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Effect of confining pressure on mechanical behavior of methane hydrate-bearing sediments


PETROLEUM EXPLORATION AND DEVELOPMENT Volume 38, Issue 5, October 2011 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2011, 38(5): 637–

640. RESEARCH PAPER

Effect of confining pressure on mechanical behavior of methane hydrate-bearing sediments
Li Yanghui*, Song Yongchen, Yu Feng, Liu Weiguo, Wang Rui
Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology, Dalian 116024, China

Abstract: Synthesized gas hydrate in lab was mixed with kaoline to form core samples, so as to simulate the hydrate-bearing sedimentary layers in the seabed. Under conditions of different confining pressure, triaxial compressive tests were conducted on hydrate-bearing sediments with different volume content of kaoline. The results show that: (1) The failure strength of gas hydrate-bearing sediments increases with the confining pressure at a low-pressure stage. The strength tends to decline gently with further increases of confining pressure. (2) The modulus of elasticity E0 keeps unchanged under various confining pressures. But the secant modulus E50 presents a great dependency on the confining pressure. Secant modulus increases to the peak and then decreases with the increase of confining pressure; (3) The internal friction angles of gas hydrate-bearing sediments are not sensitive to the volume ratio of kaoline, but its cohesion depends on the volume ratio of kaoline. The sediment strength increases with the increase of kaoline content. Key words: natural gas hydrate; mechanical property; seabed sediment; triaxial testing; safe extraction

Introduction
There are abundant marine oil and gas resources in the world. The South China Sea becomes the new deepwater oil and gas-producing area after the Gulf of Mexico and the Brazil sea area[1 2]. Large amounts of investigation data prove that, the regions where deepwater hydrocarbon exists are likely to have enriched gas hydrate resources[3 4]. Drilling operations will disturb the geologic structure of gas hydrate-bearing sediments and can lead to hydrate dissociation and decrease the geomechanical strength, resulting in the destruction of such infrastructures as drilling equipments and submarine pipelines and loss in lives and properties[5 7]. During drilling design and operations, it is essential to study the mechanical properties of gas hydrate-bearing sediments under various confining pressures to prevent the excessive deformation of gas hydrate-bearing sediments and the destruction of borehole wall. Till now, few researches are reported to investigate the effects of confining pressure on mechanical behavior of gas hydrate-bearing sediments. Hyodo et al.[8] and Vanoudheusden et al.[9] carried out a series of triaxial compressive tests on gas hydrate sand samples and unsaturated marine sedimentary soil samples in laboratory respectively. The studies indicated that the strength of gas hydrate-bearing sediments increases under confining pressure, but it is fluctuant when the temperature is

above zero. The properties of gas hydrate-bearing sediments are similar to those of frozen soils. It can be found in the literatures that the strength of frozen soil increases as confining pressure increases, until it reaches a peak value and then declines with further increase of confining pressure. The increase of confining pressure can suppress as well as induce the effect of dilatancy softening of frozen soil[10 11]. In order to verify whether the gas hydrate-bearing sediments have the same phenomena as frozen soil, and assess the stability of gas hydrate-bearing sedimentary layers, we carried out a series of triaxial compressive tests on gas hydrate-bearing sediments with different volume contents of kaolin under various confining pressures.

1

Experiment

Kaolin was used to simulate the seabed sediment. Its composition and physical properties are shown in Figure 1 and Table 1. The saturation of gas hydrate was around 30%, which was the average saturation of natural gas hydrate at seabed. The synthetic gas hydrate was mixed with kaolin in a certain volume proportion, and then cylinder samples (50 mm diameter 100 mm height) were formed in pressure molding equipment under a controlled temperature ( 10 C) and an axial pressure (10 MPa). The porosity of sample was controlled by the volume ratio of kaolin to natural gas hydrate.

Received date: 02 Dec. 2010; Revised date: 10 Jul. 2011. * Corresponding author. E-mail: li.yanghui@mail.dlut.edu.cn Foundation item: Supported by the National Major Science and Technology Project of China (2008ZX05026-004); the Major Program in the National “863” Planning of China (2006AA09209). Copyright ? 2011, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

Li Yanghui et al. / Petroleum Exploration and Development, 2011, 38(5): 637–640

Table 2

Conditions of triaxial compression tests on gas hydrate Confining pressure/MPa 1, 2.5, 3.75, 5, 6, 7, 8, 9, 10 2.5, 3.75, 5 Strain rate /(%·min 1) 1.0 1.0

Kaolin volume Temperature ratio/% /C 40 60 10 10

dium of the cylinder sample protruded in the middle and its profile looked like a drum, had no fracture surface). This paper investigated the effects of confining pressures and kaolin content on mechanical properties of gas hydrate-bearing sediment samples.
d10, d30, d60—Particle diameters with the cumulative weight percent of 10%, 30% and 60% respectively

