Cavern Wall Research Articles

Research of interlayer dip angle effect on stability of salt cavern energy and carbon storages in bedded salt rock

In order to clean and decarbonize energy, large-scale underground energy storage in salt caverns can be utilized. The construction and safety evaluations for these caverns are crucial, especially for salt caverns with significant interlayer dip angles. Aimed at the bedded salt rocks for energy and carbon storage, this study focuses on the impact of different interlayer dip angles on the stability of underground energy and carbon storage caverns in bedded salt rock formations in China. Four salt caverns with varying interlayer dip angles of 0–30° were established for stability comparison. Numerical simulations reveal that caverns with high interlayer dip angles demonstrate low volume shrinkage, general deformation resistance and small equivalent strain, while exhibiting a larger volume of plastic zones compared to caverns with lower interlayer dip angles. It is important to consider the deformation of the cavern wall rock in the down-dip direction for caverns with high interlayer dip angles. The presence of inclined interlayers affects the symmetry of the contour distribution of deformation, safety factor and equivalent strain. Furthermore, recommendations are provided for the construction of salt caverns in salt rock formations with high interlayer dip angles.

True triaxial modeling test of high-sidewall underground caverns subjected to dynamic disturbances

Seismicity resulting from the near- or in-field fault activation significantly affects the stability of large-scale underground caverns that are operating under high-stress conditions. A comprehensive scientific assessment of the operational safety of such caverns requires an in-depth understanding of the response characteristics of the rock mass subjected to dynamic disturbances. To address this issue, we conducted true triaxial modeling tests and dynamic numerical simulations on large underground caverns to investigate the impact of static stress levels, dynamic load parameters, and input directions on the response characteristics of the surrounding rock mass. The findings reveal that: (1) When subjected to identical incident stress waves and static loads, the surrounding rock mass exhibits the greatest stress response during horizontal incidence. When the incident direction is fixed, the mechanical response is more pronounced at the cavern wall parallel to the direction of dynamic loading. (2) A high initial static stress level specifically enhances the impact of dynamic loading. (3) The response of the surrounding rock mass is directly linked to the amplitude of the incident stress wave. High amplitude results in tensile damage in regions experiencing tensile stress concentration under static loading and shear damage in regions experiencing compressive stress concentration. These results have significant implications for the evaluation and prevention of dynamic disasters in the surrounding rock of underground caverns experiencing dynamic disturbances.

Integrating 1D and 3D geomechanical modeling to ensure safe hydrogen storage in bedded salt caverns: A comprehensive case study in canning salt, Western Australia

The viability of hydrogen storage in bedded salt caverns hinges on understanding the geomechanical challenges posed by the anisotropic stress states and complex geology of such environments. This study presents a comprehensive geomechanical analysis focusing on a proposed cavern within the Carribuddy Formation in Western Australia, characterized by its interbedded salt layers. This paper introduces a new geomechanical workflow, encompassing 1D and 3D modeling techniques to provide detailed changes of mechanical properties and stress state in interbedded salt formation allowing to identify the initial optimal operational pressures for underground hydrogen storage. Initial 1D models evaluated mechanical properties and in-situ stresses, while subsequent 3D simulations, enriched by data from neighboring wells, detailed the stress, strain, and displacement responses of the cavern walls to internal pressure changes. The analysis pinpointed an initial safe gas pressure range between 3000 and 4000 psi, attributing this margin to the robust characterization of the mechanical and in-situ stress of the formation. Our findings underscore the significance of high-resolution geomechanical modeling in identifying initial optimal operational pressures for hydrogen storage in salt caverns, ensuring both safety and structural integrity.

