Bedded Rock Salt Research Articles

Experimental investigation of mechanical behavior of bedded rock salt containing inclined interlayer

Uniaxial and triaxial compression tests have been carried out on specimens of rock salt, interlayers, and composite rock salt (containing inclined interlayers). In combination with failure mechanism analysis of the stratified rock structures, the variation and impact factors of bonding stresses between the layers have been investigated, and the fracture properties and failure mechanisms of inclined bedded rock salt have been revealed. The research demonstrated that the inclined interlayer influences the deformation and fracture properties of rock salt. Due to the composite mechanical characteristics, cracks primarily initiate near the interface region, and then propagate to the interlayer and the rock salt adjacent to it. The thickness of the interlayer affects the entire performance of the composite specimen and governs the development of the local fractures. Macro-cracks tend to form more easily in the test specimens that contain a medium-thick interlayer than in the ones containing only a thin interlayer. Increasing confining pressure gradually changes the overall mechanical behaviors from brittle to more ductile at higher confining pressure. The ductile behavior is characterized by differences of strain-hardening behaviors and also the geometry of cracks at different locations in the interlayer as compared to the results of uniaxial compression tests.

Experimental study of mechanical and hydraulic properties of bedded rock salt from the Jintan location

A series of triaxial compression tests, triaxial extension tests, and triaxial compression creep tests were conducted to evaluate the strength, deformability, and permeability of rock salt from the Jintan location. Based on a previous analysis, the measured data can be physically modeled very well as shown by the constitutive model Hou/Lux and indicate that China’s representative bedded rock salt from the Jintan location is characterized by good ductility and deformation properties, similar to the European high-purity rock salt. Based on a comparison of compression as well as extension failure strength determined at rock salt from Jintan location, a similarity of compression and extension failure strength could be demonstrated. Permeation flow seems to be an anisotropic process because the permeability measured at samples prior to failure by compression tests differs to those measured at samples prior to failure by extension tests. The anisotropy of damage and destruction of rock salt has a significant influence on permeability. In case of triaxial compression tests, the measured permeability increases two to six orders of magnitude because of consistent direction of injection and cracks. In case of triaxial extension tests, however, the permeability increases less with a maximum of two orders in magnitude. The test results are significant for understanding the load-bearing behavior of rock salt from Jintan location and investigations with reference to the stability and tightness of underground caverns.

Stability and tightness evaluation of bedded rock salt formations for underground gas/oil storage

The presence of interfaces has a critical influence on the stability and tightness of underground gas/oil storages. In China, these energy storages are constructed mainly in bedded salt formations and are widely distributed. Therefore, it is necessary to study the sedimentary rhythm and mechanical characteristics of the interfaces between beds. The petrologic study of core samples from exploratory wells in Yunying Salt Mine, Hubei Provence, China, reveals a sedimentary rhythm and multiple interfaces of bedded salt formations: (1) Mudstone–anhydrite–glauberite–Glauber’s salt–salt rock–mudstone are periodically deposited in sequences. Chemical deposition, the predominant formation mechanism, and mechanical deposition appear alternately and result in the formation of sedimentary multicycles; (2) depending on whether a certain component of the rhythm is lacking or not, interfaces are divided into sequential depositional interfaces and intermittent depositional interfaces. Depending on the deposition frequency, interfaces can also be classified into dominant interfaces and secondary interfaces. Depending on whether the mineral composition changes gradually or abruptly, interfaces can be divided into gradual transition interfaces and discrete discontinuous interfaces. Based on the classification of the interfaces together with scanning electron microscope analyses, the cementation state and strength of the interfaces are discussed. Both field investigations and laboratory tests show that the strengths of the chemical deposition interfaces are higher than those of the mechanical deposition interfaces. Specifically, interfaces of mudstone and rock salt are mostly weak. Taking into account the presence of weak interfaces, the strength of interbedded salt formations around caverns can be represented by a U-shaped lower-bound strength envelope curve, which means that shear failure may occur more easily at the haunches of the cavern. Therefore, when setting the range of the internal operating pressures, more attention should be paid to the shear strength of the weak interlayers and weak interfaces, in particular those at the haunches of the cavern, to ensure the stability and tightness of underground gas/oil stores.

