Salt Formations Research Articles

Joint Geophysical and Numerical Insights of the Coupled Thermal‐Hydro‐Mechanical Processes During Heating in Salt

AbstractSalt offers an optimal medium for the permanent isolation of heat‐producing radioactive waste due to its impermeability, high thermal conductivity, and ability to close fractures through creep. A thorough understanding of the thermal‐hydrological‐mechanical (THM) processes, encompassing brine migration, is fundamental for secure radioactive waste disposal within salt formations. At the Waste Isolation Pilot Plant (WIPP), we conducted joint in situ geophysical monitoring experiments during active heating to investigate brine migration near excavations. This experiment incorporated electrical resistivity tomography (ERT) alongside high‐resolution fiber‐optic‐based distributed temperature sensing within a controlled heating experiment. Additionally, discrete element model (DEM) based numerical simulations were conducted to simulate THM processes during heating, providing a more mechanistic understanding of the coupled processes leading to the observed changes in geophysical measurements. During heating, resistivity shifts near the heater were reasonably explained by temperature effects. However, in more distant, cooler regions, the resistivity decrease exceeded predictions based solely on temperature. DEM simulations highlighted brine migration, propelled by pore pressure gradients, as the likely primary factor contributing to the additional resistivity decline beyond temperature effects. The comparison between the predicted ERT responses and observations was much improved when considering the effects of brine migration based on the DEM simulations. These geophysical and simulation findings shed light on brine migration in response to salt heating, enhancing our understanding of the coupled THM processes in salt for safe radioactive waste disposal.

APPLICATION OF MUMMIFIED TURK SALTY FIBER CONCRETE AS PROTECTION AGAINST SALTY DOMES AND DESERT INTRUSION

Turk salty fiber concrete is designed to isolate retaining walls against saline materials. In 2018, TSFC mixture design was applied to the salt domes in the GOTVAND reservoir dam, SE of Iran in order to isolate them enormous salt ridges were discovered inside the dam reservoir before construction operations began in 2005 which are also identifiable through the use of geotechnical maps. The designed mixture of TSFC contains salt stone (S), water (W), cement type II (CII), bentonite (B), clay (C) and cotton or synthetic fibers (F). In the laboratory, TSFC is produced taking into account the variable of saline water. The figure ζ is defined by the proportion of the weight of the salt and water over the weight of the combined mixture of cement, bentonite, clay and fibers. The maximum value ζ has been obtained as being a bit over 6.5 for laboratory samples. It means that more than 87% of the sample weight is related to saline water and 13% is related to other materials (CII, B, C and F). The axial load test is the main index to compare resistance with friction of salt formations. The results obtained show the axial resistance of broken TSFC samples to be more 12,000 KPA. SEM images were taken to study TSFC adhesiveness between the salt and fibers. The SEM showed cubic crystallization which in itself supports the hypothesis of salty mummification due to the integrating of the salt concrete and fibers.

Impact of the compaction behavior of crushed salt on the thermo-hydro-mechanical response of a generic salt repository for heat-generating nuclear waste

Salt formations are a promising host rock for the disposal of heat-emitting nuclear waste due to their ability to heal fractures, high thermal conductivity, and near-zero permeability. This study investigates the concept of waste emplacement in salt formations by placing waste packages in drifts and backfilling them with crushed salt. The aim is to minimize fracturing and restore the initial properties of the host rock.One of the key issues in assessing the safety and performance of nuclear waste repositories in salt is determining the reconsolidation rate of the crushed salt backfill, which has been traditionally predicted using the C-WIPP model. However, this model was calibrated using data from uniaxial oedometer lab tests, where specimens with high initial porosity values have been compacted to medium porosity values. The observed compaction behavior has then been extrapolated to the low porosity range, leading to a prediction of rapid full reconsolidation of the backfill in less than 20 years. In contrast, the new C-WIPP/TUC model used in this study was calibrated for the medium porosity range, between 17% and 8%, based on more suitable triaxial lab tests. Using coupled Thermo-Hydro-Mechanical (THM) simulations with the TOUGH-FLAC simulator, it was found that the C-WIPP/TUC model predicts a slower reconsolidation rate of the backfill compared to the C-WIPP model, with an average porosity of the backfill remaining roughly 10 times that of natural salt after 10,000 years. The effects of the relatively slow reconsolidation of the crushed salt backfill on the THM behavior of the repository are also examined, highlighting the importance of accurately capturing the behavior of crushed salt for the study of the long-term integrity of a nuclear waste repository in salt.

