Experimental study of water curtain performance for gas storage in an underground cavern

Journal of Rock Mechanics and Geotechnical Engineering. 2009, 1(1): 89-96Journal of rock mechanics and Geotechnical engineeringJournalonlinewww.rockgcotech.orgCSRMEExperimental study of water curtain performance for gas storage in anunderground cavernZhongkui Li, Kezhong Wang2 Anmin Wang,Hui LiuI State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing, 100084, chinaReceived 16 April 2008: received in revised form 22 August 2009: accepted 26 August 2o 0014, chinaSchool of Civil Engineering and Architecture, Zhejiang University of Technology, Hangzhou, 3/Abstract: An artificial water curtain system is composed of a network of underground galleries and horizontal boreholes drilledfrom these galleries. Pre-grouting measures are introduced to keep the bedrock saturated all the time. This system is deployedover an artificial or natural underground cavern used for the storage of gas (or some other fluids) to prevent the gas fromscaping through leakage paths in the rock mass. An experimental physical modeling system has been constructed to evaluatthe performance of artificial water curtain systems under various conditions. These conditions include different spacings ofcaverns and cavern radii located below the natural groundwater level. The principles of the experiment, devices, design of thephysical model, calculation of gas leakage, and evaluation of the critical gas pressure are presented in this paper. Experimentalresult shows that gas leakage is strongly affected by the spacing of water curtain boreholes, the critical gas pressure, and thenumber and proximity of storage caverns. The hydraulic connection between boreholes is observed to vary with depth orocation, which suggests that the distribution of water-conducting joint sets along the boreholes is also variable. When designingthe drainage system for a cavern, drainage holes should be orientated to maximize the frequency at which they encounter majorjoint sets and permeable intervals studying in order to maintain the seal on the cavern through water pressure. Our experimentalresults provide a significant contribution to the theoretical controls on water curtains, and they can be used to guide the desigand construction of practical storage cavernsKey words: artificial water curtain; model test; storage cavern; gas pressureIntroductionHowever, underground storage caverns requirespecific geological conditions that are not availableLarge underground caverns have been used for the everywhere For example, intact rock is never completelysafe and economical storage of hydrocarbon gases, impermeable. In unlined rock caverns, gas is keptgasoline, home heating oil, jet fuel and crude oil. The from escaping by ensuring that the groundwaterneed to support the rapid industrialization of the 1970srock around the caverns exceeds theand to avoid oil crises by creating significant storagein the storage caverns [5]. High gaapabilities stimulated the construction of underground pressure can be achieved by locating a cavern at astorage caverns for such materials as crude oil and sufficient depth or by installing a"water curtain'liquefied petroleum gas(LPG), and pumped storage around the cavern.power plants [1-3. A number of underground cavernsWater curtains are an array of boreholes that arehave been constructed for oil and gas storage in Chininstalled parallel to each other over the roof of aKorea has a short construction history of large-scale cavern and around the sidewalls if necessary. with awater curtain, the groundwater pressure around a cavernUnderground storage of pressurized gases in unlined can be increased sufficiently to prevent gas leakagerock caverns has advantages over above-ground storage The idea is to make water continuously flow towardin terms of safety, environmental protection and economy [4]. the cavern from outside so that the stored gas cannever escape out of the cavern. The cavern must beDoi:10.3724SPJ1235.2009.00089sited deep enough to ensure that the hydraulic pressure*Correspondingauthor.Tel:+86-531-88320153:E-mail:wkzl@zjut.edu.cnin rock fracturSupported by the National Natural Science Foundation of China (50779024d the cavern ic always higherthan the gas pre中国煤化50539090)and the Open Research Foundation of State Key Laboratory ofHydroscience and Engineering of Tsinghua University(20080533114is maintained tCNMH Grock throughZhongkui Li et al. Journal of Rock Mechanics and Geotechnical Engineering. 