Cavern spacing of CAES cavern group in hard rock based on plastic zone and deformation analysis

Existing research on layout parameters of compressed air energy storage (CAES) cavern groups in hard rock has indicated that the key influential factors for group stability include buried depth, diameter, and cavern spacing. Among these, in contrasting to other factors, the research on cavern spacing is limited. This paper focuses on the issue of safe spacing and proposes a numerical method to determine it. By establishing a series of finite element models, the stress state of surrounding rock, changes in shape, equivalent plastic strain of plastic zone, and cavern wall deformation under different spacing have been studied. When a group of air storage caverns in rows is subjected to high internal pressure, for a small spacing, the plastic zone of the surrounding rock between two caverns is connected. Here, the size of the plastic zone, equivalent plastic strain value, and deformation of the cavern wall are relatively large, making the surrounding rock highly susceptible to damage. As the spacing increases, the horizontal width of the plastic zone, the maximum equivalent plastic strain value, the displacement changes of the cavern wall remain stable. This spacing is virtually the same as that when the plastic zone of the surrounding rock is no longer connected and can be selected as a safe spacing. This study explores the design of cavern safe spacing of underground CAES cavern groups and discusses several influencing factors, which can provide references for follow-up energy storage engineering and research.

Allowable Pillar Width for Salt Cavern Gas Storage Based on Triangular Well Layout: A Case Study in China

Salt rock, renowned for its remarkable energy storage capabilities, exists in deep underground environments characterized by high temperature and pressure. It possesses advantageous properties such as high deformability, low permeability, and self-healing from damage. When establishing a cluster of salt cavern gas storage facilities, the careful selection of ore column widths between these reservoirs is crucial for minimizing the risk of structural failure, optimizing salt rock resource utilization, and enhancing the construction and operation of gas storage reservoirs. In current practices, square triangular arrangements are commonly used in designing well layouts for reservoir groups to balance stability and economic considerations. This study, conducted in the context of the Jintan salt cavern gas storage project in Jiangsu Province, employed FLAC3D to create a finite element model for proposed gas storage configurations. A comprehensive analysis of the long-term operational safety of salt cavern gas storage with triangular well layouts was carried out. Various indices were examined, covering aspects such as cavern wall displacement, characteristics of the plastic zone, volume shrinkage, safety coefficients, seepage range, pore pressure fluctuations, and seepage volume. The study also considered the mechanical behavior of hexagonal columns within the surrounding rock during extended storage operations, leading to the optimization of allowable widths for these columns. The results indicate that, at operating pressures ranging from 6.5 to 17 MPa, the permissible column width should exceed 1.67 times the maximum cavern diameter to ensure compliance with criteria for long-term stability and containment within a square triangular layout. These findings provide valuable insights into determining the optimal allowable widths of salt cavern columns for positive triangular layouts.

Feasibility assessment of solution mining and gas storage in salt caverns: a case study of the Sanshui salt mine

The Sanshui salt mine is the sole location in the Guangdong province of South China with the potential to construct a salt cavern gas storage (SCGS) facility. Nevertheless, the gas storage construction of this mine faces significant challenges due to the presence of low‒grade salt deposits and numerous interlayers. To demonstrate the feasibility and calculate the gas storage capacity in this specific mining area, two representative salt caverns within this salt mine were simulated using a self-developed cavern-building simulation program, enabling us to accurately determine their respective volumes and shapes. Herein, the findings indicate that the combined caverns possess a total mining space volume of 1,157,000 m3, with the brine space accounting for merely 291,800 m3 (representing 25.22% of the overall mining space), and an extensive sedimentary volume of 865,200 m3 is also observed (constituting approximately 74.78% of the total mining capacity). Fortunately, this study has revealed that the sediments exhibit a porosity exceeding 40% and possess favorable permeability; consequently, countermeasures have been proposed to enhance the gas storage capacity within the pore space of these caverns, and we also utilized FLAC3D software for numerical simulation calculations to compare the stability of the cavern under different conditions of sediment pore utilization by calculating the volume loss rate, cavern wall displacement deformation, and plastic zone distribution. Moreover, the proposed method is anticipated to double the caverns’ working gas volume, increasing it from 40 million m3 to nearly 80 million m3. On the other hand, the long-term stability of caverns is numerically assessed under different pore space utilization rates of the sediments. The results also indicate that the caverns’ volume shrinkage, plastic zones, and surrounding rock displacement remain within allowable limits during 30 years of gas storage operation. The primary problem in the subsequent phase lies in effectively achieving gas injection and brine removal from the pore space of sediments while devising a methodology to extend this method to other salt caverns within similar salt mine areas. Thus, this study provides theoretical and technical guidance for the establishment of gas storage in existing salt caverns in the Sanshui salt mine and in salt mines worldwide that share similar geological conditions.

