MECHANICAL PROPERTY MEASUREMENTS OF CLAYEY SEDIMENTS CONTAINING GAS HYDRATE BY TRIAXIAL COMPRESSION TESTS

Yanghui Li1, Yongchen Song1 , Weiguo Liu1, Yu Liu1, Masahiro Nishio2, Feng Yu1,Rui Wang1 and Xiongfei Nie1

1Key Lab. of Ocean Energy Utilization and Energy Conservation of Ministry of EducationDalian University of Technology

Dalian LiaoningP.R. CHINA

2National Institute of Advanced Industrial Science and Technology (AIST)Tsukuba, Ibaraki 305-8564

JAPAN

ABSTRACTIt is essential to study the mechanical properties of marine hydrate-bearing sediments in order to have sustainable production of natural gas or oil from the reservoir under the seafloor. To acquire

more knowledge about the mechanical properties of marine hydrate-bearing sediments and assess the long-term stability of the strata, samples of clayey sediments containing gas hydrate were prepared, and tested for their mechanical properties under various conditions by triaxial compression tests. The effects of temperature and confining pressure on mechanical properties of gas hydrate-bearing sediments were analyzed. And a piecewise linear shear failure criteriaconsidering the influence of temperature was developed for gas hydrate-bearing sediments. It matches well with the experimental data, and can be taken to predict the shear failure strength of

gas hydrate-bearing sediments under subzero conditions.

Keywords: gas hydrate, mechanical properties, triaxial test, failure criterion

Corresponding author: Phone: +86-13889431602 Fax: +86-0411-84708015 E-mail: songyc@dlut.edu.cn

NOMENCLATURET Kelvin temperature [K]

τ Shear stress [MPa]σ Normal stress [MPa]σcr Critical normal stress [MPa]

INTRODUCTIONWith declining production of oil and gas from

near-shore, shallow waters, oil companies have focused their attentions to deepwater regions. So uth China Sea and Gulf of Mexico are becoming the new oil and gas-producing area [1]. The regions where oil and gas reservoirs exit are very likely to have gas hydrate resource. Gas

hydrate always acts as a seal for trapping the oil

and gas. It is stable under conditions of low temperature and high pressure. While drilling for

oil or gas hydrate, it may cause the deformation of hydrate-bearing stratum which will induce marine landslide, subsidence, and well blowout problems etc [2-5]. It is essential to consider the mechanical properties of gas hydrate-bearing sediments.Up to now, the mechanical properties of gas

hydrate-bearing sediments are not fully understood. Lee et al. [6,7] and Yun et al. [8] used tetrahydrofuran (THF) to simulate gas hydrate, and investigated the impacts of THF on the mechanical strength of sediments. Although THF hydrate has many physical properties similar to those of gas

hydrates, it is still a “proxy” for gas hydrate. Song

Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011),Edinburgh, Scotland, United Kingdom, July 17-21, 2011.

et al. [9] studied the mechanical properties of artificial methane hydrate and ice mixture by triaxial compression tests, preliminary obtained the effect of temperature, confining pressure and strain rate on the mechanical properties of specimen. Li et al. [10] used clayey sediment

containing gas hydrate as a specimen, discovered the similar phenomena as Song’s [9]. Additionally, Li studied the influence of Kaolin volume ratio on the mechanical properties. Some researchers carried out triaxial compression tests to study the strain-stress curves, the modulus, Poisson’s ratio

and the effects of saturation on these parameters. They used both synthetic sandy cores containing methane hydrates and actual marine hydrate-bearing sediments [11]. The experimental data of gas hydrate-bearing sediments is still not enough. Further exploration of mechanical properties is

required in order to assess the stability of gas hydrate-bearing sediments.In this paper, based on previous studies [10], we supplemented more triaxial compression tests under various conditions and obtained more experimental data. The effects of temperature and

confining pressure were more clarified here, a piecewise linear shear failure criteria of gas hydrate-bearing sediments was established.