Laboratory experiments on rock salt and phenomenological observations

We conducted an experimental campaign on salt samples from three different geographic origins. Short-term strain-controlled tests and constant mean stress tests, as well as long creep tests were performed. For reliable rock characterization and constitutive modeling, the experimental results should be analyzed considering heterogeneity. This is particularly important for the volumetric strains and the dilatancy onset (an important design criterion of underground facilities in salt formations). Heterogeneity can be quantified by the use of different measurement techniques, capturing the sample response at different scales. Material heterogeneity, due to the presence of impurities, can explain the differences observed between local and global measurements that cannot be justified by the heterogeneity induced by sample-piston friction. The volumetric strains measured during the isotropic phase are not negligible compared to those of the deviatoric phase, and the analysis of the isotropic phase can explain some of the differences observed among tests performed under equivalent conditions. To reduce bias, all samples should undergo the same preconditioning process. Long creep tests performed at constant stress deviator and short-term tests show that the mean pressure effect on the deviatoric response is of the same order of magnitude than natural variability. Reverse creep, a transient phenomenon that could be active when the stress deviator decreases, has a smaller amplitude than the associated elastic drop, and can be analyzed not only during creep tests, but also during unloading–reloading cycles, which are characterized by a progressive load drop, more representative of in situ conditions.

Theoretical and Technological Challenges of Deep Underground Energy Storage in China

Deep underground energy storage is the use of deep underground spaces for large-scale energy storage, which is an important way to provide a stable supply of clean energy, enable a strategic petroleum reserve, and promote the peak shaving of natural gas. Rock salt formations are ideal geological media for large-scale energy storage, and China is rich in salt rock resources and has a major shortage of energy storage space. Compared with the salt domes in other countries, the salt rock formations in China are typical lacustrine bedded salt rocks characterized by thin beds, high impurity content, and many interlayers. The development of large-scale energy storage in such salt formations presents scientific and technical challenges, including: ① developing a multiscale progressive failure and characterization method for the rock mass around an energy storage cavern, considering the effects of multifield and multiphase coupling; ② understanding the leakage evolution of large-scale deep underground energy storage caverns; ③ understanding the long-term performance evolution of large-scale deep underground energy storage caverns; ④ developing intelligent construction technologies for the deep underground salt caverns used for energy storage; and ⑤ ensuring the long-term function of deep underground energy storage spaces. The solution to these key scientific and technological problems lies in establishing a theoretical and technical foundation for the development of large-scale deep underground energy storage in China.

Application of elemental chemostratigraphy to refine the stratigraphy of the Adavale Basin, Queensland

A prerequisite to understanding the evolution and resource potential of a basin is to establish a reliable stratigraphic framework that enables the correlation of rock units across multiple depocentres. Establishing a stratigraphic model for the Adavale Basin is challenging due to its structurally complexity, lack of well penetration and its lateral changes in facies. Biostratigraphy appears broad-scale, and despite providing chronostratigraphic control for the Lower Devonian Gumbardo Formation when combined with U/Pb zircon geochronology, the rest of the Devonian succession is hampered by a lack of microfossil assemblages and their poor preservation. The aim of this study is to establish an independent chemostratigraphic correlation across the Adavale Basin using whole rock inorganic geochemistry. Within this study, a total of 1489 cuttings samples from 10 study wells were analysed by Inductively Coupled Plasma – Optical Emission Spectrometry and Inductively Coupled Plasma – Mass Spectrometry for whole rock geochemistry, in order to establish an independent chemostratigraphic zonation scheme. Based on key elemental ratios selected to reflect changes in feldspars, clay minerals and provenance, the Devonian-aged stratigraphy is characterised into four chemostratigraphic mega-sequences that encompass the Gumbardo Formation (Mega-sequence 1); the Eastwood Formation, the Log Creek Formation and the Lissoy Sandstone (Mega-sequence 2); the Bury Limestone and the Boree Salt formations (Mega-sequence 3); and the Etonvale and the Buckabie formations (Mega-sequence 4). These mega-sequences have been further subdivided into a series of chemostratigraphic sequences that can be correlated across the study wells, establishing a regional correlation framework.