2.1 Effect of confining pressures on mechanical properties of gas hydrate-bearing sediments Confining pressure is a nonnegligible factor for mechanical behavior of gas hydrate-bearing sediments due to its restriction on lateral displacement. It can be seen in Figure 3 that gas hydrate-bearing sediment samples are subjected to plastic failure under different confining pressures and the deformation enlarges with the increase of stress. The deviator stress increases gradually with increasing axial strain and finally reaches a constant value without significant peak value. Most of the samples show a strain hardening tendency with increasing strain up to the end of compression. The deviator stress corresponding to 15% strain is selected as the strength or failure stress of samples. Figure 4 shows the strength of gas hydrate-bearing sediment samples versus the confining pressure. When confining pressure 3 = 1 MPa, the samples present strain softening, and show a hardening tendency with the increase of confining pressure. The strength increases with the increase of confining pressure when 3 5 MPa. It indicates that, under low confining pressure, the increase of confining pressure can restrain the strain softening of hydrate sediment. When confining pressure 3 > 5 MPa, the sample strength decreases slowly with the increase of confining pressure and some samples

Fig. 1 Grain size distribution curve and basic parameters of kaolin Table 1 Composition SiO2 Al2O3 Fe2O3 CaO MgO The composition and content of kaolin Content /% 52.00 35.00 1.50 2.60 1.00 Composition Na2O MnO K 2O Other Content /% 0.10 0.13 1.30 6.37

All the tests were conducted by using TAW-60 low-temperature high-pressure triaxial testing device (shown in Fig. 2). The samples were consolidated for 2 hours under 10 MPa pressure (1 000 m water depth) before the constant strain rate test. The test parameters are shown in Table 2.

Stepping motor Heat exchanger

Plunger pump Sample

Hydraulic oil tank Computer

Pressure gauge Oil outlet

Thermocouple

Load cell

Thermostatic bath

Fig. 2 The schematic diagram of TAW-60 low-temperature high-pressure triaxial testing device

2

Results and discussion
Fig. 3 Strain curves of hydrate-bearing sediment samples under different confining pressures

All the gas hydrate-bearing sediment samples subjected to the triaxial compression test showed plastic deformation (the me-

Li Yanghui et al. / Petroleum Exploration and Development, 2011, 38(5): 637–640

Fig. 4 Confining pressure’s impact on the strength of hydrate-bearing sediment samples

present strain softening. It means that confining pressure can induce the strain softening of gas hydrate-bearing sediments under high confining pressure. The variation of strength with the increase of confining pressure can be divided into three regions in cyopedology: the increase region of strength, the slow decrease region of strength and sudden drop region of strength[12]. It can be found that the effects of confining pressure on gas hydrate-bearing sediments are similar to those of frozen soil. However, under higher confining pressure, the relationship shall be validated further. The stiffness of material is its resistance to deformation. The modulus of elasticity E0 is always used to describe the stiffness. From Figure 3, it can be observed that the curves are almost coincident at the period of small deformation when 3 5 MPa, which indicates that the modulus of elasticity E0 is influenced little when 3 5 MPa. The relationship between confining pressure and E0 is not verified when 3 > 5 MPa. E0 is difficult to measure; moreover, during deformation, the tangent modulus of samples is changing. Therefore, the secant modulus E50 is used to express the mean stiffness of material. Figure 5 shows the relationship between E50 of the gas hydrate-bearing sediments and confining pressure, namely a parabolic relationship. The strength of hydrate-bearing sediments is decided by the strength of hydrate and ice, interparticle friction and interaction force between particles. Confining pressure may restrict the growth of fracture, which leads to the increases in interparticle coordination and frictional resistance, and consequently increase the mechanical strength. For good gradation soils, the void between coarse grains can be filled with fine

grains during compression tests, and this chain effect of filling makes the sediments have good compaction. The gas hydrate-bearing sediments are in the phase of compaction under low confining pressure. With the increase of confining pressure, some particles will be crushed, and the structure becomes loose once one particle passes its adjacent particles, leading to a strength declination. Meanwhile, high pressure makes the ice crystals between grains melt in gas hydrate-bearing sediments[10, 13], which may improve the lubricating effect and reduce the cohesion of particles, finally decrease the friction between particles. The stiffness is affected by the interaction, contact area and friction resistance of particles, so the effects of confining pressure on stiffness are similar to those on failure strength. 2.2 Effect of kaolin content on mechanical properties of gas hydrate-bearing sediments Figure 6 shows Mohr-circles in case of kaolin volume ratios of 40% and 60%. It can be observed that internal friction angles of gas hydrate-bearing sediment samples change a little with the increase of kaolin volume ratio, but cohesion shows a high dependency on kaolin volume ratio. The failure strength increases with the increase of kaolin volume ratio. The space in sediments is occupied by ice, hydrate, unfrozen water and air. If there is any change in the content of soil, ice and hydrate in sediments, the porosity and initial density will change accordingly. The internal friction angles of gas hydrate-bearing sediments are decided by tangential friction resistance which is produced from the mosaic, occlusion and dislocation of clay particles during compaction. And the in-

