Experimental Study on Mechanical Properties of Gas Hydrate-Bearing Sediments Using Kaolin Clay

China Ocean Eng. Vol.25, No.1, pp.113- 122。2011 Chinese Occan Engincering Society and Springr-Verlag Berlin HeidelbergDOI 10.1007/513344-0110009-6Experimental Study on Mechanical Properties of Gas Hydrate-BearingSediments Using K aolin Clay'LI Yang-hui (李洋辉), SONG Yong-chen (宋永臣), YU Feng(于锋), .LIU Wei-guo (刘卫国) and ZHAO Jia-fei (赵佳飞)Key Laboratony of Ocean Energy Utilization and Energy Conservation of Ministry of Education,Dalian University of Technology, Dalian 116024, China(Received 21 January 2010; received revised form 10 September 2010; accepted 8 October 2010)ABSTRACTA triaxial system is designed with a temperature range from -20 C to 25 C and a pressure range from 0 MPa to 30MPa in order to improve the understanding of the mechanical properties of gas hydrate-bearing sediments. The mechanicalproperties of synthetic gas hydrate-bearing sediments (gas hydrate-kaolin clay mixture) were measured by using curentexperimental apparatus. The results indicate that: (1) the failure strength of gas hydrate-bearing sediments strongly dependson the temperature. The sediment' s strength increases with the decreases of temperature. (2) The maximum deviator stessincreases linearly with the confining pressure at a low-pressure stage. However, it fuctuates at a high-pressure stage. (3)Maximum deviator stress increases with increasing strain rate, whereas the stan-stress curve has no remendous changeuntil the axial strain reaches approximately 0.5%. (4) The intemal friction angles of gas hydrate-bearing sediments are notsensitive to kaolin volume ratio. The cohesion shows a high kaolin volume ratio dependency.Key words: gas hydrate; mechanical properies; marine sediment; triaxrial testing; safe extraction1. IntroductionIn recent years the extraction of gas hydrate as a potential resource of hydrocarbon energy isbeing investigated (Long et al, 2005; Pierce, 2005; Boswell, 2009), and a large amount of gas hydratein the permafrost and marine sediments has been estimated (Milkov, 2004; Dawe and Thomas, 2007).Gas hydrate is an ice-like crystalline substance that is essentially frozen methane. Its structureconsists of a gas molecule surrounded by a cage of water molecules (Ripmeester et al, 1987; Sloan,1998). It is stable under conditions of low temperature and high pressure. While drilling forexploration and exploitation, temperature and/or pressure changes can modify the gas hydrateequilibrium conditions and induce gas production. Gas may alter significantly the mechanical strengthof the marine sediments (Vanoudheusden et al, 2004), which can initiate landslides on the slope andrise, then lead to tsunami and methane leak which may influence the global climate (Dickens et al,1995; Glasby, 2003; Brown el al, 2006). Therefore, experimental test on mechanical properties of gas●This work was supported by the National High Technology Research a中国煤化工63 Progem, Crant No.2006AA09A209) the Major National S&T Program (Grant No. 20082Research DevelopmentPrugram of China (973 Program, Grant No.2009CB219507) and theMYHCNMHGhofChinCranNo.91010015).1 Coresponding author. E-mail: songyc@dlut.edu.cnLI Yang-hui et al. / China Ocean Eng. 25(1), 2011, 113-122hydrate-bearing sediments plays an important role in the safe extraction of gas hydrate.Considerable researches on the physical/chemical properties have been conducted previously inorder to allow a safe extraction of gas hydrate (Ripmeester and Ratlffe, 1988; Handa and Stupin,1992). The impact of tetrahydrofuran (THF) which is used to simulate gas hydrate on the mechanicalstrength of host sediments has long been investigated in the geotechnical literatures (Lee et al, 2007,2008; Yun et al, 2007). Although THF hydrate has many physical properties similar to those ofmethane hydrates, it is still a“proxy" for methane hydrate. Researchers further investigated themechanical properties of gas hydrate-bearing sediments using both synthetic and actual