Stability Analysis of Storage Caverns in Bedded Salt Rock Formation

Salt cavern storage is usually in bedded salt rock formation except salt dome, in particular in China. The rocks composing a bedded salt rock formation, e.g., mudstone, rock salt, interlayer, etc., often present viscoelastic-plastic behaviors, which is an important influencing factor of the long-term stability of salt caverns in it. Modelling the rheological behavior with the Druck-Prager creep model, an example of stability analysis of four salt caverns at Jintan Salt Mine of China with the finite element method is elaborated in this paper. The results show that besides the inevitable loss of effective storage room with time due to creep deformation, which decreases evidently with internal pressure but decreases slowly at a certain pressure value, the variation of operating internal pressure in each cavern can cause the change of volumes of other nearby caverns and then affect the stability of all the caverns. The internal pressure difference should be as small as possible during the operation of salt caverns.

Analysis of major risks associated with hydrocarbon storage caverns in bedded salt rock

Salt rock is internationally accepted as an ideal medium for energy storage. As an energy storage structure, the safety of hydrocarbon storage caverns in salt rock is related to the national economy and to social public security. Risk analysis is an important method of engineering safety evaluation. In this paper the major risks associated with hydrocarbon storage caverns in bedded salt rock are defined. The major risks are classified under the headings of ‘oil and gas leakage’, ‘ground subsidence’, and ‘cavern failure’, and are discussed under these topical titles. The factors leading to the major risks associated with storage caverns are identified by reviewing descriptions of major accidents of salt storage caverns around the world. Fault tree models for the three major risks are established and analyzed. Basic paths of the risk and their occurrence probability ranking are derived. The risk factors which contribute greatly to the risk are identified by calculating the importance degree of all the basic events. Finally, a comprehensive evaluation methodology for major risk loss is generated based on the analytic hierarchy process. This provides a theoretical foundation for the evaluation and prevention of major risks in the construction and operation of storage caverns in bedded salt rock.

Numerical Simulation on Open Wellbore Shrinkage and Casing Equivalent Stress in Bedded Salt Rock Stratum

Most salt rock has interbed of mudstone in China. Owing to the enormous difference of mechanical properties between the mudstone interbed and salt rock, the stress-strain and creep behaviors of salt rock are significantly influenced by neighboring mudstone interbed. In order to identify the rules of wellbore shrinkage and casings equivalent stress in bedded salt rock stratum, three-dimensional finite difference models were established. The effects of thickness and elasticity modulus of mudstone interbed on the open wellbore shrinkage and equivalent stress of casing after cementing operation were studied, respectively. The results indicate that the shrinkage of open wellbore and equivalent stress of casings decreases with the increase of mudstone interbed thickness. The increasing of elasticity modulus will reduce the shrinkage of open wellbore and casing equivalent stress. Research results can provide the scientific basis for the design of mud density and casing strength.

Theoretical Analysis of Gas and Oil Storage Cavern in Bedded Salt Rock Using a Love Function

Based upon the elastic theory, the gas and oil storage cavern in bedded salt rock formation is generalized as a spatially axisymmetric model. A Love function was built from polynomial and logarithmic functions at first, and then solved, considering the gravity, the internal gas and oil pressure of the cavern, the boundary conditions, as well as the continuity conditions at the interface between salt and non-salt media. Finally, the elastic displacement and stress components are achieved which are satisfied to the main boundary and continuity conditions.

A New Elasto-Viscoplastic Damage Model Combined with the Generalized Hoek–Brown Failure Criterion for Bedded Rock Salt and its Application

According to the requirement of the West–East Gas Transmission Project in China, the solution-mined cavities located in the Jintan bedded salt formation of Jiangsu province will be utilized for natural gas storage. This task is more challenging than conventional salt dome cavern construction and operation due to the heterogeneous bedding layers of the bedded salt formation. A three-dimensional creep damage constitutive model combined with the generalized Hoek–Brown model is exclusively formulated and validated with a series of strength and creep tests for the bedded rock salt. The viscoplastic model, which takes the coupled creep damage and the failure behavior under various stress states into account, enables both the three creep phases and the deformation induced by vicious damage and plastic flow to be calculated. A further geomechanical analysis of the rapid gas withdrawal for the thin-bedded salt cavern was performed by implementing the proposed model in the finite difference software FLAC3D. The volume convergence, the damage and failure propagation of the cavern, as well as the strain rate of the salt around the cavern, were evaluated and discussed in detail. Finally, based on the simulation study, a 7-MPa minimum internal pressure is suggested to ensure the structural stability of the Jintan bedded salt cavern. The results obtained from these investigations provide the necessary input for the design and construction of the cavern project.

