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Harmonising Rock Engineering and the Environment–Qian&Zhou(eds)©2012Taylor&Francis Group,London,ISBN978-0-415-80444-8 Geomechanical model test of underground caverns in salt rockY.R.Liu,B.Li,F.H.Guan&Q.Y angState Key Laboratory of Hydroscience and Hydraulic Engineering,Tsinghua University,Beijing,China ABSTRACT:The world’s increasing demand for natural gas has made the research on the temporal evolution of storage group damage and the corresponding protection theory of great significance.At present,researches on the overall stability and temporal evolution of underground caverns are relatively few.Relating experimental study has not been reported yet.This paper simulated the injection-production cycles of4-cavern reservoir group under different internal pressure with small block masonry.The variation of the displacement and strain of the bedrock and the cavern group is automatically recorded by the displacement meters and the strain gauges buried in the model.Cavern stability under different gas production process,the failure pressure of the caverns and the impact of mudstone interlayer on cavern stability are analyzed.Subject:Underground storageKeywords:oil reservoir,physical modelling,stability analysis1INTRODUCTIONIn recent years,the world’s demand for natural gas has increased significantly,which made energy security an impor-tant issue.Underground natural gas storage construction is an effective method to ensure the storage security and regulate the seasonal supply-demand contradiction of natural gas.Up to2004,the total number of underground gas storage in the world is610.By2008,in which some gas storages are salt cav-erns.Salt mine is the world’s major media for energy reserves because of its good sealing,low permeability and self-healing. Gas storages in Europe and the United States are mainly sin-gle caverns in thick salt dome.However,Salt rocks in China have shallow buried depth,small strata thickness and plenty of insoluble intercalations.To improve the security and space utilization of energy reserve,intensive cavern groups must be used.Thus,it is of great significance to research the tempo-ral evolution of storage group damage and the corresponding protection theory.A large number of researches on the feasibility and safety of gas storages in salt rock have been done.Berest P and Fokker P A studied the mechanical behavior of salt cavern; Favert F summarized the researches on underground gas stor-ages and forecast its development directions.Y ang Chunhe demonstrated the feasibility of underground storage construc-tion by conducting creep tests and numerical simulation of salt rock.Wu Wen put forward several stability evaluation criteri-ons including not allowed spalling,not allowed creep damage, security pillar criterion and land subsidence criterion.The experimental and theoretical research of salt rock’s mechanical properties is the base of storage group stabil-ity analysis.Many scholars abroad,such as Hansen F D, Hunsche U,SkrotzkiW and HampleA,have carried out related researches.Y ang Chunhe,Wu Wen and LiYinping et al carried out a large number of experimental research and theoreti-cal analysis.Y ang Chunhe et al summarized the results.Dai Y onghao et al performed geomechanical model test simulating single caverns of Jintan salt storage.The results of the model test and the numerical simulation are contrasted to analysis the creep deformation and stress distribution of the caverns during operation.At present,researches on the overall stability and temporal evolution of underground caverns are relatively few.The sta-bility of caverns is mainly analyzed with numerical methods. Experimental study mainly focused on the mechanical proper-ties of rock salt and the surrounding rock stability of the single cavern.Experimental study on overall stability and chain destruction of cavern groups has not been reported yet.The3D geomechanical test in this paper simulates gas injection and production of4-cavern group under different pressure.The variation of the caverns’displacement and strain under differ-ent pressure is automatically recorded by displacement meters and strain gauges buried inside the model.The allover stability and the possibility of chain damage are analyzed based on the experimental results.2SIMILITUDE THEOR Y OF RUPTURE TESTING AND SIMULATION OF SALT ROCK MASS2.1Principles of similitudeA geomechanical test is a non-linear destruction test.It must meet the requirement of similarity:1)Geometry similar:the geometry and discontinuous struc-ture of the model should be geometric similar to that of the prototype.2)Stress-strain similar:the deformation modulus,stress-strain relationship and compressive(tensile)strength of the model and the prototype should meet the similarity condi-tion.The stress-strain similar condition is matched in the whole process,as shown in Figure1.According to the result of dimensional analysis,similar conditions need to meet the followingrelations.Figure1.Stress-strain relation of prototype and model.C E,Cγ,C L,Cµ,Cε,C f,Cσ,Cτ,C C,Cδand C F are the ratio of the elastic modulus,density,geometric length,Pois-son’s ratio,strain,friction coefficient,stress,shear strength, cohesion,displacement and load.To ensure the test is normal, Cγ=1.The geometric ratio C L is the determined parameter of the test,other parameters can be obtained with equation(2),(3) and(4).2.2The masonry technology of the modelSmall block masonry is an important method of geomechani-cal model construction.Similar material is pressed into small blocks,which will be glue into a model.The small blocks are to simulate the deformation characteristics of the rock.And the bonding among the small blocks is to simulate the strength properties of fractures.Thus,small block masonry method can simulate both deformation and strength characteristics of the rocks.In the geomechanical model test,both the mechanical parameters of a single small block and the integrated deforma-tion characteristics of the masonry need to be tested to ensure the similarity.Tests show that the deformation modulus of the model is lower than that of the continuous rock by30%to50%. In order to avoid excessive deformation modulus reduction, the block size should be large enough.E.Fumagalli proposed that the masonry block size should be checked with mechan-ical tests of masonry including at least100blocks.The size of blocks used in this experiment is2cm–5cm,as shown in Figure2.Non-continuous structural surfaces,such as muddy inter-calation,etc.,are simulated