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Harmonising Rock Engineering and the Environment–Qian&Zhou(eds)©2012Taylor&Francis Group,London,ISBN978-0-415-80444-8 The state-of-the-art and research advances on the underground natural gas storage inbedded salt rocks in ChinaC.H.Y angState Key Laboratory of Geomechanics and Geotechnical Engineering,Institute of Rock and Soil Mechanics,Chinese Academy of Sciences,Wuhan,ChinaKey Laboratory for the Exploitation of Southwestern Resources and the Environmental Disaster Control Engineering,Chongqing University,Chongqing,ChinaY.P.LiState Key Laboratory of Geomechanics and Geotechnical Engineering,Institute of Rock and Soil Mechanics,Chinese Academy of Sciences,Wuhan,ChinaD.A.QuWest-east Gas Pipeline Company Gas Storage Project Department,Petro China Company Limited,Beijing,ChinaKey Laboratory for the Exploitation of Southwestern Resources and the EnvironmentalDisaster Control Engineering,Chongqing University,Chongqing,ChinaABSTRACT:The current status and development trend of natural gas pipelines and storage in China is summarized briefly. With the development of economy,the natural gas demand in China increases rapidly;the construction of long distance natural gas pipeline enters a fast development period.To support long transportation gas pipelines,a group of natural gas storages have been built or building in China.The success of the Jintan salt cavern gas storage project in old caverns validates that bedded salt rocks as well as salt domes can be utilized as host rocks of underground gas storage.Then,the advances in researches of the mechanical behaviors of deep bedded salt rocks in China are reviewed comprehensively.Some researches on the mechanical characteristics of bedded salt rocks have been present recently for feasibility studies of gas storage in deep bedded salt rocks. Those studies validate strongly the feasibility of gas storage in bedded salt rocks,and also provide useful technical supports for the construction of gas storages in salt caverns.Subject:Underground storageKeywords:rock caverns;stability analysis;rock properties;mining;numerical modeling1INTRODUCTIONIn recent years,the natural gas demand in China increases rapidly with the economic development and for the demand of environmental protection(Gong&Dong2010).By the end of2009,the total length of gas pipelines in China has reached about36thousand kilometers and the total gas trans-portation capacity has achieved over100billion cubic meters per year.The construction of the natural gas pipeline in China will develop fast in near future.In the next ten years,the total length of natural gas pipelines will reach nearly350thousand kilometers.To support long transportation gas pipelines,a group of nat-ural gas storages have been built or building in China.Accord-ing to the requirement of the West-East natural gas pipeline project in China,six existing salt caverns(‘old’cavern)in Jintan Salt Mine,Jiangsu province,were selected to be con-verted to natural gas storages after appropriate reconstruction (Y ang et al.2007).The feasibility study on gas storage in those six old salt caverns will be introduced briefly in this paper. In many countries(USA,Canada,France,Germany,etc.), underground energy storage is usually formed in salt domes rather than bedded salt formations as host rocks,while in China,salt domes suitable for energy storage have been hardly found so far.Bedded salt rocks are usually composed of many thin salt layers and non-salt sedimentary interlayers(e.g. anhydrite,shale,dolomite,glauberite,etc.)(Li et al.2007a). Theories suitable for the analysis of the stability and safety of storage in such bedded salt rocks are lacking.Some recent advances of study on the mechanical characteristics of bedded salt rocks will be introduced.2THE CURRENT STATUS OF NATURAL GAS PIPELINES AND STORAGE IN CHINAAt present,a gas transportation network system is gradually formed(Gong&Dong2010),which is characterized by the main trunk lines as the skeleton,covering the nationwide con-sumption markets by connecting the trunk lines with branch lines.The commissioning of a group of natural gas pipelines, such as the West-East natural gas pipeline and so on,signi-fies that the development speed of the gas pipeline industry in China has stridden into the world advanced level.The long distance natural gas pipelines built in China recently include Se-Ning-Lan gas pipeline,West-East natu-ral gas pipeline I and II,Shanxi-Beijing natural gas pipeline I and II,Zhong-Wu natural gas pipeline,Sichuan-East natural gas,Central Asia gas pipeline,and so on.T wo of them will be introduced as follows.The Se-Ning-Lan gas pipeline,starts from Sebei No.1 gas field in Chaidamu basin,by way of Xining in Qing-hai Province,and ends at Lanzhou in Gansu Province.Its total length is about930kilometers.With design pressure of 6.4MPa and diameter of660mm,its design throughput capac-ity is around2billion cubic meters per year.This pipeline is the first one with long distance and large diameter in Tibetan plateau.The West-East natural gas pipeline I includes trunk lines, sub-trunk