Bedded Salt Formations Research Articles

Gas tightness around salt cavern gas storage in bedded salt formations

Underground salt cavern storage has become the preferred medium for storing gas energy and strategic substances. Salt caverns are suitable for storing small molecular gases due to the low porosity and permeability of salt rocks. This paper comprehensively analyzes the physical properties of four gases - hydrogen, helium, methane, and carbon dioxide - under subsurface temperature and pressure conditions. It categorizes the flow regimes of these gases in salt rocks under geological pressure conditions based on the Knudsen number. In a typical 1000 m salt cavern, the predominant permeation flow regime of four gases in the surrounding rock is Klinkenberg flow, with helium potentially undergoing transitional flow at low operation pressures. A 3D numerical model is established for an actual salt cavern to compare the permeation and leakage characteristics of these gases within salt rock formations. Results indicate that, under identical operation conditions, the permeation range of the gases decreases in the following order: hydrogen > methane > helium > carbon dioxide. Under the cyclic operation pressures, the cumulative leakage amount of hydrogen, helium, and methane increases over time, while that of carbon dioxide initially rises and then decreases. This behavior is attributed to the fact that the reverse-permeation rate of carbon dioxide at low pressures exceeds its permeation rate at high pressures. Over 30 years cyclic operation, the leakage ratios of the gases are as follows: hydrogen (13.29 %), methane (9.34 %), helium (7.47 %), and carbon dioxide (0.93 %), with hydrogen exhibiting the highest and carbon dioxide the lowest leakage ratios. Larger permeability results in a larger permeation range, while larger porosity leads to a smaller permeation range. When the permeability of salt layer is greater than 1e-20 m2, the permeation range of hydrogen significantly increases. Gas leakage ratios increase with permeability nonlinearly and increase with porosity linearly. The impact of salt layer permeability on leakage ratios is greater than that of porosity. This study provides crucial guidance for the selection of geological formations for storing hydrogen, helium, methane, and carbon dioxide in salt caverns, as well as the investigation of gas permeation characteristics in salt layers.

Mixed Potential Sensors for the Detection of Hydrogen Leaks from Geologic Reservoirs

Large-scale hydrogen (H2) geologic storage presents a viable alternative to surface storage that is both safer and more economical. One option being considered by DOE is H2 storage in salt formations. These formations have been successfully used for gas storage for decades due to their extremely low permeability and may see significant investment as the US ramps up its R&D efforts in this field. New Mexico has a unique advantage in that bedded salt formations are located strategically in areas where excess energy production can be harnessed for hydrogen production. However, bedded salt is more permeable than domal salt, necessitating sensor deployment to monitor H2 containment. Robust sensors that can provide early leakage detection will be an essential component of the safety of H2 operating facilities. In a LANL-funded Mission Foundations Research (MFR) project, we prepared several zirconia-based mixed potential hydrogen sensors utilizing a tin-doped indium oxide electrode for H2 leak detection to test its stability in subsurface environments. Previous electrochemical sensor work has shown this type of sensor in a hydrogen safety sensor role represents a significant improvement over currently available sensors as it has an extremely stable baseline and limited cross sensitivity to other gaseous compounds.1 The feasibility of this technology will be demonstrated through a hydrogen permeation experiment to measure rates of underground hydrogen permeation between boreholes drilled underground at the DOE Waste Isolation Pilot Plant (WIPP) site. This experiment will (1) establish the use of this novel sensor in geologic H2 storage applications and (2) investigate the feasibility of H2 storage in bedded rock salt. Acknowledgement This research is supported by the Los Alamos National Laboratory, Laboratory Directed Research and Development (LDRD) Office, Missions Foundation Research Charter (Debora Wagner Program Manager). References E. L. Brosha, C. J. Romero, D. Poppe, T. L. Williamson, C. R. Kreller, R. Mukundan, R. S. Glass, and A. S. Wu, “Field Trials Testing of Mixed Potential Electrochemical Hydrogen Safety Sensors at Commercial California Hydrogen Filling Stations,” Journal of the Electrochemical Society 164 (13) B681-B689 (2017).

