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Experimental measurements of mechanical properties of carbon dioxide hydrate-bearing sediments
Marine and Petroleum Geology 46 (2013) 201e209
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Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Experimental measurements of mechanical properties of carbon dioxide hydrate-bearing sediments Weiguo Liu, Jiafei Zhao, Yuan Luo, Yongchen Song*, Yanghui Li, Mingjun Yang, Yi Zhang, Yu Liu, Dayong Wang Key Laboratory of Ocean Energy Utilization and Energy Conservation of the Ministry of Education, Dalian University of Technology, Dalian 116024, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 October 2012 Received in revised form 16 June 2013 Accepted 21 June 2013 Available online 29 June 2013
The CH4eCO2 replacement method to recover methane from hydrate-bearing sediments has received great attention because it enables the long term storage of CO2 and is expected to maintain the stability of gas hydrate-bearing sediments. In this paper, the mechanical properties of CO2 hydrate-bearing sediments were measured by a low-temperature and high-pressure triaxial compression apparatus. The strength differences between the CO2 and CH4 hydrate-bearing sediments were then analyzed to evaluate the safety of the CH4eCO2 replacement method. The strength of the CO2 hydrate-bearing sediments was found to increase as the temperature and porosity decreased and as the strain rate increased. When the confining pressure was less than 5 MPa, the strength of the CO2 hydrate-bearing sediments also increased as the confining pressure increased. However, owing to pore-ice melting and particle breakage, the strength of the CO2 hydrate-bearing sediments decreased as the confining pressure increased for confining pressures exceeding 5 MPa. The strength of the CO2 hydrate-bearing sediments was found to be larger than that of the CH4 hydrate-bearing sediments, with the strengths of the CH4 and CO2 hydrate-bearing sediments varying with the influence factors in a nearly identical fashion. The results indicate that the stability of gas hydrate-bearing sediments could be maintained using the CH4eCO2 replacement method to recover methane from these sediments. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: CH4eCO2 replacement Carbon dioxide storage Hydrate-bearing sediments Mechanical properties Strength
1. Introduction Natural gas hydrates (methane hydrates) are ice-like crystalline solid compounds, which form under conditions of low temperature and high pressure (Kvenvolden, 1995) and occur extensively on submarine continental margins and in regions of arctic permafrost (Kvenvolden and Lorenson, 2001). Owing to their enormous worldwide reserves, natural gas hydrates have attracted great attention and are considered to be one of the most promising new generation energy sources to replace fossil fuel and alleviate the global energy crisis (Kvenvolden, 1988; Kvenvolden and Lorenson, 2001). In recent decades, several methods have been proposed to produce natural gas from gas hydrate-bearing sediments, including depressurization (Ji et al., 2001), thermal stimulation (Tang et al., 2005; Yousif et al., 1991), inhibitor injection (Gayet et al., 2005; Lederhos et al., 1996; Li et al., 2007), or a combination of these methods. These methods are all based on breaking the phase equilibrium of the gas hydrate to promote CH4 hydrate
* Corresponding author. E-mail address: [email protected] (Y. Song). 0264-8172/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marpetgeo.2013.06.016
decomposition by external stimulation. However, the decomposition of the natural gas hydrate may result in the deformation of the hydrate-bearing sediment strata, which can lead to seabed subsidence or submarine landslides and even trigger earthquakes and tsunami (Brown et al., 2006; Collett and Dallimore, 2002; MacDonald et al., 1994). The CH4 released from the methane hydrate could leak into the seawater and destroy the marine ecological environment (Tyler et al., 2002) or reach the atmosphere and significantly aggravate the greenhouse effect (Glasby, 2003; Lelieveld and Crutzen, 1992). Safety issues pose a significant technical challenge to commercial exploitation of natural gas from hydrate-bearing sediments. As an innovative technology, CH4eCO2 replacement has therefore given rise to a heated discussion, and the injection of CO2 into CH4 hydrate-bearing sediments has played a central role in this debate. The injection of CO2 results in the simultaneous dissociation of methane hydrate (releasing methane) and formation of CO2 hydrate (Jung et al., 2010). The CH4eCO2 replacement method has received the most attention because (i) it potentially provides long term storage of CO2, which is considered to be the dominant contributor to global warming by the greenhouse effect (Hitchon et al., 1999); and (ii) the CH4 hydrate has the same structure as
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the CO2 hydrate, and the pore space of the hydrate-bearing sediment would therefore be refilled by CO2 hydrate, which is expected to maintain the mechanical stability of gas hydrate-bearing sediments even after recovering the CH4 gas (Geng et al., 2009; Ota et al., 2005). To evaluate the safety of recovering natural gas from hydratebearing sediments, recent studies have focused primarily on investigating the mechanical properties of CH4 hydrate-bearing sediments. Hyodo et al. (2005, 2002) first reported that the mechanical properties of CH4 hydrate-bearing sediments depend strongly on the temperature and confining pressure. It was later found that hydrate saturation and cementation also play extremely important roles in the mechanical properties of CH4 hydratebearing sediments (Clayton et al., 2010, 2005; Masui et al., 2005a, 2005b). Several constitutive models were developed to predict the stress-strain behavior of CH4 hydrate (Miyazaki et al., 2011, 2008; Yu et al., 2011a, 2011b). It was confirmed by both experiment and simulation that hydrate decomposition greatly reduces the strength of CH4 hydrate-bearing sediments (Hyodo et al., 2011, 2007; Priest et al., 2011; Rutqvist et al., 2009). Substantial progress has been made on the mechanical stability of CH4 hydrate-bearing sediments, however, few studies have been reported on the mechanical stability of CO2 hydrate-bearing sediments. It is essential to investigate the stability of CO2 hydratebearing sediments to enable the safe recovery of CH4 from natural gas hydrate-bearing sediments using CH4eCO2 replacement. In this study, we extend our previous investigation of CH4 hydrate-bearing sediments. Low temperature, high pressure triaxial testing equipment was used to determine the mechanical behavior of CO2 hydrate-bearing sediments under various conditions. The CO2 and CH4 hydrate-bearing sediments were analyzed under the same test conditions, and the differences between their strengths were evaluated, yielding preliminary results on the change in strength of hydrate-bearing sediments caused by CH4e CO2 replacement.
with a hydraulic pump that was capable of providing a maximum pressure of 30 MPa. The temperature of the pressure chamber could be adjusted over a range of 30 C to room temperature, with an accuracy of 0.5 C, by circulating the refrigerant (ethylene glycol aqueous solution) from a controllable constant temperature bath. A thermocouple and a pressure sensor were placed in the pressure chamber to monitor the temperature and pressure variations during the experiment. The computer control system operated an axial loading frame to compress the specimen with an axial load capacity of 60 KN. The experimental data were recorded automatically by a computer data acquisition system during the axial loading process. The reader is referred to Song et al. (2010) and Yu et al. (2011b) for a detailed description of the experimental setup. 2.2. Specimen preparation For comparison with the CH4 hydrate-bearing sediments, the specimen preparation method used in our previous work (Li et al., 2011a, 2011b) was also employed in this study. The CO2 hydrate was manufactured using CO2 gas and ice powder as raw materials. First, ice formed from distilled water was ground into powder using a block shaving machine. The ice powder, with mean particle size around 250 mm, was extracted using a standard 60-mesh sieve and then placed in a reactor. CO2 gas with pressure around 3 MPa was injected into the reactor, which had been stored in the refrigerator at low temperature ranging from 5 C to 10 C for about 20 h. The degree of saturation of the synthetized CO2 hydrate was about 30% according to a calculation based on the amount of methane gas from hydrate dissociation. Finally, a predetermined amount of frozen kaolin clay and CO2 hydrate were mixed, and the mixture was placed in a mold and compacted under high pressure (10 MPa) by a pressure crystal device capable of providing pressures up to 60 MPa. The size of the final cylindrical specimen was 50 mm in diameter by 100 mm in height (see Fig. 2a). 2.3. Experimental procedures
2. Experimental methods 2.1. Experimental apparatus Figure 1 shows a schematic diagram of the triaxial testing system used in this study. The system consists of a confining pressure servosystem, a temperature control system, an axial servo-system and a computer control system. The confining pressure was maintained by pumping low-viscosity silicone oil into the pressure chamber
Figure 1. Schematic diagram of the triaxial testing apparatus. 1. Axial loading frame; 2. Air pressure line; 3. Hydraulic oil tank; 4. High-pressure pump; 5. Pressure gauge; 6. Axial displacement transducer; 7. Load cell; 8. Triaxial pressure cell; 9. End caps; 10. Specimen; 11. Thermocouple; 12. Heat exchanger; 13. Computer control system; 14. Control cabinet; 15. Constant temperature bath.