Parameter design of the compressed air energy storage salt cavern in highly impure rock salt formations

Compressed air energy storage (CAES) salt caverns are suitable for large-scale and long-time storage of compressed air in support of electrical energy production and are an important component for realizing renewable energy systems. In this paper, the use of sediment voids in highly impure rock salt formations for CAES is proposed. The interaction between the insoluble sediment and the cavern wall is analyzed. A geomechanical model is established to optimize the design parameters of CAES salt caverns. The simulation results show that as the cavern height increases, the displacement decreases in the bottom sediment region and increases in the waist. As the internal air pressure increases, the safety factor increases, while the volume shrinkage and displacement decrease. The plastic zone and the strength utilization rate decrease, and the safety factor increases with increasing pillar width. Based on the above results, the long cylindrical cavern is recommended to have a height in the range of 260–280 m and a diameter of 80 m. The minimum internal air pressure is 10–12 MPa, and the maximum internal air pressure is 16–18 MPa. The pillar widths between caverns are approximately 2.0–2.5 times the cavern diameter. In this study, for a single cavern with a diameter of 80 m and a height of 260–280 m, the upper free volume (excluding sediment) and the lower sediment filled volume of a single cavern are approximately 4.6–5.0 × 105 m3 and 7.2–7.8 × 105 m3, respectively. The sediment voids used for storing compressed air increase by at least approximately 3.6 × 105 m3, which is approximately 0.8 times the upper free cavern volume. The results show that the CAES salt cavern in highly impure rock salt is feasible and has great potential.

Performance assessment of compressed air energy storage systems with and without phase change materials

In this study, two integrated hybrid solar energy-based systems with thermal energy storage options for power production are proposed, thermodynamically analyzed and comparatively evaluated. The first system uses an underground cavern to store compressed air energy. When electricity production is high during the day, a compressor set pressurizes the air and directs it to the storage unit. A heat transfer model is developed to determine temperature variations and heat losses through the cavern walls. During the insufficient solar radiation period, the compressed air inside the cavern is discharged to meet the energy needs. The second energy storage system employs a cascade latent heat storage approach to reduce heat dissipation within the cavern. The temperature of the compressed hot air gradually decreases as it passes through three-stage phase change material (PCM) units. The cascaded PCMs help reduce the temperature differences between the storage media and minimize the exergy destruction rates. Similar to the first energy storage option, the pressurized air is stored in an underground cavern. The compressed air is then discharged and passes through the latent heat storage medium in the energy recovery mode, eventually reaching the turbine inlet temperature. Finally, the high-pressure and high-temperature air drives the gas turbine and hence generator to generate electricity. The thermodynamic quantities, including energy and exergy efficiencies and exergy destruction rates, are determined for all system elements and comparatively assessed. Furthermore, a comprehensive evaluation of the thermodynamic performance criteria of these energy storage options is carried out.

Design of a mobile neutron spectrometer for the Laboratori Nazionali del Gran Sasso (LNGS)

Environmental neutrons are a source of background for rare event searches (e.g., dark matter direct detection and neutrinoless double beta decay experiments) taking place in deep underground laboratories. The overwhelming majority of these neutrons are produced in the cavern walls by means of intrinsic radioactivity of the rock and concrete. Their flux and spectrum depend on time and location. Precise knowledge of this background is necessary to devise sufficient shielding and veto mechanisms, improving the sensitivity of the neutron-susceptible underground experiments. In this report, we present the design and the expected performance of a mobile neutron detector for the LNGS underground laboratory. The detector is based on capture-gated spectroscopy technique and comprises essentially a stack of plastic scintillator bars wrapped with gadolinium foils. The extensive simulation studies demonstrate that the detector will be capable of measuring ambient neutrons at low flux levels (∼10-6 n/cm2/s) at LNGS, where the ambient gamma flux is by about 5 orders of magnitude larger.

