Yanxia Kou

The reconstruction and utilization of old salt caverns with butted wells are of great significance for accelerating the construction of large-scale underground energy storage facilities, realizing energy transformation, and achieving the “dual carbon” goals. However, the renovation work of old butted-well caverns is still in its infancy, facing technical bottlenecks in transformation methods and operational safety. This paper takes the butted-well salt cavern with a large height difference in Pingdingshan, Henan province, as the research object. Through theoretical analysis and numerical simulation, the feasibility of its reconstruction and utilization is systematically studied from the aspects of gas injection and brine discharge methods, technology parameters, and operation stability. The results show that the gas injection and brine drainage method of butted-well salt caverns is closely related to residue utilization. The “one-injection-one-discharge” method is suitable for the old butted-well salt cavern with a large height difference, considering residue utilization and economy. During gas storage, there are significant deformation differences on both sides of the cavity. The deeper cavern suffers more damage and has weaker stability compared with the shallower one, and the conventional method for determining the operating pressure based on the casing shoe has limitations. The internal pressures of this salt-cavern gas storage structure are basically equal. A new mode for determining the operating pressure of these large-height-difference butted-well salt caverns is proposed: taking the lower limit for the deeper cavern and the upper limit for the shallower one. Based on theoretical analysis, numerical simulation, and on-site pilot test insights, the renovation and utilization of old large-height-difference butted-well caverns are feasible. This study provides guidance for converting butted-well salt caverns into underground energy storage structures and accelerating the development of new-type energy storage facilities.

The conversion of abandoned butted well salt cavities into underground storage facilities holds immense significance for safeguarding energy security and improving the ecological environment. A significant barrier to the reconstruction of these old cavities is the limited comprehension of their complete morphology, caused by residue coverage. The three-dimensional seismic techniques excel in identifying complex geological structures but have a limited understanding of underground old salt cavity morphology, thus the seismic forward simulation method is utilized to study their seismic response patterns. Based on 3D seismic data, well logging data, and measured cavity shape parameters from the Yexian salt mine region in Henan Province, China, a geological model and observation system were established. The seismic response characteristics of the butted well salt cavern model, encompassing five distinct morphological attributes such as cavity spacing, cavity diameter, cavity height, sediment height, and horizontal connection channel height, were thoroughly investigated. The findings show that the cavity roof exhibits a distinctive “two peaks sandwiching a strong valley” feature, with the positions of the valley and roof remaining aligned and serving as a reliable indicator for identifying the cavity’s top surface. The width of the roof waveform exhibits an exponential amplification effect relative to the cavern width. The residue’s top surface presents an “upward-opening arc” wave peak with a downward shift that diminishes as the residue’s height increases. This peak forms a circular feature with the cavity roof reflection waveform, and the residue’s top surface is always located in the upper half of this circular waveform. The horizontal connection channel’s top and bottom surfaces exhibit contrasting reflection patterns, with the top position aligning with the reflection trough and the bottom reflection waveform shifting downward as the channel height increases. The brine cavern, residue, and bottom of the salt cavern mainly exhibit chaotic reflections. There are distinct identification characteristics on the cavity top, residue top, and connecting channel top in forward simulation. The research findings provide valuable guidance for identifying the morphology of the underground real butted well salt cavity based on 3D seismic data and accelerating the construction of underground energy storage facilities.

Utilizing voids of insoluble sediment (IS) to store gas is an effective way to improve the efficiency of salt cavern gas storage (SCGS) in China. In this study, a suitable method for predicting the void volume of insoluble sediments (VVISs) is established. This study explores three key factors affecting the VVISs through laboratory experiments. Firstly, in order to make the experimental results more in line with production realities, an analysis of the characteristics of IS in X SCGS was conducted to provide a basis for setting parameters for subsequent experiments. Secondly, experimental setups and methods for measuring the VVISs were designed. Finally, the experimental results were used to predict the VVISs in on-site cavity wells. The results indicate that the higher the proportion of quartz, illite, and large-grain particles in IS, the larger the VVISs. Under different parameters, the VVISs can account for approximately 10–40% of the IS accumulation volume. Different particle sizes can cause a variation of approximately 5–30% in the VVISs, while different mineral compositions can result in a difference of 6–23% in the VVISs. With increasing compaction pressure, the VVISs can decrease by around 5–80%. The prediction of the VVISs in on-site cavity wells shows a high degree of fit with empirical algorithms. This study can provide a reference basis for the utilization of the void space of IS in SCGS.

Salt cavern gas storage has become the key project of current and future underground gas storage (UGS) facilities construction due to their efficient peak-shaving and supply assurance capacities. The Sanshui Basin in Guangdong Province, China, is rich in salt resources and has high-purity salt rock, which is a potential area for the construction of salt cavern underground gas storage. To speed up the large-scale construction of underground gas storage in China and promote the sustainable development of the natural gas market, it is very necessary to analyze the geomechanics of the target salt layer and study the feasibility of gas storage construction. Based on comprehensive experiments of rock mechanics and thermodynamics, the strength, creep and temperature-sensitive mechanical properties of the target rock in Sanshui Basin were studied. Then, according to the geological conditions of Sanshui salt formation, a three-dimensional geological model was established to analyze the stability of salt cavern gas storage under the injection-production operation. The results show that the average tensile strength and uniaxial compressive strength of salt rock are 1.51 MPa and 26.04 MPa, respectively, showing lower strength. However, under triaxial compression, the compressive strength of salt rock increases significantly, and there is no obvious shear failure phenomenon observed. Moreover, after the peak strength, the salt rock still has a large bearing capacity. In addition, under the confining pressure of 30 MPa, the strength of salt rock decreases by 8.3% at a temperature of 60 °C compared with that at room temperature, indicating that the temperature has a low, modest effect on the mechanical properties of salt rock. The stability analysis shows that, under an injection-production operating pressure of 10–23 MPa, the displacement, plastic zone range and volume convergence rate of single cavity and cavity group are small, and the cavity shows good stability. Overall, the target salt formation in Sanshui Basin, Guangdong Province, presents a good geomechanical condition suitable for the construction of underground salt cavern gas storage. This study can provide a reference for the development and design of salt cavern UGS.