Energy analysis for damage and catastrophic failure of rocks

The development history and current state of studies on the characteristics and mechanisms of deformation and failure of rock materials were briefly reviewed from the viewpoint of energy. The main scope and the achievable objectives of the energy-based research system were expatiated. It was validated by experiments that the damage process of rocks can be well described by the rock damage evolution equation established based on energy dissipation. It was found from the uniaxial compression and biaxial compression tests that only a small proportion of the total input energy in hard rocks is dissipated before peak load and a large proportion in soft rocks is dissipated before peak load. For both hard and soft rocks, the energy dissipated after peak load accounts for a greater proportion. More energy would be required for rock failure under equal biaxial compression than under unequal biaxial compression. The total absorbed energy is different for rock failure under high-rate loading and low-rate loading. More fragmented failure pattern usually corresponds to higher energy absorption. The mesoscopic analysis on the damage and failure of bedded salt rocks showed that the energy dissipation is prominent and the total absorbed energy for rock failure is low when cracks propagate in the weak mud interlayer while it is contrary when cracks propagate in the salt rock. The energy accumulation, transfer, dissipation and release during the failure process of tunnel with impending failure under disturbance were analyzed theoretically based on the elastoplastic mechanics theory. Furthermore, the spatial distribution of energy dissipation and energy release of fractured rocks under unloading was simulated numerically. It was demonstrated that energy is likely to be released from the weakest surface under compression, which triggers the global failure of rocks.

Fractal property of spatial distribution of acoustic emissions during the failure process of bedded rock salt

A rock mechanics test system and the acoustic emission (AE) test system were employed to perform experiments on bedded rock salt samples under uniaxial compression and indirect tension. Based on the basic theory of fractal geometry, a column covering method is proposed to analyze the fractal property of the spatial distribution of acoustic emissions during the rock damage and failure process. The experimental results indicate obvious differences in failure features for different parts of a sample due to the structural differences in bedded rock salt. The consistency between the failure state and the AE spatial distribution is confirmed. The relationships of the stress and energy release with the fractal dimension of the AE spatial distribution are established. An increase in the fractal dimension corresponds to a decrease in the stress and an increase in the energy release, which is consistent with the theoretical analysis. When the fractal dimension obtained from uniaxial compression and indirect tensile tests is less than 2.20 and 2.57, respectively, the stresses in the bedded rock salt sample are over 80% of the peak stress. A great amount of accumulated damage occurs, leading to the global failure of the bedded rock salt sample. Therefore, it is possible to forecast the failure state of bedded rock salt by inspecting the variation between the stress or the energy release and the fractal dimension of the AE spatial distribution.

Study on the Seepage Law for Gas Storage in Bedded Salt Rock Caverns

Gas permeability mechanism for the laminated salt rock gas storage is very complex. The key problem is to establish the permeability model and to propose the corresponding numerical method suitable for salt rock with weak interlayer based on seepage theory in porous media. An efficient and accurate method is proposed to model the flow in fracture and matrix. The fractures are regarded as boundaries among adjacent matrix blocks. Darcy’s law governs velocities in the matrix blocks, while the flow in the fracture on boundaries is set up by accounting for the fracture’s thickness. This model can reflect the fracture flow which is different from the equivalent medium model, and its computational complexity is less than that of double porous model. Combining with the field situation of Jintan gas storage in China, the pressure distributions of storage with different cyclic pressures for gas injection and production in five years are studied. It is concluded that the effect of permeability on gas pressure distribution is very important.