The Deformation Law of a Soft-Rock Cavern by Step Excavation in a Pumped Storage Power Station

The surrounding rock stability of large underground caverns in a pumped storage power station is one of the most crucial problems in hydropower project design and construction. In the construction of hydropower projects in Southwest China, many underground soft-rock caverns in are excavated. Influenced by the high sidewall, high ground stress, large burial depth, and hydrodynamic pressure action, the deformation of the cavern is special, especially in terms of its soft-rock distribution. At present, most research of underground engineering on soft-rock stability focuses on small-scale tunnel excavations in homogeneous geological conditions, with limited studies on the large-scale excavation of deeply buried underground powerhouses in complex geological structures, featuring extensive soft-rock-layer exposure on the cavern wall. Therefore, it is particularly important to investigate the excavation method of and support measures for soft-rock caverns under complex geological conditions. Based on laboratory rock mechanics testing (petrographic analysis, uniaxial compressive strength tests, shear tests, and triaxial compression creep tests) and the distribution characteristics of the surrounding soft-rock layer of the proposed underground powerhouse, obtained from the survey, we discuss the excavation and support measurements. These include the influence of support measures on the deformation of the underground excavated cavern considering the inclination of rock layers, the impact of the excavation under supported conditions on the deformation of the underground cavern, and the correlation between the lining thickness and stress variation within the lining.

Research on the Attenuation of Blast Wave by Groups of Filled Cracks in Surrounding Rock of Cavern

This study introduces an approach for constructing groups of filled cracks in the surrounding rock to achieve anti-blasting and wave elimination protection for underground caverns. To verify the attenuation effect of the explosion wave by the filled cracks, two models, a no-crack model, and a filled-crack model, were constructed using a certain proportion of analogue materials based on the similarity theory. With the implementation of small equivalent explosion conditions, the peak pressure of the surrounding rock of the cavern and the vibration acceleration of the cavern wall under different explosion conditions were measured. To accurately judge the attenuation efficiency of the filled cracks, the peak pressure attenuation percentages between different measuring points were calculated. Furthermore, the corresponding numerical simulation analysis process was performed according to the test conditions. The results show that not only the peak pressure attenuation percentages, but also the value of vibration acceleration, were significantly reduced due to the existence of filled cracks in the model. This indicates that filled cracks in the surrounding rock of the cavern can effectively attenuate the peak pressure and vibration acceleration caused by the explosion wave, thereby achieving the protection engineering goal of anti-explosion and wave elimination.

Causes of Cold Condensation and Durability Countermeasures of Concrete Structures in Caverns

Moisture condensation often occurs in the cave wall in service, which results in the deterioration of the internal environment of the cavern and the decline of its functionality and durability. In this paper, the corrosion mechanism of concrete structure caused by the dew-condensation inside the cavern wall is analyzed, the daily temperature and humidity environment change inside and outside the cavern is measured, the influence of wind speed and ventilation condition on the dew-condensation inside the cavern is discussed, and the change rule of the external atmosphere temperature, the air temperature and humidity inside the cavern wall temperature is discussed. The results show that it is an inevitable physical phenomenon that the internal temperature of the cavern is lower than the dew point temperature, and the measures to deal with cold condensation such as sealing the cave with dry gas when no one is around, filling the cavern with air of appropriate temperature and humidity when there are people, and setting up dehumidification system are effective ways to improve the durability of the concrete structure of the cavern.

Numerical and experimental investigations of concrete lined compressed air energy storage system

Compressed air energy storage (CAES) is considered one of the critical technological approaches to bridging the gaps between clean electricity production and electricity demand. An in-situ air storage test in a shallow buried underground cavern was introduced to understand better the connection and mutual influence between aerothermodynamics and cavern safety stability in various aspects of CAES. Moreover, a corresponding finite element model of the multilayer cavity structure considering the multi-field coupling effect and rebar embedding was developed to verify the experimental results. The experimentally measured temperature, air pressure, displacement, and stress are in good agreement with the calculated results, showing the success of the modeling in this paper. The cavern temperature shows large fluctuations for the whole storage phase. The maximum temperature of the air and the cavern walls are 53 °C and 46 °C, respectively. After water circulation heat exchange, the temperature can be regulated within a reasonable range (<40 °C). The cavity walls can reverse the heat output of the air through heat exchange during the discharging phase resulting in a slower temperature drop. The concrete stress is about 1/10 of the reinforcement stress, showing that the deformation between the rebar and concrete is coordinated, and both are in the elastic deformation stage. The sealing layer, reinforced concrete lining, and rock surrounding share the internal air pressure, accounting for 2.07%, 27.55%, and 70.38%, respectively. The surrounding rock is the most important part of bearing the internal pressure, and it is an essential means to improve the safety of the cavity by carrying out anchoring and grouting of the surrounding rock. The thermodynamic processes and stress response are affected by the convection heat transfer coefficient, thermal conductivity, charging time, and leakage rate.