2009, 1(1): 89-96bores above the cavern to maintain a stable groundwater the systemlevel. The effectiveness of an unlined oil storage cavernFigure 2 shows different methods to limit oris strongly dependent on the water tightness of the rock eliminate leakage from an underground gas storagemass around the storage cavern. Thus, it is essential cavern. These methods are based on two main principlesthat all the cracks are filled with water during the of permeability and groundwater control. Permeabilityconstruction and operation of a storage cavern [6]. control means that the gas leakage is eliminated orThis method is termed as the water dynamics sealing kept at an acceptable level by ensuring that the rockmethod [7, 81mass around the storage has a sufficiently lowPrevious researches have developed the theory and permeability. The permeability coefficients of the rockapplication of water curtain installations. The extensiveImportanworks of Zimmels et al. 19-ll] have developed the controlling groundwater seepage and stress within theprinciples of water curtains, and provided useful rock. The permeability coefficient of a rock mass willsuggestions for practical installations. Lindblom et al. increase with the expansion of cracks. In faulted rock,[12, 13] advanced this work with simple numerical the permeability coefficients are different in differentsimulations. Kim et al. [14, 15] experimentally directions, i.e. anisotropic. Generally, in the directionmodeled and monitored underground LPG storage of fault, the permeability coefficient is large, whereascaverns below the groundwater table. Liu et al. [16] in the direction normal to fault, the permeabilityused finite element analysis to model gas storage in coefficient is relatively small.rock cavern under high pressure. The installation of awater curtain at the Kvilldal hydroelectric powerstation in Norway has enabled the practical evaluationeakage from a pressuriof such systems. After installation, the permeabilitycoefficients of the rock mass around the storagecaverns were studied and the rate of gas leakage wasPermeabilityGroundwatershowshorizontal water curtain boreholin the hydraulicNaturaltests. According to the results, thetight rock Grouting Lining Freezing i groundwater curtainrange from 9 to 1l cm/s. These values are slightly lowerFig 2 Methods to limit or eliminate gas leakage from athan those obtained from site investigatiopressurized underground storagePermeability coefficient tests are routinely conductedthe field. Field tests include both affusing and口 Construction tunnelpumping tests. The permeability coefficient of a rocko Water curtain tumass can be obtained by pumping water at a constantStorage cavernflow rate from a borehole and measuring the decreasein groundwater level at an observation well. Theoo ooo opermeability coefficient can be calculated by a simpleequationThe principle of groundwater control is based on theI Relative location of horizontal boreholes, water curtain fact that the presence of groundwater reduces gasnels, construction and storage cavernsleakage. The leakage reduction, or degree of groundwaterUnderground storage caverns can have multiphasecontrol, depends on the magnitude of the groundwatergas-liquid-solid coupling problems. There are two pressure as compared with the storage pressureestablished methods for studying gas-liquid and fluid- Leakage prevention by groundwater control, assolid coupling problems. One multiphase gas-liquidmentioned earlier. can be based on either the naturalsolid coupling physical modeling test of water curtain groundwater pressure or water pressure artificiallyperformance was conducted by Wong in 1994 [17enhanced through the use of a water curtain. In thisAnother physical model of the air pressure in a tunnel way, an inward hydraulic gradient is established, whichwas established by Baghbanan et al. in 1994 [18-20]. is high enough中国煤化工 grationThese kinds of tests are limited by the difficulty inThe waterCNMHGcontrolling the coupling of the gas, liquid and solids in the storage cayexueime conditions a waterZhongkui Li et al. Journal of Rock Mechanics and Geotechnical Engineering. 2009, 1(1): 89-96curtain that completely surrounds the storage cavernmay be necessary. To completely avoid leakage by 2 Design and manufacture of the modelgroundwater control, the water pressure in all potentialeakage paths, directed upward from the storage, mustWe have developed a physical modeling system thatexceed the storage pressure over at least a smallan be used to simulate many aspects of a waterdistancecurtain-controlled underground cavern including multipleGas storage based on natural groundwatercavities, different spacings of water curtain boreholesn general, does not have the economical attraction that different cavern diameters, and the position of thehigh-pressure storage has, because the pressure of the water curtain installation above the cavern. This testgas must be lower as it is related to the thickness of device system consists primarily of a test frame, thebe used to physical model(described in more detail below ) a gasincrease the groundwater pressure artificially. This supply device and a measuring device(Fig 4). Thewill allow a higher ratio between storage pressure and length, width, and height of the model are 200, 30, anddepth, and will increase operational flexibility. Experience 90 cm, respectively. Three rows of drain holes areshows that water curtains can successfully avoid gas arranged above three caverns. An air compressor isleakage at pressures up to twice the hydrostatic used to supply the gas to the test caverns through agroundwater headpressure-stabilizer tank. Barometers are used toDespite the success of earlier implementations of monitor the fluid pressures. They are installed insidewater curtains, some practical problems remain