marinesediments (Stem et al, 1996; Durham et al, 2003; Vanoudheusden et al, 2004; Hyodo et al, 2005).They indicated that the mechanical properties of methane hydrate are very different from those ofwater ice. Some uniaxial compression tests have been carried out under a series of low temperatureconditions (Stern et al, 1996; Durham et al, 2003). However, it is vital to study the mechanicalproperties of gas hydrate under various conditions such as temperature, confining pressure, strain rateand saturation. Researchers used triaxial testing apparatus to measure mechanical properties of actualand synthetic gas hydrate or gas hydrate-bearing sediments with limited success (Hyodo et al, 2002,2005; Masui et al., 2007, 2008a, 2008b). Further exploration of mechanical properties is required inorder to assess the stability of gas hydrate-bearing sediments.In this study, a puposely designed low-temperature and high-pressure triaxial testing system wassetup for the mechanical property measurements. Tests on synthetic gas hydrate-bearing sedimentcores, which were used to simulate marine sediments, were conducted and its mechanical propertiesunder various conditions were investigated.2. ExperimentThe schematic diagram of the triaxial testing system is shown in Fig. 1. The system can simulatein situ pressure and temperature conditions in a cylindrical sediment sampler (50 mm in diameter and100 mm in height). Triaxial compression tests can be carried out in the temperature ranging from-20C to 25C (normal loading capacity 60 kN, and lateral pressure loading capacity 30 MPa).Confining pressure and axial load are contolled automatically by using two EDC (Extemal DigitalController, made in DOLI Co. of Germany). The temperature of the specimen placed in pressurechamber is adjusted by circulating cell liquid from thermostatic bath and heat exchanger. Thethermocouple (with accuracy of 0.5"C) and the pressure sensor (with accuracy of0.01 MPa) are in thepressure chamber to measure the temperature of specimen and the confining pressure.The mechanical properties of synthetic gas hydrate-impacted specimens are similar to those ofactual hydrate-bearing sediments (Winters et al., 2004, 2007; Masui e1 al, 2008). Owing to the lackof drill cores of natural gas hydrate-bearing sediments, the gas hydrate in this study was manufacturedunder phase -equilibrium conditions ( 5 C, 13 MPa) by mixing ice powder with pure methane gas ina high- pressure reactor. The saturation of the gas hydrate:1 kaolin clay wasused to simulate the marine sediments. The grain size中国煤化工)arameters of thekaolin are shown in Fig. 2, in which the curve was olMYHCN M H Ggal Particle Size .Analyzer. The synthetic gas hydrate was mixed with a predetermined amount of kaolin, and thenLI Yang-hui et al. / China Ocean Eng., 25(), 2011,113- 122filled into the pressure molding device with a prescribed mass of gas hydrate-kaolin mixture. Finally,the specimen was formed under a controlled temperature (-10"C) and an axial pressure (10 MPa),meanwhile excess moiture was removed. The specimen was covered with a rubber membrane of 0.5 mmin thickness to isolate the pore pressure and confining pressure. The specimen must be consolidatedfor 2 hours before compression tests.10Specific gavity 2.60 g/cm)|鼎80 Median Diam.3.91 μmModal Diam__ 12.52 ymo叶-5) .(6"0卜2)()」占20-电610月0.1100Flg, 1.The schematic diagram of triaxial testing equipment.Grain size (m)《1 )Stepping motor, (2 )Pump, (3 )Hydraulic oil tank,(4)Pressure gauge, (5)Heat exchanger. (6)Specimen,Fig. 2. Grain size distribution curve and basic parameters(7)Thermocouple, (8)Load cel, (9)Air pressure line,of the kaolin clay.