Comparative analysis of thermodynamic and mechanical responses between underground hydrogen storage and compressed air energy storage in lined rock caverns

Underground hydrogen storage(UHS)and compressed air energy storage(CAES)are two viable largescale energy storage technologies for mitigating the intermittency of wind and solar power.Therefore,it is meaningful to compare the properties of hydrogen and air with typical thermodynamic storage processes.This study employs a multi-physical coupling model to compare the operations of CAES and UHS,integrating gas thermodynamics within caverns,thermal conduction,and mechanical deformation around rock caverns.Gas thermodynamic responses are validated using additional simulations and the field test data.Temperature and pressure variations of air and hydrogen within rock caverns exhibit similarities under both adiabatic and diabatic simulation modes.Hydrogen reaches higher temperature and pressure following gas charging stage compared to air,and the ideal gas assumption may lead to overestimation of gas temperature and pressure.Unlike steel lining of CAES,the sealing layer(fibre-reinforced plastic FRP)in UHS is prone to deformation but can effectively mitigates stress in the sealing layer.In CAES,the first principal stress on the surface of the sealing layer and concrete lining is tensile stress,whereas UHS exhibits compressive stress in the same areas.Our present research can provide references for the selection of energy storage methods.

Evaluation of the dynamic sealing performance of cap rocks of underground gas storage under multi-cycle alternating loads

The static sealing of underground gas storage(UGS),including the integrity of cap rocks and the stability of faults,is analyzed from a macro perspective using a comprehensive geological evaluation method.Changes in pore structure,permeability,and mechanical strength of cap rocks under cyclic loads may impact the rock sealing integrity during the injection and recovery phases of UGS.In this work,the mechanical deformation and failure tests of rocks,as well as rock damage tests under alternating loads,are conducted to analyze the changes in the strength and permeability of rocks under multiple-cycle intense injection and recovery of UGS.Additionally,this study proposes an evaluation method for the dynamic sealing performance of UGS cap rocks under multi-cycle alternating loads.The findings suggest that the failure strength(70%)can be used as the critical value for rock failure,thus providing theoretical support for determining the upper limit of operating pressure and the number of injection-recovery cycles for the safe operation of a UGS system.

Mineralogy,microstructures and geomechanics of rock salt for underground gas storage

Rock salt has excellent properties for its use as underground leak‐proof containers for the storage of renewable energy.Salt solution mining has long been used for salt mining,and can now be employed in the construction of underground salt caverns for the storage of hydrogen gas.This paper presents a wide range of methods to study the mineralogy,geochemistry,microstructure and geomechanical characteristics of rock salt,which are important in the engineering of safe underground storage rock salt caverns.The mineralogical composition of rock salt varies and is linked to its depositional environment and diagenetic alterations.The microstructure in rock salt is related to cataclastic deformation,diffusive mass transfer and intracrystalline plastic deformation,which can then be associated with the macrostructural geomechanical behavior.Compared to other types of rock,rock salt exhibits creep at lower temperatures.This behavior can be divided into three phases based on the changes in strain with time.However,at very low effective confining pressure and high deviatoric stress,rock salt can exhibit dilatant behavior,where brittle deformation could compromise the safety of underground gas storage in rock salt caverns.The proposed review presents the impact of purity,geochemistry and water content of rock salt on its geomechanical behavior,and thus,on the safety of the caverns.

Determination of convergence of underground gas storage caverns using non-invasive methodology based on land surface subsidence measurement

Undergroundgas storage caverns aremonitoredfor environmental safety in termsof equipmentandpotential emissions,particularly methane emissions from the underground and above-ground parts of the storage facility.Periodical measurements of land surface deformations and costly echometric measurements of convergence of individual storage facilities are carried out.The aims of environmental monitoring are:(1)to eliminate potential hazards in the shortest time,(2)assess the overall impact of intensive operation of storage facilities on the environment,(3)developmonitoringmethods relevant to environmental protection,and(4)take actions in case of failure.The paper presents a solution to the problem of determination of the convergence of underground caverns in a salt rock mass based on the results of land surface subsidence measurements carried out using the Gauss-Markov equalization algorithm.Themethod makes it possible for ongoing control of cavern volume convergence after each subsidence measurement on the ground surface and determining the actual impact of the use frequency(injection-mediumconsumption)on the convergence in time.The presentedmethodology is universal and verified on caverns located in a salt rockmass.The Gauss-Markov inversion model is the first used in this area,hence its application is significant.

