Creep Of Rock Salt Research Articles

Long-term deformation of rock salt under creep–fatigue stress loading paths: Modeling and prediction

Rock salt, due to its water solubility, low permeability, high plasticity, and damage self-healing ability, is one of the best candidate rock types for underground energy storage. Utilizing salt caves to construct compressed air energy storage (CAES) facilities can effectively enhance the utilization of renewable energy. Due to the need for peak shaving, the surrounding rock of the salt cavern will undergo discontinuous cyclic loading with varying gas injection rates and pressures, namely, alternating creep–fatigue loading. Considering the actual peak-shaving cycle of a CAES plant, long-term creep–fatigue tests of rock salt with different loading cycles and stress levels were conducted. The results indicate that in long-term creep–fatigue tests for rock salt, the lower the loading stress rate is, the greater the deformation of the rock salt. The variation in the stress limit has a greater effect on creep than on fatigue loading and unloading. The deformation rate of rock salt is influenced by alterations in the stress state. Based on the test results, according to the Norton creep model, a new creep–fatigue constitutive model for rock salt was established by defining a state variable that characterizes the level of rock hardening and introducing unloading as well as crack factors. This model can accurately describe the impact of historical loading and unloading processes on the viscoplastic mechanical characteristics of rock salt. The rock salt creep–fatigue test results were used to verify the constitutive model. A comparison of the fitting curve of the different stress loading paths with the test curve reveals good consistency, indicating that the model comprehensively considers the effects of time, load, and state on rock salt creep–fatigue, effectively describing the viscoplastic deformation characteristics of rock salt under different stress paths. These research findings provide important guidance for ensuring the stability of salt caverns used for CAES.

Effect of pillar width on the stability of the salt cavern field for energy storage

Abstract Effective planning of the cavern field involves determining the optimal pillar width between the caverns and the feasible number of caverns based on geological and mining conditions. The proper design of the pillar width is crucial to ensure the stability of the cavern field and the rational utilization of the rock salt deposit. The stability of the pillars is a complex problem influenced by various factors, including rock salt creep, changes in the cavern pressure during operational cycles, mechanical parameters, and failure criteria of the rock salt. To address this problem, the stability of the cavern field in relation to the number of caverns and pillar widths is evaluated. The evaluation is based on the following criteria: displacements, von Mises stress, strength/stress ratio, and safety factor. Three variations of pillar width and three variants of cavern fields, differing in the number of caverns, are considered. Results show that the allowable pillar width is affected by the number of caverns in the cavern field. Moreover, the stability analysis reveals uneven stress and deformation distribution in the cavern field. When the pillar width is 2.0–3.0 times the diameter of a cavern, pillars at the centre exhibit poorer stability than those at the edges of the cavern field. However, with a narrower pillar width, the highest displacements occur at the field's edges. The findings of this study provide a valuable date in the planning, design, and operation of new cavern fields for the underground storage of energy sources such as oil, natural gas, hydrogen, and compressed air in rock salt deposits.

Creep of Rock Salt Under a Large Range of Deviatoric Stresses

Creep of Rock Salt Under a Large Range of Deviatoric Stresses

Correction to: The effect of rock salt creep behavior on wellbore instability in one of the southwest Iranian oil fields

Correction to: The effect of rock salt creep behavior on wellbore instability in one of the southwest Iranian oil fields

Creep monitoring and parameters inversion methods for rock salt in extremely deep formation

When underground salt caverns are used for natural gas storage, the pressure difference between the inside and outside of the cavern will cause continuous creep deformation of the surrounding rock. Understanding the creep characteristics and constitutive model parameters of rock salt in actual formations is of great significance for guiding the construction and safe operation of gas storage facilities. In this paper, a creep contraction calculation model for slender cylindrical rock salt caverns was established firstly, based on the theory of complex variables and viscoelasticity. This model can calculate the radial displacement of the surrounding rock at any moment on the cross-section of the rock salt wellbore. Subsequently, two indirect monitoring methods for deep rock salt creep using the open hole section of the well were proposed based on this model, which involves using the cavern pressure change after wellhead closure and the liquid overflow rate after wellhead opening to indirectly reflect the deformation of underground rock salt. These two sets of field experiments can be used together to determine the values of the constitutive model parameters of rock salt creep under actual stress conditions. This theoretical framework was applied in the Ningjin Salt Cavern Gas Storage Project in China. Pressure monitoring was conducted for 150 days, and wellbore fluid overflow was measured for 45 h in a rock salt well at a depth close to 3000 m. The calculated results showed good agreement with the field monitoring data, and the parameters of the Burgers creep model for rock salt under actual conditions were determined. This demonstrates the strong feasibility of the monitoring method and the validity of the computational model. Finally, using the obtained creep parameters, a quantitative analysis of the creep contraction of the wellbore was performed considering factors such as internal pressure, in situ stress, and Poisson's ratio of the rock salt.