A brief description of the procedures and data analysis is provided here, and the reader is referred to Li et al. (2011a, 2011b) for a more detailed description. The specimen was first removed from the mold and wrapped with a rubber membrane around its perimeter. End caps were placed at the top and bottom of the specimen to prevent confining fluid from penetrating into the test specimen, which could affect the test results. The specimen was then isotropically consolidated at the 10 MPa target confining pressure for 2 h in the pressure chamber. The chamber pressure and temperature were adjusted to predetermined values and the testing was begun. The computer data acquisition system automatically recorded the stressestrain data. To minimize the dissociation of hydrate during the experiment, all of the testing processes, including the specimen preparation, were performed in a cold storage (10 C), and the time from specimen preparation to consolidation was kept as short as possible. It was assumed in the experiment that the pore space of the specimens was filled with unsaturated hydrate, and that the air content could be ignored. The specimen pore volume therefore represented the volume of hydrate and ice content. The mechanical properties of the hydrate-bearing sediments were extremely sensitive to temperature, confining pressure, strain rate and porosity. A total of 23 experiments under various test conditions, listed in Table 1, were therefore conducted in this study to investigate the effect of these factors on the mechanical properties of CO2 hydrate-bearing sediments. The experiments were carried out with porosities of 40%, 60% and 80%, temperatures of 20, 15, 10 and 5 C, confining pressures of 2.5, 3.75, 5, 7.5, 10, 12.5 and 15 MPa and strain rates of 0.1% and 1%.
W. Liu et al. / Marine and Petroleum Geology 46 (2013) 201e209 Table 1 Experimental conditions in the triaxial compression tests on artificial carbon dioxide hydrate-bearing sediments. Porosity (%)
Saturation (%)
Temperature ( C)
Confining pressure (MPa)
Strain rate (%/min)
40
30
10
1.0
40 40 60
30 30 30
5, 10, 15, 20 10 10
60 60 40, 60, 80
30 30 30
5, 10, 15, 20 10 10
2.5, 3.75, 5, 7.5, 10, 12.5, 15 5 5 2.5, 3.75, 5, 7.5, 10, 12.5, 15 5 5 5
1.0 0.1, 1.0 1.0 1.0 0.1, 1.0 1.0
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3. Results and discussion 3.1. General remarks An obvious bulging deformation can be observed at mid-height on the specimens after they have undergone the triaxial compression test, and the failure specimens take on a drum-like appearance, as shown in Figure 2b. Moreover, the specimens are not destroyed until the axial strain reaches 20%, implying that the specimens are composed of plastic material (Nash, 1998). Although the stressestrain curves vary with the test conditions, all of the stressestrain curves appear to be of hyperbolical shape, as shown in Figure 3. The stressestrain curve can be divided into three stages: the quasi-elastic stage (0 to A in Fig. 3), the hardening stage (A to B in Fig. 3) and the yield stage (B to C in Fig. 3). The deviator stress increases almost linearly as the axial strain increases, and only elastic deformation occurs in the quasi-elastic stage. Note that the quasi-elastic stage is very short, occurring only from 0% axial strain to no more than 0.5% axial strain. In the hardening stage, the deviator stress continues to increase as the axial strain increases further; however, the stress increment ratio gradually decreases. This implies that a smaller stress increment is required to produce the same axial strain increment as the axial strain increases. Plastic deformation occurs in this stage in addition to elastic deformation, and the total deformation owes mainly to plastic deformation. In the yield stage, the deviator stress maintains a nearly constant value, but the axial strain increases rapidly. In other words, the specimen is able to continue deforming under even a tiny stress increment, which implies that the specimen loses its ability to resist deformation and is completely destroyed. Owing to the shortage of test standards relating to hydrate-bearing sediments, the failure strength is based on the test standards of soil specimens and is defined as the maximum value of the deviator stress before the axial strain reaches 15%. 3.2. Effect of temperature on the mechanical properties of CO2 hydrate-bearing sediments Figure 4 shows the deviator stress as a function of the axial strain for the specimens with porosities of 40% and 60%, at
Figure 2. The CO2 hydrate-bearing samples before the experiment and after the experiment. (a) CO2 hydrate-bearing samples before the experiment. (b) CO2 hydratebearing samples after the experiment.
Figure 3. The shape of stress-strain curve of the CO2 hydrate-bearing samples subjected to the triaxial compression test.
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Figure 5. Relationship between strength and temperature for various porosities at a confining pressure of 5 MPa and strain rate of 1%/min.
Figure 4. Deviator stress-axial strain curves for different temperatures at a confining pressure of 5 MPa and strain rate of 1%/min for porosities of 40% and 60%. (a) Porosity j ¼ 40%. (b) Porosity j ¼ 60%.
temperatures of 20, 15, 10 and 5 C, a confining pressure of 5 MPa, and a strain rate of 1%/min. In the case of the specimen with a porosity of 40% at 10 C, the deviator stress increases linearly with a slope of approximately 460 MPa until the axial strain reaches approximately 0.2%. The deviator stress then continues to increase while the slope of the stress-strain curve gradually decreases until the axial strain reaches around 5%. Finally, the deviator stress reaches a peak value of nearly 6.6 MPa and the slope remains close to zero. As can be seen from Figure 4, the stressestrain curves clearly vary with temperature, and the strength increases as the temperature decreases. Figure 5 shows the relationship between the strength and the temperature for the specimens with porosities of 40% and 60%. The strength clearly increases with decreasing temperature. For the specimens with a porosity of 40%, the strength is 4.5 MPa at 5 C, and increases to 6.6 MPa at 10 C, then further increases to 8.5 MPa at 15 C and 10 MPa at 20 C. Similarly, for the specimens with a porosity of 60%, the strength increases from 3.5 MPa
at 5 C to 5.3 MPa at 10 C, 6.4 MPa at 15 C and 7.1 MPa at 20 C. The results confirm the conclusions of previous chemical research on hydrates (Sloan and Koh, 2008) that the lower the temperature, the higher the strength. Moreover, the results were consistent with those of the previous study on CH4 hydrate-bearing sediments by Hyodo et al. (2005). However, as can be seen from Figure 5, the relationship between the strength and the temperature is not linear, the strength increment ratio decreases with decreasing temperature. One possible explanation for the increase in the strength as the temperature decreases is that the hydrate is more thermodynamically stable at lower temperatures, leading to an enhancement of the intermolecular forces and making it more difficult to destroy the hydrate. Another possible explanation relates to pore-ice pressure melting, which occurs locally at grain-to-grain contacts under high stress conditions in frozen soils (Alkire and Andersland, 1973; Chamberlain et al., 1972) and in gas hydrate-bearing sediments (Li et al., 2011b). The decrease in the temperature can lower the extent of the pore-ice pressure melting at a given pressure, and the higher quantity of ice persisting in the sample can cause the bonding between the ice and sediment particles to be strengthened, further enhancing the strength of the gas hydrate-bearing sediments (Wang et al., 2004). 3.3. Effect of the confining pressure on the mechanical properties of CO2 hydrate-bearing sediments Figure 6 shows the deviator stress plotted against the axial strain for the specimens with porosities of 40% and 60%, at confining pressures of 2.5, 3.75, 5, 7.5, 10, 12.5 and 15 MPa, a temperature of 10 C, and a strain rate of 1%/min. The stressestrain curves clearly depend on the confining pressure. A higher confining pressure leads to an enhancement of the strength when the confining pressure is less than 5 MPa; however, the strength displays a downward trend as the confining pressure further increases. Figure 7 shows the relationship between strength and confining pressure for the specimens with porosities of 40% and 60%. As can be seen from Figure 7, the specimen with a porosity of 40% has a strength of 5.5 MPa under a confining pressure of 2.5 MPa, and the strength increases to 6.1 MPa under a confining pressure of
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Figure 6. Deviator stress-axial strain curves for different confining pressures at a temperature of 10 C and strain rate of 1%/min for porosities of 40% and 60%. (a) Porosity j ¼ 40%. (b) Porosity j ¼ 60%.
