Mechanical Behaviour of Salt VII
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Their complexity has increased, including new aspects of salt behavior. Development of the creep lows comprises in three steps. These lows have been applied to correlate the strain — time curve in different temperature and stress by taking into account creep mechanism map.
In last years, in third step, early models were improved to take into account more phenomena as well: A version of Munson—Dawson Munson, included inverse creep, and both the MDCF model De Vries et al. On other hand, in last decades, in order to determine the stability and therefore feasibility of an excavation stability criteria have been developed in parallel with creep laws Labaune et al. In last decades, authors who research on stability and safety of caverns have focused on 3D numerical simulation using high-performance computers to understand the behavior of salt cavern in different condition.
However, because of complexities of damage processes such as rock bursting and splitting, only using numerical methods either continuum or discontinuum does not always provide an accurate representation of the physical settings of underground engineering specially salt cavern Labaune et al.
In addition, geomechanical calculations based on suitable material law creep low are required to quantitatively assess the system stability and integrity at the specific geological conditions Minkley et al. For example, Staudtmeister and Rokahr used Finite Element Method FEM for salt cavern dimensional analysis and stability evaluation in complex loading histories. They concluded that systematic rock mechanics experiments were necessary before numerical simulation.
The material laws must be able to properly capture the spectrum of mechanical properties of salt rocks and discontinuities, ranging from viscoplastic with time-dependent softening to extremely brittle fracture. Generation and growth of cracks would be limited, when the integrity and structural stability of cavern are satisfied.
In caverns including natural gas, the large amount of deviatoric stress is neutralized by brine Halmostatic pressure in construction stage and by gas in operation stage, nevertheless the pressure resulting from natural gas would not be large enough to prevent dilation or fracturing around cavern.
So, maximum pressure optimized pressure , which relates to working gas capacity, of natural gas must be determined and considered in every injection-withdrawn cycles. Based on modeling the distribution of stress state around cavern, the dilation stress, and subsequently fracture stress, is as a function of pressure changes rate of operation cycles and time dependent characteristics of rock salt Wallner, Operational pressure range must be as exactly as possible determined at which the stability of cavern either at minimum or maximum pressure is satisfied.
De Vries et al. Therefore, understanding the time dependent and time independent behavior of rock salt at such complicated stress state around the cavern are very important, it would be more difficult because of complexity in behavior of rock salt. On other hand, design concepts and construction of salt caverns are very complicated as well.
By now, various methods of design and criteria of stability, based on either Lab or in-situ investigations, have been investigated and applied. Various stability criteria has been proposed to limit the crack generation and growth around cavern. Some of them include a comprehensive set of parameters which have been applied in engineering works.
So far, several research studies have been conducted to assess the safety of the salt caverns as underground storage systems. Table 3 gives some of researches that have been performed by authors in cavern stability field. The research results show that the stability and availability evaluation of salt caverns are complicated and deeply concerned problems so that Staudtmeister and Rokahr concluded that systematic experiments on the rock mechanics of surrounding rock are the prerequisite for the analytical analysis and numerical simulation.
Table 3 Summary of researches performed in salt cavern stability in last decades. In this paper, first, the parameters, which have influence on short term and long term behavior of salt cavern, are introduced. In second section, some of researches performed to understanding salt cavern behavior and stability are given. In third and fourth section, concepts and methods of salt cavern design, also their development process are discussed respectively. Development of stability criteria, their advantages and disadvantages are discussed in fifth section.www.cantinesanpancrazio.it/components/korahic/330-spiare-una.php
Institut für Gebirgsmechanik GmbH: Computational modelling
It is assigned that, based on investigation and discussion, the so called RD stability criterion is more comprehensive than others. Rock salt in almost homogeneous salt formation creeps in geological time scale from which principal stress becomes Hydrostatic. At such the salt mass stress redistribution and resulting in the induced deformation around cavern tends to be uniform by which long term and stability of the cavern is satisfied.
In addition, the integrity of caverns constructed in inhomogeneous salt structures no longer solely relies on the quality of the surrounding salt: they also have to rely on the interaction between different geological formations Axel, As tightness is a decisive factor for storing in salt caverns, researchers focus on the tightness properties of the caverns to check their feasibility for storing different media.