Rock salt creep: a cross-check test to cover the full relevant range of deviatoric stresses

The time-dependent response of rock salt has been mainly investigated using confined creep tests covering a differential stress range between 5 and 20 MPa. In recent years, efforts have been made to investigate the range 0.1–4.5 MPa, using dead-end drifts in underground mines to take advantage of the very stable ambient conditions (temperature, relative humidity). Up to now, the combination of experimental data in the two ranges is difficult because of the use of different salt facies, sample preparation methods, test temperatures and experimental conditions (e.g., confined vs. unconfined tests, scale of measurements). In this work, we conduct two long-term creep tests on two natural salt samples from the same origin and prepared using the same protocol. The thermo-mechanical loading path is the same for the two tests, with only a small difference in the lateral load. One test is performed in a remote drift in a salt mine and the other test is performed in a climatic chamber in the laboratory. The comparison of the results is consistent, allowing to investigate a large range of deviatoric stresses by combining results in the two facilities. The final goal of this approach is to reduce stress extrapolation by investigating the whole deviatoric range that is relevant for underground operations. Next steps include investigating in more detail the effect of intergranular fluids, testing different temperatures and performing confined tests in the mine.

A thermodynamic modeling of creep in rock salt

A number of constitutive models have been proposed along time in order to describe the mechanical behavior of rock salt. The basic formulation of these models relies, in general, on the description of deformation mechanisms in the macro or the micro scale. The present note aims at providing a framework for the development of models to describe the creep behavior of rock salts based upon continuum thermodynamics principles. This note provides the theoretical basis and, based on the proposed framework, presents the development of a two-mechanism model that describes transient and steady-state creep of rock salt. Results obtained with this model are presented as well as a discussion on the main features of the proposed formulation.

Integrity analysis of wellbores in the bedded salt cavern for energy storage

Salt cavern used for energy (natural gas, hydrogen) storage has a significant advantage in peak shaving of gas supply due to their high injection-production efficiency and fast gas injection-delivery switching speed. As the only channel for gas injection and delivery, the wellbore is easily damaged by the alternating gas pressure and is a weak component. For salt cavern underground gas storages (UGSs) in bedded rock salt, the roof collapse can also cause tensile damage to the wellbore. Long-running UGSs may suffer leakage accidents due to wellbore failures, and wellbore safety issues need to be fully considered. This paper proposes a coupled analysis of cement sheath fatigue damage and rock salt creep, and its applicability is proved by mechanical tests. A three-dimensional numerical model was established that includes a wellbore, open well section (cavern neck), and bedded salt cavern. The creep and damage constitutive equations were used to calculate rock salt creep and cement sheath damage. The results show that the creep ability of bedded rock salt is lower than that of high-purity rock salt, confirmed by the creep test results and can be described by the creep constitutive equation in this paper. The cavity shrinkage of the bedded UGSs is generally lower than expected, resulting in better wellbore integrity. The low creep capacity of the impurity-containing rock salt will reduce the cavity shrinkage and protect the airtightness of the wellbore. Based on the integrity of the wellbore, the working gas volume of the UGSs in the bedded formation can be increased. Taking the Jintan gas storage as an example, under the operating pressure of 7–17 MPa, the damaged area of the cement sheath is limited to within 0.5 m of the lower end of the wellbore. When the lower limit pressure is reduced to 6 MPa, the damaged area is limited to within 2 m of the lower end of the wellbore, and the wellbore still maintains sufficient airtightness.

Subsidence above gas storage in salt caverns predicted with viscoelastic theory

Based on the on-site monitoring data, the surface subsidence in the Jintan underground gas storage site has occurred continuously since the construction and operation of gas storage engineering. Therefore, the prediction of surface subsidence above gas storage in salt caverns is important to ensure the long-term safety of gas storage. Based on the mechanics theory, the formulas of elastic deformation of spherical and cylindrical caverns are derived. The Burgers viscoelastic model is used to describe the creep of rock salt. The viscoelastic analytical solution of the cavern deformation is obtained by using the correspondence principle in viscoelastic theory. It is assumed that the volume of the surface subsidence basin is directly proportional to the shrinkage volume of the cavern. The surface subsidence distribution curve is described by a Gaussian function. The spatiotemporal prediction of surface subsidence above gas storage in salt caverns based on viscoelastic theory is thus developed. The proposed theoretical model is used to analyze the surface subsidence above a gas storage cavern in rock salt in Jintan. The calculated subsidence is compared with the monitoring data. The accuracy of the theoretical model is verified by numerical simulation. The theoretical model can be used to predict the surface subsidence considering the solution stage in which the internal pressure changes with time. Results of this paper can provide guidance and suggestions for engineering practice of gas storage in salt caverns.