3.75 MPa and to 6.6 MPa under a confining pressure of 5 MPa. However, the strength decreases to 5.8 MPa under a confining pressure of 7.5 MPa and to 5.5 MPa under a confining pressure of 10 MPa, then continue to decrease to 5.0 MPa under a confining
pressure of 15 MPa. Similarly, for the specimens with a porosity of 60%, the strength first increases from 4 MPa to 4.7 MPa to 5.3 MPa, and subsequently decreases to 3.8 MPa and then to 3.5 MPae 3.1 MPa. Note that a similar change in the trend was reported for CH4 hydrate-bearing sediments (Li et al., 2011a, 2011b). According to previous chemical research, an increase in the confining pressure could produce an enhancement in the strength (Sloan and Koh, 2008). However, as discussed in section 3.2, high pressure conditions can induce pore-ice melting in gas hydratebearing sediments, which may decrease the strength of the sediments. The higher the pressure, the greater the extent of the poreice melting, and the further the decrease in the gas hydratebearing sediment strength. In addition, particle breakage is quite likely to occur in a high-pressure environment, leading to a decrease in the strength of the sediment. The higher the pressure, the more particles undergo breakage, and the further the decline in the strength. 3.4. Effect of the strain rate on the mechanical properties of CO2 hydrate-bearing sediments
Figure 7. Relationship between strength and confining pressure for various porosities at a temperature of 10 C and strain rate of 1%/min.
Figure 8 displays the axial strain curves as a function of deviator stress for the specimens with porosities of 40% and 60%, at strain rates of 0.1 and 1.0%/min, a confining pressure of 5 MPa, and a temperature of 10 C. It can be observed that the stressestrain curves are sensitive to the strain rate. The higher the strain rate,
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Figure 9. Deviator stress-axial strain curves for different porosities at a temperature of 10 C, confining pressure of 5 MPa and strain rate of 1%/min.
3.5. Effect of porosity on the mechanical properties of CO2 hydratebearing sediments From the discussion above, it can be seen that the strengths of the specimens with a porosity of 40% are larger than those of the specimens with a porosity of 60% under the same test conditions. Figure 9 further confirms that the strength increases with decreasing porosity by displaying the stress-strain curves of three specimens with different porosities. In addition, Figure 10 shows the relationship between strength and porosity. The specimen with 80% porosity has a strength of 4.1 MPa, while the strength increases to 5.3 MPa for the specimen with 60% porosity and to 6.6 MPa for the specimen with 40% porosity. The results are consistent with our previous studies on CH4 hydrate-bearing sediments (Li et al., 2011b). Moreover, the relationship between the strength and porosity can be seen to be linear. Note that in this study, the strength of the CO2 hydrate-bearing sediments depends primarily on the strength of the soil skeleton, because the specimens are of the “pore-filling” type without cementation as determined by the specimen preparation method. The porosity therefore decreases as Figure 8. Deviator stress-axial strain curves for different strain rates at a temperature of 10 C and confining pressure of 5 MPa for porosities of 40% and 60%. (a) Porosity j ¼ 40%. (b) Porosity j ¼ 60%.
the higher the strength. The specimen with a porosity of 40% has a strength of 4.6 MPa under a strain rate of 0.1%/min, and the strength increases to 6.6 MPa under a strain rate of 1%/min. Similarly, for the specimens with a porosity of 60%, the strength increases from 2.9 MPa to 5.3 MPa. It was also found that a higher strain rate can enhance the strength of ice (Hawkes and Mellor, 1972), frozen soil (Baker, 1979) and CH4 hydrate (Hyodo et al., 2005, 2002). During the compression process, a portion of the granular particles in the sediment slide by overcoming the frictional and the bite forces between particles. The particles can easily roll slowly across other particles at a low strain rate, because the frictional force is relatively weaker. However, as the strain rate increases, particles begin to squeeze each other, leading to an enhancement in the normal pressure between particles and thereby further enhancing the frictional force. The strength of hydrate-bearing sediments therefore increases with increasing strain rate.
Figure 10. Relationship between strength and porosity at a temperature of 10 C, confining pressure of 5 MPa and strain rate of 1%/min.
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the kaolin clay volume increases, leading to an enhancement in the strength of the CO2 hydrate-bearing sediments. 3.6. Strength comparison between CH4 hydrate-bearing sediments and CO2 hydrate-bearing sediments To assess the mechanical stability of natural gas hydrate-bearing sediments with CH4eCO2 replacement, it is necessary to analyze the strength differences between CH4 hydrate-bearing sediments and CO2 hydrate-bearing sediments. The mechanical properties of CH4 hydrate-bearing sediments were investigated in our previous studies (Li et al., 2011a, 2011b; Yu et al., 2011a). In this section, the results of triaxial compression tests of CH4 hydrate-bearing sediments (Li et al., 2011a, 2011b) and CO2 hydrate-bearing sediments are compared under the same experimental conditions. First, the failure mode of both the CO2 and CH4 hydrate-bearing sediments was a bulging deformation at mid-height on the samples. Furthermore, the stressestrain curves of both the CO2 and CH4
Figure 11. Relationship between strength and temperature at a confining pressure of 5 MPa and strain rate of 1%/min for porosities of 40% and 60% for the CH4 and CO2 hydrate-bearing sediments. (a) Porosity j ¼ 40%. (b) Porosity j ¼ 60%.
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hydrate-bearing sediments appear to be hyperbolic in shape (see section 3.1 for further details). Figure 11 shows the relationship between strength and temperature for the CH4 and CO2 hydrate-bearing sediments. The strengths of both the CH4 and CO2 hydrate-bearing sediments increase as the temperature decreases, and the shape of the trend is essentially the same for both sediments. However, the strength of the CO2 hydrate-bearing sediments is larger than that of the CH4 hydrate-bearing sediments under the same test conditions. For the specimens with a porosity of 40%, the strength of the CO2 hydratebearing sediments is 21.6%, 15.8% and 12.4% higher than that of the CH4 hydrate-bearing sediments when the test temperature is 5, 10 and 20 C, respectively. Similarly, for the specimens with 60% porosity, the strength increments are 20.7%, 17.8%, 16.0% and 16.4%, when the test temperature is 5, 10, 15 and 20 C, respectively. The relationship between strength and confining pressure for the CH4 and CO2 hydrate-bearing sediments is shown in Figure 12. The strengths of both the CH4 and CO2 hydrate-bearing sediments
Figure 12. Relationship between strength and confining pressure at a temperature of 10 C and strain rate of 1%/min for porosities of 40% and 60% for the CH4 and CO2 hydrate-bearing sediments. (a) Porosity j ¼ 40%. (b) Porosity j ¼ 60%.
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depend strongly on the confining pressure, with the form of the nearly identical trend for CH4 and CO2. The strength increases as the confining pressure increases when the confining pressure is less than 5 MPa, but the strength decreases as the confining pressure increases further. For the specimens with a porosity of 40%, the strength of the CO2 hydrate-bearing sediments is 10.2%, 15.1%, 15.8%, 16.0% and 3.8% higher than that of the CH4 hydrate-bearing sediments, when the confining pressure is 2.5, 3.75, 5, 7.5 and 10 MPa. For the specimens with a porosity of 60%, the strengths under the higher confining pressure of 12.5 MPa and 15 MPa are also compared, similarly, it can be found that the strength increments are 11.1%, 14.6%,17.8%, 15.2%, 3.7%, 4.1% and 4.1%, when the confining pressure is 2.5, 3.75, 5, 7.5, 10, 12.5 and 15 MPa. Figure 13 shows the relationship between strength and porosity for the CH4 and CO2 hydrate-bearing sediments. From Figure 13, it is evident that the strengths of the CH4 and CO2 hydrate-bearing sediments increase nearly linearly with the same slope as the porosity decreases. Moreover, it is again confirmed that the strength of the CO2 hydrate-bearing sediments is higher than that of the CH4 hydrate-bearing sediments. From the results in Figures 11, 12 and 13, it may be concluded that the strengths of the CH4 and CO2 hydrate-bearing sediments vary with all of the influence factors in a nearly identical fashion. However, the strength of the CO2 hydrate-bearing sediments is larger than that of the CH4 hydrate-bearing sediments under the same test conditions. Figure 14 shows Mohr circles of the CO2 hydrate-bearing sediments for porosities of 40% and 60%. Comparing with the Mohr circles of the CH4 hydrate-bearing sediments presented in Li et al. (2011b), it is evident that the cohesions (1.88 MPa for 40% porosity and 1.07 MPa for 60% porosity) of the CO2 hydrate-bearing sediments are nearly the same as those of the CH4 hydrate-bearing sediments (1.89 MPa for 40% porosity and 1.15 MPa for 60% porosity). However, the internal friction angles (10.09 for 40% porosity and 12.10 for 60% porosity) of the CO2 hydrate-bearing sediments are larger than those of the CH4 hydrate-bearing sediments (7.15 for 40% porosity and 8.78 for 60% porosity). These results imply that the strength difference between CO2 and CH4 hydrate-bearing sediments arises from a difference in frictional strength rather than cohesive strength. These results are intriguing because they suggest that as-yet-unknown factors may control the friction effect between gas hydrate and sediment grains. Different
Figure 14. Mohr circles of the CO2 hydrate-bearing sediments at a temperature of 10 C and strain rate of 1%/min for porosities of 40% and 60%. (a) Porosity j ¼ 40%. (b) Porosity j ¼ 60%.
phase equilibrium conditions or guest molecule characteristics could produce differences in the physical properties of the hydrate (Sloan and Koh, 2008). Furthermore, the physical properties of the soil particles had an influence on the strength in soil specimens (Terzaghi, 1943; Terzaghi and Peck, 1967), which are very similar to gas hydrate-bearing sediments. It is therefore possible that differences in the hydrate physical properties, such us surface roughness or specific surface area, may cause the strength difference between CO2 and CH4 hydrate-bearing sediments. Although its explanation requires further investigation, the result that the strength of CO2 hydrate-bearing sediments is larger than that of CH4 hydrate-bearing sediments is extremely desirable, as it indicates that the mechanical stability of gas hydrate-bearing sediments can be maintained using the CH4eCO2 replacement method to recover methane from the sediments. 4. Conclusions C,
Figure 13. Relationship between strength and porosity at a temperature of 10 confining pressure of 5 MPa and strain rate of 1%/min for the CH4 and CO2 hydratebearing sediments.
