An extensive experimental program on rock salt comprising short-term and long-term tests has been performed on samples from the same origin and prepared and preconditioned using the same protocols. The main targets are to investigate the thermo-mechanical response of rock salt under different load paths and to produce a large database on which constitutive models can be formulated and calibrated. A total of 17 tests have been conducted, including four uniaxial experiments. Temperatures range between 8 and 60 °C, and some tests last more than 2 years. The experiments cover a differential stress range relevant for underground applications. Deviators within 0.2−4.5 MPa have been investigated through uniaxial creep experiments in salt mines to take advantage of very stable ambient conditions (particularly, temperature and relative humidity). Deviators up to 35 MPa have been investigated through confined experiments in the laboratory. Additionally, a cross-check quasi-uniaxial test (confinement of 0.2 MPa) has been performed in the laboratory under conditions similar to those of the mine, and proves that experiments in both settings can be combined to extend the range of investigated stresses. However, uniaxially loaded samples show higher strain rates than confined samples. X-Ray computed tomography suggests more micro-fracturing during the former. The results obtained under confined conditions are consistent and confirm the different stress dependency of the creep rate under low and high deviators. A modified Lemaitre model is used to analyze the results. Next steps include microstructural investigations to gain insight into the dominant deformation mechanisms under different thermo-mechanical loads, allowing for more predictive constitutive models.

In gas storage operations, intensive injection and withdrawal cycles can introduce structural failure due to thermally induced stresses, leading to the detachment of overhanging blocks from salt cavern walls. This study investigates the thermomechanical mechanisms governing block detachment in underground hydrogen storage using a joint enriched finite element method (JFEM) combined with cohesive zone modelling in DISROC software. An axisymmetric cavern geometry based on the Huntorf NK1 design is modelled, incorporating three overhanging blocks and both steady-state and transient creep components in the salt mechanical response. Comparative analysis of two salt types (fast-creeping Etrez and slow-creeping Asse) under identical thermomechanical loading reveals that rapid wall cooling during depressurisation (-13.8 bar/d) induces thermal contraction and tensile stresses exceeding rock salt tensile strength (1.5 MPa). Critical fracture initiation occurs at similar to 15 degrees C regardless of salt type, with block detachment sequences nearly identical between both salts (timing difference <6 h). The results demonstrate that thermal stresses (1 MPa/degrees C cooling) dominate failure mechanisms over steady-state creep, while transient creep accelerates stress redistribution and damage localisation. These findings highlight that temperature control, rather than creep properties, governs cavern stability, emphasising the critical need for thermal monitoring and controlled depressurisation rates in underground hydrogen storage operations.

In 2014–2016, creep tests were performed in a dead-end drift of the Altaussee mine, where temperature and relative humidity experience very small fluctuations. These tests, which were several months long, proved that the creep rate of a natural salt sample is much faster in the 0.2–1 MPa deviatoric stress range than the creep rate extrapolated from standard laboratory creep tests performed in the 5–20 MPa range. In addition, the quasi-steady strain rate is a linear function of stress, and it is faster when grain size is smaller. These findings were consistent with microphysical models of pressure solution creep (rather than dislocation creep, which is the governing creep mechanism at high stresses). A gap in experimental data remained in the 1–5 MPa range, calling for a follow-up experimental program. In 2016–2019, three multi-stage creep tests were performed on salt samples from Hauterives (France), Avery Island (Louisiana, USA), and Gorleben (Germany), which had been tested in the 0.2–1 MPa range during the 2014–2016 campaign. Loads of 1.5, 3, and 4.5 MPa were applied successively on each sample for 8 months. Steady state was not reached at the end of each 8-month stage. However, tests results suggest that, in the 0.2–3 MPa range, the relationship between the strain rate and the applied stress is linear, a characteristic feature of pressure solution. For these three samples, the relationship between strain rate and deviatoric stress departs from linearity when the deviator is larger than approximately 3–4.5 MPa, pointing to a transition to dislocation creep at higher deviatoric levels.