Determination of the maximum allowable gas pressure for an underground gas storage salt cavern——A case study of Jintan,China

Increasing the allowable gas pressure of underground gas storage(UGS) is one of the most effective methods to increase its working gas capacity. In this context, hydraulic fracturing tests are implemented on the target formation for the UGS construction of Jintan salt caverns, China, in order to obtain the minimum principal in situ stress and the fracture breakdown pressure. Based on the test results, the maximum allowable gas pressure of the Jintan UGS salt cavern is calibrated. To determine the maximum allowable gas pressure, KING-1 and KING-2 caverns are used as examples. A three-dimensional(3D)geomechanical model is established based on the sonar data of the two caverns with respect to the features of the target formation. New criteria for evaluating gas penetration failure and gas seepage are proposed. Results show that the maximum allowable gas pressure of the Jintan UGS salt cavern can be increased from 17 MPa to 18 MPa(i.e. a gradient of about 18 k Pa/m at the casing shoe depth). Based on numerical results, a field test with increasing maximum gas pressure to 18 MPa has been carried out in KING-1 cavern. Microseismic monitoring has been conducted during the test to evaluate the safety of the rock mass around the cavern. Field monitoring data show that KING-1 cavern is safe globally when the maximum gas pressure is increased from 17 MPa to 18 MPa. This shows that the geomechanical model and criteria proposed in this context for evaluating the maximum allowable gas pressure are reliable.

Dynamic simulation research on injection and withdrawal performance of underground salt cavern natural gas storage

Owing to perfect impermeability,dynamics stability,flexible and efficient operation mode and strong adjustment,underground salt cavern natural gas storage is especially adapted to be used for short-term dispatch.Based on characteristics of gas flow and heat transfer,dynamic mathematic models were built to simulate the injection and withdrawal performance of underground salt cavern gas storage.Temperature and pressure variations of natural gas in gas storage were simulated on the basis of building models during withdrawal operation,and factors affecting on the operation of gas storage were also analyzed.Therefore,these models can provide theore-tic foundation and technology support for the design,building and operation of salt cavern gas storage.

Rock mass response for lined rock caverns subjected to high internal gas pressure

The storage of hydrogen gas in underground lined rock caverns(LRCs)enables the implementation of the first fossil-free steelmaking process to meet the large demand for crude steel.Predicting the response of rock mass is important to ensure that gas leakage due to rupture of the steel lining does not occur.Analytical and numerical models can be used to estimate the rock mass response to high internal pressure;however,the fitness of these models under different in situ stress conditions and cavern shapes has not been studied.In this paper,the suitability of analytical and numerical models to estimate the maximum cavern wall tangential strain under high internal pressure is studied.The analytical model is derived in detail and finite element(FE)models considering both two-dimensional(2D)and three-dimensional(3D)geometries are presented.These models are verified with field measurements from the LRC in Skallen,southwestern Sweden.The analytical model is inexpensive to implement and gives good results for isotropic in situ stress conditions and large cavern heights.For the case of anisotropic horizontal in situ stresses,as the conditions in Skallen,the 3D FE model is the best approach.

Two-and three-dimensional stability analysis of underground storage caverns in soft rock(Cappadocia, Turkey) by finite element method

Engineering design in soft rocks and its stability analysis exerts many challenges to rock engineers. Many engineering works in Turkey’s Cappadocia region must face and tackle the existing sites covered by the soft rocks. This study is aimed to examine the stability condition of a typical underground storage cavern(USC) excavated in a soft rock in this region. For this purpose, two-and threedimensional stability analyses of the USCs were performed using the finite element method(FEM).Because of the inherent difficulty in characterizing soft/weak rock masses in the region using traditional classification systems, the stability of a typical USC was evaluated by representing the rock mass condition with two distinct scenarios in FEM analysis.While these structures were unstable according to the 2D analysis conducted in RS2 software in the worstcase scenario, they were stable in the 3D analysis using RS3 software in both scenarios. Besides,feasible cover depths were examined to assess their possible effects on the factor of safety and deformation measurements. It was found that 15 m seems to be an optimal depth for excavating a typical USC in the soft rocks exposed in the region. The 3D FEM results provide valuable information to optimize the future planning and preliminary design of USCs.