The effect of rock salt creep behavior on wellbore instability in one of the southwest Iranian oil fields

The effect of rock salt creep behavior on wellbore instability in one of the southwest Iranian oil fields

Failure analysis for gas storage salt cavern by thermo-mechanical modelling considering rock salt creep

Salt cavern is ideal vessel for underground gas storage due to the high deliverability. During cyclic gas operations of injection-and-withdrawal, the temperature change in salt cavern imposes thermal stress on cavern wall. The temperature change, together with the pressure change in salt cavern, causes the effective stress on cavern wall to alter, leading to variation of creep rate. In this study, a coupled thermo-mechanical model is proposed for failure analysis of salt cavern in rock salt prone to creep deformation. Six cases that consider rock salt with different creep tendency have been conceived for the coupled thermo-mechanical model to investigate the stability of the salt cavern in terms of cavern convergence and failure indices. The results indicate that the affected region by cyclic pressure and temperature is up to 10 m inside the rock salt from the cavern wall. It is also revealed from the results that largest displacement occurs on the top of the cavern, indicating that the cavern top is most liable to deformation damage. Although convergence of cavern fluctuates over the injection and withdrawal sessions, long-term convergence of cavern depends on the creep tendency of rock salt rather than the cyclic loading; rock salt with stronger creep tendency leads to larger cavern convergence. The results also demonstrate that there is stress concentration on the top and bottom of the cavern; the cavern wall is likely subject to shear failure according to Mohr‒Coulomb failure criteria, however, the cavern is unlikely to fail due to expansion according to Drucker‒Prager failure criteria. Both Mohr‒Coulomb and Drucker‒Prager failure indices are smaller in the cases where rock salt has stronger creep tendency. The proposed thermo-mechanical model provides an approach for evaluation of long-term stability of underground salt caverns.

Determination of permeability for hydrocarbon release due to excavation-induced stress redistribution in rock salt

Due to a stress redistribution after the excavation of an underground tunnel for radioactive waste disposal, an Ed/DZ (excavation disturbed/damaged zone) will be generated in the near field of the opening, resulting in significant changes in the hydraulic and mechanical properties of the rock mass in the zone. Initially more or less randomly distributed hydrocarbons at grain boundaries in rock salt, which sometimes can only be observed with ultraviolet light, can then be mobilised and migrate at a potentially significant rate towards the excavation.Within the international cooperative project DECOVALEX 2019, the migration mechanism of such fluid inclusions in rock salt is being studied intensively. A multi-scale modelling strategy has been developed. A macroscale coupled hydro-mechanical modelling of an underground excavation was performed to determine hydraulic and time-dependent deviatoric stress conditions, by taking into account the rock salt creep behaviour. Under the obtained macro-scale constraints, micro-scale modelling of a pathway dilation along halite grain boundaries was performed using different model strategies: a) coupled hydromechanical modelling with a consideration of hydraulic pressure-induced dilatant deformation, b) nonlinear dynamic model taking account of fluid migration, stress-dependent grain boundary wetting and shear-induced dilatancy of salt, and c) phase-field modelling of flow pathway propagation. The permeability increase resulting from the pathway dilation is estimated to be as high as two orders of magnitude. Based on the permeability determined, a series of pressure build-ups measured from a borehole with a high hydrocarbon release rate, a total of 430 build-ups within a monitoring time of 938 days, can be simulated with a macro-scale compressible flow model accounting for different zones around the opening.