Statistical evaluation of five failure criteria for intact salt rock

Five multiparameter empirical criteria were exclusively evaluated by comparing them with the strength data covering various stress conditions to find out which failure criterion best fits the test data and describes the mechanical behavior of the salt rock sequence (halite, bedded composite specimens and anhydrite interlayers). Full-scale comparison of all criteria for the three rock types was conducted based on five standard statistics calculated from least squares curve-fitting, which measures both the goodness of fitting and the quality of future prediction. The results indicate that all five nonlinear criteria with a basic power form are efficient in predicting the strength trend in the low tension area as well as in the high compression area of the soft rocks. The parameters obtained for the bedded rock salt are somewhat in the ones for the “pure” rocks and are even closer to those obtained for the halite. The generalized Hoek-Brown criterion is proven to perform best to two rock strength data followed by one for the Bieniawski empirical criterion, thus is the best candidate for the analysis of the salt rock. The Sheorey empirical criterion consistently achieves an intermediate performance for all the three rocks. It seems that the superiority of the poly-axial criteria (the Mogi 1967 criterion and the N-type criterion) over the former three triaxial criteria no longer exists when applied to the conventional triaxial strength data. Besides, the method of tension cut-off was proposed to solve the ambiguity problem of the two poly-axial criteria in the tension field in the plane of the major (σ1) and minor principal stress (σ3).

Numerical Simulation of Gas Leakage in Bedded Salt Rock Storage Cavern

Storage of hydrocarbons in underground salt caverns created by solution mining has been commonly and successfully used for several years in developed country. Sealing performance is an important technique and safety index for salt rock gas storage. Considering the unique stratigraphic characteristics of the bedded salt rocks in China, interlayer has great influence on the tightness of salt carven. It is likely that stored natural gas will escape along the interface of interlayer. It is the key problem to establish the permeability model and to provide corresponding numerical simulation which is suitable for bedded salt rock. In the paper, based on seepage theory in porous media, evolvement equation of gas permeability is established. The salt cavern in Jintan salt mines is taken as engineering background; during the operating period, the pressure distribution and gas leak distance at interface and layers with different operating pressures are studied. It is demonstrated that the gas infiltration velocity along the damaged interface is much faster than the salt rock and mudstone interlayer, and damaged interface is the main gas leakage path. Moreover, with the bigger operation pressure, natural gas spreads much faster at the beginning years. The simulation results prove scientific basis for the design and reasonable control of operating pressure and tightness evaluation of bedded salt rock storage cavern.

Comparative study of four failure criteria for intact bedded rock salt

Damage smear method (DSM) is adopted to study trans-scale progressive rock failure process, based on statistical meso-damage model and finite element solver. The statistical approach is utilized to reflect the mesoscopic rock heterogeneity. The constitutive law of representative volume element (RVE) is established according to continuum damage mechanics in which double-damage criterion is considered. The damage evolution and accumulation of RVEs are used to reveal the macroscopic rock failure characteristics. Each single RVE will be represented by one unique element. The initiation, propagation and coalescence of meso-to macro-cracks are captured by smearing failed elements. The above ideas are formulated into the framework of the DSM and programed into self-developed rock failure process analysis (RFPA) software. Two laboratory-scale examples are conducted and the well-known engineering-scale tests, i.e. Atomic Energy of Canada Limited's (AECL's) Underground Research Laboratory (URL) tests, are used for verification. It shows that the simulation results match with other experimental results and field observations.

Evidence of potash salt formation in the Pliocene Sedom Lagoon (Dead Sea Rift, Israel)