(l0)Themmostatic bath, (1l )Computer.In order to reduce the dissociation of gas hydrate during the experiment, the hydraulic oil wascooled down aforehand, and all the testing processes including specimen preparation were completedwithin the cold storage (- -10"C). The time that specimens exposed to the air must be as short aspossible and therefore the experimental procedure of specimen molding, parameters calibration(height, mass) and specimen installation is preferably completed in less than 10 minutes. Tests werecarried out under various conditions with temperature ofT= -5, -10, and -20C, confining pressuresofσ3= 2.5, 3.75, 5, 7.5 and 10 MPa, kaolin volume ratios of ψ = 20%, 40%, and 60%, and strain ratesof 0.1% and 1.0 %/min, as shown in Table 1.Table 1 Experimental conditions of triaxial compression tests on synthetic gas hydrate-bearing sedimentsTemperatureConfining pressureStrain rateKaolin volume ratioSaturation(C)( MPa)( /min)20%30%-55.00 .1.0%2.50, 5.00, 10.00-205.0040%‘-S-102.50, 3.75, 5.00 7.50, 10.000.1%, 1.0%2060%中国煤化工0.1%, 1.0%MHCNMHG116u1 Yang -hui er al. / China Ocean Eng, 25(), 2011, 113- 1223. Results and DiscussionAIll the gas hydrate-bearing specimens subjected to triaxial compression test showed plasticdeformation (the medium of the cylinder sample protruded in lateral and its profile looked like adrum). In the paper, the influencing factors on mechanical properties of gas hydrate-bearingsediments were investigated.3.1 Effect of Temperature on Mechanical Properties of Gas Hydrate-Bearing SedimentsTemperature plays an important role in gas hydrat-bearing sediments. It not only decides theformation or dissociation of gas hydrate but also its mechanical properties. When the confiningpressure is 5 MPa, a series of triaxial compression test were undertaken under various temperaturesby using dfferent kaolin volume ratis of 20%, 40%, and 60% as shown in Fig. 3. It shows that thedeviator stress increased gradually with increasing axial strain and finally reached a constant valuewith a reasonably asymptotic behavior. A significantly continuous hardening tendency up to the endof testing is shown when the axial strain is more than 15%. Moreover, the failure strength of gashydrate-bearing sediments strongly depends on the magnitude of temperature. The temperature dropleads to the increase of the mechanical strength. However, the sifness (the slope of the initialdeviator stress strain relationship for the sediments) is a constant irrespective of temperature.6虿8I101205Axial strain (%)(a) Kaolin volume ratio ψ= 20%(b) Kaolin volume ratio ψ= 40%0r9上Fig. 3. Relationship between deviator stress and axial strainunder various temperatures at constant strain rate1.0 %/min, confining pressure 5 MPa.15中国煤化工(c) Kaolin volume ratio中= 60%MYHCNMHGLI Yang-hui et al. / China Ocean Eng., 25(1), 2011,113- 122117Fig. 4 presents the maximum deviator stress and the dependency as a function of temperature atvarious kaolin volume ratios of 20%, 40%, and 60%. The maximum deviator stress is defined as thepeak value during the compression tests until the axial strain reaches 15%. The result indicates thatthe maximum deviator stress clearly increases with the decreasing of temperature, which is wellpresented in accord with the previous chemically based rescarches. The relationship between themaximum deviator stress and temperature is linear. The strength increment ratio increases with theincrease of kaolin volume ratio.Flg. 4. Relationship between maximum deviator stress andtemperature under various kaolin volumc ratios atconstant strain rate I .0%/min and confiningConfrung preure sMPpressure 5 MPa.Stusnrte 1 0%/mnK olun volume nabo:+-20 -25 .Temperature (C)Previous experimental results show that pore-ice pressure melting occurs locally at grain-to graincontacts under high stresses in the frozen soils (Chamberlain et al, 1972; Alkire and Andersland, 1973), andthis phenomenon also happened in gas hydrate-bearing sediments. The water and ice contents are affectedsignificantly by the temperature change. The water content decreases with the decline of temperature. The icecontent increase leads to the strengthening of bonding between ice and sediment particles, and then furtherenhances its capability of resistance to deformation (Wang et al, 2004).3.2 Effect of Confining Pressure on Mechanical Properties of Gas Hydrate-Bearing SedimentsFig.5 