Theoretical research on gas seepage in the formations surrounding bedded gas storage salt cavern

When constructing salt cavern gas or petroleum storage in lacustrine sedimentary salt formations rich in mudstone interlayers, the influence of the sealing performance of interlayers and salt-mud interface on the storage tightness should be considered adequately. In order to reveal the gas seepage in deep formations surrounding bedded salt cavern underground storage, a leakage analysis model was established based on the characteristics of a low dip angle and the interbedded structure of bedded rock salt. The gas seepage governing equations for one-dimensional and plane radial flow were derived and solved. A gas seepage simulation experiment was conducted to demonstrate the accuracy and reliability of the theoretical calculation results. The error of the seepage range was approximately 6.70%, which is acceptable. The analysis and calculation results indicate that the motion equation of gas in deep formations satisfies a non-Darcy's law with a threshold pressure gradient and slippage effect. The sufficient condition for the gas flow to stop is that the pressure gradient is equal to the threshold pressure gradient.The relationship between the leakage range and operating time is a positive power function, that is, the leakage range gradually increases with time and eventually stabilizes. As the seepage range increases, the seepage pressure decreases sharply during the early stage, and then decreases gradually until the flow stops.Combining the research results with engineering applications, three quantitative evaluation indexes named the maximum admissible leakage range, leakage volume and leakage rate are proposed for the tightness evaluation of gas storage salt cavern during their operating stage. These indexes can be used directly in actual engineering applications and can be compared with the key design parameters stipulated in the relevant specifications. This work is expected to provide theoretical and technical support for the gas loss and tightness evaluation of gas storage salt caverns.

Experimental study on repair characteristics of damaged rock salt of underground gas storage

Damage in rock salt has significant implication on permeability, which affects the tightness of underground salt cavern gas storage in further. During the leaching of a salt cavern, the brine with formation temperature and pressure can promote the self-healing of rock salt in the excavation damage zone (EDZ). Laboratory tests were conducted to study the promoting effect. The permeability of two intact rock salt specimens was tested. Then they were damaged into two kinds of the state respectively through uniaxial compression. After that, they were put in saturated brine (with a temperature of 50℃ and pressure of 12 MPa, which we called the repair environment in this paper) for 7 d. Finally, the permeability and mechanical properties were obtained after the damaged specimens being repaired. The results show that the permeability of intact rock salt is below 10^-19 m^2;the permeability increases by more than two orders because of damage;the permeability decreases significantly after being repaired, which can be comparable to its intact state. Discussions of the repair mechanisms are presented (especially the mechanism of recrystallization), which may help to provide significant guidance for the study of the tightness and stability of gas storage facilities in China.

Failure transition of shear-to-dilation band of rock salt under triaxial stresses

Great potential of underground gas/energy storage in salt caverns seems to be a promising solution to support renewable energy.In the underground storage method,the operating cycle unfortunately may reach up to daily or even hourly,which generates complicated pressures on the salt cavern.Furthermore,the mechanical behavior of rock salt may change and present distinct failure characteristics under different stress states,which affects the performance of salt cavern during the time period of full service.To reproduce a similar loading condition on the cavern surrounding rock mass,the cyclic triaxial loading/unloading tests are performed on the rock salt to explore the mechanical transition behavior and failure characteristics under different confinement.Experimental results show that the rock salt samples pre-sent a diffused shear failure band with significant bulges at certain locations in low confining pressure conditions(e.g.5 MPa,10 MPa and 15 MPa),which is closely related to crystal misorientation and grain boundary sliding.Under the elevated confinement(e.g.20 MPa,30 MPa and 40 MPa),the dilation band dominates the failure mechanism,where the large-size halite crystals are crushed to be smaller size and new pores are developing.The failure transition mechanism revealed in the paper provides additional insight into the mechanical performance of salt caverns influenced by complicated stress states.

Rock engineering problems related to underground hydrocarbon storage

Oil and gas can be stored underground by a variety of means,such as in depleted oil and gas fields,in aquifers,in rock salt caverns,in unlined mined rock caverns,in lined shallow caverns and abandoned mines.Different types of underground storages require different geological and hydrogeological conditions and are associated with different rock engineering problems.However,the common issue is to ensure the gas-and oil-tightness of storage caverns.In other words,the stored oil and gas must not escape from the storage caverns.This may be realized by different means according to the types of storages and the sites geological conditions.There are basically two approaches of gas leakage control,i.e.permeability control and hydrodynamic containment.The latter involves the use of a water curtain system in many cases,which creates an artificial hydraulic boundary condition and helps to establish the required groundwater condition when needed.In addition to the common problems,the underground storage of liquefied petroleum gas(LPG) requires special attentions to the opening of rock joints,which result from the tensile thermal stress induced by the low storage temperature.Great care must be taken in choosing abandoned mines for oil and gas-storage since it is quite rare that the natural site conditions can meet the usual requirements,in particular for the gas tightness.The paper provides a general description of the gas leakage control for underground oil and gas storage projects,and addresses various rock engineering problems associated with selected types of storages in detail.