La Revue Française de Géotechnique (RFG) est une publication scientifique parrainée par les comités français de mécanique des sols, de mécanique des roches, de géologie de l'ingénieur et des géosynthétiques

In this paper, some remarkable aspects of transient mechanical behaviour of salt caverns are discussed. The salt reverse creep was observed in some laboratory and in-situ tests following a significant unloading. The salt mass stress evolution with time is responsible for a very long transient behaviour of salt caverns and hydro-fracturing at a pressure much smaller than that expected with elastic behaviour.

L’étude de la stabilité mécanique des cavités salines servant au stockage de l’hydrogène est un sujet important dans le contexte de la transition énergétique. Alors que l’hydrogène émerge comme un vecteur énergétique clé pour le stockage d’énergie décarbonée, comprendre les réponses mécaniques des cavités salines autour des pressions d’exploitation est essentiel. Cet article présente une analyse détaillée des effets des pressions maximales et minimales constantes sur une période de 30 ans sur la perte de volume par fluage, la contrainte effective, le facteur de sécurité des cavités salines vis-à-vis de l’apparition de la dilatance ainsi que la subsidence en surface. En utilisant une modélisation géomécanique couplée, cet article explore l’intégrité structurelle d’une cavité soumise à des conditions de chargement extrêmes sur une longue période, fournissant des informations essentielles pour le dimensionnement, l’exploitation et la maintenance des futurs sites de stockage souterrain d’hydrogène gazeux.

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.

ABSTRACT: From 1996 to 2021, uniaxial creep tests were performed on salt samples in dead-end drifts of the Varangéville (France) and Altaussee (Austria) mines to take advantage of constant temperature and hygrometry. The applied loads were from 0.05 MPa (relative) to 4.5 MPa, i.e., much smaller than the loads currently applied during standard creep tests performed at the laboratory. Main conclusions are: (1) Steady state is reached after a long period (longer than 8 months). (2) Cumulated transient creep is relatively large (3) Strain rates are faster (than extrapolated from high stresses) by 4-5 orders of magnitude (4) Steady state strain rate is a linear function of the applied stress (approximately) in the σ < 3 MPa domain (5) Strain rate is a decreasing function of grain size (6) The transition between the linear (n = 1) and the non-linear (n = 3 to 5) behavior seems to range between 3 MPa and 4.5 MPa (7) No creep is observed in a very dry environment (8) No threshold for salt creep (or smaller than 0.05 MPa) is observed (9) In the small stress domain, reverse creep is observed. 1. INTRODUCTION It has been suspected for long (Spiers et al., 1990; Urai and Spiers, 2007) that, in the small deviatoric stress domain (σ < 3 MPa), the governing mechanism for salt creep was pressure solution — rather than dislocation creep. A consequence should be that creep rate in this domain is much faster - by several orders of magnitude - than extrapolated from tests performed in the high stress domain. In addition, creep rate should be a decreasing function of grain size; it should be a linear function of the applied stress, and the presence of a small amount of brine at the grains interface should be a necessary condition for active creep. These statements were based on theoretical arguments, geological evidence and the results of tests performed on artificial salt.

The KEM-17 project of the Dutch State Supervision of Mines presented a critical review of concepts of cavern abandonment and related science. It recommended that analyses of cavern abandonment are done as an integrated project, addressing (i) micro-scale physical processes, (ii) cavern scale models based on field scale experiments and numerical models, (iii) the salt dome scale, to model the far field of the salt caverns. The Cavern Closure Consortium (CCC) project is based on this, focusing on the Haaksbergen and Heiligerlee cavern fields in the Netherlands. We build on (i) innovative deformation experiments, integrated with state of the art microstructural analysis to define constitutive equations for deformation and permeation, focusing on the poorly understood domain below 8 MPa differential stress, (ii) numerical finite element models of the cavern field combined with cavern-scale field experiments to define the closure parameters and temperature evolution of the cavern, and (iii) state of the art numerical models at the scale of the whole salt pillow or salt dome to define the "ist-Zustand". All these contributions are closely integrated and will lead to much improved prediction of the evolution of the caverns after closure and abandonment.