Modeling and Parameter Optimization of Multi-Step Horizontal Salt Cavern Considering Heat Transfer for Energy Storage

Horizontal salt caverns represent a prime choice for energy storage within bedded salt formations. Constructing multi-step horizontal salt caverns involves intricate fluid and chemical dynamics, including salt boundary dissolution, cavern development, brine flow, heat transfer, and species transportation. In this paper, the influence of heat transfer and turbulent flow is considered in developing a 3D multi-physics coupled flow model for the construction of multi-step horizontal salt caverns. The feasibility and accuracy of the model are verified by comparisons with the field data of the Volgograd horizontal salt cavern. The effects of turbulent flow and heat transfer on the dissolution process are thoroughly analyzed. By analyzing the characteristics of the flow field, the brine concentration distribution, and cavern expansion, the results indicate a steady rise in cavity brine concentrations throughout the leaching phases, with the previously formed cavities continuing to enlarge during subsequent leaching stages, albeit at a diminishing rate of expansion. Furthermore, the results reveal that a larger injection flow rate results in a larger cavern volume, whereas higher injection concentrations result in smaller cavern volumes. While the step distance has a minimal impact on cavern volume, identifying the optimal step distance remains crucial. This analysis of construction parameters aims to provide valuable insights into the design and engineering practices involved in developing multi-step horizontal salt caverns for energy storage purposes.

Research on the helium seepage mechanism in the strata surrounding bedded salt cavern helium storage and its tightness evaluation

To improve the safety and scale of helium storage, large-scale or strategic helium storage in underground salt caverns can be implemented. Salt caverns leached from bedded salt formations have been considered as potential underground storage sites for helium in China. However, the salt formations are typical lacustrine sedimentary rocks with an interbedded structure, thus the sealing feasibility of such salt caverns must be evaluated before they can be used. To this end, the helium seepage equation in deep strata and permeability evolution function are derived with the help of experimental results and theoretical analysis. Based on these, a helium seepage model considering slippage effect and threshold pressure gradient is proposed to evaluate the sealing properties of salt caverns for helium storage. The analysis results indicate that it is feasible to use bedded salt mine formations for helium storage when the formations permeability parameters and reservoir design parameters meet the long-term tightness evaluation criteria, including permeability, porosity, salt pillar width, salt roof thickness and operating pressure and duration. More importantly, specific recommendations are provided for the operation parameters, design parameters and formation permeability thresholds of salt caverns for ensuring helium storage safety while maintaining stable cavern operations. The threshold permeability of the target salt formations should be approximately 1 × 10−18 m2 to ensure tightness. The minimum and maximum operating pressure are recommended to be 10 MPa and 18 MPa respectively for 50 years of this helium storage. The salt pillar width should be 300 m, and the salt roof thickness should be 40 m.

Analysis of pressure interval/injection and production frequency on stability of large-scale supercritical CO2 storage in salt caverns

Carbon dioxide (CO2) is the primary greenhouse gas, with annual emissions reaching up to 54 billion tons globally, leading to an unprecedented acceleration of global warming. In conjunction with carbon capture, utilization, and storage (CCUS), which represents the most advanced and efficient technology for energy conservation and emission reduction, as well as the innovative concept of CO2 storage in salt caverns, this paper investigates the industrial application potential of supercritical CO2. Herein the feasibility of large-scale storage of supercritical CO2 in salt caverns was investigated, focusing on the stability aspect associated with such storage. Specifically concentrating on the impact of upper and lower injection/production pressures and frequencies on the stability of carbon storage in salt caverns, this study utilizes numerical simulation methods to simulate the creep process that occurs in China's bedded salt formations. Numerical simulation results showed that: 1) From the perspective of salt cavern stability, the volume shrinkage, creep and damage degree of surrounding rock during operation all fall within an acceptable range. Consequently, it is feasible to implement carbon storage in these salt caverns for a during of 100 years. 2) Shortening the range of internal pressure will enhance the stability of salt cavern for carbon storage. Furthermore, different pressure intervals have an impact on the creep concentration position within the cavern, primarily resulting in a transformation between internal contraction of surrounding rock and sediment accumulation caused by floor creep by altering the lower pressure limit. 3) The injection and production frequency of salt cavern carbon storage (SCCS) is characterized by a low occurrence rate (<10 cycles). The stability of the cavern can be enhanced by increasing the injection and production frequency, nevertheless, it is important to note that internal pressure remains the primary factor influencing cavern stability compared to the injection and production frequency.

