Experimental investigation of mechanical behavior of bedded rock salt containing inclined interlayer
Introduction
Rock salt has excellent ductility [1], [2] and good self-healing features when damaged [3], as well as extremely low permeability [4], [5]. Therefore, salt formations are commonly used as host rocks for storage of crude oil or natural gas [6], [7], and are also considered for permanent disposal of radioactive and other, e.g. chemical, wastes [8], [9]. For the most effective utilization of storage caverns in salt formations, the ideal physical and mechanical characteristics compared with other rock types, such as sandstone, limestone, granite, etc., have been regarded as a prerequisite for the licensing of a repository [10], [11]. More recently, pre-evaluations have been promoted to study the feasibility of utilizing salt formations to store compressed air or hydrogen by taking advantage of the almost impermeable characteristics and rheological responses of rock salt [12], [13].
The salt formations used for energy storage consist of either bedded rock salt [14], [15] or salt domes [16]. The mechanical properties of rock salt vary greatly due to differences in the sedimentary deposition environments, in which they were formed, sediment components, crystal geometries, content and distribution of impurities, tectonic histories experienced, etc. [15]. Especially in China, the rock salts available for caverns are mainly of deep-water lacustrine deposits, which are characterized by containing numerous insoluble nonsaline interlayers (anhydrite, glauberite, mudstone, shale, dolomites, etc.), thin and low-grade salt layers, and variable complex structures. So the bedded rock salts in China are typically thinly interbedded formations. In addition, such formations of bedded rock salts are usually tightly correlated with graben structure basins, thus a difference of deposition exists between center and edges and then results in the formation of inclination, folds, minor faults, etc., which further complicates the structures of the formations. Compared to the salt domes with massive salt real estate, the total geo-sections of bedded rock salts in China available for cavern construction are usually less than 100–150 m thick, and their complex geo-structure increases further the difficulty of cavern construction.
Due to the difference in mechanical behavior of the adjacent interlayers and salt layers, especially after the excavation of a cavern, inharmonious deformation and also shear stress will be induced in the interface regions, which may result in slippage cracks developing along the interfaces [17], [18]. Then the stability and tightness of the storage cavern may be seriously affected. DeVries et al. [14] studied the roof stability of storage caverns in bedded salt formations. Minkley et al. [19] established constitutive models to describe the mechanical behavior of rock salt with imbedded weakness planes. This model includes not only the hardening/softening behavior and dilatancy effects for rock salt, but also the displacement-and viscosity-dependent shear strength softening for salt bearing bedding planes. Yang and Li [20], [21] established 2-D and 3-D Cosserat-like models for bedded rock salt, which take into account the compatibility of the meso-displacement between two different layers and also the bending-effect. In China, these models have been gradually used as an appropriate constitutive theory and failure criterion for rock salt imbedded with numerous thin interlayers [22].
The bedded salt mines used for study and for construction involve mainly horizontal interlayer lithology [15], [16], [17], [19], [20], [21], [22], [23]. In more recent years, several rock salt mines in China with stratum dip angles of 10–30° have been selected as Strategic Storage Candidate Sites to construct deep storages cavern clusters. Mechanical properties of the rock salt are required for designing the caverns with the necessary stability, tightness and safety. However, the experimental and theoretical information resources available for bedded rock salt, especially the bedded rock salt with inclination, are still limited.
Generally, the dip angle of geological materials will influence their engineering features, which is a well recognized phenomenon in slope [24] and tunnel projects [25]. It may behave similarly for deep underground storage facilities in salt formations. In addition, due to the coupling effect of inclination and the mechanical characteristic differences of the rock salt versus the nonsaline interlayers, higher shear stress concentration may occur near the interface between the layers. This phenomenon may result in shear slippage, mechanical damage, and even cause new seepage channels to develop along these interface regions. Therefore, it is of urgent necessity to conduct experimental and theoretical investigations on the properties of these typical lithotypes and to include in these studies the fact that these formations frequently are dipping.
Based on the research background and project demands stated above, we conducted research to study the features of the inclined rock salt, especially the mechanical behavior of the interfaces and interlayers, with the expectation to supply some basic information support for the gas storage clusters to be constructed in such formations. The test matrix pursued in this paper has the main purpose of determining the deformation and fracture characteristics of inclined bedded rock salt. The results and analyses of the experiments are presented. We use the term “rock salt” to refer specifically to halite, and halite with impurities; we use the term “composite rock salt” to refer to the special lithotype consisting of rock salt and interlayer; and we use “interlayer” to refer to the nonsaline stratum intersecting or overlying the cavern.
Section snippets
Specimen preparation
In order to study the deformation and fracture characteristics and also the crack initiation mechanism of interfaces in bedded rock salt with inclined interlayers, elemental components’ analysis, as well as uniaxial and triaxial compression tests, have been carried out on eighteen specimens. All the specimens were obtained from the same pilot well, whose coring depth ranges from 1400 to 1800 m, from the Pingdingshan Salt Mine in He׳nan Province of China. Because of the crystallization matrix of
Typical stress–strain curves of uniaxial compression tests
Nine specimens have been tested in uniaxial compression, three for each lithotype. Due to the limited amount of core available, only the composite rock salt containing medium-thick interlayer specimens (thickness~10–40 mm) was selected for uniaxial compression tests. This group consists of two pure rock salt specimens (S-1, S-2) and one impure rock salt specimen (S-3).
From the uniaxial tests, it is discovered that the pure rock salt specimens are the weakest, having the lowest elastic modulus
Stress–strain curves of triaxial compression tests
The triaxial compression tests included in this paper have been carried out on 10 specimens, whose physical information is listed in Table 1. Due to the limited number of composite rock salt specimens containing thin or medium-thick interlayer, the triaxial compression tests have only been conducted under confining pressure of 5 MPa (two specimens) and of 10 MPa (eight specimens). The differential stress–axial strain curves of three lithotypes are shown in Fig. 8. The testing results of each
Discussion and engineering applications
Based on the analysis of the uniaxial and triaxial compression experiments, the thinner the interlayer, the weaker the constraint affecting the salt region. The thinner the interlayer, the more obvious the ductility of the overall performance. And the possibility of initiation of cracks near the interlayer is reduced in such formations. The thicker the interlayer, the stronger the constraint affecting the salt region and the more obvious the brittleness of the overall performance. The
Semi-theoretical and semi-qualitative analysis of the fracture mechanism
Due to the composite material properties of bedded rock salt, the complex relationship of stress and strain results in the interface becoming the most likely zone where cracks may first initiate and propagate [18], [22], [26], [34]. The geometries and characteristics of cracks near the interface will directly influence the tightness and stability of the caverns, because the mechanical properties of the interface have been damaged by the cracks. In this way, the analysis of the interfaces’
Conclusions
Based on the experimental and theoretical analyses presented, some probable useful conclusions have been made as follows:
- 1.
The inclined interlayers obviously affect the deformation and fracture properties of the composite rock salt. One consequence is to cause different fracture types at the top, middle and bottom locations of interlayers.
- 2.
The interface is the most likely zone where cracks may initiate first. The propagation of the crack tips near the interface causes the curvature of extended
Acknowledgment
The authors acknowledge the financial support from National Basic Research Program of Science Foundation of China (973 Program) (Nos. 2009CB724602 and 2009CB724603), the National Natural Science Foundation of China (Nos. 51274187 and 41272391) and the Youth Science Foundation of China (No. 51304187).
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