Rock salt possesses characteristics of low porosity, low permeability, reasonable short-term mechanical strength and stiffness and a propensity to creep stably under deviatoric stresses. Rock salt is soluble in water (~1:7 volume ratio for salt and saturated brine), allowing caverns to be dissolved, and salt caverns possess good safety characteristics (environmental and physical security). Studies in the last seven decades have led to commissioning of dissolved salt caverns for the storage of liquids, gases and even solid wastes [1
]. In several countries (USA, Germany, etc.), salt mines are used to store radioactive wastes. In addition to high safety, dissolved caverns with adequate borehole connections may have large volumetric capacity, fast injection and withdrawal speeds and low operating costs [7
]. Underground energy storage (oil, gas, compressed air) has been implemented in very thick salt or salt domes [8
]. The USA, Germany, France, Canada and other countries have established underground oil or gas storage, used for commercial short-term or seasonal storage, or as national strategic energy reserves (as in the USA Strategic Petroleum Reserve). Because of the demand for commercial and strategic energy storage, the implementation of large-volume salt cavern underground storage in China has begun.
The key to salt cavern storage security is to ensure the extremely low permeability of the rock salt so as to effectively block the leakage of oil and gas. This is straightforward in thick, clean, deep deposits. However, the rock salt in China we are working with has several characteristics, such as shallow depth, low salt thickness, high impurity content (hard gypsum mudstone, gray mudstone, salt mudstone, sandy mudstone), and so on [9
]. The physical and mechanical characteristics of these deposits are complex and challenging to measure. Therefore, it is an important design aspect to carefully study the layered rock salt, especially the permeability characteristics, and to develop an understanding using field data and laboratory tests to develop a model.
The permeability characteristics of rock salt have been extensively studied. Field tests show that rock salt permeability is generally less than 10−17
]. Beauheim and Roberts [13
] created a conceptual model for far-field Salado hydrology involved permeability in anhydrite layers and at least some impure halite layers. They note that the pure and most impure salt have negligible permeability because of low porosity and a lack of porosity inter-connection. Their research indicated that in the near-field of an opening in salt rock (excavation damaged zone (EDZ)), dilation, creep and shear can increase the permeability (this should be most severe at layer interfaces between salt and non-salt rocks because of deformation incompatibility). They report typical average permeability values for anhydrite of approximately 10−18
, and for pure halite less than 10−20
]. Popp [14
] combined gas permeability and P and S wave velocity measurements under hydrostatic and triaxial loading conditions on rock salt specimens from the Gorleben salt dome and the Morsleben salt mine. Isotropic loading markedly decreased permeability, tending toward the in situ matrix permeability (<10−20
), with a concomitant wave velocity increase because of progressive closure of grain boundary cracks. The experiments show that permeability change is not only a function of dilatancy, but also of microcrack linkage [14
]. Allemandou and Dusseault [15
], using before-and-after CAT-scans on 100-mm cores, showed explicit evidence of damage as a thick external annulus of slightly higher porosity (microcracks along grain boundaries), explaining why permeability is so sensitive to isotropic stress in the laboratory. Their results also showed large effects of increased stiffness (>50%) and unconfined compressive strength (>15%) in specimens that had been re-stressed to their in situ stress (annealed) for 72 h. Indeed, sampling damage and slow grain boundary annealing likely account for a substantial amount of the experimental scatter in laboratory measurements of permeability and transient creep in salt and can be taken as evidence that properties are altered in the EDZ in the ground.
In summary, during the investigation of the physical and mechanical properties of interlayers in bedded salt deposits and their effects on storage caverns, it is found that the porosity and permeability of salt and shale are minuscule and of the same order, and values for anhydrite may be somewhat greater. Specimen damage is a significant issue and must be recognized during test programs. In bedded salt storage caverns, anhydrite interlayers may present a greater risk of being a leakage path than shale interlayers for several reasons—creep incompatibility, stiffness and higher intrinsic permeability—so evaluation and testing are needed.