Geomechanics of salt towards natural barriers for the abandonment of pre-salt wells in Brazil

Well plug and abandonment (P&A) is a topic within the context of decommissioning of great importance for the industry and the environment. The trend for carbon capture and storage (CCS) is an additional encourager given the importance of the hydraulic isolation of abandoned wells. Favorable properties such as creep and essentially null permeability generally observed in the salt formations that overlay the Pre-salt reservoirs reinforce expectations that competent geological barriers can be formed. This work presents a literature review on P&A, salt as a barrier, and the geomechanical behavior of salt. Numerical analyses considering the thermomechanical behavior of salt and the repercussion on hydraulics are carried out in a conceptual model of a Pre-salt well to investigate the formation of a geological barrier. Methods to stimulate salt creep are tested by adopting feasible ranges. Results have shown that: i) the well can close due to creep and stimulation methods accelerate closure significantly; ii) well integrity is maintained during its closure; and iii) the reduction in confinement as the well creeps results in an increase in porosity and permeability. The confinement is more influential on permeability than porosity. In the most critical scenario, the permeability was almost 106 times the initial value when the well was closed. These findings add value to the research and industry community to form a knowledge base for future P&A of Brazilian Pre-salt wells and serve as a guide for further advanced and specific analyses.

Long-term sinking of nuclear waste canisters in salt formations by low-stress creep at high temperature

Rock salt has a self-sealing capacity, low permeability, and high thermal conductivity, making it a potential host for heat-generating nuclear waste. The feasibility of nuclear waste disposal within salt formations has been investigated mostly for small-sized canisters. Geologic disposal of larger-sized canisters originally designed for spent fuel storage and transportation has lately been examined as a cost-effective alternative. This raises questions about their long-term vertical movement due to their weight and high decay heat. Low-stress creep governs this movement; however, most salt constitutive models do not incorporate it. In this paper, the Norton and the WIPP creep models are compared with the Lux/Wolters/Lerche (LWL) model and a simpler model that combines linear and Norton creep laws (named combined creep model). The LWL and combined creep models consider pressure solution creep, though all incorporate dislocation creep. The models are first applied to creep tests under various stress levels. The LWL and the combined creep models results fit the experimental data well in both high and low stress ranges, whereas the Norton and WIPP models results only fit in higher-stress ranges. The different models are further applied for analyzing long-term canister movement. A sinking rate of -4.4times 10^{-7} mm/year was predicted using the Norton and WIPP models versus -2.1times 10^{-2} mm/year and -3.1times 10^{-2} mm/year using the LWL and the combined creep models, respectively. This comparative study confirms that creep models calibrated exclusively against high-deviatoric stress data might result in an inaccurate estimation of waste packages sinking rate in salt formations.