The Dead Sea is a Ca-chloride evaporitic lake close to halite saturation. Carnallite (KMgCl 3·6H 2O) is the next mineral predicted to precipitate and is currently produced in solar ponds on the shores in Israel and Jordan. The particular composition of the Dead Sea has been attributed to the contribution of Pliocene residual brines formed during the deposition of the Sedom formation that currently reappear as hydrothermal flows. The chemical composition of these returning flows suggests that Sedom brines reached the potash-forming (sylvite and/or carnallite) stage. The Sedom formation is a thick (more than 2000 m) evaporite succession made up of bedded salt rock accumulated during the Pliocene when an arm of the Mediterranean sea seeped through the Dead Sea Rift forming the Sedom Lagoon. The present study is focused on the upper part of the Sedom formation (Mearat Sedom member) where detailed petrology, bromine contents in salt, Cryo-SEM-EDS microanalysis of fluid inclusions in halite, and isotopic compositions ( δ 34S and δ 18O in sulfates, and δ 37Cl in salt) were investigated. Although halite is the main evaporite mineral, minor sylvite and carnallite occurrences in the form of early diagenetic nodules, cements and/or authigenic crystals are commonly present. The geochemical modeling of the primary fluid inclusion composition in halite points to the evaporation of seawater or mixtures of seawater and continental water, with a very minor (< 1.5%) proportion of CaCl 2-rich and MgCl 2-rich brines similar to those discharging into the present-day Dead Sea. This suggests that the refluxing of residual brines was already active in Sedom times. A cyclic evolution of the basinal brines can be deduced from the composition of fluid inclusions together with the bromine contents in halite and the sulfate isotopes. Thus, stages of dilution involving fresh marine and/or continental waters alternated with stages of high concentration when the saturation of the lagoon brine with respect to the potash salts was reached. Potash minerals could crystallize on the shores or in the marginal zones of the lagoon and could be dissolved and recycled during the subsequent dilution stages.

A new Cosserat‐like constitutive model for bedded salt rocks

AbstractSalt rocks are commonly used as geologic host rocks for storage of gas and crude oil, and are being considered for the disposal of radioactive waste. Different from the salt rock domes in many countries, the salt rock formations in China are usually laminar with many alternating layers, i.e. rock salt, anhydrite, and/or mudstone. Considering the unique stratigraphic characteristics of these salt rocks, a new Cosserat‐like medium constitutive model is proposed in order to facilitate efficient modeling of the mechanical behavior of these formations. In this model, a new representative volume element, containing two different layers, is employed to simulate the compatibility of the meso‐displacement between two different layers and also the bending effect. A new method for the deformation and failure analysis of bedded salt rocks is derived therefrom. Having the macro‐average stresses, the conventional stresses in the different layers can be obtained in sequence. The conventional stresses can then be utilized in a routine way for the strength and failure analysis. For the initial numerical modeling, the new Cosserat‐like medium is reduced to a transversely isotropic one. The simplified constitutive model for layered media is then implemented into FLAC3D codes. A test sample validates that the results by using the numerical model are in good agreement with that by using the built‐in model, and the mesh size for the new model is reduced greatly. Finally, an application for the stability of oil storage caverns in deep thinly bedded salt rocks is carried out. The effects on convergence of storage caverns and on the failure of surrounding rock due to the presence of the mudstone interlayers (hard phase) are discussed in detail. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Dissolution and Seepage Coupling Effect on Transport and Mechanical Properties of Glauberite Salt Rock

Potential high rates of aqueous dissolution are characteristic of salt rocks, and solute and mass flux through a soluble porous medium are functions of solute concentration gradients and pressure gradients. Due to different dissolution properties for different mineral components in glauberite salt rock, an interaction between mineral dissolution and solvent seepage arises, driven by the hydraulic pressure gradient in the rock. The originally almost impermeable glauberite rock becomes an increasingly permeable porous medium with dissolution, changing the transport and mechanical properties because of the progressive removal of solid sodium sulfate (Na2SO4), one of the constitutive components of glauberite salt rock. Glauberite is often found in bedded salt rock deposits, and the mineral glauberite has economic value and has been mined for many years in China. More economic and safe technologies, such as controlled solution mining, are inherently attractive. Thus, investigations into relevant physical and mechanical properties of glauberite in the context of solution mining have value, and to clarify glauberite behaviour, a series of experiments were performed. It is observed through experiments that the permeability of the rock mass during dissolution of glauberite is a function of the dissolution duration and the hydraulic pressure gradient applied to the system. For example, in laboratory tests, after 49, 53 and 70 h of dissolution, the relationships between permeability (k—cm2) and pressure gradient (Δp—MPa across the specimen of length 100 mm) of the glauberite specimens were observed to be k = 0.24 for a Δp of 0.10, k = 0.30 for a Δp of 0.12, and k = 0.41 for a Δp of 0.18, with the empirical functional relationship becoming gradually steeper with pressure. Also, the triaxial compression (mechanical) characteristics of glauberite salt rock change substantially after a period of dissolution: the compressive strength under a confining stress of σ3 = 2.0 MPa changes initially from 46 to 11 MPa after 70 h of dissolution and seepage. Along with strength degradation, the Young’s modulus (stiffness) changed from 4.6 to 0.5 GPa. Evidently, coupled dissolution and seepage rate greatly impact both transport and mechanical properties of the rock as fabric evolves in a time-dependent manner.

