Artificial caverns in salt rock formations play an important role in the net‐zero energy transition challenge, both for covering short‐term fluctuations in energy demand and serving as safe locations for long‐term underground gas storage both for hydrogen and natural gas. Geophysical tools can serve for monitoring geomechanical changes in the salt cavern during selection and development, and during gas storage/extraction activities, but the use of common geophysical monitoring techniques has been very limited in this area. Here, we present experimental work on physical and transport properties of halite rocks within the energy storage context and assess the potential of seismic and electromagnetic data to monitor gas storage activities in salt formations. First, we analysed the stress‐dependency of the elastic and transport properties of five halite rocks to improve our understanding on changes in the geological system during gas storage operations. Second, we conducted two dissolution tests, using cracked and intact halite samples, monitored with seismic (ultrasonic P‐ and S‐waves velocities and their attenuation factors) and electromagnetic (electrical resistivity) sources to evaluate (i) the use of these common geophysical sensing methods to remotely interpret caverning development and (ii) the effect of structural discontinuities on rock salt dissolution. Elastic properties and permeability showed an increasing trend towards rock sealing and mechanical enhancement with increasing pressure for permeabilities above 10−21 m2, with strong linear correlations up to 20 MPa. In the dissolution tests, the ultrasonic waves and electrical resistivity showed that the presence of small structural discontinuities largely impacts the dissolution patterns. Our results indicate that seismic and electromagnetic methods might help in the selection and monitoring of the caverning process and gas storage operations, contributing to the expected increase in demand of large‐scale underground hydrogen storage.

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 p PLoS One PLoS ONE 440 plosone 101285081 plos PLoS ONE 1932-6203 PLOS PMC6261542 PMC6261542.1 6261542 6261542 30485293 10.1371/journal.pone.0207058 PONE-D-17-15715 1 Research Article Earth Sciences Geomorphology Topography Landforms Caves Physical Sciences Physics Classical Mechanics Continuum Mechanics Fluid Mechanics Fluid Dynamics Flow Rate Engineering and Technology Energy and Power Fuels Fossil Fuels Natural Gas Physical Sciences Materials Science Materials Fuels Fossil Fuels Natural Gas Physical Sciences Physics Classical Mechanics Continuum Mechanics Fluid Mechanics Fluid Dynamics Bubbles Physical Sciences Physics States of Matter Fluids Physical Sciences Physics Classical Mechanics Continuum Mechanics Fluid Mechanics Fluid Dynamics Fluid Flow Research and Analysis Methods Bioassays and Physiological Analysis Biochemical Analysis Bioelectrochemical Analysis Amperometry Earth Sciences Geology Gas injection and brine discharge in rock salt gas storage studied via numerical simulation The process of gas injection and brine discharge in salt rock gas storage http://orcid.org/0000-0003-4935-219X Liu Jianjun Conceptualization Methodology Project administration 1 2 * Wang Yingjie Formal analysis Software Writing – original draft 1 Xie Kai Resources 3 Liu Yichen Data curation Visualization 1 1 School of Geoscience and Technology, Southwest Petroleum University, Chengdu, China 2 State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock & Soil Mechanics, Chinese Academy of Sciences, Wuhan, China 3 School of Civil Engineering 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. Introduction 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 ]. The brine is discharged out of the storage after the salt rock is dissolved by clear water injected through the pipeline, and then a cavity with a certain volume and shape is formed underground. Therefore, it is of great significance for the construction of natural gas storage by controlling the position of pipe string and the rate of brine velocity. 10.1371/journal.pone.0207058.g001 Fig 1 A schematic diagram of gas injection and brine discharge. 1 Research background of gas storage 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 ]. In the process of gas injection and brine discharge, the cavity brine and injected natural gas coexist under the high-pressure condition in the rock salt cavern; the main parameters of the research are the gas injection pressure, the pressure of the brine discharge row, the halogen flow rate, and the sonar measurement data of cavity shape. The influence of the depth of the brine injection pipe and the gas injection rate on the removal efficiency is studied mainly by focusing on a two-dimensional finite element model to simulate the results of brine discharge and the effluent halogen effect [ 26 – 28 ], but the actual cavity is much more complicated than the established two-dimensional model. 2 Fluid dynamics theory in the process of gas injection 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. In the process of gas injection, the gas movement in the cavern is an active situation because the natural gas is continuously injected into the gas storage volume. 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 m 3 /h, 80 m 3 /h and 60 m 3 /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.