Stability evaluation of salt cavern hydrogen storage and optimization of operating parameters under high frequency injection production

Hydrogen energy storage plays a crucial role in peak shaving of renewable energies, and underground salt caverns are ideal locations for large-scale hydrogen storage. The widespread utilization of hydrogen leads to wide-ranged injection production frequency (F) for salt cavern hydrogen storage (SCHS). To address the hydrogen demand under various scenarios, numerical simulation was employed to demonstrate the influence of F on the stability of SCHS at different buried depth (D), and the key operating parameters under high-frequency injection and production were optimized. A 3D geomechanical model was established for bedded salt formations, and evaluation criteria were proposed to quantitatively characterize the stability of SCHS. The stability of SCHS initially improves and then deteriorates with F at different D, and the optimal F is 6 times/year. The effect of F will be boosted as D increases. The recommended internal pressure (P) of SCHS at 900, 1700 and 2500 m under high-frequency injection and production is (0.25–0.85)σz, (0.35–0.85)σz, (0.5–0.85)σz MPa, respectively. And when P is reasonable, the pillar width (W) at different depths greater than 2 times the maximum diameter of salt cavern can meet the stability requirements. The stress difference and stress ratio of the surrounding rock increase with D, causing an increase in volume shrinkage and safety factor. The changes in volume shrinkage and plastic volume correspond well with the change of P when F is less than 6 times/year, while then it ceases to exist. The volume shrinkage growth model was established, and it quantitatively indicates that the small or large F is unfavorable to the stability of SCHS. The variation in the SCHS stability with F attributes to the different effects of creep and fatigue of salt rock.

Stability and optimization of small-spacing two-well (SSTW) gas storage salt caverns in bedded salt formation

Many rock salts in China have a bedded structure, consisting of several thin interlayers and salt layers. To eliminate the limitations of the conventional single-well gas storage caverns, the small-spacing two-well (SSTW) salt caverns can be utilized. In this paper, under the geological conditions of Pingdingshan salt formation in Henan province, China, a geomechanical model was established to evaluate the stability of the SSTW salt cavern. The deformation, volume shrinkage rate, and safety factor were used for the cavern stability evaluation. The optimal cavern roof shape, two-well spacing, and cavern roof height were determined by analyzing the salt cavern stability under constant pressure conditions. Based on the optimal salt cavern morphology, the stability of the salt cavern and interlayers under various injection and withdrawal cycle frequencies are investigated. The results show that the rampant arch-shaped SSTW salt cavern has good feasibility in Pingdingshan salt formation. The cavern roof vertical displacement and volume shrinkage decrease with the increase of cavern roof height, and it is suggested that the ratio of cavern roof height to cavern height is at least 0.26. With the continuous increase of the two-well spacing, the cavern volume shrinkage also increases. It is recommended that the ratio of the two-well spacing to the single cavern radius should be no more than 0.625. The increase in cycle frequency reduces the duration of salt caverns under low-pressure conditions, thus improving the mechanical stability of the cavern and interlayers. This paper provides design and operation parameters for the SSTW salt cavern in Pingdingshan salt formation and can also be a reference for the construction of other SSTW salt caverns.