To prevent climate change, Europe and the world must shift to low-carbon and renewable energies. Hydrogen, as an energy vector, provides viable solutions for replacing polluting and carbon-emitting fossil fuels. Gaseous hydrogen can be stored underground and coupled with existing natural gas pipe networks. Salt cavern storage is the best suited technology to meet the challenges of new energy systems. Hydrogen storage caverns are currently operated in the UK and Texas. A preliminary risk analysis dedicated to underground hydrogen salt caverns highlighted the importance of containment losses (leaks) and the formation of gas clouds following blowouts, whose ignition may generate dangerous phenomena such as jet fires, unconfined vapor cloud explosions (UVCEs), or flashfires. A blowout is not a frequent accident in gas storage caverns. A safety valve is often set at a 30 m depth below ground level; it is automatically triggered following a pressure drop at the wellhead. Nevertheless, a blowout remains to be one of the significant accidental scenarios likely to occur during hydrogen underground storage in salt caverns. In this paper, we present modelling the subterraneous and aerial parts of a blowout on an EZ53 salt cavern fully filled with hydrogen.

Abstract Rock salt is a very complex material. Simulation of the non-linear and time-dependent mechanical behavior of salt caverns requires advanced constitutive models, relevant sets of parameters and accurate numerical computations. No well-known software packages were designed initially for salt caverns; they had to be adapted by the end user due to the unusual behavior of salt compared to other rocks. Most of the time, cavern thermodynamics, salt geomechanics and hydraulics cannot be calculated simultaneously. Some software packages are limited only to rock/soil mechanics and thermal computations —they include no cavern thermodynamics. However, coupling cavern thermodynamics and rock mechanics is essential when considering problems such as fast cycling in a gas cavern, the storage of hydrogen or compressed air, or for modeling of the long-term behavior of caverns. A case-by-case study must be performed for all new projects, taking into account relevant creep and also considering sets of parameters that were calibrated accurately in the laboratory — or better, from on-site tests performed in an existing borehole or cavern. This paper introduces the main features of salt-rock mechanics and salt-cavern thermodynamics. A simple example of a cycled hydrogen cavern experiencing a blowout is presented. Introduction Storage of gaseous and liquid hydrocarbons in salt caverns is a mature technology. More than 2000 caverns are operated worldwide. Storage of electricity as compressed air in salt caverns (CAES) is possible; two caverns have been operated for this purpose since the 1970s, and a couple of new projects are under development (Koopmans et al., 2022). There also are many ongoing projects related to hydrogen storage (Fig. 1). Salt caverns initially were designed, created and operated using very limited modeling tools. The complexity of rock-salt behavior and of cavern thermodynamics appeared gradually.

In current economic and environmental contexts, the optimization of long, horizontal well completion and the maximization of individual well performance are becoming increasingly important. The challenge is to be able to start improving the production efficiency while designing an adapted completion for each well without compromising the project economy. The cost-effective formation evaluation technique described in this paper allows rapid identification of dynamic heterogeneities along the reservoir after the drilling of a horizontal well. This key information then can be used to optimize well completion and treatment. This new approach, called WTLog, combines well testing and logging techniques and was introduced initially for the optimization of unconventional well completion (Jacques et al., 2019 and Manivannan et al. 2019). The log begins by circulating a low-viscosity liquid that can be injected in the formation through the mud cake. The brine circulation operation is run at the end of the drilling phase, after reaching TD of the drain while maintaining a constant wellhead pressure at the wellhead. The constant pressure control can be applied without a specific additional choke device when Managed Pressure Drilling (MPD) is used to drill the formation section. The inlet and outlet flowrates are measured accurately, and their difference corresponds to the apparent formation-injection rate. The depth of the interface between the two liquids inside the borehole is estimated from the flowrates and pressure measured at the wellhead. Combining these data allows derivation of the low-viscosity/liquid-injection profile along the open hole. A permeability log then can be derived by inversion. Well Test Logging has been applied successfully for the first time on two horizontal wells in a conventional carbonate reservoir. The interpretation results were benchmarked to static conventional openhole logs and validated against the data log obtained by the dynamic production log tool (PLT) performed after well start-up. This technique opens new perspectives for optimizing well completion in these carbonate-fractured formations for which porosity logs might not be a good permeability indicator and where conductive fractures seen on image logs are not always indicative of future production.