The Uncertain Bright Future of Underground Hydrogen Storage

The most obvious obstacles to a big ramp-up in global hydrogen production are well known. They include technological breakthroughs to bring down production costs along with new sources of demand from the power and transportation sectors. Less obvious is that a small army of reservoir engineers, geologists, and other subsurface experts will be needed to understand where and how tomorrow’s hydrogen hubs will store their clean-burning fuel. Bulk storage on the surface is considered by many experts to be simply out of the question. That means large hydrogen projects will need a subsurface component, and some think depleted oil and gas fields—with an emphasis on the latter—may fit the bill. Saline aquifers are being eyed for the role too. But as this all suggests, no one has ever attempted to use these formations for hydrogen storage. Just four shallow salt formations, three salt domes in Texas and one salt field in the UK, represent the totality of the world’s hydrogen underground storage (HUS) capacity. Research is underway to expand HUS in salt formations but that will not solve for the fact that they are not a geologic option for many locations where big industrial players are hoping to produce hydrogen. This includes most of Europe and most of the US outside of its Gulf Coast states. By contrast, deeper sedimentary structures of various flavors are in no short supply but lack any material field experience that might help jumpstart the de-risking of storing several Bcf of hydrogen. The upstream industry’s extensive experience in operating what are the closest analogues—natural gas storage and carbon capture and storage (CCS)—will help that process but there are new challenges when it comes to injecting the universe’s smallest molecule into porous media. Topping the list is hydrogen’s strong propensity to migrate inside a reservoir (laterally and vertically) along with the potential for troublesome chemical and biological reactions. Hydrogen may also be clean burning but it offers only about a third of the energy density as methane, which means it needs roughly three times the storage volume to deliver the same energy output to a gas-fired power plant. Among those working to bring clarity to such issues is Mojdeh Delshad, a reservoir engineer and professor at The University of Texas at Austin. Her latest research involved using commercial reservoir simulators to model what would happen if selected gas fields and saline aquifers in the US used for CCS or natural gas storage were instead used to store hydrogen. “We wanted to know about the challenges of hydrogen, which because of its properties—very low density, very low viscosity—is going to move in the reservoir much more quickly than CO2 and methane. And that’s exactly what we found, which means we’re going to have to do something differently with hydrogen storage in order to capture and produce what is injected,” said Delshad.

Hydrogen geo-storage in rock salt formations: petrophysical characterisation and geomechanical testing of a potential host formation in Western Australia

Hydrogen geo-storage in rock salt formations: petrophysical characterisation and geomechanical testing of a potential host formation in Western Australia

Viscoelastic analysis of surface deformation above salt caverns for gas storage based on semi-infinite space

The construction of salt caverns for energy storage is of strategic significance, but the operation of underground gas storages in salt formations will cause surface deformation. The research and analysis of surface deformation is an important part of the stability evaluation of salt caverns for gas storage. From the perspective of classical mechanics theory, the analytical solution of surface deformation above salt caverns for gas storage is deduced. The surface displacement caused by the contraction of a salt cavern is regarded as the boundary displacement caused by the surface force acting on a spherical cavern in a semi-infinite space. The equivalent conversion of load form is carried out based on the mirror principle. The vertical subsidence and horizontal displacement of the surface of the elastic semi-infinite space is obtained. According to the elastic-viscoelastic correspondence principle, the viscoelastic solution of the surface displacement of the semi-infinite body, regarded as Maxwell body, can be obtained. A numerical model of a salt cavern for gas storage is established based on the engineering practice. The results of the analytical solution and the numerical simulation are compared and analyzed to verify the accuracy of the analytical solution. The analytical solution was applied to predict the surface displacement above a group of caverns, and the influence of the layout of salt caverns and cavern center spacing on the surface displacement is analyzed. The research results show that the regular triangular layout is slightly better than the square layout. The larger the cavern center spacing, the smaller the maximum surface subsidence. The proposed theoretical prediction model can provide a theoretical basis for controlling surface deformation for the construction of gas storage caverns.

Deep Well and Ultra-Deep Well Drilling and Completion Technology Optimization and Application of Kuche Piedmont Area