lines,branch lines,underground gas storages,tie lines and their branch lines.Tarim gas field in Xinjiang Province is the main resource of this project;it feeds mainly the gas markets of theY angtze River Delta.The total length of this pipeline is about3835kilometers.With design pressure of10MPa and diameter of1016mm,its annual transportation capacity can reach17billion cubic meters.To ensure safety of the long transportation gas pipelines, a group of natural gas storages have been built or building. Usually,natural gas is stored in a number of different ways: (1)depleted reservoirs in oil and/or gas fields,(2)aquifers, and(3)salt cavern formations.The Jintan salt cavern gas stor-age project,as a supporting project of the West-East natural gas pipeline I,belongs to the West-East gas pipeline com-pany of Petro-China.Now the total storage gas is about0.05 billion cubic meters,stored in5salt caverns which are con-verted from existing depleted brine production caverns(Y ang et al.2007).In June,2010,the JZ-1salt cavern began to be injected with gas,which is the first one leached for the pur-pose of gas storage rather than for brine production.There will be57salt caverns to be built by Petro-China in this area in the next10years.Recently,the Yunying salt cavern gas storage project in central China is under feasibility study, which is the supporting project of the West-East natural gas pipeline II.3THE FEASIBILITY STUDY ON GAS STORAGE IN DEPLETED SALT CA VERNSAccording to the requirement of the West-East natural gas pipeline project I,six depleted salt production caverns,West-1,West-2,East-1,East-2,Gang-1and Gang-2(Figure1)in Jintan Salt Mine were selected to be natural gas storages after appropriate reconstruction.3.1Sonar measure of the cavernsTo satisfy the requirement of structure and volume of gas stor-age,the preferred salt caverns for gas storage were selected from30old solution-mined caverns.Among the about15 preferred caverns,only six caverns,namely West-1,West-2, East-1,East-2,Gang-1and Gang-2caverns,were chosen as research objects to evaluate the usability for gas storage(Y ang et al.2007).The in-situ sonar measures of the cavern boundary were carried out.As an example,the results of sonar measure of East-1cavern are shown in Figure2.West-1and West-2caverns are located at Xizhiqiao well district of Jintan Salt Mine,and the distance between the two caverns is about104.43m;East-1and East-2caverns are located at Donggang well district,and the distance between them is about119.97m;Gang-1and Gang-2caverns belong to Donggang well district too,and their space is about104.65m. Based on their ground space and maximum radius,the min-imum pillar width between Gang-1and Gang-2caverns was estimated to be about16.7m.Figure1.3D perspective drawing of6reconstructed brine caverns and2observationcaverns.Figure2.the sonar measure results of East-1cavern.3.2Mechanical experiments on salt rocksTo investigate the mechanical properties of salt rocks,a series of laboratory test including uniaxial compression,triaxial compression and creep experiment by the XTR01Electric-fluid Serving Compression Machine,were carried out(Liang et al.2007).The rock cores were taken from Jintan Salt Mine. The test results show that the elastic modulus of salt rocks range from18.5to21.8GPa.The salt rocks present obvi-ous strain softening in uniaxial compression test,and exhibit strain hardening and dilatability step by step with increase of confining pressure.The results of triaxial creep test under confining pres-sure5∼20MPa indicated that the rock salt in Jintan Salt Mine behaves good creep property.The confining pressure influences the creep properties of salt rocks remarkably.The presence of a larger confining pressure can retard the arrival of steady creep stage and accelerating creep stage.Certainly,the differential stress is the determinate factor for creep behavior of salt rocks.The larger axial pressure induced the larger strain rate which accelerates the coming of the second and the third creep stage(as shown in Figure3).3.3Conclusion on feasibility of gas storage in depleted saltcavernsBased on the experimental analysis of the mechanical and creep characteristics of salt rocks and mud rocks,numerical study on deformation and stability of these caverns was also carried out.For investigating the feasibility of sealing of the old salt caverns,brine-pressing cavern tests on two salt cavernsFigure3.The Typical creep strain-time(day)curves of salt rocks. were performed.It were concluded that the six salt caverns are suitable to be utilized as gas storages,and that•The creep characteristics of the salt rocks is close to that of the salt rocks mixed with minor mud,while the steady state creep velocity of the mud rocks mixed with minor salt is lower,only one eighth of that of salt rocks.This indicates that the salt rock layers control mainly the creep deformation of salt caverns.•The breakthrough pressures of the six salt caverns are among6.7to21.3MPa.This denotes that the sealing abil-ity of the caverns in bedded salt rocks formation is good enough to be utilized as gas storages.