Investigation of rock salt layer creep and its effects on casing collapse

Casing collapse is one of the costly incidents in the oil industry. In the oil fields of southwest Iran, most casing collapses have occurred in Gachsaran formation, and the halite rock salt layer in this formation may be the main cause for these incidents because of its peculiar creep behavior. In this research, triaxial creep experiments have been conducted on Gachsaran salt samples under various temperatures and differential stresses. The main purpose was to determine the creep characteristics of Gachsaran rock salt, and to examine the role of creep in several casing collapses that occurred in this formation. Results indicated that the halite rock salt of Gachsaran formation basically obeys the power law; however, its creep parameters are quite different from other halite rocks elsewhere. The time-dependent creep of Gachsaran rock salt exhibits strong sensitivity to temperature change; however, its sensitivity to variation of differential stress is rather low. The numerical simulation of the rock salt creep in a real oil well demonstrated the importance of creep and reservoir conditions on the safety factor of the tubing related to casing collapse.

Stresses and Temperature Affecting Acoustic Emission and Rheological Characteristics of Rock Salt

Synchronized acoustic emission and strain measurements were carried out in rock salt samples subjected simultaneously to different levels of uniaxial mechanical and incrementally increasing temperature effects. Methodological and hardware support of such measurements is described. Experimental dependences are obtained, which reflect changes in shear strains and acoustic emission activity of samples as functions of time and temperature for different axial stresses. As the stresses increase, rock salt transits to the stage of progressive creep at lower temperatures. The transition to each subsequent stage of the temperature effect is accompanied by an increase in the steepness of shear strains and activity-average acoustic emission. The patterns of changes in these parameters at the stages of steady and progressive creep of rock salt are analyzed. The advantages of using acoustic emission measurements to predict rock salt failure due to progressive creep, as well as their importance for solving the problem on estimating salt rocks properties in real thermobaric conditions for the construction and operation of underground gas storages are noted.

Long term creep closure of salt cavities

We quantify the long term creep closure of salt cavities under in-situ conditions. We use an incompressible Carreau viscosity model to treat combined dislocation and pressure solution creep of rock salt. We build three material models based on the published compilations of experimental data. Analytical expressions are given for the maximum closure velocity under combined pressure and shear loads in the far field. A non-linear finite element model is used to study the apparent viscosity around a circular hole. In the low stress regime, depending on the grain size, creep rate can increase several orders of magnitude by the action of pressure solution. The influence of grain size dominates over all other parameters such as temperature or dislocation creep parameters. In general, pressure solution is dominant in cold fine-grained salts under low loads, while dislocation creep is dominant in warm coarse-grained salts under high loads. In the dislocation creep regime, the hole closure rate is n (stress exponent) times more sensitive to the far field shear stress than to the pressure differences. Significant hole closure can be reached in less than one day in very fine-grained salts under in-situ loads in the order of 1MPa. A thorough understanding of salt grain size and water availability is needed to properly assess the relative contribution of dislocation and pressure solution creep in hole closure. A direct relationship relating grain size with deformation mechanism cannot be established in general. The closure of a cavity in a typical salt having a 10mm grain size can either be dominated by pressure solution or dislocation creep depending on the temperature and salt type. Best practices to prevent hole closure include increasing hole pressure, and avoiding, when possible, the salt layers which are known to be fined-grained and under high deviatoric stress.

Failure mechanism of bedded salt formations surrounding salt caverns for underground gas storage

Understanding the failure mechanism of bedded salt formations surrounding salt caverns is of great importance for underground gas storage. However, laboratory mechanical experiments of cores alone are insufficient to determine the mechanical properties of bedded salt formations, because the stress state of the cores varies spatially, and man-made damage might have occurred during the coring process, especially at the interfaces. Therefore, both physical simulation experiments and numerical analyses are needed to better understand the failure mechanism of bedded salt formations surrounding salt caverns. According to the physical simulation tests, the uniaxial and triaxial compressive strength curves of bedded salt rocks appear to be U-shaped as the dip angle changes, implying that shear failure may occur more easily at the top and bottom haunches of the cavern than elsewhere. Numerical analyses show that plastic zones occur initially at the top and bottom haunches of the cavern, which is accordance with the physical test results and theoretical analyses. For the two simulated models, the plastic zones in the interlayers tend to expand towards the model boundary to induce the instability of the salt cavern, particularly at the middle of the cavern with soft and weak interlayers after years of creep. Conversely, the plastic zones in the rock salt begin to occur at the top and bottom haunches of the cavern and then expand gradually to other places, albeit with a limited scope. The results suggest that the creep of rock salt can lead to the failure of interlayers in bedded salt formations, thereby affecting the stability of salt caverns.