In recent years, the exploration and development of China's Tarim Basin has moved towards ultra-deep and complex areas. With the increase in well depth and the influence of the geological characteristics of the Kuche Piedmont area, a series of drilling problems for 8000m-level ultra-deep wells in this area have been caused. As a result, a series of new drilling and completion technologies have been formed: ①Based on the tower standard I type well structure, the improved tower standard II-B\II-C type well structure is used to solve the drilling of two sets of salt formations. Difficulties; ②Optimizing UDM-2 oil-based drilling fluid, achieving an average hole diameter expansion rate of only 6.82%, reducing lost circulation and other complications; ③Designing and optimizing a new type of multi-edge PDC bit to significantly increase the average ROP; ④Application The industry-leading pre-measurement resistivity technology successfully predicts the salt bottom; ⑤The application of fine controlled pressure cementing technology to achieve effective sealing of high-pressure saline formations. In recent years, the Kuche Piedmont Block in the Tarim Basin has continued to tackle key problems in the application of ultra-deep well drilling and completion technologies, and has successfully reached a new level of 8000m, which has played an important role in supporting the development of deep oil and gas exploration and development business.

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.

Комплексная цифровизация, реализуемая при освоении запасов Гремячинского месторождения калийно-магниевых солей

The paper reviews the main trends in digitalization of the production processes implemented during the development of the Gremyachinskoe potassium-magnesium salt deposit. The main objective of the digital projects is to maintain competitiveness in the current market conditions. When implementing the integrated digital transformation project, the EuroChem-VolgaKaliy created a clear system of criteria to control the efficiency of the business processes and intriduced instrumental control over the actual duration of the technological cycle. Comparing more complete information on personnel with production metrics data creates objective ratings of face teams and foremen, which makes it possible to reduce production cycle time and increase average daily production volumes by 26%. An important area for further implementation of the project will be upgrading the IT-infrastructure, robotization of the office tasks, e.g. automatic generation of production requests, transfer of messages between the related divisions with no manual entry of information. After the information array is formed, it is planned to introduce the dependency analysis of production indicators with definition of equipment breakdown parameters. This will help to reduce the total number of downtime cases and increase the productivity of the mine as a whole, and will ensure a significant reduction in duration of unscheduled emergency repairs. A promising trend in activities of the Gremyachinskoe mining-and-processing integrated works is application of artificial intelligence in planning of mining operations. In order to solve these tasks it is envisaged to create 3D models, to simulate various methods and technologies of mining salt formations, to determine the design parameters of the mining method with account of the actual morphology and thixotropy of the formations.

Drilling fluid for stabilizing the wellbore in compound-salt formation based on multiple-synergism-method

The complex features of the compound-salt formation result in wellbore instability during drilling operations. The mechanism of wellbore instability in compound-salt formation is studied by the analysis of mineral compositions, microstructures, physical-chemical properties, and interface mechanical properties of compound salts from different areas. The results show that the existing micro-scale pores and fractures, creep and dissolution of salt, hydration of gypsum-mudstones are the key causes of wellbore instability in compound-salt formation. In order to stabilize the wellbore in compound-salt, we propose the drilling strategies based on the synergism of multiple methods of strengthening the wellbore by the sealing and consolidating effect of drilling fluid, inhibiting the dissolution of salt and the hydration of gypsum-mudstones, balancing the water activity between rocks and drilling fluid, providing effective stress to support the wellbore. Based on the effect of water activity on the shale hydration swelling and dispersion properties, the design basis of the safe window of water activity for drilling fluids is determined. According to the wellbore instability problems when drilling in compound-salt formations in M Oil Field in the Middle East, H-S Oil Field of Uzbekistan, and Xinjiang Uygur Autonomous Region of China, the drilling fluids are designed respectively based on the proposed wellbore stabilizing technology. The field applications show that the drilling fluid technology greatly improves the wellbore stability in compound salt formations and reduces downhole problems, thereby improving drilling efficiency.

