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Procedia Engineering
ProcediaProcedia Engineering 00 (2011) Engineering 26000–000 (2011) 1872 – 1877 www.elsevier.com/locate/procedia
First International Symposium on Mine Safety Science and Engineering
Study on mechanical properties of gas hydratebearing porous media Baoyong ZHANG a,b,c,*, chengyuanpinga, Qiang WUb a
National Engineering Research Center for Coal Gas Control, China University of Mining & Technology, Xuzhou, 221008, China. b Department of Safety Engineering,Heilongjiang Institute of Science & Technology,Harbin 150027,China. c Guangzhou Center for Gas Hydrate Research，Chinese Academy of Sciences, Guangzhou 510640, China.
Abstract Gas hydrate-bearing porous media are natural or artificial porous deposits (soil sediments or coal) that contain gas hydrate in their pores. The mechanical properties of gas hydrate-bearing porous media are very necessary for researches on gas production and hydrate-based technology. This paper summarized the advances of the research on the mechanical properties of the gas hydrate deposit, from the experimental and numerical aspects. Also, this paper points out that research on the mechanical properties of synthesized gas hydrate porous media (coalbed seam) is necessary for coal and gas outburst prevention and mitigation. First, the theories about strength and deformation are introduced; second, a simple constitutive model for gas hydrate-bearing sediments is referred; third, the experimental and numerical researches are discussed; finally, this paper provides some suggestions for future study on mechanical properties of gas hydrate – bearing porous media.
© 2010 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and/or peer-review under responsibility of [China Academy of Safety Science and Technology, China University of Mining and Technology(Beijing), McGill University and Wollongong University]
Keywords: Gas hydrate-bearing porous media; mechanical properties; gas hydrate production; coal and gas outburst.
1 Introduction Gas hydrates are crystalline substances composed of water and gas, in which a solid water-lattice accommodates gas molecules in a cage-like structure, or clathrate. Gas hydrates are widespread in permafrost regions and beneath the sea in sediment of outer continental margins. These ice-like structures possess strongly contrasting physical properties compared with the hosting sediments [1]. Over the past
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1877-7058 © 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. doi:10.1016/j.proeng.2011.11.2379
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decades, there have been considerable researches on the physical and chemical properties of methane hydrate as well as the methane exploitation, however the mechanical properties of methane hydrate have not been fully investigated yet [2], which is necessary to understand the stability around the porous media. The gas hydrates bearing sediments (GHBS) are the focus of a special attention as a potential energy resource in the next decades. Recently, the methane hydrate bearing sediment attracts interest from various fields of scientist and engineers. The research on the mechanical properties of GHBS is very necessary for geo-hazard prevention (deep water drilling, slope instability, landslide), and global climate change, especially for gas production from GHBS. Besides the gas-hydrate bearing sediments, the mechanical properties of gas-hydrate containing coal seam is also very important. Wu[3] proposed a new method to prevent coal and gas outburst, using hydrate-based technology. The application of this technology also requires the analysis of the mechanical properties of the coal containing gas hydrates. For all of the viewpoints, the author would like to point out the importance of the mechanical behavior of the material and call attention to this matter. Especially, the gas production from methane hydrate requires knowledge about the mechanical properties in many scenes. 2 Theory about strength and deformation of of gas hydrate-bearing porous media 2.1 Definition of strength and deformation parameters Waite et al. [4] presents the detailed information of strength and deformation of GHBS. Sediment strength and the extent to which sediment deforms under a load are critical inputs for the analysis of potential failures around wells [5-6] and for evaluating seafloor stability over larger length scales [7-8]. Sediment strength is a combination of the cohesive resistance, c, and effective stress–dependent frictional resistance described by the friction angle, Φ, which includes resistance to sliding between particles, particle re-arrangement, and particle crushing. The two contributions to shear strength are captured in the Coulomb failure criterion which relates the shear stress at failure, τ f , to the normal effective stress, σ n′ , acting on the failure plane, (1) τ f = c +σ n′ tan Φ
This failure criterion plots as a straight line in τ − σ n′ space. The state of stress at a point in equilibrium within the test sample plots as a circle in τ − σ n′ space called the Mohr circle of stress. The sediment reaches failure when the Mohr circle becomes tangent to the Coulomb failure envelope, shown in Fig.1.
Fig. 1 Mohr-Coulomb failure diagram[4]. The strength parameters c and Φ can be measured in the laboratory using triaxial compression [10,11] tests in which a cylindrical specimen is subjected to and effective confining stress σ 3′ = σ 3 − Pp and then brought to failure by increasing the axial effective stress σ 1′ = σ 1 − Pp (Figure 1, inset). Triaxial test data obtained at different effective confining stress levels are combined to define the linear Coulomb failure envelope from which c and Φ are derived.
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Fig.2 illustrates the shear resistance and dilation mechanisms occurring at different levels of hydrate saturation in the pore space. Effective stress measurements are preferred when interpreting shear behavior, but at high hydrate saturations for which pressure in the occluded pores cannot be measured, interpretations must be based on total stress measurements[4].
