plateau in Figure 8 however, in Figure 7a the CH4 recovery is still increasing with time. This has an effect on the CO2 sequestration behavior. Because CH4 recovery plateaus in Figure 8, the potential for CO2 sequestration decreases, however in the tests shown in Figure 7a; we still have more CH4 being released which increases the potential for CO2 sequestration causing an increase in CO2 sequestration values as opposed to Figure 8.



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4.2.2 No heating (0 W) and high heating (100 W) tests at 155 ml/min and 1000 ml/min Figures 9 and 10 show the comparison of the results comparing 0 W and 100 W for 155 ml/min (Figure 9) and 1000 ml/min injection rate (Figure 10). Evident from Figure 9, the addition of 100 W of heat to the CO2 injection at 155 ml/min significantly improves the methane recoverability. However, the number of moles of CO2 sequestered is nearly 20 in both these tests. This can be attributed to the injection rate of the CO2 being the limiting parameter. In other words, the methane is generated at much higher rate during hydrate dissociation than the CO2 injection rate, this process is limited by the availability of gaseous phase CO2. One advantage of this is that the majority of CO2 injected is sequestered in the reactor and the gas leaving the reactor is highly concentrated in methane, which reduces the efforts to further purify the methane. It is also important to note the 100

W

heating

produces

methane

at

much

faster

rate

than

the

0

W

heating.

Figure 9 Cumulative moles of CH4 recovered and CO2 sequestered for tests 1 and 3


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Figure 10 Cumulative moles of CH4 recovered and CO2 sequestered for tests 2 and 4 In Figure 9, the number of moles of methane recovered and CO2 sequestered track together (without heating) during the test indicating that the rate of injection of CO2 nearly matched the rate at which methane was being released from the reactor. Figure 10 shows the data for CH4 recovered and CO2 sequestered for test 2 (0 W) and test 4 (100 W) both at 1000 ml/min CO2 injection. Significant improvement in the amount of methane recovered and CO2 sequestered is observed in Figure 10 for the high heating rate. The rate at which methane is released is higher for 100 W heating compared to no heating. At a constant CO2 injection rate increasing the heating rate causes an increase in the methane recovery and a decrease in the amount CO2 sequestered. The addition of heat dissociates the hydrates increasing the recovery of methane; however it also increases the temperature of the sediment beyond the CO2 hydrate stability temperatures resulting in a decrease in the amount of CO2 sequestered. This is in contrast with Figure 14 as discussed later where, at the 100 W heating rate more free water is generated which combines with CO2 and causes more CO2 sequestration at 1000 ml/min as compared to the 155 ml/min CO2 injection rate. 4.2.3 Low heating (20 W) and high heating (100 W) tests at 155 ml/min and 1000 ml/min Figure 11 and Figure 12 show the plots for low and high heating rates at both low and high injection rate respectively. In Figure 11 and 12, the 100 W test produced significantly higher methane recovery than 20 W tests, as shown by comparing solid circles and solid triangles.

Figure 11 Cumulative moles of CH4 recovered and CO2 sequestered for tests 3 and 5 It is evident from Figure 11 that more CO2 was sequestered than methane recovered for the 20 W heating test. However, less CO2 was sequestered than methane recovered for the test with 100 W heating. Comparison of 17 ACS Paragon Plus Environment


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Figures 11 and 12 for the amount of CO2 sequestered indicates that for tests with a 100 W heating, 1000 ml/min enables more CO2 sequestration as compared to 155 ml/min injection. This is directly correlated to the flow rate of CO2 injection being the limiting factor. That is CO2 could be sequestered more than the 155 ml/min injection rate and perhaps more than the 1000 ml/min rate. In the case of higher CO2 injection flow rate the possibility of combination of CO2 with water molecules increases yielding the greater amounts of CO2 sequestered.

Figure 12 Cumulative moles of CH4 recovered and CO2 sequestered for tests 4 and 6 4.3 Study of overall CO2 sequestration Figure 13 shows the moles of CO2 sequestered with time for all the 6 tests. The solid and dotted lines in Figure 13 show the theoretical trend if all the CO2 injected was sequestered in the system at low and high CO2 injection rates respectively. Comparison of the data points with these theoretical lines gives an idea of how efficient is the CO2 sequestration.