In this work, a low temperature, high pressure triaxial testing apparatus was designed. Triaxial compression tests were conducted
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to determine the effects of the temperature, confining pressure, strain rate and porosity on the mechanical properties of CO2 hydrate-bearing sediments. In addition, the triaxial tests results for CH4 and CO2 hydrate-bearing sediments were compared. The strength of the CO2 hydrate-bearing sediments was found to increase as the temperature and porosity decreased and as the strain rate increased. The strength increased with increasing confining pressure when the confining pressure was less than 5 MPa; however, it decreased when the confining pressure was increased further. In addition, the strength of the CH4 and CO2 hydrate-bearing sediments followed the same trends as the influence factors were varied. However, the strength of the CO2 hydratebearing sediments was larger than that of the CH4 hydrate-bearing sediments for pressures between 2.5 MPa and 15 MPa and temperatures between 5 C and 20 C. These results indicate that the stability of gas hydrate-bearing sediments can be maintained using the CH4eCO2 replacement method to recover methane from the sediments. The experimental data and results provide a preliminary understanding of the variation of the mechanical behavior of hydratebearing sediments with physical properties such as the temperature, confining pressure, and porosity. However, the situ CH4eCO2 replacement reaction is a complicated process involving many reaction mechanisms. It is therefore necessary to fully assess the mechanical stability of hydrate-bearing sediments which have undergone the situ CH4eCO2 replacement process, and this goal will be the focus of our future research. Acknowledgments This study was supported by the Major National S&T Program (No. 2011ZX05026-004), the Major State Basic Research Development Program of China (973 Program) (No. 2009CB219507), and the Natural Science Foundation of China (Grant No. 51006017). References Alkire, B.D., Andersland, O.B., 1973. The effect of confining pressure on the mechanical properties of sand-ice materials. Journal of Glaciology 12 (66), 469e481. Baker, T.H.W., 1979. Strain rate effect on the compressive strength of frozen sand. Engineering Geology Ground Freezing 13 (1e4), 223e231. Brown, H.E., Holbrook, W.S., Hornbach, M.J., Nealon, J., 2006. Slide structure and role of gas hydrate at the northern boundary of the Storegga Slide, offshore Norway. Marine Geology 229 (3e4), 179e186. Chamberlain, E., Groves, C., Perham, R., 1972. The mechanical behavior of frozen earth materials under high pressure triaxial test conditions. Geotechnique 22 (3), 469e483. Clayton, C., Priest, J.A., Best, A.I., 2005. The effects of disseminated methane hydrate on the dynamic stiffness and damping of a sand. Geotechnique 55 (6), 423e434. Clayton, C., Priest, J.A., Rees, E., 2010. The effects of hydrate cement on the stiffness of some sands. Geotechnique 60 (6), 435e445. Collett, T., Dallimore, S., 2002. Detailed analysis of gas hydrate induced drilling and production hazards. In: The 4th International Conference on Gas Hydrates (ICGH2002), Yokohama, Japan. Gayet, P., et al., 2005. Experimental determination of methane hydrate dissociation curve up to 55 MPa by using a small amount of surfactant as hydrate promoter. Chemical Engineering Science 60 (21), 5751e5758. Geng, C., Wen, H., Zhou, H., 2009. Molecular simulation of the potential of methane reoccupation during the replacement of methane hydrate by CO2. The Journal of Physical Chemistry A 113 (18), 5463e5469. Glasby, G., 2003. Potential impact on climate of the exploitation of methane hydrate deposits offshore. Marine and Petroleum Geology 20 (2), 163e175. Hawkes, I., Mellor, M., 1972. Deformation and fracture of ice under uniaxial stress. Journal of Glaciology 11 (61), 103e131. Hitchon, B., Gunter, W.D., Gentzis, T., Bailey, R.T., 1999. Sedimentary basins and greenhouse gases: a serendipitous association. Energy Conversion and Management 40 (8), 825e843. Hyodo, M., et al., 2002. Triaxial compressive strength of methane hydrate. In: The Twelfth (2002) International Offshore and Polar Engineering Conference, Kitakyushu, Japan. Hyodo, M., Nakata, Y., Yoshimoto, N., Orense, R., 2007. Shear behaviour of methane hydrate-bearing sand. In: The Seventeenth (2007) International Offshore and Polar Engineering Conference, Lisbon, Portugal.
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Hyodo, M., Yoneda, J., Nakata, Y., Yoshimoto, N., 2011. Strength and dissociation property of methane hydrate bearing sand. In: The 7th International Conference on Gas Hydrates (ICGH 2011), Edinburgh, Scotland, United Kingdom. Hyodo, M., Yukio, N., Norimasa, Y., Toshiro, E., 2005. Basic research on the mechanical behavior of methane hydrate-sediments mixture. Soils and Foundations 45 (1), 75e85. Ji, C., Ahmadi, G., Smith, D.H., 2001. Natural gas production from hydrate decomposition by depressurization. Chemical Engineering Science 56 (20), 5801e5814. Jung, J.W., Espinoza, D.N., Santamarina, J.C., 2010. Properties and phenomena relevant to CH4eCO2 replacement in hydrate-bearing sediments. Journal of Geophysical Research 115 (B10), B10102. Kvenvolden, K.A., 1988. Methane hydrate e a major reservoir of carbon in the shallow geosphere? Chemical Geology Origins of Methane in the Earth 71 (1e 3), 41e51. Kvenvolden, K.A., 1995. A review of the geochemistry of methane in natural gas hydrate. Organic Geochemistry 23 (11e12), 997e1008. Kvenvolden, K.A., Lorenson, T.D., 2001. The global occurrence of natural gas hydrate. In: Paull, C.K., Dillon, W.P. (Eds.), Natural Gas Hydrates: Occurrence, Distribution, and Detection, Geophysical Monographs, pp. 3e18. Washington, DC. Lederhos, J.P., Long, J.P., Sum, A., Christiansen, R.L., Sloan, E.D., 1996. Effective kinetic inhibitors for natural gas hydrates. Chemical Engineering Science 51 (8), 1221e 1229. Lelieveld, J., Crutzen, P.J., 1992. Indirect chemical effects of methane on climate warming. Nature 355, 339e342. Li, G., Li, X., Tang, L., Zhang, Y., 2007. Experimental investigation of production behavior of methane hydrate under ethylene glycol injection in unconsolidated sediment. Energy & Fuels 21 (6), 3388e3393. Li, Y., Song, Y., Liu, W., Liu, Y., Nishio, M., 2011a. Mechanical property measurements of clayey sediments containing gas hydrate by triaxial compression tests. In: The 7th International Conference on Gas Hydrates (ICGH 2011), Edinburgh, Scotland, United Kingdom. Li, Y., Song, Y., Yu, F., Liu, W., Zhao, J., 2011b. Experimental study on mechanical properties of gas hydrate-bearing sediments using kaolin clay. China Ocean Engineering 25 (1), 113e122. MacDonald, I.R., et al., 1994. Gas hydrate that breaches the sea floor on the continental slope of the Gulf of Mexico. Geology 22 (8), 699e702. Masui, A., Haneda, H., Ogata, Y., Aoki, K., 2005a. Effects of methane hydrate formation on shear strength of synthetic methane hydrate sediments. In: The Fifteenth (2005) International Offshore and Polar Engineering Conference, Seoul, Korea, pp. 364e369. Masui, A., Haneda, H., Ogata, Y., Aoki, K., 2005b. The effect of saturation degree of methane hydrate on the shear strength of synthetic methane hydrate sediments. In: The Fifth International Conference on Gas Hydrates (ICGH2005), Trondheim, Norway. Miyazaki, K., et al., 2008. Variable-compliance-type constitutive model for methane hydrate bearing sediment. In: The 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia Canada. Miyazaki, K., Aoki, K., Tenma, N., 2011. A nonlinear elastic constitutive model for artificial methane-hydrate-bearing sediment. In: The 7th International Conference on Gas Hydrates (ICGH 2011), Edinburgh, Scotland, United Kingdom. Nash, W.A., 1998. Schaum’s Outline of Theory and Problems of Strength of Materials, fourth ed. McGraw-Hill, New York. Ota, M., et al., 2005. Replacement of CH4 in the hydrate by use of liquid CO2. Energy Conversion and Management 46 (11e12), 1680e1691. Priest, J., Sultaniya, A., Clayton, C., 2011. Impact of hydrate formation and dissociation on the stiffness of a sand. In: The 7th International Conference on Gas Hydrates (ICGH 2011), Edinburgh, Scotland, United Kingdom. Rutqvist, J., Moridis, G.J., Grover, T., Collett, T., 2009. Geomechanical response of permafrost-associated hydrate deposits to depressurization-induced gas production. Journal of Petroleum Science and Engineering 67 (1e2), 1e12. Sloan, E.D., Koh, C.A., 