Hydrogen tightness evaluation in bedded salt rock cavern: A case study of Jintan, China

Hydrogen is regarded as one of the most important energy sources for the future. Safe, large-scale storage of hydrogen contributes to the commercial development of the hydrogen industry. Use of bedded salt caverns for natural gas storage in China provides a new option for underground hydrogen storage (UHS). In this study, the physical properties of multicomponent gases in UHS and salt rock are reviewed and discussed, along with the flow of hydrogen in the surrounding salt rock. Mathematical models of the two-phase multicomponent flow of the gas–brine system in the UHS were established. A numerical model of a simplified elliptical salt cavern was built to simulate the migration of the gas–brine system in the UHS. The hydrogen tightness of the UHS was evaluated through simulation with different storage strategies, salt rock and interlayer permeabilities, and gas components. The results indicate that: (1) Cyclic injection and withdrawal facilitate hydrogen leakage, which is accelerated by increasing the frequency. (2) The huff-n-puff of hydrogen gas in the injection and withdrawal cycles forces the gas into pore space and enhances the relative permeability of the gas phase. The migration of hydrogen and brine weakens the hydrogen tightness. Brine saturation is an important index for evaluating the hydrogen tightness of UHS. (3) The leakage rate of UHS increases with an increase in the permeability of the salt rock and interlayer and the total thickness of the interlayers. The average permeability Kwa weighted by the thickness of layers for the bedded salt formation is proposed to integrate three variables to facilitate field application of the simulation results. The critical Kwa is less than 3.02 × 10−17 m2 if the recommended annual hydrogen leakage rate is less than 1%. (4) The difference between hydrogen and other gas species is another important factor in the leakage rate and should be considered. This study provides theoretical guidance for evaluating the feasibility of UHS in salt caverns and site selection in China.

Stability and economic evaluation of multi-step horizontal salt caverns with different step distances in bedded salt formations

A multi-step horizontal salt cavern (MSHSC) for energy storage has been drawing more attention, using retractable water-injection tubes to mine larger storage spaces between two boreholes. The retract distance per step best determines the shape of the cavern roof, and thus has an important influence on the stability of MSHSCs. This paper discusses the impact of the step distance on the stability and economy of MSHSCs. The geometry of MSHSCs with different step distances is obtained using our previously developed solution mining model. Geo-mechanical models are developed based on the geometry and the geo-conditions of Jintan salt mine in China. Then a series of static creep analyses are performed using FLAC3D software. Results show that smaller step distances mean higher stability risks during long-term operation. As the step distance is reduced, both the volume shrinkage and plastic zone volume increase. In the displacement contours after 30 years of operation, the large displacement zone increases significantly with decreasing step distance. However, the cavern volume and expected revenue increase with decreasing step distance, contrary to the stability parameters. Economic analysis with integrated stability considerations is conducted to give a balanced step distance. Volume shrinkage is considered in the calculation of effective cavern volume. Plastic zone volume is reflected in the space cost of MSHSCs, since it influences the thickness of the cavern top plate and the pillar width between adjacent caverns. Further, a comprehensive indicator of economic cost/expected revenue per unit volume of rock salt versus time is proposed, calculated, and compared. According to the comparison results, a medium step distance of 100 m or 125 m is recommended for the design of MSHSCs in a given salt layer, with the stability and economic considerations best balanced.

Research on anchoring effect and failure mechanism of hard interlayers in the cavern pillar of underground gas storage in bedded salt formations

The stability and tightness of pillars are the key to ensure the safe operation of the salt cavern group for underground gas storage (UGS). Even in the case of a wide pillar, the hard interlayer in the center of the pillar may still produce plastic collapse early in the operation. In this research, referring to the mechanism of full-length grouting-bonded bolts and surrounding rock, it is speculated that there is a similar effect between hard interlayer and rock salt in the pillar. After analyzing the distribution law of shear stress at the interface and the distribution of additional radial stress in hard interlayer, it is concluded that the hard interlayer has an anchoring effect on the pillar, and the surrounding rock salt has a tensile force on the hard interlayer. The pulling effect on the hard interlayer reduces the radial stress in the center of the pillar, which is the direct cause of the plastic collapse here. Finally, through the above -mentioned research, it can be concluded that the hard interlayer in the center of the pillar can be prevented from being damaged or even penetrated by two ways: arranging the interlayer far from the center of the cavern and avoiding the interlayer being too thin.