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Figure 13 Overall CO2 sequestration profiles for all the tests Open markers correspond to the tests at 1000 ml/min CO2 injection whereas solid markers correspond to the tests at 155 ml/min CO2 injection. As can be seen from Figure 13, the rate at which CO2 is being sequestered into the reactor tracks along solid or dotted line and then begins to diverge from those lines in the later part of the test. It can be seen that the amount of CO2 sequestered was relatively lower for 100 W heating tests for both low as well as high injection tests. The highest amount of CO2 was sequestered for the test at low heating and low injection rate (shown in solid squares) considering the end of the test values. Test 4 at high heating and high injection (100 W and 1000 ml/min) continued for almost 3 times the duration of the low heating and unheated tests. It can be explained on the basis of the criteria used for terminating the test. The tests are terminated as soon as the effluent stream reaches 95 % CO2. Due to higher value of heating rate more CH4 was recovered and the duration of the test increased due to CH4 dominated effluent stream. This provided: 1) more time for CH4CO2 exchange and 2) generated more free water that allowed additional CO2 hydrate formation. 4.4 Comparison of CO2 injection tests with only heat input tests Heating tests for hydrate dissociation, which used the same parameters and procedure for achieving initial hydrate saturation, were done for both low and high heating rates to discover the effect of addition of CO2. These are tests 7 and 8 in Table 1 (test 7 used the 100 W heat rate and test 8 used the 20 W heat rate). Both tests made use of the 50 % hydrate saturation yet there was no CO2 injected. Tests 7 and 8 were terminated when the flow rate of CH4 leaving the reactor fell below the lower detection limit (75 ml/min) of the flow meter. Figure 14 shows the amount of methane recovered in three different tests, all of which made use of 100 W heating rate. 19 ACS Paragon Plus Environment


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The three tests were carried out at 1000, 155 and 0 ml/min of CO2. As shown in Figure 14, the highest amount of methane was recovered for the test using 100 W heating and a 155 ml/min CO2 injection rate. Comparison of test 7 with test 3 shows that the addition of 155 ml/min of CO2 increases the methane recoverability significantly.

Figure 14 Methane recovered at three different CO2 injection rates at 100 W On the other hand, injecting CO2 at 1000 ml/min caused a decrease in the amount of CH4 recovered as compared to pure heating test. This is due to the free water available for heat convection. As we supply heat to the sediment at 100 W, methane hydrates dissociate and free water is available for both recombination with CO2 and for convection of heat away from heat source. However, in the case of 1000 ml/min CO2 injection, the majority of the free water combines with CO2 to form CO2 hydrate, as a result the water available for convection of heat away from the heat source is reduced which causes a reduction in the rate of methane hydrate dissociation. On the other hand, in the case of 155 ml/min injection rate, most of the free water remains available for heat convection away from the heating source and this causes an increase in the methane recoverability. This is in contrast with Figure 10 where 100 W heating rate brings the sediment temperature beyond the stability zone of CO2 hydrates as compared to the test at 0 W. At 100 W the amount of CO2 sequestered was 13 moles at 155 ml/min as compared to 40 moles at 1000 ml/min.


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Figure 15 Methane recovered at three different CO2 injection rates at 20 W Figure 15 shows the amount of methane recovered at three different CO2 injection rates at 20 W heating rate. As can be seen, the highest amount of methane was recovered in the test with no CO2 injection. Injection of CO2 at 1000 ml/min here gives a higher rate of methane release in the beginning of the test. Again this is related to the amount of free water available. The lower heating rate of 20 W releases less free water in this case compared to the tests with 100 W. In the 155 ml/min injection tests, this small amount of free water immediately combines with the CO2 and this reduces the probability of CH4 replacement or CH4 hydrate dissociation. On the other hand, the 1000 ml/min injection test pushes the gas phase methane out from the system which increases the rate at which methane is recovered in test 6 as compared to test 5. 5. Temperature-Pressure behavior during the dissociation and its impact on thermodynamic equilibrium It is important to discuss the effect of temperature-pressure conditions in the reactor during dissociation. Each thermocouple in the reactor showed unique temperature during the dissociation process and it has significant impact on the CO2 hydrate formation since this process is dependent mostly on T & P. Consider the following graphs (Figure 16 and Figure 17) for Test 4 (100 W, 1000 ml/min CO2 injection) and Test 7 (100 W, 0 ml/min CO2 injection) respectively. Figure 16 compares the temperature at thermocouple location B1 and Figure 17 compares the temperature at location B3 for these two tests. These two tests are selected to represent the effect of injecting CO2 during the dissociation. Test 4 is conducted by injecting CO2 at 1000 ml/min whereas test 7 is conducted without any injection of CO2. It should be noted that thermocouples B1 and B3 are selected to study the thermodynamic equilibrium conditions when we go from region lower in CO2 concentration to the region higher in CO2 concentration. Location B1 is located away 21 ACS Paragon Plus Environment