Then, creep closure of salt cavern is one of most considerable issue in cavern stability and tightness which depends on geo-mechanical characteristics of rock salt and stress state around cavern. However, in condition where the stress state is not monotonic i. Also, based on investigation of uniaxial compression tests, Liu et al. Also, it was deduced that the reason of longitudinal cracks is because of deformation inconsistency between almost hard mud stone and rock salt at such the condition the lateral deformation of rock salt becomes larger than Cohesion. Otherwise, in triaxial stress states, the difference deformations are limited such that the longitudinal cracks would not be visible.
So, the mechanical properties of rock salt vary either bedded or dome greatly due to differences in the environments where they were formed, sediment components, crystal geometries, content and distribution of impurities, tectonic histories experienced, etc. Hou, ; De Vries et al. Based on various Lab investigation have been performed to understand the behavior of rock salt, it is concluded that the behavior is very complicated which depends on stress amount and history, temperature, moisture, etc.
But, authors have the same viewpoint in following features: Salt behaves like a fluid in the sense that it flows even under small deviatoric stress. Salt is a non-Newtonian fluid and its strain rate is proportional to a rather high power of applied deviatoric stress which means that the creep rate of a cavern is a highly non-linear function of its internal pressure or, more precisely, of the gap between the lithostatic pressure at cavern depth and its internal pressure Berest et al.
On the other hand, few well-documented field data are available. Some deep natural gas storages have experienced large volume losses; well known cases include the Eminence salt dome gas storage in Louisiana Baar, and the Kiel gas cavern in Germany Kuhne et al.
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Several shut-in tests or brine flow measurements have been performed in different sites. The results are, in general, heavily influenced by brine thermal expansion Berest et al. Based on Lab and in-situ investigations, various authors has proposed variant concepts. In s, based on investigation of pillar models, Dreyer proposed a concept by which it could be possible to determine the minimum internal pressure to stabilize cavern geomechanically.
Dreyer introduced a relationship between Maximum Permissible Pressure MPP and failure of cavern by which the structural stability of salt cavern was met. Based on the MPP, the minimum internal pressure of cavern was determined so that minimum plastic deformation would be induced around the cavern, whereas, the minimum thickness of pillar between two caverns was investigated by LPZ.
Based on the low amount of cohesion and tensile strength of rock salt, it was concluded that in the late the plastic zones would be appeared around the cavern even in low deviatoric stresses which was in contrast to PCR, so, it was concluded that as an unsuitable concept. Changing the design concept into the PCR encountered the high rate of creep closure of the cavern.
Up to s, more than natural gas and oil filled cavern was successfully constructed using Maximum Permissible Pressure Dreyer concept in Germany. In this concept, maximum pressure is determined as a function of the pressure gradient in cavern. Nevertheless, one of most important disadvantage of these concepts was that the second or induced stress state were not considered.
In s, the team headed by K. In this concept, the stress components running perpendicular to the internal pressure are used as the design parameter. These have to be a certain amount higher than the internal pressure. The zone that surrounds the whole cavern, and which satisfies this criterion, must also have a specific thickness to guarantee the integrity of the cavern. Figure 3 shows the development of Design Concepts in historical view. Various methods have been proposed by authors Fig.
Earlier, experimental methods were applied but they were unable to consider all parameters and characteristics included in integrity and stability of cavern. On other hand, Analytical Methods, because of their complexity, only have been applied in caverns including simple geometry. It is assumed in CM, salt has no micro-crack and behaves ductile in which volume changes during creep deformation and failure resulting from macroscopic creep rupture.
Based on Lab and in-situ investigations, attitude on behavior of rock salt has been changed, authors concluded that as loading rate and stress intensity reaching a certain level, mainly intercrystalline micro-fractures open; with increasing stress or deformation the micro-fractures grow and connect as well. Nevertheless, the CM is not suitable to describe such the behavior of rock salt. Based on the Lab investigation, dilation increases the permeability, it means that the mechanical damage affects on hydraulic behavior of rock salt. On other hand, temperature and pressure of stored material change as a part of thermo-dynamical behavior of stored material during injection and production.
The knowledge of the Thermo-Hydro-Mechanical THM material behavior of the in-situ rock salt is a main assumption to proof the static stability and tightness of salt which not considered in earlier methods. Salt caverns progressively close because salt deforms continuously creeps when subjected to shear stress resulting from the difference between the cavern pressure acting on the walls of the cavern and the in situ stress in the surrounding salt.