Tightness evaluation and countermeasures for hydrogen storage salt cavern contains various lithological interlayers

Large-scale energy storage methods are required for shaving peak of renewable energy. Because of the good compactness of salt rock, it is proposed to store hydrogen in salt rock caverns to solve this problem. China has abundant salt rock resources, which provide good choices for large-scale hydrogen storage in salt formation. But the salt formations of China are mainly of bedded salt rock, which contain interlayers with various lithologies and different permeability and porosity. Therefore, the tightness of hydrogen storage in different bedded salt formations of China should be investigated. In order to demonstrate the tightness performance of salt cavern hydrogen storage and provide references for site selection and storage management, a series of lab-experiments, theoretical and numerical simulations, and discussions were conducted. Firstly, the porosity and permeability of interlayers with different lithologies were tested and analyzed. Then, a geological model was established, five cases of hydrogen leakage simulation are designed, these cases stand for the typical bedded salt formations of China. In each case, the interlayers have different lithologies, permeability and porosity. The COMSOL Multiphysics software was used to simulate the hydrogen leakage of cavern under different cases, and the leakage range, pore pressure and leakage amount of hydrogen were analyzed. The results show that interlayers are the main positions of hydrogen leakage, and with the decrease of the interlayers' porosity and permeability, the tightness of cavern will be better. It is suggested that when hydrogen is stored in a single cavern, the permeability of interlayers should be less than 1E-17 m2, while when stored in a group of caverns, it should be less than 1E-18 m2. At the same time, a reflux zone is found in the surrounding rock of storage cavern, hydrogen in the reflux zone will flow back to cavern, which is not conducive to its operation. Leakage coefficient of the surrounding rock of cavern is defined as α, and it is proposed only when α ≤ 1.18E-20 m2, can the cavern meet the tightness requirements of hydrogen storage. Finally, some countermeasures are put forward for cavern utilization in bedded salt rock. This research provides guidance and basis for the site selection of hydrogen storage in salt caverns, and promotes the development of underground salt cavern energy storage in China.

The influence of cavern length on deformation and barrier integrity around horizontal energy storage salt caverns

Horizontal cavern structures are discussed as a means to constructing energy storage facilities in bedded salt formations. During their design, the spacing of injection and discharge boreholes for solution mining is an important variable determining cavern length, shape and construction cost, but also influencing the long-term mechanical behavior during the storage of oil or gas. Increasing the borehole spacing has been suggested in order to increase cavern capacity and construction rate while decreasing construction cost but its mechanical consequences have yet to be explored. In this paper, cavern geometries constructed with different borehole spacings are obtained using a solution mining simulation model. Static creep analyses have been conducted based on these geometrical models to discuss the impact of the maximum cavern length on the mechanical behavior of the caverns. The results show that the creep deformation of the horizontal cavern induces limited volume change depending on operating pressures. Increasing the length of the cavern initially increases cavern roof displacement and volumetric convergence, but plateaus after a length 600 m, i.e. 5 to 6 times the cross-sectional diameter. A similar behavior is observed in terms of selected integrity criteria. The results indicate the principal possibility to access larger storage volumes and higher construction speed by increasing the borehole spacing to longer than 5 to 6 times the designed cross-sectional diameter.