2D Monte Carlo Simulation of Cocrystal Formation Using Patchy Particles

Cocrystals of Active Pharmaceutical Ingredients (APIs) are an attractive therapeutic alternative to salt formations. However, due to the molecular scale processes involved, the earliest stages of cocrystal formation remain poorly understood. In this paper, some light is shed on the thermodynamics and kinetics of co-crystallization. Importantly, to mimic the molecular scale processes of cocrystal formation, we use 2D Monte Carlo simulations and a computational model with short-range attraction and a mixture of two types of patchy particles (PPs) monomers. Each type possesses four patches, grouped in two by two, and each couple of patches is characterized by its specific placement on the circumference of the monomer and corresponding patch strength (a strong and narrow or weak and wide interaction). The spatial placement of the patches on both PPs monomers (alternating periodically through 60 and 120 degrees and vice versa) selected by us shows the emergence of both rhombohedral (metastable) and trihexagonal (stable) Kagome-like structures. The Kagome-like structures are preceded by formation of two types of trimers involving strong bonds only, or mixed trimers of strong and weak bonds, the later serving as building blocks for the finally generated Kagome patchy cocrystal, after prolonged simulation times. The step-by step process governing the cocrystal formation is discussed in detail, concerning the temperature interval, concentrations of PPs, the specific patch geometry and patch anisotropy as well. It is to be hoped that an understanding of the mechanisms of co-crystallization can help to control practical cocrystal synthesis and the possible phase transformations.

Unconventional Bradenhead Cementing in the Delaware Basin Case History

SummaryThis Delaware Basin case history describes a bradenhead cementing method that improves zonal isolation and top of cement (TOC). Common solutions for cementing depleted formations include stage tools, low-density slurries, and reverse circulation. For this bradenhead method, cement is pumped through the wellhead valves and down the annulus to a target loss zone. Field data, laboratory data, and the engineering model support the improvement bradenhead cementing provided across depleted formations.The depleted Brushy Canyon Formation serves as an injection interval for a two-stage bradenhead cement job. This approach isolates formations below the Brushy Canyon with a conventional first stage, while achieving TOC with the second stage pumped down the annulus. Fracture gradients of the injection interval and casing shoe constrain the cement design. To achieve adequate cement placement, the design phase includes the pressure calculations required to start flow and injection for the bradenhead stage. Reactive spacers are utilized to prevent annulus fluid level drop in extreme cases. A benchmarked cement bond log (CBL) quantifies the results and validates this unconventional cementing method.More than 50 two-stage bradenhead cement jobs were executed with a TOC success rate of 100% in intermediate and production hole applications. CBL data obtained after cementing the intermediate casing confirm the two-stage bradenhead as a placement method. In areas constrained by casing shoe pressures, CBL data identified potential areas of shallow injection. These data resulted in a modification of the fluids pumped. Use of these re-engineered fluids reduced injection initiation pressures and improved cement bonding across salt and anhydrite formations. Cementing costs were reduced by eliminating stage tools, annular casing packers, and low-density cement slurries. Pumping the bradenhead job offline (off the rig’s critical path) reduces cost further. These features support bradenhead cementing as an effective solution for wellbore sections with a potential injection zone.The regulatory requirement to place cement above depleted formations resulted in many innovative cementing methods. Stage tools, low-density slurries, and reverse circulation are attempts to avoid formation breakdown, but these methods increase cost and have limited reliability and integrity. Bradenhead cementing takes advantage of the depleted interval to aid in cement placement. Although counterintuitive, bradenhead cementing has proved to be a cost-effective solution that improves zonal isolation and eliminates integrity risks associated with stage tool failures.

Potential for Underground Storage of Liquid Fuels in Bedded Rock Salt Formations in Poland

The paper aims to give a universal methodology for assessing the storage capacity of a bedded rock salt formation in terms of the operational and strategic storage facilities for liquid fuels. The method assumes the development of a geological model of the analyzed rock salt formation and the determination of the salt caverns’ size and spacing and the impact of convergence on their capacity during operation. Based on this method, the paper presents calculations of the storage capacity using the example of the bedded rock salt formations in Poland and their results in the form of storage capacity maps. The maps show that the analyzed rock salt deposits’ storage capacity in northern Poland amounts to 7.1 B m3 and in the Fore-Sudetic Monocline to 10.5 B m3, in the case of strategic storage facilities. The spatial analysis of the storage capacity rasters, including determining the raster volumes and their unique values, allowed us to quantify the variability of the storage capacity in the analyzed rock salt deposits.

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.

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.