Rock salt caverns are considered one of the best hosts to store oil, natural gas, radioactive and toxic wastes due to their low permeability, self-healing characteristics and wide distribution on the Earth. Stored nuclear waste in rock salts will radiate for many years. Therefore, the thermal energy and also temperature in the host environment will increase depending on time. In this study, P-wave velocity (Vp), Brazilian tensile strength (σt), uniaxial compression strength (σc) of Çankırı rock salt were investigated under different temperatures ranging from 20°C to 250°C since the temperature * E-mail: ntunar@hacettepe.edu.tr 23 3 2023 2023 18 3 430135 e0283435 7 12 2022 8 3 2023 23 03 2023 24 03 2023 25 03 2023 © 2023 Nazlı Tunar Özcan 2023 Nazlı Tunar Özcan https://creativecommons.org/licenses/by/4.0/ This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Rock salt caverns are considered one of the best hosts to store oil, natural gas, radioactive and toxic wastes due to their low permeability, self-healing characteristics and wide distribution on the Earth. Additionally, X-ray micro-computed tomography technique was employed to observe the microstructure and determine the porosity of rock salt samples depending on the temperature. The V p and the σ t of Çankırı rock salt decrease with increasing temperatures of samples whereas the σ c increases. The ductility of rock salt tends to increase with augmented temperature and the axial strain at the ultimate stress level is 2.96% at 20°C whereas it reaches up to 6.29% at 250°C. The AE activity of rock salt generates at the early stages of loading and AE count prominently increases with the increasing temperature of samples. Therefore, the stress levels of crack initiation (σ i ) and crack damage (σ cd ) thresholds were reached earlier than the previous one with each temperature increment. According to X-ray micro-CT images of rock salts, the number of cracks increased markedly in thermally treated rock salt samples and therewith the porosity increases from 1.12% to 2.73% with an increase in temperature from 50°C to 250°C. The author received no specific funding for this work. Introduction Rock salt is recently employed in the storage of petroleum and natural gas as well as nuclear waste repositories due to its very low permeability, visco-plastic deformation behavior and self-healing characteristics. Using rock salt as a host rock has economic importance in terms of enabling the storage of resources that will meet energy needs. It also provides crucial worldwide environmental safety by isolating hazardous wastes. Nuclear wastes continue to radiate for many years due to long radioactive half-life. Thereby, the thermal energy in the hosting medium increases over time. Soppe et al. [ 1 ] determined that the temperature of the rock salt in the close vicinity of the waste container gets warmed up to 165°C. Similarly, the temperature of a rock salt storage cavern can increase up to 140°C during the injection and withdrawal processes of natural gas in relation to the injection rate, pressure, and maximum storage volume [ 2 , 3 ]. Consequently, it is essential to reveal the effects of temperature increase on the geo-mechanical and physical properties of surrounding rock salt. There are many studies [e.g. 4 – 12 ] on the mechanical properties and long term behaviour of rock salt whereas the studies investigating the effect of temperature on these properties are limited. Liang et al (2006) determined that the thermal effect has significant impacts on the properties which control the repository quality of rock salt and suggested that more research is required on the physical and geo-mechanical characteristics of rock salt at high temperatures. Li et al. [ 14 ] conducted thermo-mechanical coupled triaxial compression tests on two kinds of pure rock salt and impure (interbedded) rock salt specimens at temperatures 53°C and 65°C. The researchers indicated that the σ c and failure mode of rock salt is sensitive to temperature changes and the σ c decreases “AE count per 2 sec.” and “axial stress” vs. “cumulative AE count” graphs, the AE activity starts rapidly from the early stages of loading and increases continuously until the peak stress is reached. In addition, cumulative AE count is properly increased with increasing axial stress levels. The AE counts recorded at the same stress level prominently increase as the temperature of samples increases ( Fig 10A2–10F2 ). This result shows that temperature has a strong effect on the AE characteristics of rock salt regarding ductile behavior increasing with temperature. While the σ ci and σ cd thresholds of the non-heated samples are 3.9 MPa (16.8% of the peak stress) and 15.6 MPa (67.2% of the peak stress), the σ ci and σ cd thresholds of the samples heated up to 250°C decreased dramatically to 0.98 MPa (3.6% of the peak stress) and 9 MPa (33.1% of the peak stress). Eventually, the cracking processes of rock salt samples initiates at the earlier phase of the loading with the increasing temperature. 10.1371/journal.pone.0283435.g009 Fig 9 The experimental setup of uniaxial compressive strength test and AE monitoring. On the other hand, the cracks in the internal structure of rock salt can be reduced or recovered in time by self-healing characteristics under appropriate temperature, pressure and humidity conditions. Thus, the elapsed time between the thermal treatment of samples and testing, and the environmental conditions have also an effect on the uniaxial compressive strength. According to the stress-strain curves in Fig 8 , the rock salt is becoming more ductile at a higher temperature of samples. In this study, the amount of axial strain measured under the same stress levels increased with the increasing temperature.