•The numerical analysis showed that the reduction of the cavern volume can be restricted by the increase of the inter-nal pressure.Therefore,for lengthening the service lives of the salt caverns,it is suggested that the continuous running time of caverns under lower pressure should be reduced as soon as possible.•The safe distance between the tube-in-tube shoe and the cavern top are8m for West-1,12m for West-2and6m for the other four caverns respectively,according to design age limit,20years.•The brine-pressing cavern tests on West-1and West-2shafts validated the sealing ability of the two shafts,and the steady creep parameters by invention analysis agreed well with those from the indoor tests on salt rock cores.4RESEARCH ADV ANCES ON THE MECHANICALBEHA VIOR OF DEEP BEDDED SALT ROCKS4.1Compression tests of bedded salt rocksThe deformation and failure characteristics of the rock salt with interlayer under uniaxial and triaxial compression stress state were studied(Li et al.2007a).The core samples are from Yunying Salt Mine in Hubei province.The bedded salt rocks are almost composed of the rock salt layers and the anhydrite interlayers.The rock samples were divided into three types. The first sample type is rock salt samples,which is grey-brown and coarse-grained,containing a little of glauberite inclusion.Its main chemical compositions are NaCl,Na2SO4 and CaSO4.The second is anhydrite interlayer,containing 17.50%∼23.62%NaCl and36.24%∼42.62%water insolu-ble matter.The third is rock salt samples containing anhydrite interlayer of various thicknesses.The photos of three kinds of typical core samples are shown in Figure4.Some conclusions were drawn out after comparing the uniaxial compression test results.The uniaxial compression strength,Y oung’s modulus and elastic limit of theanhydrite Figure4.Three kinds of typical coresample.Figure 5.Stress-strain curves of three types under confining pressure of5MPa.are the highest while that of the pure rock salt are the lowest, and that of the rock salt containing interlayer are in between. The uniaxial compression strength of the rock salt containing inter-layer,increases with the volume fraction of the interlayer (Li et al.2007a).The test results indicated that the strength and the compres-sion moduli of samples increase with the confining pressure. Meanwhile,with the confining pressure increasing,deforma-tion properties of the rock transforms from brittle to ductile gradually.Under the same confining pressure,when reaching the maximum stress,the axial strain of the rock salt is the largest,that of the anhydrite is least,and that of the bedded salt rock is midterm.The pure rock salt also presents obvious strain hardening property.A stress-drop phenomenon was observed in the case of uniaxial compression and triaxial compression under low con-fining pressure,and it even appears several times in a test as shown in Figure5.It implies well the effects of hard interlayer to the damage of the salt rock.However,the stress-drop phe-nomenon was not observed in the test of which the confining pressure is higher.The failure patterns of three types in the uniaxial compres-sion test are all tensile failure with multiple splitting planes along axial direction.However,the failure of the composite rock is remarkable different from the other two.The intrin-sic uniaxial compression strength of the anhydrite is higher than that of the rock salt,but the interlayer phase fractures first.The cracks then extend to the interface of two materials and result in the cleavage of salt rocks as shown in Figure6. The failure patterns of these rocks at triaxial compression are apparent different too.The anhydrite samples fracture along an oblique shearing plane,while the salt rock samples exhibit obvious plastic failure characteristics.The failure of the com-posite rock samples are rather complex.The anhydrite phaseFigure 6.Uniaxial compression failure pattern of rock sample with anhydriteinterlayer.Figure 7.The force diagram of the direct shear sample.fractures by shear stress first and then the cracks propagate to the salt phase.4.2Mechanical properties tests of the bedded salt rockinterfaces For bedded salt rocks,its failure and damage characteristics relate closely with the mechanical properties of interfaces between rock salt and anhydrite,and the mechanical prop-erties of interfaces have remarkable effect on the stability of storage caverns built in bedded salt rocks.The previous com-pression tests (Li et al.2007a)and creep tests (Chen et al.2006)indicate that no interface slide appears in the compos-ite rock samples under compression tests.For the purpose of deep knowing the mechanical properties of interfaces,direct shear tests (Li et al.2007b),Brazilian tests and microscopic analysis of interfaces (Liu et al.2006)were performed.4.2.1Direct shear testThe samples for direct shear tests are from Yunying Salt Mine too.In order to compare the shear strength of the interfaces between salt and anhydrite,pure rock salt and pure anhy-drite,three kinds of samples were all included in the direct shear tests.The rock cores were cut to be