Fig. 2 Mechanisms controlling the shear strength of hydrate-bearing sediments. Sediment grains are white circles, hydrate is black, and water is blue (Waite et al. [4] modified from Yun et al. [9]) 2.2 A simple constitutive model for natural gas hydrate bearing sediments The stress-strain relationship under a constant strain rate 0.1% per minute can be represented by variable compliance-type constitutive model [12].
{
}
m3 dλ∗ = a1 (λ∗ − 1) − m1 + a3λ* ⋅ σ ∗n , dt λ ε σ 1 λ∗ = , λ = , λ0 = ,σ ∗ = Ei λ0 σ σ c0
(2)
Where Ei is the initial elastic modulus and σ c 0 means peak strength. The parameter n represents the degree of time-dependency or visco-elasticity of material. In this model, compliancy λ , i.e. inverse of elastic modulus into a simulation code for elastic analysis, the model is easy to introduce into a numerical simulation of mechanical behaviour. The parameter Ei depends on methane hydrate saturation. These parameters were determined based on previous triaxial compression tests with constant strain rate. Since the time-dependency or viscoelasticity of sand is considered to be stronger in the presence of methane hydrate, the parameter n would depend on methane hydrate saturation. According to laboratory triaxial compression tests with alternative strain rate, the value of n would be estimated by the following equation,
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1 n= −1 log 2 (0.0019 * S h + 1.0065)
(3)
Where S h shows methane hydrate saturation. The other parameters in (2), such as a1 , m1 , a3 and m3 , were referred to the previously proposed procedure. Determined all of the parameters in (2), the
stress-strain relation under constant strain rate 0.1% per minute can be calculated by (2). Variable-compliance-type constitutive model is simple and can be applicable to various timedependent behaviors. Hence, the validity of the model can be appreciated by creep tests or other laboratory experiments. 3 Experiments on mechanical properties of gas hydrate-bearing sediments Over the past decades, gas hydrate researches mainly focus on their physical and chemical properties, as well as the methane exploitation, however, few experiments have been conducted on the mechanical properties of gas hydrate-bearing media (GHBM). In recent years, triaxial test apparatus was widely used to determine mechanical characteristics of natural and artificial gas hydrate-bearing[2]. Yamamoto [13] pointed out the importance of the mechanical behavior of the methane hydrate bearing sediments and summarized the experimental and numerical works on this subject. Winters et al. [14] found that the presence of gas hydrate and other solid pore-filling material, such as ice, increased the sediment shear strength. The magnitude of that increase is related to the amount of hydrate in the pore space and cementation characteristics between the hydrate and sediment grains. For consolidation stresses associated with the upper several hundred meters of sub-bottom depth, pore pressures decreased during shear in coarse-grained sediment containing gas hydrate, whereas pore pressure in fine-grained sediment typically increased during shear. The presence of free gas in pore spaces damped pore pressure response during shear and reduced the strengthening effect of gas hydrate in sands. Hyodo et al. [15] performed a series of tests on artificial methane hydrate produced in sand, using low temperature and a developed high confining pressure triaxial compression technique. The specimens used were prepared by producing an artificial methane hydrate in the pore of sand sediments. On the basis of the experiments results, they obtained the influences of the temperature, back pressure, effective confining pressure and methane hydrate saturation on the mechanical properties of the methane hydratebearing sands. Additionally, the variations of the volumetric strain of the sands are examined with the effective confining pressure, shear stress, and critical void ratio in the methane dissociation process. Masui et al. [16] carried out tri-axial compression test on the hydrate bearing sediment and Toyoura sand containing synthetic gas hydrate. They found that the tri-axial compressive strength of the hydrate bearing sediment increases with increasing the pore saturation of gas hydrate. The mechanical strength of synthetic hydrate bearing sediment is similar to that of natural hydrate bearing sediments, with notable differences in the stress-strain and volumetric strain-strain relationships. The different initial void ratios and particle size distributions of these sediments lead to different deformation characteristics. Yun [9] showed the effect of the host sediment type (sand, clay, silt) and hydrate saturation using THF hydrate. Clayton et al. [17] investigated the effects of depositing varying quantities of methane hydrates on their shear and bulk modulus, and damping, over a range of isotropic effective stress. Results are compared with those obtained on the same sand without hydrate bonding and after dissociation. Song [18] carried out a series of triaxial shear tests on artificial methane hydrate specimen and found that when the confining pressure was less than 10MPa, the increase of confining pressure leaded to the enhancement of shear strength. Zhang et al. [19] carried out shear tests on the synthetic ice-bearing fine silty sand, tetrahydrofuran (THF) hydrate-, CO2 hydrate-, and methane hydrate-bearing fine silty sands. The experimental results show that these four deposits behave as plastic failure. The larger the confined
