Date Published: November 28, 2018
Publisher: Public Library of Science
Author(s): Jianjun Liu, Yingjie Wang, Kai Xie, Yichen Liu, Stuart Prescott.
Underground gas storage in rock salt is of great importance for peak-shaving and emergency gas supply. This paper addressed an actual rock salt underground gas storage facility in Jiangsu province, China, as the research project and carried out the following research centered on a detailed geological model, a salt cavern model and the process of gas injection and brine discharge. First, based on the theory of gas-liquid two-phase flow, the authors established a relationship between brine flow and natural gas bubbles under high pressure in the process of brine discharge. Second, the effect of pipe depth on the gas injection and brine discharge was simulated. The objective was mainly to choose the best combination of pipe depth and rate of brine discharge flow based on analysis of the relationship between the brine discharge pipe depth and the flow rate of the residual brine, and the optimal rate was given according to different distances. Third, the effect of residual brine on the gas injection and brine discharge was analyzed. The relationship curves between the maximum velocity on the surface of brine and the distance from the lower end of the brine discharge pipe to the bottom of the gas storage were obtained, and reasonable rates were suggested under different actual working conditions.
There are many advantages to salt cavern underground gas storage in rock salt: for example, the creep of rock salt formations is good, the permeability of rock salt formations is low, the structure of rock salt formations is complete, the hydrogeological conditions are relatively simple, and the caprock is well separated. Rock salt is readily soluble in water, which can reduce construction costs. Therefore, salt cavern gas storage is performed in water-soluble rock salt deposits and has become the most widely used type of natural gas reserve in the world [1–7].
Based on the economic and strategic importance of the underground rock salt reserves, in recent years, research on salt caverns has become more mature in many countries [13–17]. At home and abroad, this field has made great progress, mainly in the numerical simulation and physical test models of the characteristics of the rock salt and the structural stability and long-term stability of rock salt. In the first rock salt discussion meeting, R.W Jessen  proposed to measure the shape of the salt cavern at the stage of gas injection and brine discharge and obtain the shape of the salt cavern through the interpretation of seismic data. In 1986, Reda  developed a cylindrical cavern through an experiment to simulate the cavity process, which was used to simulate the process of cavity formation by injecting water into the cavity. Based on the mass energy equation of natural gas, Hagorrt  established a mathematical model in 1993 to predict intracavity pressure and temperature changes.
In the process of gas injection, the high-pressure brine injected into the pipe is a type of compressible unsteady research object, and the brine in the cavern is a type of incompetent stationary research object. Therefore, the fluid dynamics equations in the process of gas injection can be established according to the laws of mass, momentum and energy conservation of fluid mechanics.
The comprehensive treatment of the well depth ranges from 12.45–1175.0 m, which consists of the strata of several formations: the Dong Tai Formation, upper San Duo Formation, middle San Duo Formation, lower San Duo Formation, upper Dai Nan Formation, lower Dai Nan Formation, and Fu Ning Formation, as shown in Table 1.
The purpose of the research on the effect of gas injection and brine discharge is mainly to select the best combination of pipe depth and rate of brine discharge flow, based on the analysis of the relationship between the brine discharge pipe depth and the flow rate of the residual brine.
In theory, the closer the bottom of the brine discharge pipe is, the less residual brine in the gas storage; however, the pipes cannot simply go down to the bottom of the cavern due to the safety and stability of the pipes. In this section, the four models of the lower part of the storage volume are all based on the actual working conditions when the distance from the lower end of brine discharge pipe to the bottom of the cavern is 1 meter. The movement trend of the fluid was calculated at the different flow rates when the depth of residual brine was 1 meters, 2 meters, 3 meters and 4 meters. According to the rule of brine flow rate optimization, Table 3 is the maximum flow rate on the residual brine surface for different depths of brine, and Fig 6 is the velocity profile of the corresponding brine flow.
When the rate of brine flow is held constant, the maximum velocity of the gas-liquid interface increases with the continuous deepening of the column, and its rate of change rate is increasing. For the same depth of brine pipe, the maximum velocity of the brine interface increases, and its increase is basically the same. When the discharge pipe is 1 meter from the bottom of the cavern, the effect of the residual brine content on the gas injection efficiency is discussed considering the factors of practical engineering. When the depths are 4 meters, 3 meters and 2 meters in the rock salt gas storage, the corresponding reasonable rates of brine discharge flow are 100 m3/h, 80 m3/h and 60 m3/h, respectively. When the depth is only 1 meter in the gas storage volume, it is reasonable to discharge the brine at a flow rate less than 20 m3/h. Due to the limitation of the quantity of gas storage at the present stage, the accuracy of velocity, distance and depth control can not be very high, the conclusions obtained in this study can only be verified in the existing gas storage cavity. But the conclusions of this research can still be the guideline in the engineering practice of natural gas storage.