Progress and Prospect of Geophysical Research on Underground Gas Storage: A Case Study of Hutubi Gas Storage,Xinjiang,China

Hutubi Underground Gas Storage(UGS) is an important part of China’s West-East Gas Pipeline Project. Its production safety also plays an essential role in ensuring peaking,emergency and reserve of natural gas in the region. Geophysical observations and research conducted on Hutubi UGS provide significant support for understanding the operation status and ensuring safe operation of the UGS. Since the beginning of the construction of Hutubi UGS and its production,several works including gas field observation experiments,rock experiments and numerical simulations have been conducted. Preliminary observational results show that during the initial operation stage of the UGS,the "breathing phenomenon"in caprock and microseismicity in the vicinity are closely related to the operation of the gas storage. However,rock experiments and numerical simulations show that these activities may gradually weaken with the multi-cycle operation of gas storage. The impact of the operation of Hutubi UGS in the surrounding areas is gradually weakening,and its operation tends to be stable. Implementing long-term and multi-method geophysical observations is able to provide us a better understanding of the relationship between the operation of UGS and regional geological hazards. On this basis,the corresponding geomechanical model can be established to form an effective risk management mechanism for gas storage operation. Thus,it is of great significance to understand its operation status,monitor storage conditions,guide production and operation,and ultimately guarantee the safe production of the gas storage.

Experiments on mechanical properties of salt rocks under cyclic loading

The primary purpose of underground gas storages is to provide gas for seasonal consumptions or strategic reserve.The periodical operations of gas injection and extraction lead to cyclic loading on the walls and surrounding rocks of gas storages.To investigate the mechanical behaviors of different host rocks in bedded salt deposit,laboratory experiments were conducted on the samples of rock salt,thenardite,glauberite and gypsum.The mechanical properties of rock samples under monotonic and cyclic loadings were studied.Testing results show that,under monotonic loading,the uniaxial compressive stress（UCS） of glauberite is the largest（17.3 MPa）,while that of rock salt is the smallest（14.0 MPa）.The UCSs of thenardite and gypsum are 16.3 and 14.6 MPa,respectively.The maximum strain at the peak strength of rock salt（halite） is much greater than those of the other three rocks.The elastic moduli of halite,thenardite,glauberite and gypsum are 3.0,4.2,5.1 and 6.8 GPa,respectively.Under cyclic loading,the peak strengths of the rock specimens are deteriorated except for rock salt.The peak strengths of thenardite,glauberite and gypsum decrease by 33.7%,19.1% and 35.5%,respectively;and the strains of the three rocks at the peak strengths are almost the same.However,the strain of rock salt at the peak strength increases by 1.98%,twice more than that under monotonic loading.Under monotonic loading,deformation of the tested rock salt,thenardite and glauberite shows in an elastoplastic style.However,it changes to a ductile style under cyclic loading.Brittle deformation and failure are only observed for gypsum.The results should be helpful for engineering design and operation of gas storage in bedded salt deposit.

Deformation Analysis of LRC Underground Gas Storage

The risk during construction and in the operation of the underground gas storage （UGS） was analyzed. One of most important risk which should be prevented is large deformation or destruction of the steel lining. The specific deformation of the steel lining needs to be inside the acceptable value. This paper presents lined rock cavern （LRC） concept and specific deformations, which can occur under operation of underground gas storage. Analysis is performed with different （3D model and axis symmetrical） FEM models and analytical model. We made a comparison between analytical calculation and FEM calculation. Concrete wall is mechanically not regarded as reinforced concrete structure which means that concrete will crack. Finally, we determined the minimum value of Young＇s modulus, which satisfies the condition of maximum deformation of steel lining.

Dynamic subsidence prediction of ground surface above salt cavern gas storage considering the creep of rock salt

A new model is proposed to predict the dynamic subsidence of ground surface above salt cavern gas storage during the leaching and storage, which takes into account the creep of rock salt. In the model, the extended form of Gaussian curve is adopted to figure out the shape of subsidence areas. The corresponding theoretical formulas are derived. In addition, parameters are studied to investigate the surface subsidence as a function of the salt ejection rate, internal pressure, buried depth, diameter, height, running time, etc. Through an example, the subsidence of the salt cavern gas storage located at Jiangsu of China obtained by the new model was compared with those by Peter A F formula, Schober & Sroka formula and FLAC3D through simulation. The results showed the proposed model is precise and correct, and can meet the actual engineering demands. The surface subsidence is equidirectional with the increase of salt ejection rate, depth, diameter, height, and running time, but reverse to the increase of internal pressure. The depth, diameter, running time and internal pressure have great effects on the subsidence, whereas the salt ejection rate and height have little influences on it.