shows the axial strain-dependence curves of deviator stress under different confiningpressures at a temperature of -10C and various kaolin volume ratios of 20 %, 40 %, and 60 %. Themaximum deviator stress increases with the increase of confining pressure when 03≤5 MPa. Thestiffness keeps basically unchanged under such condition. However, the maximum deviator stressinitially drops and then increases when 03 > 5 MPa. There is no signifcant regulation at a highpressure stage.Fig. 6 shows the maximum deviator stress plotted against the confining pressure. The maximumdeviator stress increases linearly with the confining pressure at a low-pressure stage, fluctuates at ahigh-pressure stage. The critical point between the low and high pressure stages is not determined atdifferent kaolin volume ratios in the present test, for which further studies are recommended.The increase of deviator stress was caused by the in中国煤化工at a low pressurestage according to the previous studies (Hyodo et_dies were foundTH。 CNM HGIra hg-ressureexplaining the relationship between the deviator stress anu ..... prv. .... a118LI Yang hui er al. / China Ocean Eng. 25(), 2011, 113- 122stage. A high pressure condition can lead to the pore ice melting at grain-to-grain in gas hydrate-bearingsediments, which may decrease the strength of sediments. Furthermore, the particle breakage may alsodecrease the strength of the sediments, which is more likely to happen at a high-pressure stage. Particlesare aranged closely in dense sediments and therefore a part of the granular particles have to roll acrossother parts during high-pressure compression tests. The process of this roll-over must overcome a strongbite force, thus the sediments show high shear strength. The structure becomes loose once particles passthe others, and causes a strength declination.650.28 °01020Axial strain (%)15(@) Kaolin volume ratio甲= 20%(b) Kaolin volume ratio ψ= 40%岂3Flg. 5. Relationship between deviator stress and axial strainunder various confining pressures at constant strainrale 1.0%/min and temperature -10 c.(c) Ksolin volume ratio ψ= 60%AoF! stFlg. 6. Relationship between maximum deviator stress andconfining pressure under various kaolin volumeratios at constant strain rate 1.0%/min andtemperature-I0 C.中国煤化工02678101213MHCNMHGConfining prcssure (MPs)LI Yang-hui el al. / China Ocean Eng, 25(), 2011, 113-1221193.3 Effect of Strain Rate on Mechanical Properties of Gas Hydrate-Bearing SedimentsThe mechanical properties of ice and the gas hydrate-bearing sediments strongly depend on strainrate (Hawkes and Mellor, 1972) as shown in Fig. 7. The peak deviator stress increases with theincreasing strain rate, whereas there is no tremendous change until the axial strain reachingapproximately 0.5%. It means that the stiffness is a constant irrespective of strain rate. In addition, thecurves show that the deviator stress has a significant hardening tendency at the strain rate of 1.0%/min,and softening tendency at the strain rate of 0.1 %/min. The results agree with the extrapolation of thosetests on ice and frozen soil as reported by Baker (1979).4+3-/152Axial strain (%)(C) Kaolin volume ratio甲= 40%(b) Kaoin volume ratio ψ= 60%Fig. 7. Relationship between deviator stress and axial strain under diferent strain rate (0.19%/min, 1.0%/min) at constanttemperature -10 c, confining pessre 5 MPa and kaolin volume ratio 40% and 60%, rspectively.3.4 Effect of Kaolin Volume Ratio on Mechanical Properties of Gas Hydrate-Bearing SedimentsFig. 8 shows the stress-strain curves of triaxial compression tests at kaolin volume ratio of 20%,40% and 60%. As the kaolin volume ratio increases, the maximum deviator stress increases, whereas thestiffiness is unchanged.Fig. 9 shows Mohr-circle in case of kaolin volume ratios of 40% and 60%. It can be observed thatthe intemal friction angles of gas hydrate-bearing sediments are not sensitive to the kaolin volume ratio.The high kaolin volume ratio dependency of cohesion is unambiguous. The cohesion of sediments withkaolin volume ratio 60% is approximately 65% larger than that of the kaolin volume ratio of 40%. Thereis a litle change on the intemal friction angle.Fig. 8. Relationship between deviator stress and axialstrain under various kaolin volume ratios atconstant temperature -10C and confiningpressure 5 MPa.