2008. Clathrate Hydrates of Natural Gases, third ed. CRC Press, New York, USA. Song, Y., Yu, F., Li, Y., Liu, W., Zhao, J., 2010. Mechanical property of artificial methane hydrate under triaxial compression. Journal of Natural Gas Chemistry 19 (3), 246e250. Tang, L.G., Xiao, R., Huang, C., Feng, Z.P., Fan, S.S., 2005. Experimental investigation of production behavior of gas hydrate under thermal stimulation in unconsolidated sediment. Energy & Fuels 19 (6), 2402e2407. Terzaghi, K., 1943. Theoretical Soil Mechanics. Wiley, New York. Terzaghi, K., Peck, R.B., 1967. Soil Mechanics in Engineering Practice. John Wiley and Sons, New York. Tyler, P.A., German, C.R., Ramirez-Llodra, E., Van Dover, C.L., 2002. Understanding the biogeography of chemosynthetic ecosystems. Oceanologica Acta 25 (5), 227e241. Wang, D., Ma, W., Chang, X., 2004. Analyses of behavior of stressestrain of frozen Lanzhou loess subjected to K0 consolidation. Cold Regions Science and Technology 40 (1e2), 19e29. Yousif, M.H., Abass, H.H., Selim, M.S., Sloan, E.D., 1991. Experimental and theoretical investigation of methane-gas-hydrate dissociation in porous media. SPE Reservoir Engineering 6 (1), 69e76. Yu, F., Song, Y., Li, Y., Lam, W., 2011a. Analysis of stressestrain behavior and constitutive relation of methane hydrate-bearing sediments with various porosity. International Journal of Offshore and Polar Engineering 21 (2), 1e7. Yu, F., Song, Y., Liu, W., Li, Y., Lam, W., 2011b. Analyses of stress strain behavior and constitutive model of artificial methane hydrate. Journal of Petroleum Science and Engineering 77 (2), 183e188.
Contents lists available at SciVerse ScienceDirect
Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Experimental measurements of mechanical properties of carbon dioxide hydrate-bearing sediments Weiguo Liu, Jiafei Zhao, Yuan Luo, Yongchen Song*, Yanghui Li, Mingjun Yang, Yi Zhang, Yu Liu, Dayong Wang Key Laboratory of Ocean Energy Utilization and Energy Conservation of the Ministry of Education, Dalian University of Technology, Dalian 116024, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 October 2012 Received in revised form 16 June 2013 Accepted 21 June 2013 Available online 29 June 2013
The CH4eCO2 replacement method to recover methane from hydrate-bearing sediments has received great attention because it enables the long term storage of CO2 and is expected to maintain the stability of gas hydrate-bearing sediments. In this paper, the mechanical properties of CO2 hydrate-bearing sediments were measured by a low-temperature and high-pressure triaxial compression apparatus. The strength differences between the CO2 and CH4 hydrate-bearing sediments were then analyzed to evaluate the safety of the CH4eCO2 replacement method. The strength of the CO2 hydrate-bearing sediments was found to increase as the temperature and porosity decreased and as the strain rate increased. When the confining pressure was less than 5 MPa, the strength of the CO2 hydrate-bearing sediments also increased as the confining pressure increased. However, owing to pore-ice melting and particle breakage, the strength of the CO2 hydrate-bearing sediments decreased as the confining pressure increased for confining pressures exceeding 5 MPa. The strength of the CO2 hydrate-bearing sediments was found to be larger than that of the CH4 hydrate-bearing sediments, with the strengths of the CH4 and CO2 hydrate-bearing sediments varying with the influence factors in a nearly identical fashion. The results indicate that the stability of gas hydrate-bearing sediments could be maintained using the CH4eCO2 replacement method to recover methane from these sediments. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: CH4eCO2 replacement Carbon dioxide storage Hydrate-bearing sediments Mechanical properties Strength
1. Introduction Natural gas hydrates (methane hydrates) are ice-like crystalline solid compounds, which form under conditions of low temperature and high pressure (Kvenvolden, 1995) and occur extensively on submarine continental margins and in regions of arctic permafrost (Kvenvolden and Lorenson, 2001). Owing to their enormous worldwide reserves, natural gas hydrates have attracted great attention and are considered to be one of the most promising new generation energy sources to replace fossil fuel and alleviate the global energy crisis (Kvenvolden, 1988; Kvenvolden and Lorenson, 2001). In recent decades, several methods have been proposed to produce natural gas from gas hydrate-bearing sediments, including depressurization (Ji et al., 2001), thermal stimulation (Tang et al., 2005; Yousif et al., 1991), inhibitor injection (Gayet et al., 2005; Lederhos et al., 1996; Li et al., 2007), or a combination of these methods. These methods are all based on breaking the phase equilibrium of the gas hydrate to promote CH4 hydrate
* Corresponding author. E-mail address: [email protected] (Y. Song). 0264-8172/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marpetgeo.2013.06.016
decomposition by external stimulation. However, the decomposition of the natural gas hydrate may result in the deformation of the hydrate-bearing sediment strata, which can lead to seabed subsidence or submarine landslides and even trigger earthquakes and tsunami (Brown et al., 2006; Collett and Dallimore, 2002; MacDonald et al., 1994). The CH4 released from the methane hydrate could leak into the seawater and destroy the marine ecological environment (Tyler et al., 2002) or reach the atmosphere and significantly aggravate the greenhouse effect (Glasby, 2003; Lelieveld and Crutzen, 1992). Safety issues pose a significant technical challenge to commercial exploitation of natural gas from hydrate-bearing sediments. As an innovative technology, CH4eCO2 replacement has therefore given rise to a heated discussion, and the injection of CO2 into CH4 hydrate-bearing sediments has played a central role in this debate. The injection of CO2 results in the simultaneous dissociation of methane hydrate (releasing methane) and formation of CO2 hydrate (Jung et al., 2010). The CH4eCO2 replacement method has received the most attention because (i) it potentially provides long term storage of CO2, which is considered to be the dominant contributor to global warming by the greenhouse effect (Hitchon et al., 1999); and (ii) the CH4 hydrate has the same structure as
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the CO2 hydrate, and the pore space of the hydrate-bearing sediment would therefore be refilled by CO2 hydrate, which is expected to maintain the mechanical stability of gas hydrate-bearing sediments even after recovering the CH4 gas (Geng et al., 2009; Ota et al., 2005). To evaluate the safety of recovering natural gas from hydratebearing sediments, recent studies have focused primarily on investigating the mechanical properties of CH4 hydrate-bearing sediments. Hyodo et al. (2005, 2002) first reported that the mechanical properties of CH4 hydrate-bearing sediments depend strongly on the temperature and confining pressure. It was later found that hydrate saturation and cementation also play extremely important roles in the mechanical properties of CH4 hydratebearing sediments (Clayton et al., 2010, 2005; Masui et al., 2005a, 2005b). Several constitutive models were developed to predict the stress-strain behavior of CH4 hydrate (Miyazaki et al., 2011, 2008; Yu et al., 2011a, 2011b). It was confirmed by both experiment and simulation that hydrate decomposition greatly reduces the strength of CH4 hydrate-bearing sediments (Hyodo et al., 2011, 2007; Priest et al., 2011; Rutqvist et al., 2009). Substantial progress has been made on the mechanical stability of CH4 hydrate-bearing sediments, however, few studies have been reported on the mechanical stability of CO2 hydrate-bearing sediments. It is essential to investigate the stability of CO2 hydratebearing sediments to enable the safe recovery of CH4 from natural gas hydrate-bearing sediments using CH4eCO2 replacement. In this study, we extend our previous investigation of CH4 hydrate-bearing sediments. Low temperature, high pressure triaxial testing equipment was used to determine the mechanical behavior of CO2 hydrate-bearing sediments under various conditions. The CO2 and CH4 hydrate-bearing sediments were analyzed under the same test conditions, and the differences between their strengths were evaluated, yielding preliminary results on the change in strength of hydrate-bearing sediments caused by CH4e CO2 replacement.