Over 30 years of research on crushed salt as a barrier material: fundamental findings and open questions

Abstract. Repositories for high-level radioactive waste in geological formations require knowledge on thermal, mechanical and fluid transport properties of the whole repository system, including the engineered barriers and backfill materials. For about 30 years, crushed salt has been considered the most suitable geotechnical barrier material to backfill cavities and encapsulate radioactive waste in rock salt repository sites (e.g., Czaikowski et al., 2020). Over time, when the surrounding cavity walls converge by the creep of salt, it can become strongly compacted and safely encapsulates radioactive waste from any fluid flow. Hence, crushed salt has been characterized in detail for its physical material properties and its response to environmental controls (stress, temperature and moisture). This characterisation provides a basis for long-term numerical simulations (e.g., Liu et al., 2018), which verify so-called safety cases in radioactive waste disposal. Displacement-controlled oedometric compaction tests mimic the long-term in situ behaviour of crushed salt. The tests show that it can be compacted to a state comprising physical rock properties similar to natural rock salt. In general, compaction is easier with an increase in humidity and temperature (e.g., Stührenberg, 2007; Kröhn, et al., 2017). Triaxial test series address the compactions' response to differing confining pressures and help to identify generalized constitutive equations for crushed salt. Both BGR procedures, the oedometric and the triaxial compaction, are verified by the German accreditation body (DAkkS). Figure 1 illustrates the history of oedometric tests at the BGR laboratory since 1993, which examined crushed salt from various origins and differing temperature conditions. Most tests focused on material from the Asse mine, revealing the compactions' response to the materials' humidity and to brine flow. Moreover, systematic test series with synthetic grain size distributions and bentonite additives provided a basis for barrier material design. More recent tests on bedded salt formations (e.g., Teutschenthal and Sondershausen mines) allow the differentiation from characteristics from domal salt deposits (e.g. Gorleben). The current research continues the history of oedometric and triaxial tests, but has a new focus on late compaction stages with marginal remaining porosities (&lt;5 %). The approach of systematic material characterization under best-controlled conditions essentially benefits from the international research collaboration in the KOMPASS project (Czaikowski et al., 2020). The aim of its current phase two is to synthetically generate, identify and quantify dominant grain-scale deformation processes in response to changes in environmental controls. Subsequently, these laboratory results will be embedded in numerical models on the long-term in situ rheology of crushed salt.

Suitability of flat bedded salt formations in Germany as the site for a repository for heat-producing radioactive waste

Abstract. The search for a site of a final repository for highly radioactive waste in Germany was renewed when the Site Selection Act (StandAG) came into force in 2013. In Germany the development of concepts for a final repository and safety analyses for a repository in a salt dome was prioritized for many decades, whereas repository concepts in clay and crystalline rock were first considered only two decades ago. The aim of a comparative site selection procedure is to find a site before 2031, which provides the best possible safety for the enclosure of highly radioactive waste over a time period of 1 million years. The fundamental safety-related consideration is the enclosure of toxic waste in a so-called containment effective geological area (ewG). The main aspect of a long-term proof of safety is logically the systematic proof of safe long-term enclosure of the deposited waste. The approach developed within the framework of appropriate research projects (e.g. Eickemeier et al., 2013) is essentially based on the proof of geotechnical integrity of the ewG as the fundamental geological barrier as well as the geotechnical barriers. Due to their unique characteristics, including imperviousness and plastic deformability, salt rocks have been used for decades in Germany and worldwide in mining and especially for energy storage. Whereas halite in salt domes (type steep​​​​​​​ inclined salt) is distributed particularly in northwest Germany, flatly deposited salt rock (type flat bedded salt​​​​​​​) dominates in middle Germany and salt pillows (type salt pillows) in parts of northeast Germany. Both types of “bedded salt”​​​​​​​ widely reflect in their lateral extension sedimentation-related deposition conditions, apart from diagenetically related alterations. Beginning with the presentation of the host rock-specific boundary conditions of the various rock salts, this article focuses on the appropriate procedures for the proof of integrity of the geological barrier rock salt, based on the available experiences, corresponding reference studies and analogous examples. In the results it is shown that repository concepts in bedded salt formations and especially in the constellation of salt pillows provide substantial safety-related advantages due to a site-specific multibarrier system with alternate deposition of salt and saliferous clay as well as an intact overlying rock covering.