cylinders,the lengths of which range from 100to 120mm,and then the cylinders were embed-ded into casting concrete.When machining samples of the composite rock,the interfaces of rock salt and anhydrite were laid in the horizontal middle of the cylinders to be shear planes in the tests.As demonstrated in Figure 7,the samples for direct shear test were put in the shear box,and were applied shear force (a couple of force Q)until failure under normal forces P of 5kN,10kN,20kN and 30kN respectively.Shear strength of bedded salt rock,rock salt and pelitic anhydrite were obtained by using regression analysis.ThetestFigure paring of three types of rock samples before and after Brazilian test.results (Li et al.2007b)showed that the shear strength param-eters c and φof interfaces are 4.08MPa and 45.9◦respectively,which are near those of salt rock and of anhydrite,even slightly higher.This indicated that the interfaces between rock salt and anhydrite are bonded perfectly,and they are not weak faces as expected.Obviously,this ‘strong interface’property is bene-ficial to stability and feasibility of sealing of the caverns in the bedded salt rocks,and it makes energy underground storage in deep bedded salt rocks feasible as well in salt domes.4.2.2Brazilian testFor the energy storages built in the bedded salt rocks,the slip and open along the interfaces between rock salt and anhydrite may result in leakage of oil or gas.Therefore,the tensile strength of the interface in bedded salt rocks is a key mechanical parameter for feasibility of sealing of the storage.Generally,for the common rock,the interfaces are weak faces with lower tensile strength,even can not bear tensile load.Although the results of compression tests and direct shear tests indicate that interfaces in the bedded salt rocks ofYunying Salt Mine are not weak faces,the tensile strength test is still neces-sary because it is more direct study method for the mechanical properties of the interfaces.The samples of Brazilian tests included eight rock salt samples,eight anhydrite samples,and five composite rock samples.The test results showed that,the average tensile strength of rock salt is 1.04MPa,about 5.6%of its average uniaxial compression strength;the average tensile strength of anhydrite is 2.20MPa,about 9.2%of its average uniaxial compression strength.The failure patterns (Figure 8)of there rock samples were similar to that of common rocks.The fracture initiated at the middle of the disk and then broke the sample to be two semicir-cle parts.However,the fractured face of the composite samples was not along the interface as expected but along a plane near the interface in the half part of rock salt.Therefore,the tests indicated again that the interfaces have strong adhesive strength,and even the tensile strength of it is slightly larger than that of rock salt (Liu et al.2006).4.2.3Electron microscope scanning test of interfaceIn order to explain the reasons of strong interface from micro-cosmic view,electron microscope scanning tests of bedded rock salt have been carried out by Liu et al.(2006).Some typical samples of bedded rock salt are shown in Figure 9.Figure 9.T ypical samples of bedded rocksalt.Figure 10.Electron microscope scanning photos of interfaces.There are two types of interfaces in bedded rock salt:The one is a sharp interface,and the other is a mingle transitional zone.The electron microscope scanning photos of sharp interface are shown in Figure 10(a).Near the sharp interface,the size of halite particles (thick plate shape)is large,and there are some pelitic mineral filled in the gaps of halite particles;the size of anhydrite particles (brick or strip shape)is small and these particles are spaced closely.The particles of anhydrite and rock salt embed tightly each other.The electron microscope scanning photos of mingle inter-faces are shown in Figure 10(b).The particles of anhydrite and rock salt have no obvious interface,and the spaces between larger particles are filled with some pelitic mineral.These indicated microscopically that the interface between halite and anhydrite is not a week one but a transitional zone,of which the particles of rock salt and anhydrite mingle sufficiently and tightly.4.3A cosserat-like constitutive model for bedded ralt rocksand its applications In most of the literatures (Dawson &Cundall 1995;Adhikary &Dyskin 1998;Forest et al.2001),rock masses are traversed by multiple planes of weakness which separate them into multiple blocks or layers.And the mechanical prop-erties of each layer are same.The discontinuities or the weak bedding planes are usually characterized as joints with linear or nonlinear normal and tangential stiffness but with vanish-ing thickness.The bedded rock considered in reference (Li et al.2009)is identified as a bilaminated layered (two-phase)medium formed by two layers of homogeneous components.The bedded salt rock,which is a kind of typical bedded rocks,is a composite of anhydrite-halite or mudstone-haliterock.Figure 11.Cosserat medium representative unit of layered rock mass.For inhomogeneous or discontinuous medium,when the dimensions