with a hydraulic pump that was capable of providing a maximum pressure of 30 MPa. The temperature of the pressure chamber could be adjusted over a range of 30 C to room temperature, with an accuracy of 0.5 C, by circulating the refrigerant (ethylene glycol aqueous solution) from a controllable constant temperature bath. A thermocouple and a pressure sensor were placed in the pressure chamber to monitor the temperature and pressure variations during the experiment. The computer control system operated an axial loading frame to compress the specimen with an axial load capacity of 60 KN. The experimental data were recorded automatically by a computer data acquisition system during the axial loading process. The reader is referred to Song et al. (2010) and Yu et al. (2011b) for a detailed description of the experimental setup. 2.2. Specimen preparation For comparison with the CH4 hydrate-bearing sediments, the specimen preparation method used in our previous work (Li et al., 2011a, 2011b) was also employed in this study. The CO2 hydrate was manufactured using CO2 gas and ice powder as raw materials. First, ice formed from distilled water was ground into powder using a block shaving machine. The ice powder, with mean particle size around 250 mm, was extracted using a standard 60-mesh sieve and then placed in a reactor. CO2 gas with pressure around 3 MPa was injected into the reactor, which had been stored in the refrigerator at low temperature ranging from 5 C to 10 C for about 20 h. The degree of saturation of the synthetized CO2 hydrate was about 30% according to a calculation based on the amount of methane gas from hydrate dissociation. Finally, a predetermined amount of frozen kaolin clay and CO2 hydrate were mixed, and the mixture was placed in a mold and compacted under high pressure (10 MPa) by a pressure crystal device capable of providing pressures up to 60 MPa. The size of the final cylindrical specimen was 50 mm in diameter by 100 mm in height (see Fig. 2a). 2.3. Experimental procedures
2. Experimental methods 2.1. Experimental apparatus Figure 1 shows a schematic diagram of the triaxial testing system used in this study. The system consists of a confining pressure servosystem, a temperature control system, an axial servo-system and a computer control system. The confining pressure was maintained by pumping low-viscosity silicone oil into the pressure chamber
Figure 1. Schematic diagram of the triaxial testing apparatus. 1. Axial loading frame; 2. Air pressure line; 3. Hydraulic oil tank; 4. High-pressure pump; 5. Pressure gauge; 6. Axial displacement transducer; 7. Load cell; 8. Triaxial pressure cell; 9. End caps; 10. Specimen; 11. Thermocouple; 12. Heat exchanger; 13. Computer control system; 14. Control cabinet; 15. Constant temperature bath.
A brief description of the procedures and data analysis is provided here, and the reader is referred to Li et al. (2011a, 2011b) for a more detailed description. The specimen was first removed from the mold and wrapped with a rubber membrane around its perimeter. End caps were placed at the top and bottom of the specimen to prevent confining fluid from penetrating into the test specimen, which could affect the test results. The specimen was then isotropically consolidated at the 10 MPa target confining pressure for 2 h in the pressure chamber. The chamber pressure and temperature were adjusted to predetermined values and the testing was begun. The computer data acquisition system automatically recorded the stressestrain data. To minimize the dissociation of hydrate during the experiment, all of the testing processes, including the specimen preparation, were performed in a cold storage (10 C), and the time from specimen preparation to consolidation was kept as short as possible. It was assumed in the experiment that the pore space of the specimens was filled with unsaturated hydrate, and that the air content could be ignored. The specimen pore volume therefore represented the volume of hydrate and ice content. The mechanical properties of the hydrate-bearing sediments were extremely sensitive to temperature, confining pressure, strain rate and porosity. A total of 23 experiments under various test conditions, listed in Table 1, were therefore conducted in this study to investigate the effect of these factors on the mechanical properties of CO2 hydrate-bearing sediments. The experiments were carried out with porosities of 40%, 60% and 80%, temperatures of 20, 15, 10 and 5 C, confining pressures of 2.5, 3.75, 5, 7.5, 10, 12.5 and 15 MPa and strain rates of 0.1% and 1%.
W. Liu et al. / Marine and Petroleum Geology 46 (2013) 201e209 Table 1 Experimental conditions in the triaxial compression tests on artificial carbon dioxide hydrate-bearing sediments. Porosity (%)
Saturation (%)
Temperature ( C)
Confining pressure (MPa)
Strain rate (%/min)
40
30
10
1.0
40 40 60
30 30 30
5, 10, 15, 20 10 10
60 60 40, 60, 80
30 30 30
5, 10, 15, 20 10 10
2.5, 3.75, 5, 7.5, 10, 12.5, 15 5 5 2.5, 3.75, 5, 7.5, 10, 12.5, 15 5 5 5
1.0 0.1, 1.0 1.0 1.0 0.1, 1.0 1.0
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3. Results and discussion 3.1. General remarks An obvious bulging deformation can be observed at mid-height on the specimens after they have undergone the triaxial compression test, and the failure specimens take on a drum-like appearance, as shown in Figure 2b. Moreover, the specimens are not destroyed until the axial strain reaches 20%, implying that the specimens are composed of plastic material (Nash, 1998). Although the stressestrain curves vary with the test conditions, all of the stressestrain curves appear to be of hyperbolical shape, as shown in Figure 3. The stressestrain curve can be divided into three stages: the quasi-elastic stage (0 to A in Fig. 3), the hardening stage (A to B in Fig. 3) and the yield stage (B to C in Fig. 3). The deviator stress increases almost linearly as the axial strain increases, and only elastic deformation occurs in the quasi-elastic stage. Note that the quasi-elastic stage is very short, occurring only from 0% axial strain to no more than 0.5% axial strain. In the hardening stage, the deviator stress continues to increase as the axial strain increases further; however, the stress increment ratio gradually decreases. This implies that a smaller stress increment is required to produce the same axial strain increment as the axial strain increases. Plastic deformation occurs in this stage in addition to elastic deformation, and the total deformation owes mainly to plastic deformation. In the yield stage, the deviator stress maintains a nearly constant value, but the axial strain increases rapidly. In other words, the specimen is able to continue deforming under even a tiny stress increment, which implies that the specimen loses its ability to resist deformation and is completely destroyed. Owing to the shortage of test standards relating to hydrate-bearing sediments, the failure strength is based on the test standards of soil specimens and is defined as the maximum value of the deviator stress before the axial strain reaches 15%. 3.2. Effect of temperature on the mechanical properties of CO2 hydrate-bearing sediments Figure 4 shows the deviator stress as a function of the axial strain for the specimens with porosities of 40% and 60%, at
Figure 2. The CO2 hydrate-bearing samples before the experiment and after the experiment. (a) CO2 hydrate-bearing samples before the experiment. (b) CO2 hydratebearing samples after the experiment.
Figure 3. The shape of stress-strain curve of the CO2 hydrate-bearing samples subjected to the triaxial compression test.
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Figure 5. Relationship between strength and temperature for various porosities at a confining pressure of 5 MPa and strain rate of 1%/min.
