ABSTRACT
Salt rocks are commonly used as the ideal geologic host rocks for storage of gas and crude oil, and are being considered for the disposal of radioactive waste. However, the sediment characteristics of salt rocks in China are greatly different with that in the many other countries. According to the requirement of the West-East Gas Transmission Pipeline Project in China, the existing salt caverns in Jintan Salt Mine, Jiangsu province, were selected to be natural gas storages after appropriate reconstruction. In this paper, the feasibility studies of the gas storage for the construction of Jintan gas storage project were introduced. The geological investigation of the host salt rock formation, the experimental analysis on the mechanical and creep characteristics of salt rocks and mud rocks, numerical study on deformation and stability of these caverns, and in-situ test in brine-filled cavern tests for two salt caverns were presented, respectively. It was concluded that the six salt caverns, Xi-1, Xi-2, Dong-1, Dong-2, Gang-1 and Gang-2 caverns, were suitable to be utilized as gas storages. The conclusions about usability and some suggestion for the old cavern reconstruction and running were as follows:
The mechanical and creep test showed that the creep characteristics of rock salt and rock salt containing mudstone were similar. Under differential stress 15.0 MPa, the steady creep strain rate of salt rocks was (2.5~5.0) × 10−5/h, that of salt rocks containing mudstone was (1.9~3.8)×10−5/h, and that of mud rocks containing salt was (0.5~1.0)×10−5/h. The steady creep strain rates of the three rocks were in the same order, so the cavern cap rocks would not have to suffer incompatible creep deformations.
The breakthrough pressures of the cavern roof rocks were between 6.68~21.25 MPa, which provided the caverns a good sealability. The six caverns were far from the faults, so, these faults would not affect the stability and sealability of gas storage.
The numerical analysis results of cavern stability and deformation showed that, with the increase of internal pressure, the maximum displacement and the volume reduction of salt caverns decrease gradually. Increasing internal pressure could restrain obviously the cavern volume reduction, so, in order to prolong the service life of gas storage, it was strongly recommended that the running time under low internal pressure should be reduced as soon as possible.
Allowable operational pressure difference between the adjacent caverns depended on the design service life and the minimum operational pressure. The results showed that the cavern stability could be guaranteed if the operational pressure was between 6 to 14.5 MPa. For safety, it was suggested that the operational pressure difference between two adjacent caverns should not exceed 3.0 MPa.
Based on the data of volume reduction rates of each cavern under different pressure reduction rate, a volume reduction rate of 0.015%/day was suggested to be the ultimate limit during a gas extraction phase. The maximum pressure reduction rate should not exceed 0.55 MPa/day during gas extraction phases.
400The results of brine pressure and discharge tests of Xi-1 cavern and Xi-2 cavern showed that the two caverns were sealedwell, and they could withstand the maximum internal pressure up to 15.43 MPa. Under differential stress of 5~10 MPa, the creep rate of rock salt 2.36×10−5/h obtained from laboratory tests agreed well with the result 2×10−5/h by inversion analysis of the cavern pressure test. Therefore, from the numerical simulation of a former gas storage running process, it was reasonable to adopt the above-mentioned creep parameters of the salt rocks from the Jintan Salt Mine.