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from the CO2 injection port, whereas location B3 is closest to the CO2 injection port; as shown in inset in Figure 16 and Figure 17. As a result, thermocouple location B1 and B3 represent two extreme cases and are chosen for study of thermodynamic equilibrium conditions during the experiment.

Figure 16 Temperature profile during dissociation of 50 % saturated hydrate sediment at 100 W heating rate. Solid circle represents temperature at location B1 with CO2 injection rate at 1000 ml/min whereas open circle represent temperature at location B1 without any CO2 injection. Solid line shows CH4 hydrate equilibrium temperature at 3.3 MPa, whereas dotted line shows CO2 hydrate equilibrium temperature at 3.3. MPa. Figure 16 shows that temperature at location B1 settles at a higher value in the case of CO2 injection test as opposed to the test where there is no CO2 injection. This difference in the temperature at location B1 is the additional heat released due to the formation of CO2 hydrates at this location. It can be seen that this is not true for the initial 9 hours of the tests. For the first 9 hours, the temperature at location B1 is higher for the test without CO2 injection than for the test with CO2 injection. This makes sense because when there is no CO2 injection, the heat front reaches location B1 at a faster rate due to the availability of more free water compared to the test when CO2 is being injected. In the case of CO2 injection test free water released in the CH4 hydrate dissociation is combined with injected CO2 for reformation of CO2 hydrates. This creates more resistance for heat transfer and hence also explains the trend observed in first 9 hours. After 9 hours however, CO2 hydrates tend to grow in the outer parts of the reactor (away from the injection port). This causes CO2 hydrate formation near location B1 giving rise to an increase in temperature at location B1 as compared to the test without any CO2 injection. 22 ACS Paragon Plus Environment

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Figure 17 Temperature profile during dissociation of 50 % saturated hydrate sediment at 100 W heating rate. Solid circle represents temperature at location B3 with CO2 injection rate at 1000 ml/min whereas open circle represent temperature at location B3 without any CO2 injection. Solid line shows CH4 hydrate equilibrium temperature at 3.3 MPa, whereas dotted line shows CO2 hydrate equilibrium temperature at 3.3. MPa. Similar trend is observed at location B3 in Figure 17 where the temperature at location B3 is higher for the first 20 hours in the case of the test without any CO2 injection. After 20 hours however, the temperature reached at location B3 in the CO2 injection test is higher than that reached in the test without CO2 injection. This difference in temperature indicates additional heat is released during formation of CO2 hydrates. 6. Carbon neutrality aspects Carbon neutrality index, which is the ratio of gm/hour of carbon sequestered (in the form of CO2) over the gm/hour of carbon recovered (in the form of CH4) was calculated for all the CO2 injection tests. To allow for a fair comparison, the carbon neutrality index was calculated at 18 % methane recovered for all the tests. Table 2 below shows the value of carbon neutrality index at 18 % methane recovered and end of test percent recovery & sequestration factor.