with dehydrated gypsum.3EXPERIMENTAL SIMULATION3.1Simulation rangeThis test simulates4underground reservoirs;each one is simulated as an ellipsoid with the same size:the length of the semi-major axis is84m,and the length of the semi-minor axis is36m.The ellipsoid centers are in the same horizon-tal plane,located at the vertices of a square with side length 144m.At the bottom of the cavern is sediment area with a height of56m.So the actual height of the cavern is112m. 24m above the center of the cavern is a muddy intercalation with a thickness of2m.the buried depth of the caverns is 1000m.Figure2.Blocks of varioussizes.Figure3.Longitudinal profiles of themodel.Figure4.Cross sectional drawing of the model.The size of the test cell is1.5m×1.5m×1.4m,as shown in Figure5.Schematic diagram of the whole test system is shown in Figure6.3.2Loading systemThe test load is divided into two parts:First,simulate the weight load of776m of rock;second,simulate the pressure variation during the gas injecting and production.The weight load can be calculated by formula(5).Load with self-made jack,using the pad between the model and the jack as transition as shown ih Figure7.Figure 5.Steel testcell.Figure 6.Schematic diagram of experimentaldevice.Figure 7.Loading device for weight stress.The internal pressure is simulated with silicone balloons.The balloons are put into the cavern before capping,as shown in Figure 8.The balloon is connected to the filling equipment outside with the high-strength hose buried in the model,as shown in Figure 9.The balloon is connected with a balance cylinder.With the help of the high precision barometer con-nected with the balance cylinder,the internal pressure of the balloon can be precisely controlled.3.3Data measurement systemThe relative displacement between the cavern walls,the abso-lute displacement and the strain on the cavern wallareFigure 8.Silicone balloons in thecavern.Figure 9.Gaschannel.Figure 10.The layout of displacement meter and the strain gauge (longitudinal profile).measured.The layout of displacement meter and the strain gauge is shown in Figure 10and Figure 11.There are also some internal displacement meters arranged in the mudstone interlayer to measure the interlayer disloca-tion.The self-made displacement meter is made from the man-ganese copper,with strain gauges pasted on both sides.The absolute displacement is fixed outside the model with dis-placement rod.To avoid the impact of the model on the displacement rod,the rod is wrapped with casing pipe,and the contact of the rod and the casing should be avoided.The displacement meter rod and casing are made of plastic pipe and steel pipe.3.4Test scheme5kinds of cycles with different maximum pressure were carried out,that is,12MPa,18MPa,24MPa,28MPa and 36MPa.For each cycle,the loading process was as follows:1)Weight simulation:simulate the weight of 776m rocks above the model with surface force.The loading step wasFigure11.The layout of displacement meter and the strain gauge (cross section).2MPa,added from0MPa to7.78MPa.The load–unload–load process was adopted in order to compact the model.2)4caverns production.Internal pressure of4caverns variedat the same time.The load step was1MPa.3)3caverns production.Internal pressure of3caverns variedat the same time.The load step was1MPa.The inter-nal pressure of all4caverns was raised to the maximum pressure by1MPa.Then3of them began to decompress.4)2adjacent caverns production.Internal pressure of2adja-cent caverns varied at the same time.The load step was 1MPa.The internal pressure of all4caverns was raised to the maximum pressure by1MPa.Then2adjacent caverns began to decompress.5)Single cavern production.The load step was1MPa.The internal pressure of all4caverns was raised to the maximum pressure by1MPa.Then1cavern began to decompress.6)2diagonal caverns production.The load step was1MPa.The internal pressure of all4caverns was raised to the maximum pressure by1MPa.Then2diagonal caverns began to decompress.4RESULTS ANAL YSIS4.1Cavern stability analysis under different gasproduction process.Displacement curves of displacement meter1and9under dif-ferent internal pressures are shown in Figure12and Figure13. Take the positive direction of the x-axis as displacement meter 1’s positive direction,and the negative direction of the y-axis as displacement meter9’s positive direction.The figures showed that gas production of single cavern is the most dangerous gas production process.The risk level order of5different gas production processes in the sequence of descending was:single cavern,2diagonal caverns,2adjacent caverns,3caverns,4caverns.4.2The failure pressure of the cavernsThe internal pressure and displacement curve of displacement meter1during gas injection is shown in Figure14.The failure pressure is28MPa.Figure12.Displacement curves of displacement meter9under different internal pressures(12MPa).Figure13.Displacement curves of displacement meter1under different internal pressures(18MPa).Figure14.The internal pressure and displacement curve of dis-placement meter1during gas inject ion(18MPa).Other internal pressure displacement curves show that displacement2,3,9and10were not damaged,while dis-placement4,5,7and8were damaged at28MPa.4.3The impact of mudstone interlayer on cavern stability The curve of mudstone interlayer relative displacement and the internal pressure(at the position of displacement meter 9)is shown in Figure15.The dislocation of the mudstone interlayer with the maximum pressure36MPa is very small.Figure15.The curve of mudstone interlayer relative displacement and the internal pressure(at the position of displacement meter9). Thus the impact of mudstone interlayer on cavern stability is limited.5CONCLUSIONThis experiment simulated the injection-production cycles of 4-cavern reservoir group with different internal pressure.The variation of the displacement and strain of the bedrock and the cavern group is automatically recorded by the displace-ment meters and the strain gauges buried in the model.The following conclusions can be drawn:1)The risk level order of different gas production processes inthe sequence of descending was:single cavern,2diagonal caverns,2adjacent caverns,3caverns,4caverns.2)The curves of the internal pressure and the displacementduring the production showed that the critical pressure of the model is28MPa.3)The dislocation of the mudstone interlayer with the max-imum pressure36MPa is very small.Thus the impact of mudstone interlayer on cavern stability is limited. 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