Stormont and Daemen [16
] used a pressure pulse method for rock salt with a permeability below 10−17
and found that the permeability within the EDZ is 10−16
, whereas the permeability of intact salt is less than 10−21
. Wu et al. [17
] tested the permeability of rock salt under different osmotic pressures and compared results using the Klinkenberg effect and the quasi-static pressure method. Yan et al. [18
] analysed carbonate strata storage permeability and established relative models for excavation radius, permeability and porosity. Chen et al. [19
] used the equivalent boundary gas percolation model to study the gas pressure distribution in the surrounding rock within five years under different injection pressure conditions in a salt cavern natural gas storage and found that the permeability of bedding surfaces between rock salt and non-salt interlayers has an important influence on the reservoir pressure distribution.
Other similar cases involving materials from salt cavern gas storage core holes have been studied in China to generate models that can allow some generalization of the results. Taking the rock salt underground oil and gas storage facility in Jintan, Jiangsu Province, as a prototype, Zhang et al. [20
] developed a reservoir medium geomechanical model for permeability. Ren et al. [21
] developed similar materials for cavity experiments, and based on similarity, Jiang et al. [22
] developed an artificial model material of rock salt with interlayers.
There are few studies on the permeability characteristics of mudstone-rock salt mixes in the laboratory or in the field, so it is difficult to provide reliable guidance for the construction and maintenance of the pressure integrity for caverns in salt strata with non-salt interlayers. A representative stratigraphy of salt caverns in China is shown in Figure 1
In this paper, we report on the development of a model material of rock salt with mud and then study and analyze its permeability. We took a storage cavern mudstone interlayer from Jintan and pure rock salt as our basic ingredients and made two kinds of synthetic specimens—mixed and layered—and then carried out permeability tests. These synthetic specimens can be made with any mudstone content, and once a specimen is made, it is straightforward to study the relationships between permeability and mudstone content. Indeed, the permeability testing reveals a reasonably regular relationship between the permeability of rock salt with mudstone and mudstone content, and we then explained the experimental phenomena and results. These results may help provide some methods and guidance for the study of the tightness of layered rock salt gas storage facilities in China.
(1) During the experiment, it was found that the mudstone is more compressible than rock salt, and the compressibility of rock increased with the increase of porosity.
(2) The pseudo-pressure seepage equation has extensive application value, so the specimen permeability was calculated by the pseudo-pressure method. The Klinkenberg permeability of the synthetic specimens was obtained by curve-fitting. The permeability of synthetic pure rock salt was 6.93 × 10−20 m2 and its porosity 3.8%, and the permeability of synthetic pure mudstone was 2.97 × 10−18 m2, with a porosity 17.8%. The test results were close to the natural specimens, and it shows that this synthetic material permeability model is a reasonable analogue of use in engineering.
(3) Comparing permeability test results for the two kinds of synthetic specimens, it was found that the permeability of the mixed specimens was about 40% higher than that of the layered rock salt specimens at the same mudstone content. This suggests that in salt cavern gas storage cases, the layered rock salt is tight at the top and bottom of the cavity. The permeability of the transition zone of rock salt and mudstone around the cavity is higher, and the transition zone would be the permeable channel to be concerned with; although the permeabilities are still very low, and there would also be the beneficial effect of capillary blockage of the brine-filled channels in the field case.
(4) We also found that for both kinds of specimens, there was a strong exponential relationship between Klinkenberg permeability and mudstone content: when the mudstone content is below 40%, the Klinkenberg permeability increases only slightly with mudstone content, whereas above this threshold, the Klinkenberg permeability increases significantly.
Note: The formation mechanisms of natural rock specimen and synthetic specimen were different. Natural rock was formed in the long-term diagenesis, while the synthetic specimens in this paper were formed in ultra-high pressure. The porosity and permeability of synthetic specimens were similar to the natural specimens, but the natural rock (mudstone/salt) had different pore space structures compared to the synthetic specimens. Therefore, the approach, respectively the outcome (permeability/porosity relationship), can only be a first attempt at investigation.