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Table 2 Carbon neutrality index along with the overall recovery and sequestration factors Test

Carbon

%

Sequestration

Parameters

Neutrality

Recovery Factor

Index

Test 1- 0 W, 155 ml/min

0.97

24 %

0.97

1.8

18 %

1.8

0.04

99 %

0.14

0.57

60 %

0.68

0.87

31 %

1.4

1.31

22%

1.34



Test 4- 100 W, 1000 ml/min



Referring to the Table 2 above, Test 2 and Test 6 gave a carbon neutrality index of higher than 1, indicating more number of moles of CO2 were sequestered than CH4 recovered. It should be noted that both these tests were at the high CO2 injection flow rate of 1000 ml/min. However, the % recovery of methane in both the tests was significantly lower than other tests in the testing matrix. Alternatively, tests 3 and 4 gave lower values of carbon neutrality index but the percentage recovery taking into account the entire duration of the tests was relatively higher. Therefore a choice has to be made between sequestration efficiency and the recovery efficiency to get optimized results. Results from test 4 for example show how a compromise can be made to get a balance between methane recovery and the carbon dioxide sequestration. Test 4 resulted in a very good methane recovery of 60 % and with a commensurate carbon neutrality index of 0.57. 24 ACS Paragon Plus Environment

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7. Effect of heating rate on carbon sequestration and thermal efficiency Although an increase in recovery efficiency with an increase in heating rate is not surprising, the effect of heating rate on carbon sequestration and thermal efficiency should be considered. The prospect of the heating rate is threefold in this work. First, the effect of increasing the heating rate on recovery efficiency as already discussed, second the effect of increasing the heating rate on CO2 sequestration and third the effect of increasing the heating rate on thermal efficiency of the process. To understand the second and third points in detail we will consider specific examples. To understand the effect of the heating rate on carbon sequestration, Table 2 is presented in graphical form in Figure 18

Figure 18 % Recovery efficiency and Carbon neutrality index plotted as a function of heating rate. Left Y axis shows % Recovery Efficiency and right Y axis shows Carbon Neutrality Index with heating rate on X axis. In Figure 18, dotted lines correspond to % recovery efficiency (plotted on left Y axis) whereas solid lines correspond to the Carbon Neutrality Factor (plotted on right Y axis). An important observation from this graph is that although recovery efficiency increases with increasing heating rate, the carbon neutrality index decreases with increasing heating rate. As a result it is important to realize that it may not be desired to only increase the heating rate. Instead an optimized approach is required that will give significant recovery while sequestering carbon dioxide. To consider the effect of the heating rate on third factor (thermal efficiency), an example from a previously published work has been given. Fitzgerald et al 27 used the same experimental setup and conducted hydrate dissociation tests using the thermal stimulation method (no CO2 was injected in this work). The



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experimental parameters used in this work were two different initial hydrate saturations (10 % and 30 % by pore volume) and two different heating rates (20 W and 100 W). Table 3 summarizes the results from this work Table 3 Summary of thermal efficiency values from Fitzgerald et al 27

Hydrate Saturation

Heating Rate

Efficiency at

32 %

10.5 % 10.5 % 30 %

20 W

100 W

87.8 % 79 %

20 W

100 W

41 %

90 %

23% released

Observed in Table 3, an increase in the thermal efficiency by increasing heating rate is true only in the case of 30 % hydrate saturation. In the case of 10 % initial hydrate saturation; increasing the heating rate from 20 W to 100 W caused a decrease in the thermal efficiency from 79 % to 41 %. This indicates that the heating rate is not the only factor when considering the hydrate dissociation process, it also depends on the initial hydrate saturation as well. The thermal efficiency analysis is not done in this work because there are two different driving forces for hydrate dissociation – heat and CO2 injection. Thermal efficiency only considers the heating factor and hence is not a fair comparison. In conclusion, the recovery efficiency is just one factor that dominates the hydrate dissociation process. There are other important factors such as carbon sequestration and thermal efficiency that must be taken into consideration. Importantly before making any choice the process needs to be optimized based on all three factors.


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8. Conclusion A large-scale laboratory reactor of 59.3 liters was designed and used to simulate the hydrate formation and dissociation in the permafrost sediment. A test matrix was setup using 50% initial hydrate saturation and different heating rates and CO2 injection rates. Test with input of thermal energy gave higher values of recovery efficiency as compared to baseline tests where no heat was supplied for the dissociation. Adding heat at the rate of 20 watts resulted in an increase in recovery efficiency to 22 % and 31 % for 1000 and 155 ml/min CO2 injection rates respectively. Similarly, addition of heat at 100 watt increased recovery efficiency to 60 % and 99 % for CO2 injection rates of 1000 and 155 ml/min respectively. Higher values of heating rates favor higher values of methane recovery efficiency while decreasing the amount of CO2 sequestered and vice versa. The heating rate has an important effect on the hydrate dissociation process. Recovery efficiency, carbon sequestration and thermal efficiency of the process are dominated by the heating rate. It was possible to sequester CO2 at a rate that is an order of magnitude higher than what would be generated by combustion process of methane. Successful demonstration of methane gas recovery from hydrate phase combined with CO2 sequestration was done. The results obtained in this study could be useful as a guideline for selecting methods and parameters for carbon neutral energy production from CH4 gas hydrates.