Figure 4. Deviator stress-axial strain curves for different temperatures at a confining pressure of 5 MPa and strain rate of 1%/min for porosities of 40% and 60%. (a) Porosity j ¼ 40%. (b) Porosity j ¼ 60%.
temperatures of 20, 15, 10 and 5 C, a confining pressure of 5 MPa, and a strain rate of 1%/min. In the case of the specimen with a porosity of 40% at 10 C, the deviator stress increases linearly with a slope of approximately 460 MPa until the axial strain reaches approximately 0.2%. The deviator stress then continues to increase while the slope of the stress-strain curve gradually decreases until the axial strain reaches around 5%. Finally, the deviator stress reaches a peak value of nearly 6.6 MPa and the slope remains close to zero. As can be seen from Figure 4, the stressestrain curves clearly vary with temperature, and the strength increases as the temperature decreases. Figure 5 shows the relationship between the strength and the temperature for the specimens with porosities of 40% and 60%. The strength clearly increases with decreasing temperature. For the specimens with a porosity of 40%, the strength is 4.5 MPa at 5 C, and increases to 6.6 MPa at 10 C, then further increases to 8.5 MPa at 15 C and 10 MPa at 20 C. Similarly, for the specimens with a porosity of 60%, the strength increases from 3.5 MPa
at 5 C to 5.3 MPa at 10 C, 6.4 MPa at 15 C and 7.1 MPa at 20 C. The results confirm the conclusions of previous chemical research on hydrates (Sloan and Koh, 2008) that the lower the temperature, the higher the strength. Moreover, the results were consistent with those of the previous study on CH4 hydrate-bearing sediments by Hyodo et al. (2005). However, as can be seen from Figure 5, the relationship between the strength and the temperature is not linear, the strength increment ratio decreases with decreasing temperature. One possible explanation for the increase in the strength as the temperature decreases is that the hydrate is more thermodynamically stable at lower temperatures, leading to an enhancement of the intermolecular forces and making it more difficult to destroy the hydrate. Another possible explanation relates to pore-ice pressure melting, which occurs locally at grain-to-grain contacts under high stress conditions in frozen soils (Alkire and Andersland, 1973; Chamberlain et al., 1972) and in gas hydrate-bearing sediments (Li et al., 2011b). The decrease in the temperature can lower the extent of the pore-ice pressure melting at a given pressure, and the higher quantity of ice persisting in the sample can cause the bonding between the ice and sediment particles to be strengthened, further enhancing the strength of the gas hydrate-bearing sediments (Wang et al., 2004). 3.3. Effect of the confining pressure on the mechanical properties of CO2 hydrate-bearing sediments Figure 6 shows the deviator stress plotted against the axial strain for the specimens with porosities of 40% and 60%, at confining pressures of 2.5, 3.75, 5, 7.5, 10, 12.5 and 15 MPa, a temperature of 10 C, and a strain rate of 1%/min. The stressestrain curves clearly depend on the confining pressure. A higher confining pressure leads to an enhancement of the strength when the confining pressure is less than 5 MPa; however, the strength displays a downward trend as the confining pressure further increases. Figure 7 shows the relationship between strength and confining pressure for the specimens with porosities of 40% and 60%. As can be seen from Figure 7, the specimen with a porosity of 40% has a strength of 5.5 MPa under a confining pressure of 2.5 MPa, and the strength increases to 6.1 MPa under a confining pressure of
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Figure 6. Deviator stress-axial strain curves for different confining pressures at a temperature of 10 C and strain rate of 1%/min for porosities of 40% and 60%. (a) Porosity j ¼ 40%. (b) Porosity j ¼ 60%.
3.75 MPa and to 6.6 MPa under a confining pressure of 5 MPa. However, the strength decreases to 5.8 MPa under a confining pressure of 7.5 MPa and to 5.5 MPa under a confining pressure of 10 MPa, then continue to decrease to 5.0 MPa under a confining
pressure of 15 MPa. Similarly, for the specimens with a porosity of 60%, the strength first increases from 4 MPa to 4.7 MPa to 5.3 MPa, and subsequently decreases to 3.8 MPa and then to 3.5 MPae 3.1 MPa. Note that a similar change in the trend was reported for CH4 hydrate-bearing sediments (Li et al., 2011a, 2011b). According to previous chemical research, an increase in the confining pressure could produce an enhancement in the strength (Sloan and Koh, 2008). However, as discussed in section 3.2, high pressure conditions can induce pore-ice melting in gas hydratebearing sediments, which may decrease the strength of the sediments. The higher the pressure, the greater the extent of the poreice melting, and the further the decrease in the gas hydratebearing sediment strength. In addition, particle breakage is quite likely to occur in a high-pressure environment, leading to a decrease in the strength of the sediment. The higher the pressure, the more particles undergo breakage, and the further the decline in the strength. 3.4. Effect of the strain rate on the mechanical properties of CO2 hydrate-bearing sediments
Figure 7. Relationship between strength and confining pressure for various porosities at a temperature of 10 C and strain rate of 1%/min.
Figure 8 displays the axial strain curves as a function of deviator stress for the specimens with porosities of 40% and 60%, at strain rates of 0.1 and 1.0%/min, a confining pressure of 5 MPa, and a temperature of 10 C. It can be observed that the stressestrain curves are sensitive to the strain rate. The higher the strain rate,
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Figure 9. Deviator stress-axial strain curves for different porosities at a temperature of 10 C, confining pressure of 5 MPa and strain rate of 1%/min.
3.5. Effect of porosity on the mechanical properties of CO2 hydratebearing sediments From the discussion above, it can be seen that the strengths of the specimens with a porosity of 40% are larger than those of the specimens with a porosity of 60% under the same test conditions. Figure 9 further confirms that the strength increases with decreasing porosity by displaying the stress-strain curves of three specimens with different porosities. In addition, Figure 10 shows the relationship between strength and porosity. The specimen with 80% porosity has a strength of 4.1 MPa, while the strength increases to 5.3 MPa for the specimen with 60% porosity and to 6.6 MPa for the specimen with 40% porosity. The results are consistent with our previous studies on CH4 hydrate-bearing sediments (Li et al., 2011b). Moreover, the relationship between the strength and porosity can be seen to be linear. Note that in this study, the strength of the CO2 hydrate-bearing sediments depends primarily on the strength of the soil skeleton, because the specimens are of the “pore-filling” type without cementation as determined by the specimen preparation method. The porosity therefore decreases as Figure 8. Deviator stress-axial strain curves for different strain rates at a temperature of 10 C and confining pressure of 5 MPa for porosities of 40% and 60%. (a) Porosity j ¼ 40%. (b) Porosity j ¼ 60%.
the higher the strength. The specimen with a porosity of 40% has a strength of 4.6 MPa under a strain rate of 0.1%/min, and the strength increases to 6.6 MPa under a strain rate of 1%/min. Similarly, for the specimens with a porosity of 60%, the strength increases from 2.9 MPa to 5.3 MPa. It was also found that a higher strain rate can enhance the strength of ice (Hawkes and Mellor, 1972), frozen soil (Baker, 1979) and CH4 hydrate (Hyodo et al., 2005, 2002). During the compression process, a portion of the granular particles in the sediment slide by overcoming the frictional and the bite forces between particles. The particles can easily roll slowly across other particles at a low strain rate, because the frictional force is relatively weaker. However, as the strain rate increases, particles begin to squeeze each other, leading to an enhancement in the normal pressure between particles and thereby further enhancing the frictional force. The strength of hydrate-bearing sediments therefore increases with increasing strain rate.
Figure 10. Relationship between strength and porosity at a temperature of 10 C, confining pressure of 5 MPa and strain rate of 1%/min.
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the kaolin clay volume increases, leading to an enhancement in the strength of the CO2 hydrate-bearing sediments. 3.6. Strength comparison between CH4 hydrate-bearing sediments and CO2 hydrate-bearing sediments To assess the mechanical stability of natural gas hydrate-bearing sediments with CH4eCO2 replacement, it is necessary to analyze the strength differences between CH4 hydrate-bearing sediments and CO2 hydrate-bearing sediments. The mechanical properties of CH4 hydrate-bearing sediments were investigated in our previous studies (Li et al., 2011a, 2011b; Yu et al., 2011a). In this section, the results of triaxial compression tests of CH4 hydrate-bearing sediments (Li et al., 2011a, 2011b) and CO2 hydrate-bearing sediments are compared under the same experimental conditions. First, the failure mode of both the CO2 and CH4 hydrate-bearing sediments was a bulging deformation at mid-height on the samples. Furthermore, the stressestrain curves of both the CO2 and CH4
Figure 11. Relationship between strength and temperature at a confining pressure of 5 MPa and strain rate of 1%/min for porosities of 40% and 60% for the CH4 and CO2 hydrate-bearing sediments. (a) Porosity j ¼ 40%. (b) Porosity j ¼ 60%.
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hydrate-bearing sediments appear to be hyperbolic in shape (see section 3.1 for further details). Figure 11 shows the relationship between strength and temperature for the CH4 and CO2 hydrate-bearing sediments. The strengths of both the CH4 and CO2 hydrate-bearing sediments increase as the temperature decreases, and the shape of the trend is essentially the same for both sediments. However, the strength of the CO2 hydrate-bearing sediments is larger than that of the CH4 hydrate-bearing sediments under the same test conditions. For the specimens with a porosity of 40%, the strength of the CO2 hydratebearing sediments is 21.6%, 15.8% and 12.4% higher than that of the CH4 hydrate-bearing sediments when the test temperature is 5, 10 and 20 C, respectively. Similarly, for the specimens with 60% porosity, the strength increments are 20.7%, 17.8%, 16.0% and 16.4%, when the test temperature is 5, 10, 15 and 20 C, respectively. The relationship between strength and confining pressure for the CH4 and CO2 hydrate-bearing sediments is shown in Figure 12. The strengths of both the CH4 and CO2 hydrate-bearing sediments
Figure 12. Relationship between strength and confining pressure at a temperature of 10 C and strain rate of 1%/min for porosities of 40% and 60% for the CH4 and CO2 hydrate-bearing sediments. (a) Porosity j ¼ 40%. (b) Porosity j ¼ 60%.