to bholes arranged around the three caverns to monitorsolved. For example, the influence of the spacing of changes in the water pressure field under differentwater curtain boreholes and the influence of water working conditionscurtain invalidation on gas tightness is poorly understoodTherefore. we have designed a set of tests to evaluatethe performance of a water curtain system in arunderground gas storage cavern. The key point of ourgxperiment is to add a water curtain across anidentified fluid migration path to improve the sealability:of the cavern. we will present construction details forhe water leakage control system, hydraulic test results,and a revised instrumentation programThe seal on the unlined rock cavern can be definedor andpply device; b-Barometer: c-Gas supplby its hydrogeological design. The water prepipe; d-Artificial water curtain holes; e-Unlined test cavern; f-Measuringessure atpoints for pressure field; g-Reaction setting: h-Reaction frame: i-Drain holethe water curtain should be increased to the levelPressurizing system for water curtain; k-Micrometer gaugewhere the boundary conditions of a cavern remainFig 4 Physical model test device for a water curtain systemunchanged. The pressure around the cavern should beThrough a series of material modeling tests(e.glarger than that inside the cavern to have the flowdirected inward to the cavern as shown in Fig 3. Thedensity and void ratio tests), it was found that thmaterial strength increased and deformation reducedcondition for the tightness (or sealability) can besharply when the relative density of materials changedexpressed using the following equation [21, 22from 0.7 to 0.8. Therefore, a relative density of 0.8H>P+F+swas used to manufacture the model. The model waswhere H is the hydraulic head at the crown, P is the made of layered and compacted sand. Each layer ismaximum pressure head inside the cavern, F is the 5 cm thick and was tamped 10 times to pack the layershape factor of safety, and S is the factor of safetydown. During assembly, the recordingwere installed in the model at the designed positionsFor instance, the So-called high-accuracy mini-typemulti-point extensometers are installed inside themodel. Subsequently the model was saturated withH>P+F+Swater. The model was then left to stabilize for about24 hours. Finally, pressure sensors, a measuring pipeand the water curtain pipes were installed. Copper pipesd inside themodel: others中国煤化工Fig 3 Conceptual view of the tightness of the cavernoutside the moCNMHGZhongkui Li et al. Journal of Rock Mechanics and Geotechnical Engineering. 2009, 1(1): 89-96In this paper, the water curtain tests include singl3 Testing the water curtain systemdouble or triple storage caverns. During the experimentthe water curtain boreholes are open to be filled withJoints discontinuities and fractures are commonpressurized water at a range of pressures: 0.3, 0.4, 0.5most rocks. The simulation of these irregularities in a 0.6 and 0.7 m of hydraulic heads. The spacing of waterrock mass model is important. However, their irregulacurtain holenature makes this a difficult procedure [23, 24. TheWe will show that gas leakage rises as the waterkey point is that all of the fractures must be filled withcurtain pressure is reduced or the interval spacing ispressurized water. For the practical engineeringincreased. In general, water curtain invalidation willdevelopment of a water curtain system, it is necessarybe shown to occur in unlined caverns and at high gasto ensure that the water curtain covers and encirclesthe storage caverns, assumed in this case to betunnel-shaped. This is accomplished by building a4 Leakage calculatenetwork of water curtain boreholes above the cavernsith borehole axes parallel to the caverns axes. If4.1 Gas leakage calculationneeded, other boreholes can be positioned beside theIn this test, the general equation of state for an idealgalleries, extending several meters downward (Fig. 5)gas is used to calculate the leakage of gas mass or bulkSuch a water curtain system is generally linked by volumealleries that can access the diagonal and horizontalpV=nRT(2)water curtain borehole systemwhere pa is the gas pressure (MPa),V is the bulkvolume(m), R is the universal gas constant of 8.13J/(mol- K), T is absolute temperature(293. 15+ 20)Kand n the molar amount of gas (mol). In turn,n=m/M, where m is the air mass (kg) and M is theWagas molar mass(kg/mol) For air, its molar mass Mairgallery Water curtainborehole0.0289 kg/mol. Submitting n into(2), we obtain&VMGas mass can be calculated by Eq 3), and from thatStorage caverngas leakage can be obtainedFig 5 Relative position of storage caverns and sidelong4.2 Air leakage calculationboreholes from water galleryWith the exception of gas that leaks through thejoints and fractures of the bedrock around the cavernsIn our physical model, the water curtain boreholeswere spaced at intervals of 0.5 or 0.8 m, as shownhere is also some gas that will dissolve in thepressurized water. The loss of gas through dissolutiondiagrammatically in Fig. 6. During excavation, thesen water is given by Henrys lawboreholes were pressurized to 0.2 MPa. They act as themain water curtain system to prevent gas from leakingX(4)into fissures due to a fall in the groundwater levelHowever, when the facilities begin operation (i.e. thewhere X is the amount of the gas dissolved( thebalanced molar component), p' is the component ofction of gas), the water curtain boreholes will beactual gas pressure, H is Henry's constant of actual gascned to the curtain gallery, which will be filled withwater with a pressure less than 0.2 MPathat varies with gas pressure and temperature [2Henrys law only gives the gas loss that dissolves inwater at a steady state. We assume that the measurementsO Open water curtain boreholeswe make for this research are done in a steady state,and therefore the dissolved bulk gas in water is●o●●0●o●0●0●0●。