Fig. 5 Confining pressure’s impact on the secant moduli of hydrate-bearing sediment samples

Fig. 6 Mohr-circles and the strength envelope of gas hydratebearing sediment samples

Li Yanghui et al. / Petroleum Exploration and Development, 2011, 38(5): 637–640

terlocking force in clayey particles, hydrate and water ice accounts for small proportion. The surface roughness of particles keeps constant when kaolin volume ratio changes. Thus the internal friction angles change a little with different kaolin volume ratios. However, the change in content of kaolin may alter the porosity, cementation state and initial density of sediments. When the content of kaolin decreases, the porosity may increase, and the effective section area for bearing load resisting deformation is decreased, so the strength of sediments decreases. When the initial density increases, the grain spacing deceases and hydrated film thickness becomes thin; the electrochemical force (or cohesion) between particles and the strength of sediments may increase.

[2]

Huang Changwu. Deep sea: a new area of international energy competition. Petroleum Exploration and Development, 2010, 37(1): 31.

[3]

Macdonald I R, Guinasso N L, Sassen R, et al. Gas hydrate that breaches the sea floor on the continental slope of the Gulf of Mexico. Geology, 1994, 22(8): 699–702.

[4]

Rutqvist J, Moridis G J, Grover T, et al. Geomechanical response of permafrost-associated hydrate deposits to depressurization-induced gas production. Journal of Petroleum Science and Engineering, 2009, 67(1/2): 1–12.

[5]

Cameron I, Handa Y P, Baker T H W. Compressive strength and creep behavior of hydrate-consolidated sand. Canadian Geotechnical Journal, 1990, 27(2): 255–258.

3

Conclusions

[6]

Brown H E, Holbrook W S, Hornbach M J, et al. Slide structure and role of gas hydrate at the northern boundary of the Storegga Slide, offshore Norway. Marine Geology, 2006, 299(3/4): 179–186.

The failure strength increases with the increase of confining pressure under low confining pressure. However, when the confining pressure further increases, the strength of samples decreases slowly. The strength variation under higher confining pressure conditions is to be studied further. The modulus of elasticity E0 keeps unchanged under low confining pressures. The secant modulus E50 and confining pressure present a parabolic relationship. The secant modulus increases to a peak and then decreases with the increase of confining pressure. The internal friction angle of gas hydrate-bearing sediments is not sensitive to the volume ratio of kaolin. However, its cementation and initial density depend on the volume ratio of kaolin greatly, which affects the cohesion of gas hydratebearing sediments.

[7]

Glasby G P. Potential impact on climate of the exploitation of methane hydrate deposits offshore. Marine and Petroleum Geology, 2003, 20(2): 163–175.

[8]

Hyodo M, Nakata Y, Yoshimoto N, et al. Basic research on the mechanical behavior of methane hydrate-sediments mixture. Soils and Foundations, 2005, 45(1): 75–85.

[9]

Vanoudheusden E, Sultan N, Cochonat P. Mechanical behaviour of unsaturated marine sediments: experimental and theoretical approaches. Marine Geology, 2004, 213(1/2/3/4): 323–342.

[10] Chamberlain E J, Groves C, Perham R. The mechanical behavior of frozen earth materials under high pressure triaxial test conditions. Geotechnique, 1972, 22(3): 469–483. [11] Parameswaran V R, Jones S J. Triaxial testing of frozen sand. Journal of Glaciology, 1981, 27(95): 147–156. [12] Ma Wei, Wu Ziwang, Sheng Yu. Effect of confining pressure on strength behaviour of frozen soil. Chinese Journal of Geotechnical Engineering, 1995, 17(5): 7–11. [13] Alkire D B, Andersland B O. The effect of confining pressure on the mechanical properties of sand-ice materials. Journal of Glaciology, 1973, 12(66): 469–481.

References
[1] Sun Qing, Lian Lian. Key technologies with development of exploration and exploitation of the deep-water oil and gas and gas hydrates in south China sea. Periodical of Ocean University of China, 2005, 35(6): 1049–1052.


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