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depend strongly on the confining pressure, with the form of the nearly identical trend for CH4 and CO2. The strength increases as the confining pressure increases when the confining pressure is less than 5 MPa, but the strength decreases as the confining pressure increases further. For the specimens with a porosity of 40%, the strength of the CO2 hydrate-bearing sediments is 10.2%, 15.1%, 15.8%, 16.0% and 3.8% higher than that of the CH4 hydrate-bearing sediments, when the confining pressure is 2.5, 3.75, 5, 7.5 and 10 MPa. For the specimens with a porosity of 60%, the strengths under the higher confining pressure of 12.5 MPa and 15 MPa are also compared, similarly, it can be found that the strength increments are 11.1%, 14.6%,17.8%, 15.2%, 3.7%, 4.1% and 4.1%, when the confining pressure is 2.5, 3.75, 5, 7.5, 10, 12.5 and 15 MPa. Figure 13 shows the relationship between strength and porosity for the CH4 and CO2 hydrate-bearing sediments. From Figure 13, it is evident that the strengths of the CH4 and CO2 hydrate-bearing sediments increase nearly linearly with the same slope as the porosity decreases. Moreover, it is again confirmed that the strength of the CO2 hydrate-bearing sediments is higher than that of the CH4 hydrate-bearing sediments. From the results in Figures 11, 12 and 13, it may be concluded that the strengths of the CH4 and CO2 hydrate-bearing sediments vary with all of the influence factors in a nearly identical fashion. However, the strength of the CO2 hydrate-bearing sediments is larger than that of the CH4 hydrate-bearing sediments under the same test conditions. Figure 14 shows Mohr circles of the CO2 hydrate-bearing sediments for porosities of 40% and 60%. Comparing with the Mohr circles of the CH4 hydrate-bearing sediments presented in Li et al. (2011b), it is evident that the cohesions (1.88 MPa for 40% porosity and 1.07 MPa for 60% porosity) of the CO2 hydrate-bearing sediments are nearly the same as those of the CH4 hydrate-bearing sediments (1.89 MPa for 40% porosity and 1.15 MPa for 60% porosity). However, the internal friction angles (10.09 for 40% porosity and 12.10 for 60% porosity) of the CO2 hydrate-bearing sediments are larger than those of the CH4 hydrate-bearing sediments (7.15 for 40% porosity and 8.78 for 60% porosity). These results imply that the strength difference between CO2 and CH4 hydrate-bearing sediments arises from a difference in frictional strength rather than cohesive strength. These results are intriguing because they suggest that as-yet-unknown factors may control the friction effect between gas hydrate and sediment grains. Different
Figure 14. Mohr circles of the CO2 hydrate-bearing sediments at a temperature of 10 C and strain rate of 1%/min for porosities of 40% and 60%. (a) Porosity j ¼ 40%. (b) Porosity j ¼ 60%.
phase equilibrium conditions or guest molecule characteristics could produce differences in the physical properties of the hydrate (Sloan and Koh, 2008). Furthermore, the physical properties of the soil particles had an influence on the strength in soil specimens (Terzaghi, 1943; Terzaghi and Peck, 1967), which are very similar to gas hydrate-bearing sediments. It is therefore possible that differences in the hydrate physical properties, such us surface roughness or specific surface area, may cause the strength difference between CO2 and CH4 hydrate-bearing sediments. Although its explanation requires further investigation, the result that the strength of CO2 hydrate-bearing sediments is larger than that of CH4 hydrate-bearing sediments is extremely desirable, as it indicates that the mechanical stability of gas hydrate-bearing sediments can be maintained using the CH4eCO2 replacement method to recover methane from the sediments. 4. Conclusions C,
Figure 13. Relationship between strength and porosity at a temperature of 10 confining pressure of 5 MPa and strain rate of 1%/min for the CH4 and CO2 hydratebearing sediments.
In this work, a low temperature, high pressure triaxial testing apparatus was designed. Triaxial compression tests were conducted
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to determine the effects of the temperature, confining pressure, strain rate and porosity on the mechanical properties of CO2 hydrate-bearing sediments. In addition, the triaxial tests results for CH4 and CO2 hydrate-bearing sediments were compared. The strength of the CO2 hydrate-bearing sediments was found to increase as the temperature and porosity decreased and as the strain rate increased. The strength increased with increasing confining pressure when the confining pressure was less than 5 MPa; however, it decreased when the confining pressure was increased further. In addition, the strength of the CH4 and CO2 hydrate-bearing sediments followed the same trends as the influence factors were varied. However, the strength of the CO2 hydratebearing sediments was larger than that of the CH4 hydrate-bearing sediments for pressures between 2.5 MPa and 15 MPa and temperatures between 5 C and 20 C. These results indicate that the stability of gas hydrate-bearing sediments can be maintained using the CH4eCO2 replacement method to recover methane from the sediments. The experimental data and results provide a preliminary understanding of the variation of the mechanical behavior of hydratebearing sediments with physical properties such as the temperature, confining pressure, and porosity. However, the situ CH4eCO2 replacement reaction is a complicated process involving many reaction mechanisms. It is therefore necessary to fully assess the mechanical stability of hydrate-bearing sediments which have undergone the situ CH4eCO2 replacement process, and this goal will be the focus of our future research. Acknowledgments This study was supported by the Major National S&T Program (No. 2011ZX05026-004), the Major State Basic Research Development Program of China (973 Program) (No. 2009CB219507), and the Natural Science Foundation of China (Grant No. 51006017). References Alkire, B.D., Andersland, O.B., 1973. The effect of confining pressure on the mechanical properties of sand-ice materials. Journal of Glaciology 12 (66), 469e481. Baker, T.H.W., 1979. Strain rate effect on the compressive strength of frozen sand. Engineering Geology Ground Freezing 13 (1e4), 223e231. Brown, H.E., Holbrook, W.S., Hornbach, M.J., Nealon, J., 2006. Slide structure and role of gas hydrate at the northern boundary of the Storegga Slide, offshore Norway. Marine Geology 229 (3e4), 179e186. Chamberlain, E., Groves, C., Perham, R., 1972. The mechanical behavior of frozen earth materials under high pressure triaxial test conditions. Geotechnique 22 (3), 469e483. Clayton, C., Priest, J.A., Best, A.I., 2005. The effects of disseminated methane hydrate on the dynamic stiffness and damping of a sand. Geotechnique 55 (6), 423e434. Clayton, C., Priest, J.A., Rees, E., 2010. The effects of hydrate cement on the stiffness of some sands. Geotechnique 60 (6), 435e445. Collett, T., Dallimore, S., 2002. Detailed analysis of gas hydrate induced drilling and production hazards. In: The 4th International Conference on Gas Hydrates (ICGH2002), Yokohama, Japan. Gayet, P., et al., 2005. Experimental determination of methane hydrate dissociation curve up to 55 MPa by using a small amount of surfactant as hydrate promoter. Chemical Engineering Science 60 (21), 5751e5758. Geng, C., Wen, H., Zhou, H., 2009. Molecular simulation of the potential of methane reoccupation during the replacement of methane hydrate by CO2. The Journal of Physical Chemistry A 113 (18), 5463e5469. Glasby, G., 2003. Potential impact on climate of the exploitation of methane hydrate deposits offshore. Marine and Petroleum Geology 20 (2), 163e175. Hawkes, I., Mellor, M., 1972. Deformation and fracture of ice under uniaxial stress. Journal of Glaciology 11 (61), 103e131. Hitchon, B., Gunter, W.D., Gentzis, T., Bailey, R.T., 1999. Sedimentary basins and greenhouse gases: a serendipitous association. Energy Conversion and Management 40 (8), 825e843. Hyodo, M., et al., 2002. Triaxial compressive strength of methane hydrate. In: The Twelfth (2002) International Offshore and Polar Engineering Conference, Kitakyushu, Japan. Hyodo, M., Nakata, Y., Yoshimoto, N., Orense, R., 2007. Shear behaviour of methane hydrate-bearing sand. In: The Seventeenth (2007) International Offshore and Polar Engineering Conference, Lisbon, Portugal.
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