●0●calculated by Eg. (4)hand results中国煤化工Fig 6 Sketch of water curtain test for a typical cavern arrangement. 5.1 Natural(sCNMHGZhongkui Li et al. Journal of Rock Mechanics and Geotechnical Engineering. 2009, 1(1): 89-96In practical engineering, a rock mass is characterized gas pressure is lower than the natural water pressureby faults, joints and bedding planes. Groundwater flothe gas leakage reduces rapidly.in rock masses is complicated by these discontinuities andAt the same working pressure, gas leakage willtherefore it is difficult to accurately consider all such increase as the number of the caverns increases(Fig.7)factors in groundwater models. Fortunately, host rocks This is because as the number of caverns increases, thefor gas storage are generally selected to be hard and number of leakage seepage paths rises and their routesmassive with few fractures. A simplified groundwaterbecome shortermodel can then be used to numerically analyze theThe process curves for increasing pressures aregroundwater flow Gas tightness design and hydraulic important because they can be used to indicatemodeling in such settings can be performed bypressure at whichleakage accelerates In generalcontinuum approaches [26-29]. Although the influencetwo straight lines can be used to describe them(Fig 8)of a single fissure on groundwater flow in fractureda+ bps(rock masses might not be negligible, the analysqyp8(p2≥ps)results presented in this study are based on followinwhere a. b c and d are the constants related to numbetassumptions for bulk mediaane(1)The groundwater flow obeys Darcy's lawpressure, i.e. the working gas pressure corresponding to(2) The medium is continuous, heterogeneous andthe inflexion in Fig. 7 or 8. When the pressure is greater(3) The groundwater flow is steady, i. e. groundwaterconditions are constant(5) Rock around the storage caverns is saturateds0 s Critical gas pressure→ith waterFigure 7 shows the relationship curves betweendifferent working gas pressures and gas leakage forsingle, double and triple cavern models. The resultsshow that the process curves can be divided into threesegments. For the first segment, gas leakage has no0.00.10.20.3040.50.60.70.80.91.0ng gas pressuresignificant change. For the second segment, gas Fig 8 Relationship curve between cavern working gas pressureleakage increases slightly as the gas pressure increasesand gas leakage at natural water levelFor the third segment, gas leakage increases quickly5.2 Influence of the pressure differenThe inflexion points on the curves indicative of gasleakage rate increases are distinct. For single doubleThe difference between the water curtain pressureand triple cavern models, the inflexion points occur at u the gas pressure is called the water curtain0.6, 0.5 and 0.4 m, respectively(Fig.7)pressure difference, Pa. This important factorinfluences gas tightness [30-32]. For example, in thecase of the triple cavern model, when Pa is largerthan 0.2 m, gas leakage from the caverns is modeled tobe nearly zero( Fig 9). When Pa falls below -0 2 m,Triple cavern modelobvious leakage can be found. When Pa falls belowDouble cavernSingle cavernDouble cavern model三0.4Single cavern model0.00.10.20.3040.50.60.70.80.91.0Cavern working gas pressure (m)Fig7 Hysteresis curves between cavern working gas pressureand gas leakage for rising and falling pressures0.0le curves reportingecrease in中国煤化工204working pressure are quite different from those with Fig, 9 relatioCNMH Gain differentialworking gas pressure increasing. When the working pressure and gasZhongkui Li et al. Journal of Rock Mechanics and Geotechnical Engineering. 2009, 1(1): 89-960.0 m, the gas leakage increases quickly and air when the spacing is closer. Although the water curtainbubbles escape rapidly from the top of the model. pressure difference is the same, the degree of capillaryCentralized seepage paths develop in the range from saturation is smaller for more widely spaced boreholes10 to 20 cm below the top of the modelthan for narrowly-spaced ones. This means that, forIn comparing the curves of the single, double and the overall rock mass, the permeability is larger. Thattriple cavern models(Fig 9), note that the curves of is why the amount of gas leakage is higher in systemsthe single and double cavern models are fairly similar with greater borehole spacings. The determination ofto each other, in that their departure from the line ofsuitable