The development of carbonate reservoirs of a giant field, Offshore Abu Dhabi, requires long horizontal wells to maximize productivity, but at the risk of unwanted gas and water channeling through its inherent heterogeneities. Conformance can be enhanced with dedicated segmented completions (blank sections, Inflow Control Device, Autonomous Inflow Control Device, etc.) or selective acid stimulation (diverter, Limited Entry Liner), which are increasingly implemented to extend well life, and eventually well value. If these technologies have matured, success depends heavily on the quality of the formation knowledge prior to completion. As of today, conventional logs provide the basic ground, but they lack dynamic information, whereas production logging results are obtained too late, when the well is already completed. Initially introduced for the optimization of unconventional well completions (see Jacques et al, URTEC 2019), the Well Testing Logging (WTLog) offers the advantage to record a log of mobility, at the end of drilling the openhole, enabling a favorable timing to influence adapted completion and stimulation design. Contrasted viscosity brines are sequentially circulated through the drill pipes at a constant rate and back-produced from the casing at constant pressure. The fluids interface travels in the drain from the TD to the casing shoe, and the measurement of the differential formation seepage is interpreted into an injectivity profile. Combined with rate fall-off phase analysis, permeability and skin logs are derived. Lasting a few hours and realized with conventional rig equipment (such as cement pumps, coriolis flowmeters, Managed Pressure Drilling system), it is a nonintrusive, safe, and ultimately low-cost operation. Forward, it can replace costly logging, when aimed at characterizing heterogeneities. Within a year, the two first WTLog pilots of the Middle East were successfully designed and carried out. They targeted two appraisal wells in distinct undeveloped reservoirs (Cretaceous and Upper Jurassic formations) which benefited from rich acquisition programs (Image log, Production log) to benchmark and qualify this technology. After an explanation of the technology principles, this paper describes the design, operations, and results of these pilots. It then focuses on the petrophysical consolidation of the matrix/fracture characterization. It concludes by sharing the learnings and offers insight to what extent it is a promising technology to be applied in Middle East carbonate reservoir developments.

Ensuring the stability and integrity of underground gas storage salt caverns is a very complicated subject due to the non-linear and time-dependent behavior of rock salts under complicated thermal and mechanical loading conditions. For this reason, pressure and temperature fluctuations in the caverns and their surrounding strata must be integrated into the analysis and the numerical tools that are used for this purpose. LOCAS, a 2D axisymmetric finite-element code, dedicated to the stability analysis of underground salt spaces, was applied to assess the effects of various operating and geometrical parameters on the cavern behavior. In this paper, we aimed to give an overall assessment of the behavior of the salt caverns used for natural gas storage. In this work, some specific loading scenarios were considered first, followed by thorough parametric and sensitivity analyses to reveal the impacts of the geometrical parameters and operational parameters involved on the behavior of salt caverns using the modern stability criteria. The findings showed that the onset of dilation was more likely to happen within the first cavern life cycle when pressure dropped to the minimum level. As for the potential of tension occurrence in the surrounding rock, this is more likely to happen by increasing the number of operation cycles, especially in the upper one-third of the cavern wall. Finally, it was seen that the cavern depth and minimum cavern internal pressure had even more important influences than the others on the salt cavern behavior.

Tightness is a fundamental prerequisite to any underground storage. In storage salt caverns, a safe maximum admissible pressure must be selected to avoid product loss. The tensile strength of salt is small, and cavern pressure must be kept lower than geostatic pressure or, more precisely, lower than the least compressive stress at the cavern wall. The vertical stress can be assessed through density logs. The redistribution of stresses in the rock mass, due to the visco-plastic nature of rock salt, must be taken into account. A couple of cases in which a hydraulic connection between one cavern and another cavern, or between a cavern and the edge of a salt dome, are known. These connections originated in geological anomalies rather than in the creation of a fracture. There exists a pressure threshold, lower than the geostatic pressure, for which micro-fracturing and an increase in salt permeability occur, vindicating the position that a safety margin is needed when selecting the maximum pressure. Well tightness is important as well; it depends on several factors, among which are the quality of the cement, and the maximum fluid pressure in the cavern and along the access well. A tightness test is mandatory. The Nitrogen Leak Test is the most common such test. A review of selected gas-storage sites shows that, in most cases, the maximum admissible gradient at the casing shoe is 0.018 MPa/m (0.8 psi/ft), and up to 0.019 MPa/m (0.85 psi/ft) in some American states, values that are consistent with the considerations listed above.