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.

Stability Analysis of a Typical Salt Cavern Gas Storage in the Jintan Area of China

Using underground space to store natural gas resources is an important means by which to solve emergency peak shaving of natural gas. Rock salt gas storage is widely recognized due to its high-efficiency peak shaving and environmental protection. Damage and stress concentrations inside the cavern injection during withdrawal operations and throughout the storage facility life have always been among the most important safety issues. Therefore, accurate evaluation of the stability of rock salt gas storage during operation is of paramount significance to field management and safety control. In this study, we used the finite element numerical analysis software Flac3D to numerically simulate large displacement deformations of the cavern wall during gas storage—in addition to the distribution of the plastic zone of the rock around the cavern and the surface settlement—under different working conditions. We found that the maximum surface settlement value occurred near the upper part of the cavern. The surface settlement value increased as a function of creep time, but this increase leveled off, that is, a convergence trend was observed. The value was relatively small and, therefore, had little impact on the surface. The application of gas pressure inhibited the growth of the plastic zone, but on the whole, the plastic zone’s range increased proportionally to creep time. For the 20-year creep condition, the deformation value of the cavern’s surrounding rock was large. Combined with the distribution of the plastic zone, we believe that the cavern’s surrounding rock is unstable; thus, corresponding reinforcement measures must be taken.

Compaction and restraining effects of insoluble sediments in underground energy storage salt caverns

Compared with the salt domes formed by marine deposits abroad, the salt formations in China have bedded strata of lacustrine deposition, which contain rock salts and nonsalt interlayers. During the leaching phase, rock salts will dissolve to form a cavern that is used for storing oil or natural gas, whereas the nonsalt interlayers will soften and detach from the cavern walls, accumulating to the cavern bottom. These sediments will restrain the cavern walls and increase the working capacity because of the pore space in the sediments. Therefore, the stress and porosity of the sediments are key parameters for the assessment of the compaction and restraining effects of insoluble sediments. In this work, a mechanical element model of the sediments is proposed to predict the stress and porosity of the sediments in a cylindrical salt cavern. The depth of the sediments is introduced to analyze the compaction effect. The influencing factors of the equations of stress and porosity are then discussed using different friction coefficients, lateral stress coefficients, and hydraulic radii. To investigate the restraining effect of the sediments on the stability of the salt cavern, coupled numerical simulations are carried out using the discrete-continuous coupled method. Comparing the numerical simulation results of the salt cavern with and without sediments, the porosity of the sediments decreases, and the effective stress increases with creep time. The increasing rates of deformation and shrinkage gradually decrease because of the presence of sediments, which is favorable to improving the stability of the salt cavern. The numerical simulation results of the salt cavern with different variables indicate that the shrinkage of the cavern and porosity of the sediments are not sensitive to the sediment density, ball friction, or wall friction. This study can provide a reference for predicting the stress and porosity of the sediments and for investigating the stability of salt caverns with sediments.