Impact of rock salt creep law choice on subsidence calculations for hydrocarbon reservoirs overlain by evaporite caprocks

AbstractAccurate forward modeling of surface subsidence above producing hydrocarbons reservoirs requires an understanding of the mechanisms determining how ground deformation and subsidence evolve. Here we focus entirely on rock salt, which overlies a large number of reservoirs worldwide, and specifically on the role of creep of rock salt caprocks in response to production‐induced differential stresses. We start by discussing available rock salt creep flow laws. We then present the subsidence evolution above an axisymmetric finite element representation of a generic reservoir that extends over a few kilometers and explore the effects of rock salt flow law choice on the subsidence response. We find that if rock salt creep is linear, as appropriate for steady state flow by pressure solution, the subsidence response to any pressure reduction history contains two distinct components, one that leads to the subsidence bowl becoming narrower and deeper and one that leads to subsidence rebound and becomes dominant at later stages. This subsidence rebound becomes inhibited if rock salt deforms purely through steady state power law creep at low stresses. We also show that an approximate representation of transient creep leads to relatively small differences in subsidence predictions. Most importantly, the results confirm that rock salt flow must be modeled accurately if good subsidence predictions are required. However, in practice, large uncertainties exist in the creep behavior of rock salt, especially at low stresses. These are a consequence of the spatial variability of rock salt physical properties, which is practically impossible to constrain. A conclusion therefore is that modelers can only resort to calculating bounds for the subsidence evolution above producing rock salt‐capped reservoirs.

FEASIBILITY STUDY OF UNDERGROUND SALT CAVERNS IN WESTERN NEWFOUNDLAND: EXPERIMENTAL AND FINITE ELEMENT INVESTIGATION OF CREEP-INDUCED DAMAGE

Underground caverns in rock salt deposits are the most secure disposal method and a type of gas-storing facility. Gas storage plays a vital role in ensuring that a strategic relationship is secured between an established energy infrastructure provider and a midstream energy company. The Fischells Brook area is a pillow-shaped body of salts located in the St. George's Bay area of southwest Newfoundland, which has three layers of salt beds, and is capable of excavating caverns for the storage purposes. The development of cavern facilities requires the stability analysis through numerical models and experimental facilities. This work was motivated to examine the engineering feasibility of the salt cavern characteristics in this area, and to investigate its stability under creep behavior. An experimental test facility was developed to investigate the constitutive parameters governing the creep of rock salt, and the constitutive parameters were implemented into a developed finite element model to investigate the stability of the cavern over a 5-year period. Also a stress-based dilatancy failure envelope was developed to interpret the results of the numerical model, and to conduct sensitivity analyses for different design scenarios. The design recommendations developed in this study will be implemented as a key part of an engineering feasibility study for underground caverns in salt deposits in western Newfoundland.

Steady-State Creep of Rock Salt: Improved Approaches for Lab Determination and Modelling

Actual problems in geotechnical design, e.g., of underground openings for radioactive waste repositories or high-pressure gas storages, require sophisticated constitutive models and consistent parameters for rock salt that facilitate reliable prognosis of stress-dependent deformation and associated damage. Predictions have to comprise the active mining phase with open excavations as well as the long-term development of the backfilled mine or repository. While convergence-induced damage occurs mostly in the vicinity of openings, the long-term behaviour of the backfilled system is dominated by the damage-free steady-state creep. However, because in experiments the time necessary to reach truly stationary creep rates can range from few days to years, depending mainly on temperature and stress, an innovative but simple creep testing approach is suggested to obtain more reliable results: A series of multi-step tests with loading and unloading cycles allows a more reliable estimate of stationary creep rate in a reasonable time. For modelling, we use the advanced strain-hardening approach of Gunther–Salzer, which comprehensively describes all relevant deformation properties of rock salt such as creep and damage-induced rock failure within the scope of an unified creep ansatz. The capability of the combination of improved creep testing procedures and accompanied modelling is demonstrated by recalculating multi-step creep tests at different loading and temperature conditions. Thus reliable extrapolations relevant to in-situ creep rates ( $$10^{-9}$$ to $$10^{-13}$$ s $$^{-1}$$ ) become possible.

Parametric assessment of salt cavern performance using a creep model describing dilatancy and failure

The present paper discusses a systematic approach to study the time-dependent performance of underground storage caverns in rock salt. An elasto-viscoplastic constitutive model capable of describing dilatancy, short-term failure and long-term failure during creep of rock salt was implemented in a Lagrangian finite element formulation. Using the finite element formulation, a series of numerical analyses was performed to investigate the influence of the cavern geometry and non-salt interbeds on the performance of underground storage caverns excavated in rock salt in the framework of large displacements. The attained results demonstrated the capability of the proposed approach in performance assessment of the underground caverns.