Supporting Information Reproducibility of Hydrate Formation Tests

Acknowledgments The authors acknowledge the insightful feedback and assistance from Mr. Jeffrey LeBlanc from City College of New York.



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International Energy Statistics - EIA. Web. 06 Jan. 2016.

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(13) Liu Y.; Strumendo M.; Arastoopour H. Simulation of methane production from hydrates by depressurization and thermal stimulation. Ind. Eng. Chem. Res. 2008, 48(5) 2451-64. (14) Wang, Y.; Feng, J.-C.; Li, X.-S.; Zhang, Y.; Li, G. Analytic Modeling and Large-scale Experimental Study of Mass and Heat Transfer during Hydrate Dissociation in Sediment with Different Dissociation Methods. Energy 2015, 90, 1931–1948. (15) Wang, Y.; Feng, J.-C.; Li, X.-S.; Zhang, Y.; Li, G. Large Scale Experimental Evaluation to Methane Hydrate Dissociation below Quadruple Point in Sandy Sediment . Appl. Energy 2016, 162, 372–381. (16) Zhao, J.; Kun X.; Yongchen S.; Weiguo L.; Weihaur L.; Yu L.; Kaihua X.; Yiming Z.; Xichong Y.; Qingping Li. A review on research on replacement of CH4 in natural gas hydrates by use of CO2. Energies 5, 2 2012: 399-419. (17) Deusner, C.; Nikolaus B.; Elke K.; Matthias H. Methane production from gas hydrate deposits through injection of supercritical CO2. Energies 5, 7 2012: 2112-2140. (18) Song, K. Y.; Gl F.; F. Fleyfel; R. Martin; John L.; R. Kobayashi. Solubility measurements of methane and ethane in water at and near hydrate conditions. Fluid Phase Equilib. 128, 1 1997: 249-259. (19) Lekvam, K.; Bishnoi P.R. Dissolution of methane in water at low temperatures and intermediate pressures. Fluid Phase Equilib. 131, 1 1997: 297-309. (20) Collett, T. S. U.S. Geological Survey Professional Paper, Issue 1570; U.S. Government Printing Office, 1993, University of Chicago, Digitalized Feb 23, 2001. (21) Taladay, K. B.; Moore F. G. Concentrated Gas Hydrate Deposits in the Kumano Forearc Basin, Nankai Trough, Japan. Methane Hydrate Newsletter 15 (Apr. 2015): n. pag. Web. 05 Sept. 2016. . (22) Tsypkin, G. G. Effect of decomposition of a gas hydrate on the gas recovery from a reservoir containing hydrate and gas in the free state. Fluid Dyn. 40, 1 2005: 117-125. (23) Moridis, G. J. Numerical studies of gas production from methane hydrates. Proc. - SPE/CERI Gas Technol. Symp. Society of Petroleum Engineers, 2002.



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(24) Castaldi M.; Zhou Y.; Yegulalp T. Down-hole combustion method for gas production from methane hydrates. J. Pet. Sci. Eng. 2007, 56(1) 176-85. (25) Zhou Y.; Castaldi M.J.; Yegulalp T.M. Experimental investigation of methane gas production from methane hydrate. Ind. Eng. Chem. Res. 2009, 48(6) 3142-9. (26) Fitzgerald G.; Castaldi M.; Zhou Y. Large scale reactor details and results for the formation and decomposition of methane hydrates via thermal stimulation dissociation. J. Pet. Sci. Eng. 2012 94 19-27. (27) Fitzgerald, G. C.; Castaldi, M. J. Thermal Stimulation Based Methane Production from Hydrate Bearing Quartz Sediment. Ind. Eng. Chem. Res. 2013, 52 (19), 6571–6581.


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10. TOC Graphic


Thermally Assisted Dissociation of Methane Hydrates and the

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