of microstructures,such as spacing of rock joint plane,thickness of layer,dimension of crack or inclusion,are very small compared to the dimensions of the engineer-ing structures,homogenization methods are often used (Yin et al.2007).However,the gradient of the stress field becomes large when a chamber is excavated in bedded salt rock.Then,the influence of bending effect must be considered.In order to consider the bending effect caused by micro displace-ment compatibility,a 2-dimension Cosserat-like constitutive model was established for the bedded salt rocks by introduc-ing Cosserat theory.Considering displacement compatibility between adjacent layers with mismatch mechanical proper-ties in three dimensions,the 2-dimension Cosserat constitutive model was extended to three dimensions case (Li et al.2009).The representative unit (Figure 11)is the composite of materials A and B.The 3-Dimensional Cosserat-like medium constitutive relationship can be deduced based on the classical continuum theory,assuming that:•T wo different isotropic materials are bonded perfectly without sliding,opening or penetration;•The four side surfaces remain planar when the materialundergoes tensile or bending deformation;•The deformation is small.A new three-dimensional Cosserat-like medium constitu-tive model,in which the influence of bending is taken into account,is proposed for bilaminated layered salt rocks asfollow:where,the eight independent nonzero elements can be defined completely in terms of the elastic modulus and the volume fractions of two layers (Li et al.2009).Based on 3-Dimensional Cosserat-like medium constitutive model (Li et al.2009)an interface program of this model for FLAC 3D is developed on theVisual C ++platform byYin et al.(2007).The program of the new constitutive model has been validated through a test sample,so it is reliable to apply it in practical engineering.The asymptotic damage-failure causedFigure12.V olume reduction ratios of the oil storage caverns as a function of the different internal pressures.by the sequent damage of the individual layer with different elastic modulus can be considered while the element number needed for computation is very small.This provides a base for numerical simulation of the stability of the gas/oil storage caverns built in the bedded salt rock mass.According to the engineering geological condition,3-Dimension computational models were established for ideal-ized proposed salt caverns of ZK1075,ZK1083and ZK1099 using FLAC3D(Yin et al.2007).The anhydrite interlayers with thickness of2meters or more were modeled explicitly, while the thinner layers are modeled implicitly in the Cosserat medium.Figure12shows that volume reduction ratios of the oil storage caverns as a function of the different internal pres-sures after the caverns ZK1075,ZK1083and ZK1099being constructed.From the figures,we can see that the volume reduction ratios decrease with the increase of the internal pressures.For compression,the case of cavern ZK1083was cal-culated by using the user-defined model and the inbuilt Mohr-Coulomb model of FLAC3D respectively(Yin et al. 2007).The plastic zones,the displacement vector distribu-tion of the surrounding rock and the volume reduction ratios were compared in order to analyze the influences of the anhy-drite interlayers on the stability of salt caverns.The maximum wall displacement of the cavern,7.44cm,was almost2times larger than the result obtained by using Cosserat model.It is because that the reinforcement effects of the minor interlayers are taken into account when using the user-defined Cosserat model.This indicated that the hard anhydrite layers restrain the deformation of the adjacent salt rocks.Certainly,this rein-forcement effect of the hard interlayers is beneficial to the stability of the surrounding rocks of the storage cavern.5CONCLUSIONSWith the development of economy,the natural gas demand in China increases rapidly.The construction of long distance natural gas pipeline enters a fast development period,and a gas transportation network system is gradually formed.To support long transportation gas pipelines,a group of natural gas storages have been built or building in China.Different from other countries(USA,Canada,France,Germany,etc.), in China,underground energy storage is usually constructed in bedded salt formations rather than salt domes as host rocks, because salt domes suitable technically and economically for energy storage have not been found so far in China.Some researches on the mechanical characteristics of bedded salt rocks have been present recently for feasibility studies of gas storage in deep bedded salt rocks and the success of the Jintan salt cavern gas storage project in old caverns and also new cav-erns by solution mining validates that bedded salt rocks as well as salt domes can be utilized as host rocks of underground gas storage.Those studies validate strongly the feasibility of gas storage in bedded salt rocks,and also provide useful technical supports for the construction of gas storages in salt caverns. 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