borehole spacings is therefore critical for theno leakage is at about the same point (i.e. whereefficient enclosure of an underground cavern with afalls below 0.0 m). To ensure that the gascompletely contained in the underground caverns, the 5.4 Water curtain invalidationwater curtain pressure difference must be larger thanWater curtain invalidation is of an extreme state butthe fluctuant value (like those identified for the threepossibility cannot be ruled out for practical undergroundcases above)of the water seepage pressurestorage caverns. When water curtain invalidation5.3 Influence of the spacing of water curtain occurs, the gas will leak freely, and the water curtainboreholesmust be improved to return the system to a normalIn practical engineering applications, one of the working state. The most likely remediation measureshat could be undertaken involve the enhancement ofconcerns in the construction and operation of a nevcavern is the stability of neighboring caverns [33-35]the water curtain pressure(by increasing the head) orFor this reason, the caverns should be adequatelythe number of boreholes used in the water curtainseparated. In addition, the dewatering of the rock massFigure ll shows a relationship curve that schematicallyaround an existing cavern should be prevented bydescribes the evolution of such a process from theconstructing the vertical water curtain system aroundpoint of gas leakage due to water curtain invalidationthe access galleries and shaft. Our preliminary designto the restoration of the water curtain pressure at somelater timeproposed a separation distance between caverns of00+ 25 )m [36]. However, in our model test, thesegeometrical dimensions were found to be too large,0.35regardless of the dimensions of the caverns or thewater curtain boreholesFigure 10 shows the relationship curves between品0.20Pa and gas leakage for two arrangements of the watercurtain boreholes (represented by 1 and 2 in Fig. 10)0.10In arrangement l. all of the water curtain boreholes areFailureRestorein a functioning state. In arrangement 2. some of thewater curtain boreholes are closed. The test shows thatTime(h)the separation of the water curtain boreholes can Fig l1 Schematic curve of leakage vs. elapsed time during awater curtain failure event that is subsequently restoregreatly affect the relationship curve At the same watercurtain pressure, a greater spacing of boreholes can make 5.5 Critical gas pressurethe relationship curves increase more rapidly than thatThe critipressure regimes from tight pressure regions. Once theSingle cavern model Iworking gas pressure increases above the critical gasDouble cavern model iTriple cavern model 1pressure, gas leakage will increase rapidly. Figure 12Single cavern model 204shows the relationship curves between the square ofL Double cavern model 2Triple cavern model 2the cavern working gas pressure and gas leakageWhen the square of the cavern pressure reaches 0.5 mebout 0.7 mAccording to Muskat theory, if the seepage area andpermeability coefficient are assumed to be constant,080.60.40.20.0-0.2-0,4Pressure difference(m)the radial gasprorata with the square ofFig 10 Relationship curves between gas leakage and watergas pressure中国煤化ht-line fitscurtain pressure.to the experimeCNMHGZhongkui Li et al. Journal of Rock Mechanics and Geotechnical Engineering. 2009, 1(1): 89-96show that the critical gas pressure can change. For06— Fitting linesexample, at the same working gas pressure, the critical■ Single cavern mode● Double cavern modelgas pressure will change as the numbers of the caverns▲ Triple cavern modelw Gas leakage in jointare increased(3)Inside the caverns, the gas working pressure orthe critical gas pressure increases as the water curtainessurecreasesAs the water curtain pressucannot be increased without limit. it is essential toensure optimization of cost and benefit in the performance0.00.20.40.60.81.0L2141.6nd deFig 12 Relationship curves between gas leakage and the square4) Under the same gas pressure or water curtainof cavern working gas pressurepressure difference, gas leakage will increase as theWhen the working gas pressure exceeds the criticalnumber of caverns increases. Increasing the number ofcaverns causes an increase in the number of leakagegas pressure, the water curtain begins to fail and gasleakage increases rapidly In our moopaths but the degree of capillary saturation in the rockmass decreasesthe single storage cavernmodel, but also for the double and triple cavern(5)In practical engineering applications, the watercurtain pressure or gas pressure of the cavern canmodels. Note that the critical gas pressure will changewith a change of the water curtain pressure(Fig. 13).borehole spacing and storage caverns dimensionan underground storage cavern, these possible dynamicchanges must be consideredThese modeling results provide some importantreference values for theoretical evaluations of water0.7mcurtain systems. They are also of significance for the→0.8mdesign and construction of practical projects-1.0mrencesch. In: proceed0.80.6040.20.0-0.2-04the of Korea-Japan Symposium on Rock Engineering. 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