Ground Subsidence Prediction Model and Parameter Analysis for Underground Gas Storage in Horizontal Salt Caverns

Due to a great demand of natural gas or oil storage in these years, horizontal caverns were proposed to fully use bedded salt formations of China. Under the same geological and operating conditions, the horizontal cavern would shrink more than traditional pear-shaped cavern, which might bring larger ground subsidence and affect the safety of storage facilities. A new prediction model was proposed in this paper for the time-dependent ground subsidence above horizontal caverns. The proposed model considered the impurity of bedded salt formations and simplified the horizontal cavern to an ideal cylinder. The shape of the subsidence trough was determined by the probabilistic integration method, and corresponding calculation formulas for the tilt, curvature, horizontal displacement, and horizontal strain were derived. Based on the assumption that the subsidence volume at the ground was proportional to the reduced volume of horizontal cavern, a formula for the reduced volume over time was established. FLAC3D was introduced to simulate the ground subsidence, and the results show that the proposed prediction model agreed well with the simulation results. Finally, the proposed prediction model was used to analyze the impacts of different stratigraphic parameters and design parameters. The results mainly show that, as the draw angle increases, the subsidence trough becomes deeper and narrower; as the depth of the cavern increases, the maximum subsidence first increases and then decreases, and the subsidence trough gradually becomes round; with the increase of the purity, the subsidence gradually decreases; with the increase of the creep properties and the stress exponential constant, the maximum subsidence first increases rapidly and then slowly approaches the limit; increasing the brine extraction velocity can shorten the cavern construction period and then reduce excessive ground subsidence; the subsidence decreases nonlinearly with the increase of internal pressure; with the increase of the cross section diameter and length of the horizontal cavern, the subsidence presents a significant nonlinear increase. In addition, unlike the traditional pear-shaped cavern, under the same conditions, the ground subsidence above the horizontal cavern according to this newly proposed model is much larger, and the ground subsidence contour line is no longer a standard circle. The findings of this study can help for better understanding of the prediction of ground subsidence above salt caverns and also provide a reference for the design and construction. However, the proposed prediction method is ideal and theoretical and should be further improved by engineering practice in the future.

Pillar stability of salt caverns used for gas storage considering sedimentary rhythm of the interlayers

In view of the low permeability and good damage self-healing properties of rock salt, salt caverns are thought to be ideal media to hold natural gas. However, widely distributed salt formations in China were formed bedded by lacustrine sedimentation with significant sedimentary rhythm characteristics. The various interlayers in bedded salt formations may cause instability of the cavern pillar due to their uncoordinated deformation, thus threatening the safety of the whole gas storage caverns. Combining theoretical analysis and numerical simulation, the influence of different characteristics of interlayers on the stability of salt cavern pillars were detailed analyzed. Some main conclusions were summarized as follows: 1) In a sedimentary rhythm, the rock salt and the soft interlayers can be considered as one category, and the hard interlayers can be treated as another category. Compared with the hard interlayers, rock salt and the soft interlayers contributes more to the shrinkage of salt cavern; 2) With the decrease of pillar width or the increase of operating time, the principal stress curves (both σ1 and σ3) in the interlayers along the line between the centers of the two caverns generally experience the process of "Bimodal type - Saddle type - Unimodal type"; 3) The entire plastic zone expansion process in hard interlayers of the cavern pillar shows the periodic law of "interfaces - middle interlayer - interfaces - middle interlayer"; 4) As to site selection of salt caverns for gas storage, bedded salt formations with too many interlayers should be avoided. If the interlayers cannot be avoided, the salt cavern should be constructed with interlayers located far away from the cavern center.