Underground Gas Storage Process Optimisation with Respect to Reservoir Parameters and Production Equipment The storage of natural gas in geological structures such as depleted fields, aquifers and salt caverns plays an important role in a gas supply system as it balances the fluctuation of gas demand and price. Hydraulic loss due to fluid flow through gas storage production equipment and an interfering effect from nonequal productivity index of storage wells may have an important influence on gas storage performance. An integrated mathematical model is developed based on underground gas storage facility production data. Using this model, the hydraulic loss is determined. A real test case that consists of a gas storage reservoir linked to the surface facility is analysed. The mathematical model uses an experimentally determined pressure drop coefficient in chokes. The base case scenario created using real gas storage facility data enables the achievement of a good history match with the given parameters of the gas storage reservoir.

Abstract This article deals with comparative technical and economic aspects of conventional and some nonconventional methods of storing gas. Conventional gas storage was first begun by injection and subsequent production of gas in a depleted gas field in Ontario, Canada in 1915. Conventional methods also include storage in depleted in oil fields and aquifers. Aquifer storage was first introduced into the United States with the injection of gas into the Galesville aquifer at Herscher, Ill. in 1953. Nonconventional methods include storage of gas in coal mines, mined salt caverns steel pipe and earth strata with artificial caprock and lateral confinement created by impermeable chemical grouts. Another method is storage of liquified gas in frozen earth or mined caverns. The growth and status of gas storage in the U.S. and Western Europe is summarized and technical and economic factors are related to the probable future direction and growth of storage in these areas. Introduction Major markets for natural gas in the U. S. and Western Europe often consume more gas during the four coldest winter months than during the remainder of the year. Peak winter demand usually exceeds three times the average summer consumption rate. Unless some form of near-market gas storage is used, large enough pipelines must be installed from producing fields to handle this peak winter demand. The resulting pipeline load factor, defined as average yearly flow rate divided by maximum or design rate, is the

The presence of evaporitic formations in sedimentary basins, often dominated by the salt mineral halite, is of great influence on the structural style developed during tectonic events. On a somewhat smaller scale, salt rocks often host a variety of deep solution mined caverns, which are increasingly finding use for strategic storage of energy resources in the form of gaseous or liquid fuels and as vessels for off-peak energy storage in compressed gas. This is in addition to the use of convention