Measurements of gamma ray, cosmic muon and residual neutron background fluxes for rare event search experiments at an underground laboratory

Ambient radiation background contributed by the penetrating cosmic ray particles and the radionuclides present in the rock materials have been measured at an underground laboratory at Jaduguda, Jharkhand, India, located inside a mine at 555 m depth. The laboratory is being set up to explore rare event search processes, such as direct dark matter search, neutrinoless double beta decay, axion search, supernova neutrino detection, etc., that require specific knowledge of the nature and extent of the radiation environment in order to assess the sensitivity reach and also to plan for its reduction for the targeted experiment. The gamma ray background, which is mostly contributed by the primordial radionuclides and their decay chain products, have been measured inside the laboratory and found to be dominated by rock radioactivity for Eγ≲3MeV. Shielding of these residual gamma rays for the experiment was also evaluated. The cosmic muon flux, measured inside the laboratory using large area plastic scintillator telescope, was found to be: (2.257±0.261±0.042)×10−7cm−2s−1, which agrees reasonably well with simulation results. The neutron background flux has been measured for the radiogenic neutrons and found to be: (1.61±0.03)×10−4cm−2s−1 for no threshold cut. Detailed GEANT4 simulation for the radiogenic neutrons and the cosmogenic neutrons have been carried out. Effects of multiple scattering of both the types of neutrons within the surrounding rock and the cavern walls were studied and the results for the radiogenic neutrons are found to be in reasonable agreement with experimental results. Neutron fluxes contributed by those neutrons of cosmogenic origin have been reported as function of the energy threshold.

Stability analysis for compressed air energy storage cavern with initial excavation damage zone in an abandoned mining tunnel

Compressed air energy storage (CAES) is a buffer bank for unstable new energy sources and traditional power grids. The stability of a CAES cavern is a key issue to cavern safety. However, the stability of a cavern from an abandoned mining tunnel has not been well studied. This paper investigates the stability of the CAES cavern from an abandoned mining tunnel. Firstly, an initial heterogeneous excavation damage zone (EDZ) in the surrounding rock mass is partitioned based on mining and operation processes. Then, a thermal-hydraulic-mechanical coupling model is formulated for the CAES cavern with a sealing layer and implemented within COMSOL Multiphysics. After verification with available data in literature, this coupling model is used to investigate the influence of partitioned EDZ heterogeneity on the displacement and stability of cavern. It is found that the displacement at the cavern wall increases with operation cycle. The principal stress decreases with operation cycle in the high-damage zone but remains stable in the low-damage zone. The principal stress difference changes first and then becomes stable in the disturbance zone. Larger cavern radius, heat transfer coefficient, initial pressure of cavern, temperature of injected air, and faster gas injection rate induce bigger displacement. These results on the stability of a CAES cavern can be a basis for the repair of an existing cavern.

Hydro-mechanical continuum modelling of fluid percolation through rock salt

Salt caverns for storing renewably produced hydrogen underground pose a promising option to reduce CO2 emissions by improving the utilization of volatile renewable energy sources. Rock salt limits the infiltration of hydrogen into the storage cavern walls due to its favourable low permeability. However, pressure-driven percolation may create discrete pathways for the fluid and damage the cavern. Depicting this effect accurately in simulations may be crucial for assessing the long-term stability and integrity adequately. By introducing an extension to a deformation-dependent permeability model by Alonso, Olivella and Arnedo (2006) we can capture the non-linear feedback between salt deformation, permeability development and fluid migration using continuum models. Based on a quasi-isotropic arrangement of potential flow paths the model is able to reproduce pressure-driven percolation of hydrofrac experiments on rock salt specimens in terms of breakthrough pressure as well as preferential path orientation and locality. The model is implemented in the scientific open-source framework OpenGeoSys.

Inverse Simulation for Salt Cavern Wall Collapse during Cycling Loading: A Field Case Study

Salt cavities used as underground gas storage have received increased attention for serviceability and sustainability considerations during gas cycling loading. However, traditional stability analysis overlooks the thermal effects induced by the operation and cannot predict the potential risk. In a field case in Jintan, Jiangsu Province of China, the thermal response to gas injection-and-withdrawal process of the walls of Cavern L was explored. A Bonded-Block Model (BBM) based on a 3DEC platform is proposed in inverse simulation for the field case’s cavity collapse process. By comparing the field case with simulation results, the proposed thermal-mechanical coupling model is validated for predicting the risk of cavity instability. The simulation results show that the thermal effect induces the collapse of the surrounding rock, that is, thermal tensile stress causes thermal cracks on the surface of the cavity.