Experimental and theoretical research on the debrining process in sediments for a gas storage salt cavern

The sediments account for a large part of the total mined volume of the cavern in high impurity salt mines. This paper was aimed at the scientific problem of gas storage in the pore space of the sediments, and expanded the storage capacity of salt cavern in high impurity salt mines. Debrining in the mined space of salt caverns (contained sediments) is a vital step to complete the gas storage. To experimentally observe the debrining process, debrining experiments are carried out using a self-developed experimental device. The debrining process can be divided into four typical stages, e.g., rapid increase of injection gas pressure, discharging the upper brine, i.e. the brine free of sediments, discharging the brine in sediments, and rapid decrease of injection gas pressure. A mathematical model is proposed for calculating debrining parameters of a salt cavern including sediments based on the pressure equilibrium principle at the gas-brine interface. A numerical method is established for calculating the total debrining time of irregular salt caverns including sediments. The effective cross-sectional area of debrining process is calculated by using the porosity of the sediments. A linear function of the porosity of the transition zone of the sediment surface is put forward. Therefore, the porosity of the upper brine space can gradually transition to the sediments, avoiding the error of sudden increase of the gas-brine interface and debrining rate, which is consistent with the practical debrining process. The porosity of sediments is considered to be a decaying exponential function of depth based on the accumulation of sediment particles. The crosssectional area at any depth can be obtained through the interpolation of the sonar scanning data. The Jianghan salt cavern is selected for the applications of the proposed model, and the descending speed and depth of the gas-brine interface, and injection gas pressure with debrining time are analyzed. Results show that the greater the debrining rate, the shorter the time to reach the final sediment surface. The descending speed of the gas-brine interface shows a U-shaped curve, which is significantly affected by the cavern shape and porosity of sediments. The increment of the debrining rate has little effect on the ultimate injection gas pressure but has a significant effect on the increasing rate of injection gas pressure. These results can provide a reference for enhancing gas storage by using the sediment pore space of the mined volume and thereby expanding the effective gas storage volume of salt caverns.

Automated summary

The debrining process in high impurity salt mines involves four stages: rapid increase of injection gas pressure, discharging the upper brine, discharging the brine in sediments, and rapid decrease of injection gas pressure. A mathematical model based on the pressure equilibrium principle is proposed to calculate debrining parameters, and a numerical method is established to calculate the total debrining time. The results show that the debrining rate affects the time to reach the final sediment surface and the descending speed of the gas-brine interface.