Physical simulation of flow field and construction process of horizontal salt cavern for natural gas storage

Underground salt caverns provide appropriate space for storing natural gas. However, salt caverns have to be constructed in bedded salt formations in China. The traditional construction method of salt cavern encounters serious problems in such formations. Instead, horizontal salt caverns are proposed as a possible alternative for gas storage. Horizontal caverns can be created by “multi-step retreating” (MSR) method. Two laboratory tests were conducted to understand the shape development and flow field of such cavern. In one test, we measured fluid velocity distribution in a simulated salt cavern environment by particle image velocimetry technology. In another test, the construction process of a salt cavern was simulated through MSR method. Results show that the flow field in horizontal cavern can be divided into five regions. The flow characteristics and rock salt dissolution properties in these regions are different. A semi-cylindrical cavern with similar cross section shape along the horizontal direction is obtained through the physical experiment. This study provides important guidance for analyzing the flow field and construction process of the horizontal salt cavern.

Assessment of the potential for underground hydrogen storage in bedded salt formation

Salt formations of an appropriate thickness and structure, common over the globe, are potential sites for leaching underground caverns in them for storage of various substances, including hydrogen. Underground hydrogen storage, considered as underground energy storage, requires, in first order, an assessment of the potential for underground storage of this gas at various scales: region, country, specific place.The article presents the results of the assessment of the underground hydrogen storage potential for a sample bedded salt formation in SW Poland. Geological structural and thickness maps provided the basis for the development of hydrogen storage capacity maps and maps of energy value and heating value. A detailed assessment of the hydrogen storage capacity was presented for the selected area, for a single cavern and for the cavern field; a map of the energy value of stored hydrogen has also been presented. The hydrogen storage potential of the salt caverns was related to the demand for electricity and heat. The results show the huge potential for hydrogen storage in salt caverns.

Stability of the bedded key roof above abandoned horizontal salt cavern used for underground gas storage

A large number of salt caverns by solution mining are abandoned widely. After reasonable reformation, some of these caverns are expected to store energies, such as petroleum, natural gas, and compressed air energy. Different from other countries, salt formations of China are mainly bedded with lower thickness. In order to improve the mining efficiency of bedded salt formations, solution mining with double-well convection is widely adopted, which brings more horizontal caverns with larger dimensions and also larger roofs. When these horizontal caverns are transformed into underground energy storage, the layered roofs are easily suffered from unbalanced deformation and failure, and then become the channels of oil or gas leakage. Therefore, a series of theoretical analysis and numerical simulation were carried out to reveal the failure mechanism and stability of the bedded roof, and then some meaningful conclusions are acquired. Firstly, in the overlying strata, the argillaceous anhydrite layer is the key roof in the selection of the abandoned horizontal cavern for energy storage. Secondly, according to plate theory, the invalid of the bedded key roof needs to be treated into failure and instability respectively. Thirdly, the analysis on the vertical and horizontal stress in the key roof shows that it does experience a second type of instability. Finally, in order to maintain the stability of bedded key roof, the main influence factors require careful and comprehensive consideration, including the length of the long side, the thickness of the protective salt layer, and the internal pressure.

Study on the mechanism of roof collapse and leakage of horizontal cavern in thinly bedded salt rocks

In the thinly bedded salt formations, due to the limited thickness of the salt strata along with the fast upward dissolution, the collapse of the non-salt roof of the cavern often occurs, leading to the leakage of brine and instability of the cavern as well as other undesirable geological consequences. To investigate the gypsum-salt interbedded roof collapse, the failure mechanism and the control methods of Dainan siltstone, which is the indirect roof of the horizontal salt caverns in Zhaoji Salt Mine, were thoroughly investigated. Through the drilled cores from this mine, a series of contrast tests such as SEM, EDS, XRD, soaking, and nuclear magnetic properties were carried out to study the physical properties of the salt layer and non-salt layer of the cavern roof and to determine the cavern roof collapse and leakage mechanism. Using the Comsol software, the model of a horizontal cavern roof leakage was established, and the leakage range and leakage of brine at different time were analyzed, as well as the safety problems caused by it. Finally, some suggestions have been provided for the leakage control of the cavern roof in bedded salt cavern.