EXPERIMENTAL APPARATUS AND TES T CONDITIONSThe triaxial testing system, sample preparation device, parameters of kaolin clay and the procedure of experiment were introduced in the previous work [9,10]. Figure 1 is the schematic diagram of triaxial testing system.

Figure 1 The schematic diagram of triaxial testing system

1.Stepping motor 2.Pump 3.Hydraulic oil tank 4.Pressure gauge 5.Heat exchanger 6.Specimen 7.Thermocouple 8.Load cell 9.Air pressure line 10.Thermostatic bath 11.Computer

Tests were carried out under the various conditions

with temperature of T = -1, -3, -5, -10, -15 and -20

℃, confining pressures of σ3 = 1, 2.5, 3.75, 5, 6, 7,

8, 9, 10, 12.5 and 15MPa, kaolin volume ratios 40%, and strain rates of 1.0 %/min, as shown in Table 1.

Temperature

(℃)

Confining pressure(MPa)

Strain rate(/min)

-1, -3 5 1.0%

-51, 2.5, 3.75, 5, 7.5,

101.0%

-101, 2.5, 3.75, 5, 6, 7, 8, 9, 10, 12.5, 15

1.0%

-151, 2.5, 3.75, 5, 7.5,

101.0%

-20 5 1.0%

Table 1 Experimental condition of triaxial compression tests on gas hydrate-bearing sediments

RESULT AND DISCUSSIONGas hydrate is one kind of metastable material in nature, and it is stable under the conditions of low temperature and high pressure. The changes of temperature or pressure not only decide the formation or dissociation of gas hydrate but also

its cementing status, moisture content, and pore structure etc, which may alter the mechanical properties of hydrate-bearing sedimentssignificantly [12-14]. In this study, it is considered that the pore space of samples is full filled with gas hydrate and ice powder.

Stress-strain behavior of gas hydrate-bearing sedimentsThe deviator stress versus axial strain curves are shown in Figure 2. It can be seen that the curves are all presented as a shape of hyperbola. The

deviator stress increased gradually with increasing axial strain and finally reached a constant value without significant peak value. A significant hardening tendency with increasing strain is revealed up to the end of compression. Figure 3shows the relationship between deviator stress and

1

2

3

4

5 6 7

8

9

11

10

axial strain under various confining pressures. The curves also show a hardening tendency with increasing strain.

Figure 2 Relationship between deviator stress and axial strain under various temperatures

Figure 3 Relationship between deviator stress and axial strain under various confining pressures

Effect on strength and secant modulusThe failure strength is defined as the peak value for the deviator stress during the compression until reaching 15% of axial strain. Figure 4 shows the maximum deviator stress and the dependency on temperature. The result indicates that the

maximum deviator stress clearly increases with decreasing of temperature, and it appears to be linear with temperature when the temperature is not low enough. With further reduce of temperature, the increase of maximum deviator

stress has slowed down tendency, and finally reaches a constant value. Li et al. [10] said the relationship between failure strength and temperature is linear, which is not exact when the temperature is quite low.

Figure 4 Relationship between maximum deviator stress and temperature

Figure 5 shows the maximum deviator stress

plotted against the confining pressure. Maximum deviator stress increases linearly with the confining pressure at first, and presents a slow descending trend with the further increase of confining pressure. It means that confining pressure may not only inhibit but also induce the

strain softening effect. This phenomenon also exits in cyopedology [15].

Figure 5 Relationship between maximum deviator stress and confining pressure

The stiffness of material is its resistance to deformation. The modulus of elasticity E0 is always used to describe the stiffness. While E0 is difficult to measure and impacted greatly by experimental errors, the secant modulus E50 is used to express the mean stiffness of material. Figure 6

and 7 shows the E50 of the gas hydrate-bearing sediments and the dependency on temperature and confining pressure respectively. They indicate that E50 increases linearly with the drop of temperature. The secant modulus and confining pressure presents a parabolic relationship.