中国大规模盐穴储氢需求与挑战

氢能是来源广泛且低碳清洁的能源,大力发展氢能产业是实现双碳目标和应对全球能源转型的重要举措。在氢能“制备―储存―运输―应用”全产业链中,储氢难问题长期制约着氢能产业高质量发展。盐穴储氢具有成本低、规模大、安全性高和储氢纯度高等突出优势,是未来氢能大规模储备的重要发展方向,也是我国能源低碳转型的重大战略需求。综合调研了我国制氢产业和氢能消费现状,分析了我国盐穴储氢的需求。调研了国外利用盐穴储存天然气和氢气的技术及工程现状,总结了我国盐穴储气库发展和建设历程。对比了利用盐穴储存天然气、氦气、压缩空气和氢气的异同点,提出我国盐穴储氢面临三大科技挑战:层状盐岩氢气渗透与生化反应、盐穴储氢库井筒完整性管控、储氢库群灾变孕育与防控。研究成果明确了我国氢气储备需求的快速增长趋势和大规模盐穴储氢的重点攻关方向。

我国盐穴地下储库建设的挑战与对策研究——从“金坛模式”到“XX模式”

我国盐穴地下储库建设起源于江苏金坛盐矿并获得成功,初步构建了我国盐穴储库建设首个技术标准体系。但是,我国各地盐矿的品位、埋深、矿层厚度和夹层特性等地质条件与金坛盐矿相差甚远,盐穴储库建设面临诸多挑战,不能照搬“金坛模式”。着眼于盐穴储库地质设计、造腔工程、注气排卤方案等方面,首先总结盐穴储库建库的“欧美模式”,以及基本上借鉴“欧美模式”发展形成的我国“金坛模式”的基本特征,然后结合我国已开展和正在开展的其他多座盐矿的盐穴储库建设的研究和工程实践,提出几种基于“一地一议、因矿制宜”原则的典型盐穴储库建库的“XX模式”。“XX模式”意味着是不同于金坛模式并有极大突破和变革的新型建库模式,是应对我国各地盐矿地质复杂性所带来挑战的技术方案和新思路。最后,对于下一步应对盐矿复杂地质条件带来的挑战需要突破和创新的关键理论和技术进行了展望。

MICP技术在地下水封油库渗控注浆中的应用潜力

地下水封油库在建设及运营过程中洞室涌水量大小是影响洞库水封安全和运营成本的重要因素。目前地下水封油库工程建设中采用传统的注浆方式难以将围岩渗透性降低到设计要求,因此洞室涌水量得不到很好的控制。微生物诱导碳酸钙沉淀(MICP)技术具有注浆黏度低、环境适应性强、生成矿化物质稳定等显著优点,引起了学术界与工程界广泛的兴趣和研究。本文首先分析了地下水封油库涌水量控制中存在的相关问题,然后阐述了MICP技术对裂隙渗流控制的机理、影响因素以及对裂隙渗透性的影响效果,最后探讨了MICP技术在地下水封油库中应用的关键问题。通过分析表明,MICP作为地下水封油库渗流控制的一种辅助注浆方式,可以较为准确地控制围岩渗透性变化,达到对洞室涌水量有效控制的目的。

基于地下水化学特征的水封油库水幕系统有效性评价

水幕系统有效性是地下水封油库安全稳定运行的重要基础条件,然而目前评价水幕系统有效性的方法并不高效便捷。以我国首个大型地下水封油库工程为背景,采集油库运行期现场水样和岩样进行水质检测分析和电镜扫描试验,获取库址区地下水水化学类型和岩石的矿物成分组成特征。采用数理统计方法,基于地下水水化学特征获得了不同部位地下水之间的水力联系,开展水幕系统有效性评价。研究表明:油库运行初期库址区地下水主要为HCO^(3)—Na•Ca型水,围岩中的钾长石、钠长石和钙长石发生水化学反应,地下水中的K^(+)、Na^(+)、Ca^(2+)和HCO_(3)^(-)浓度总体呈上升趋势,pH值总体呈下降趋势;Cl^(-)浓度低于造成钢筋腐蚀的浓度,地下水对洞库支护系统无明显腐蚀作用;洞库周边监测孔地下水与水幕供水的水化学特征相似,说明水幕系统与油库围岩之间存在较好的水力联系,形成良好的水封效果。该研究可为判断地下水封油库运行情况提供重要依据,并为评价水幕系统有效性提供了一种科学方法。