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pressure, the higher the strength. The deposits with the same saturation of gas hydrate have different strengths when the kinds of hydrates are different. Wang and Lu [20] summarized the advances of the research on the mechanical properties of the gas hydrate deposit. 4 Numerical simulations on mechanical properties of gas hydrate-bearing sediments Production of gas from low permeability hydrate bearing zone requires some sort of well stimulation techniques such as the hydraulic fracturing. To design such measures, for examples, fracture propagation simulations, the failure mechanism of the hydrate-bearing sediment is essential [21]. Coupled model of the deformation of gas hydrate bearing sediments and gas hydrate dissociation with fluid and heat flow have been studied by Klar and Soga [22](2005) and Rutqvist and Morridis [6]. In the models, displacement of solid, pore pressure, and hydrate saturation are major variables to solve with temperature, gas saturation etc. The two phase flow (water, gas) and three-phase (water-gas-solid) involved porous-elastic formulation are essential part and heat flow is necessary to consider. Failure criteria and constitutive law that are influenced by variable hydrate saturation in the model. Brugada et al. [23] performed .a series of Discrete Element Method (DEM) simulations of triaxial compression tests to study the influence of methane hydrate saturation on the stress-strain relationship, the volumetric response and on the macroscopic geomechanical properties such as friction and dilation angle. The simulations showed that for the pore-filling case, the hydrate contribution to the strength of the sediment is of a frictional nature, rather than of a cohesive nature. Until now, only few numerical simulations are carried out on the mechanical properties of GHBS, due to the complexity of the problem, and the late start. The numerical research on GHBS can be the references for the future investigations on the mechanical properties of coalbed seam containing gas hydrate. Therefore, coal and gas outburst can be effectively prevented. 5 Conclusions The study on methane hydrate bearing sediments is early stage, and we just start understanding the nature of the material qualitatively. Knowledge of the mechanical properties of gas hydrate-bearing porous media is necessary for gas production from GHBS and for hydrate-based technology. In-situ and laboratory experiments should be performed to be the basis for the development of the numerical study. Also, numerical simulation technology should be developed to quantitatively evaluate the mechanical behavior of the gas hydrate bearing porous media. Additionally, it is very necessary to establish the constitutive model for it. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (50874040, 50904026), Guangzhou Center for Gas Hydrate Research-Chinese Academy of Sciences (CASHYD0907s1), Scientific Research Fund of Heilongjiang Provincial Education Department(11551420). References [1] G. Guerin, and D. Goldberg: Journal of Geophysical Research, 107, (2002), B5, EPM 1-1-EPM 1-12. [2] Y.C. Song, F. Yu, Y.H. Li, et al: Journal of Natural Gas Chemistry, 19, (2010), 246-250. [3] Q.Wu, X.Q. He: Journal of China University of Mining and Technology, 13(1), (2003), 7-10.
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[4] W.F. Waite, J.C. Santamarina, D.D.Cortes, et al: Reviews of Geophysics, 47, (2009), 1-38. [5] A. Masui, K. Miyazaki, H. Haneda, et al: 6th International Conference on Gas Hydrates, Chevron, (2008). [6] J. Rutqvist, G.J. Moridis: Offshore Technology Conference, Am. Assco. of Pet. Geol., Houston, Tex, (2007). [7] M.F. Nixon, and J.L.H. Grozic: Can. Geotech., J., 44, (2007), 314-325. [8] N. Sultan, P. Cochonat, J.P. Foucher, et al: Mar. Geol., 213, (2004a), 379-401. [9] T.S. Yun, J.C. Santamarina, and C. Ruppel: J. Geophys. Res., (2007), 112, B04106. [10] ASTM Standard, D 4767-04: ASTM Int., West Conshohocken, Pa, (2004). [11] ASTM Standard, WK3821: ASTM Int., West Conshohocken, Pa, (2006). [12] A. Masui, K. Miyazaki, H. Haneda, et al: Offshore Technology Conference, Am. Assco. of Pet. Geol., Houston, Tex, (2007). [13] K. Yamamoto, B. Maney, L. Kiewiet: Proc. 5th Int. Conf. On Gas Hydrate, Vol.3, 922 (Paper ref.3031), (2005). [14] W.J. Winters, I.A. Pecher, W.F. Waite, et al: American Mineralogist, 89, (2004), 1221-1227. [15] M. Hyodo, Y. Nakata, N. Yoshimoto, et al: Proc. 17th Int. Offshore and Polar Engrg. Conf., Lisben, (2007), 1326-1333. [16] A. Masui, H. Haneda, Y. Ogata, et al: Proc. 15th Int. Offshore and Polar Engrg. Conf., Soeul, (2005)364-369. [17] C.R.I. Clayton, J.A. Priest, A.I. Best. Geotechnique, 55, (2005), 423-434. [18] Y.C. Song, F. Yu, Y.H. Li, et al.: Journal of Natural Gas Chemistry, 19, (2010), 246-250. [19] X.H. Zhang, S.Y. Wang, Q.P. Li,et al: Rock and Soil Mechanics, 31, (2010), 3069-3074. [20] S.Y. Wang, and X.B. Lu: Advances in Mechanics, 39, (2009), 176-188. [21] T. Ito, A. Igarashi, K. Suzuki, et al. Proc. Offshore Technology Conference, Texas, (2008). [22] A. Klar, K. Soga: Poromechanics III, Balkma, London, (2005), 653-660. [23] J. Brugada, Y.P. Cheng, K. Soga: Granular Matter, 12, (2010), 517-525.
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