Abstract
Knowledge about the permeability of surrounding rock (salt rock and mudstone interlayer) is an important topic, which acts as a key parameter to characterize the tightness of gas storage. The goal of experiments that test the permeability of gas storage facilities in rock salt is to develop a synthetic analogue to use as a permeability model. To address the permeability of a mudstone/salt layered and mixed rock mass in Jintan, Jiangsu Province, synthetic mixed and layered specimens using the mudstone and the salt were fabricated for permeability testing. Because of the gas “slippage effect”, test results are corrected by the Klinkenberg method, and the permeability of specimens is obtained by regression fitting. The results show that the permeability of synthetic pure rock salt is 6.9 × 10−20 m2, and its porosity is 3.8%. The permeability of synthetic mudstone rock is 2.97 × 10−18 m2, with a porosity 17.8%. These results are close to those obtained from intact natural specimens. We also find that with the same mudstone content, the permeability of mixed specimens is about 40% higher than for the layered specimens, and with an increase in the mudstone content, the Klinkenberg permeability increases for both types of specimens. The permeability and mudstone content have a strong exponential relationship. When the mudstone content is below 40%, the permeability increases only slightly with mudstone content, whereas above this threshold, the permeability increases rapidly with mudstone content. The results of the study are of use in the assessment of the tightness of natural gas storage facilities in mudstone-rich rock salt formations in China.
Highlights
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
Jintan purepure rockrock salt salt as our basic ingredients and made and as our basic ingredients and madetwo twokinds kindsof of synthetic synthetic specimens—mixed specimens—mixed and layered—and carried out permeability tests. These synthetic synthetic specimens specimens can 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
Note: Y is the mass of the rock salt; N is the mass of the mudstone; C is the mass of cement; W is the mass of brine
Summary
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. 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. 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 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.
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