Experimental investigation of mechanical properties of bedded salt rock

Because of salt cavern utilization for liquid, gas and solid waste storage, salt rock mechanical properties are needed for assessments of facility, stability and safety. Bedded salt deposits are widespread and used as much or more than diapiric salt bodies as storage facility hosts, but experimental data on the mechanical properties of bedded salt rock with impurities are far less common than data available on relatively pure diapiric salt rocks. Through laboratory uniaxial and triaxial compression experiments on rock salt (halite), interlayers (anhydrite) and bedded composite specimens (anhydrite–halite and mudstone–halite), differences in mechanical properties of the various lithologies are explored. In the composite specimens, the weakest or the most deformable component governs the behavior. Also, the properties of bedded composite lithology specimens tend to be in between the property ranges of the “pure” lithologies. The elastic modulus of the bedded salt rock increases from 5.3 to 24.1 GPa with an increase in the confining stress from 0 to 15 MPa, with some evidence of sample damage. The ductile transition for halite at the strain rates used is at about σ 3∼10 MPa. With increasing σ 3, the anhydrite–halite composite lithology deformation showed strain hardening and a strong trend to ductile behavior as the halite bands tended to dominate the behavior. Strain incompatibility effects exist along interfaces between creeping and non-creeping phases in anhydrite–halite composite lithologies. Mudstone–halite rocks tended to be extremely weak, compared with all other specimens.

Effect of temperature variation on a rock salt deformation – a case study

In situ deformation measurements using extensometers installed in bedded rock salt at 650 m below the surface at the Waste Isolation Pilot Plant (WIPP) were analysed extensively. The strain rate curves varied periodically suggesting that seasonal temperature variation affects the deformational behaviour of the underground excavations. The effect of different parameters including opening's geometry, stand-up time, and distance from the opening on the strain rate changes due to a temperature variation were investigated with a new approach using a curve-fitting equation. For understanding the measured deformational behaviour of the underground excavations at WIPP, a temperature analysis was carried out using the Heating 7.2 program. The actual temperature variation with time in the rock mass around the underground excavation could be successfully simulated with the program.

Dilation of anisotropic rock salt: Evidence from Mount Sedom diapir

The mechanical behavior of bedded rock salt is studied using constant strain rate biaxial tests performed on 24 bedded rock salt samples from Mount Sedom diapir. Careful measurements of elastic parameters in unload‐reload loops indicate that they are influenced by confining pressure but are not sensitive to anisotropy. The yield stress is also influenced by confining pressure but not by anisotropy. The stress‐strain curve therefore indicates isotropy up to the yield stress. Initiation of dilation marks the onset of dependence of volumetric strain on anisotropy. Major principal compressive stress at dilation (σ1,d) is at maximum when compression is normal to bedding planes (β=0°), the stress decreases with increasing values of β, and minimum values are measured when compression is parallel to bedding planes (β=90°). Beyond onset of dilation point the relative dilation with respect to stress difference (V = |Δεν/Δ(σ1 ‐ σ3)|; σ1 &gt; σ1,d; MPa−1) increases with decreasing confining pressure and with increasing β; namely, dilation is intensified with decreasing confining pressure and as the major principal compressive stress direction becomes parallel with bedding plane orientation. Current models for compression‐dilation boundary ignore anisotropy and therefore provide site specific solutions but fail to describe general rock salt behaviour. A new empirical model for compression‐dilation boundary in anisotropic rock salt, developed for a data set from Mount Sedom, predicts that stress at onset of dilation decreases with decreasing confining pressure and with increasing value of β. The rate of change of σ1,d with respect to β decreases with decreasing confining pressure, and the rate of change of σ1,d decreases with increasing confining pressure for all values of β.