A creep test performed on a Landes salt sample during one year and a half is described. During the first year, a 0.6 MPa axial load is applied to the sample. At the end of this one-year phase, strain rate (9x10(-12)s(-1)) is much faster than the strain rate extrapolated from high-stress tests. Steady state strain rate is not reached. In an attempt to reach steady state strain rate "from below", a 0.9 MPa load is applied during two days before restoring the initial load (0.6 MPa). After the load is restored, reverse creep is observed first (strain rate sign changes before vanishing to zero after a few hours). Then, strain rate increases to reach 5 x 10(-12)s(-1) after five months, slower than the strain rate before the load change. Commonly accepted constitutive laws can explain this effect, which provides a lower and an upper bound for steady state strain rate. This note presents a method to determine such bounds.

The objective of this paper is to assess the creep law of natural salt in a small deviatoric stress range. In this range, creep is suspected to be much faster than what is predicted by most constitutive laws used in the cavern and mining industries. Five 2-year, multistage creep tests were performed with creep-testing devices set in a gallery of the Altaussee mine in Austria to take advantage of the very stable temperature and humidity conditions in this salt mine. Each stage was 8-month long. Dead loads were applied, and vertical displacements were measured through gages that had a resolution of 12.5 nm. Loading steps were 0.2, 0.4, and 0.6 MPa, which are much smaller than the loads that are usually applied during creep tests (5–20 MPa). Five salt samples were used: two samples were cored from the Avery Island salt mine in Louisiana, United States; two samples were cored from the Gorleben salt mine in Germany; and one sample was cored from a deep borehole at Hauterives in Drôme, France. During these tests, transient creep is relatively long (6–10 months). Measured steady-state strain rates ( $$\dot {\varepsilon }$$  = 10−13–10−12 s−1) are much faster (by 7–8 orders of magnitude) than those extrapolated from relatively high-stress tests (σ = 5–20 MPa). When compared to n = 5 within the high-stress domain for Gorleben and Avery Island salts, a power-law stress exponent within the low-stress domain appears to be close to n = 1. These results suggest that the pressure solution may be the dominant deformation mechanism in the steady-state regime reached by the tested samples and will have important consequences for the computation of caverns or mines behavior. This project was funded by the Solution-Mining Research Institute.

In wells producing water, oil, gas or geothermal energy, or in access wells to hydrocarbon storages, it is critical to evaluate the permeability of the formation as a function of depth. Continuous permeability logs in these wells are typically derived using tools that measure electrical, nuclear, magnetic or acoustic signals, using empirical relations that are often formation dependent. The permeability logs derived using these empirical relations often show significant differences when compared to the permeabilities obtained from core samples or well tests. A new technique is proposed in this paper in which the open hole is scanned with an interface between two fluids with a large viscosity contrast. The injection rate into the formation depends on interface location and well pressure history. An inverse problem is solved to estimate permeability as a function of depth from the evolution of flow rates with time. During the test, the well is equipped with a central tube, typically a drill string, and the scanning is done by injecting in the central tube a liquid that is different from the liquid in the annulus, at a constant wellhead pressure. Injection and withdrawal rates are measured at the tubing and the annulus wellheads, respectively; the difference between these two rates gives the formation injection rate. Interface location is also estimated from the flow rates and pressure at the wellhead and an injection profile in the open hole is derived. A permeability log is derived from this injection log by considering a radial, monophasic flow in each layer and same skin value for all formation layers. Initial formation pressure and storativity, estimated from other logs, are also used as inputs. The sensitivity of the permeability log to these inputs is estimated using analytical expressions. The proposed methodology is applicable to oil or water bearing formations drilled using oil or water-based muds, respectively. A continuous permeability log is estimated from the synthetic test data using the proposed interpretation workflow; it shows a correlation of 0.95 (on a scale of 0 to 1) when compared to the input permeability log. A laboratory model that mimics a multi-layered formation is used to study the repeatability of the technique and the validity of the uniform skin assumption by creating a mudcake at the inner radius. Four consecutive tests were performed on the same set of samples and the interpreted permeability logs are compared to the benchmark permeability log; correlations are greater than 0.94.