Figure 6 Secant modulus E50 as a function of temperature

Figure 7 Secant modulus E50 as a function of confining pressure

Piecewise linear shear failure criteria for gas

hydrate-bearing sedimentsShear failure criteria are important to tell when the material yield and shear failure happens. The mechanical properties of gas hydrate-bearing

sediments are similar with that of frozen soil. Maet al. [15] proposed a polynomial strength yield criteria for the frozen soil based on the mean effective stress, the cohesion and friction angle in the octahedral plane. For soils, there are many strength criteria such as Drucker criteria [16] and

Von Mises-Botkin criteria. But they are not very applicable for gas hydrate-bearing sediments. In this paper, a piecewise linear shear failure criteriawas proposed with the reference to Mohr-Coulomb criteria based on the experimental data.Define the top of the Mohr circle is the failure

point of the specimen, then adopt piecewise linear fitting by least-square algorithm method. The results are plotted in Figure 8, and the fitting coefficients are summarized in Table 2.

τ＝a1＋b1σ σ ≤ σcr (1)

τ＝a2＋b2σ σ ≥ σcr (2)

τ is the shear stress of gas hydrate-bearing

sediments.σ is the normal stress of gas hydrate-bearing sediments.σcr is the critical normal stress when the τ reaches the peak value.

Figure 8 Mohr-circles and fitting strength lines of gas hydrate-bearing sediments

T (K) a1

b1

a2

b2

σcr

268.15 0.807 0.103 1.696 -0.036 6.463

263.15 1.147 0.151 2.732 -0.086 7.229

258.15 1.607 0.139 3.627 -0.122 7.756

Table 2 Fitting coefficients of fitting strength line

Based on the fitting coefficients in Table 2, the relationship between a1, a2, b1, b2, σcr and temperature can be obtained, and replotted in Figure 9 and 10, respectively. The obtained empirical relationship between a1, a2, b1, b2, σcr and

temperature (T) is:

a1＝22.24－0.08T (3)

b1＝1.09－0.004T (4)

a2＝53.5－0.19T (5)

b2＝－2.35＋0.009T (6)

σcr＝41.17－0.13T (7)

T is in the unit of Kelvin.

Figure 9 Relationships between coefficients and temperature

Figure 10 Relationship between critical normal stress and temperature

Substitute a1, a2, b1, b2, and σcr into equation (1) and (2), the obtained piecewise linear shear failure criteria becomes to this:

τ＝(22.24－0.08T)＋(1.09－0.0036T)σ

σ ≤ σcr (8)

τ＝(53.5－0.019T)＋(－2.35+0.0086T)σ

σ ≥ σcr (9)

σcr＝41.17－0.13T (10)

The unit of τ, σ and σcr is MPa, the unit of T is Kelvin.Based on equation (8), (9) and (10), a comparison is made between the actual values and predicted values. The comparison results are showed in

Figure 11. The proposed shear failure criteria can match very well with the experimental data, and can be taken to predict the failure strength of gas hydrate-bearing sediments in certain conditions. The shear failure criteria is still not clear under above zero and higher confining pressure

conditions, which further studies are demanded.

Figure 11 The piecewise linear shear failure criteria of gas hydrate-bearing sediments

CONCLUSIONTo acquire more knowledge about the mechanical properties of gas hydrate-bearing sediments,

samples of clayey sediments containing gas hydrate were formed synthetically in laboratory, and tested for their mechanical properties under various conditions using triaxial compression apparatus. The results can be summarized as following conclusions: As the temperature

decreases, the failure strength of gas hydrate-

bearing sediments increases linearly. However, with further reducing of temperature, the upward trend in failure strength decreases, and finally approaches to a definite value; The failure strength of gas hydrate-bearing sediments increases linearly with the confining pressure at first, and presents a

slow descending trend with the further increase of confining pressure; The secant modulus E50 presents a great dependency on the temperature and confining pressure; A piecewise linear shear failure criteria is obtained, and it can be taken to predict the failure strength of gas hydrate-bearing

sediments under sub-zero conditions.

ACKNOWLEDGEMENTThis work was supported by a grant from the National High Technology Research and

Development Program of China (863 Program) (No.2006AA09209), the Major National S&T Program (No.2008ZX05026-004), the Major State Basic Research Development Program of China (973 Program) (No.2009CB219507).

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