中国煤化工管20FYHCNMHG120u Yang-hui et al. / China Ocean Eng, 25(), 2011, 113- 1224l0-11MPLpr.西205十234567891011120123456789101112σ (MPa)σ(MPa)(a) Kaolin volume ratio ψ = 40%(b) Kaolin volume ratio ψ = 60%Fig. 9. Mohr-cireles of gas hydrate -bearing sediments with dfferent kaolin volume ratis.The intemal friction angles of gas hydrat-bearing sediments are decided by tangential frictionresistance. Since the interlocking force of the sediments is huge, the existence of gas hydrate has smalleffect on the increase of interlocking force. And the surface roughness of particles keeps constant withkaolin volume ratio changing. Thus internal friction angles change a ltte with different kaolin volumeratios. The contents of kaolin clay, hydrate and water ice changing may alter the porosity, cementationstate and initial density of sediments (Lu et al., 2008). The porosity decreases with the increase of kaolinvolume ratio, whereas initial density increases with it. The decline of porosity may increase the effectivesection area for absorbing load and resistance of deformation, which could enhance the stength ofsediments. When the initial density increases, the interparticle pore spacing deceases and the hydratedfilm thickness becomes thin which may enhance the electrochemical force among the particles. Theresult indicates that the kaolin volume ratio is an important factor to influence the strength of sediments.4. ConclusionsTo acquire more knowledge about the mechanical properties of gas hydrate-bearing sediments, apurposely designed low-temperature and high-pressure triaxial testing system was set up for themechanical property measurements. A series of tests were carried out on synthetic gas hydrate-bearingsediments instead of actual marine sediments. The following conclusions are drawn based on theexperimental results.(1) The failure strength of gas hydrate -bearing sediment strongly depends on the magnitude oftemperature. The lower the termperature drops, the larger the strength is.(2) The maximum deviator stress increases linearly with confining pressure at a low-pressure stage, andfluctuates at a high-pressure stage. The critical point of the low and high pressure stages is notdetermined.(3) The maximum deviator stress increases with the increasing strain rate, whereas there is notremendous change until the axial strain reaches 0.5%.中国煤化工(4) The intermal friction angles of gas hydrate-bearing :MYHC N M H Go kaolin volumeLI Yang-hui el al. / China Ocean Eng., 25(1), 2011, 113- 122ratio. However, the cohesion shows a high kaolin volume ratio dependency.The experimental data and results can be used to establish the constitutive model of gashydrate-bearing sediments, and to develop a numerical model for gas hydrate safe extraction. Furtherinvestigations on the effect of high-pressure (25 MPa) and relationships of stress-strain arerecommended for future studies.ReferencesAlkire, D. B. and Andersland, B. 0., 1973. The effect of confining pressure on the mechanical properties ofsand-ice materials, J. Glaciol, 12(66): 469~481.Baker, T. H. W., 1979. Strain rate efeet on the compressive strength of frozen sand, Eng. Geol, 13(1-4);223~231.Boswell, R., 2009. Is Gas Hydrate Energy Within Reach?, Science, 325(5943): 957- 958.Brown, H. E, Holbrook, W. S., Hormbach, M. J. and Nealon, J, 2006. Slide structure and role of gas hydrate atthe northem boundary of the Storegga Slide, offshore Norway, Mar. Geol, 229(3-4): 179~186.Chamberlain, E.. Groves, C. and Perham, R., 1972. The mechanical behaviour of frozen carth materials underhigh-pressure triaxial test conditions, Geotechnique, 22(3): 469~483.Dawe, R. A. and Thomas, S., 2007. 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