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Article

Assessing the Feasibility of Using Coastal Salt Caverns for Strategic Energy Storage from Safety and Economic Perspectives

by
Zuer Li
1 and
Qihang Li
2,*
1
School of Marxism, Changzhou Institute of Technology, Changzhou 213032, China
2
School of Resources and Safety Engineering, Chongqing University, Chongqing 400044, China
*
Author to whom correspondence should be addressed.
Sustainability 2026, 18(10), 4949; https://doi.org/10.3390/su18104949
Submission received: 9 April 2026 / Revised: 7 May 2026 / Accepted: 13 May 2026 / Published: 14 May 2026
(This article belongs to the Special Issue Technologies of Energy Storage, Carbon Capture, Utilization and Storage)
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Abstract

To support the strategy of building a strong maritime nation, oil and gas resources need to be shifted from inland to coastal areas, and large-scale strategic reserves must be established to meet national security and energy security requirements. Currently, the primary method for offshore gas storage involves onshore steel tanks, which suffer from high costs and limited capacity. The offshore sediment-type salt cavern gas storage is a high-quality alternative solution; however, its long-term stability and economic viability remain to be studied. The feasibility of gas storage in an abandoned cavern of a coastal, low-grade salt mine was simulated using ANSYS Parametric Design Language (APDL) and FLAC3D 7.0, and the cost–benefit comparisons were conducted among abandoned salt caverns, newly constructed single- and double-well salt caverns, and onshore storage tanks. The results show that, without utilizing the sediment storage space, the gas storage capacity is reduced and surrounding rock deformation is increased. On the other hand, the sediment’s supporting effect can mitigate creep deformation and enhance cavern stability. In addition, increasing the operating cycle frequency can significantly reduce volume shrinkage, roof subsidence, and the extent of the plastic zone. Economic analysis shows that the estimated construction cost for repurposing coastal sediment-type salt caverns is approximately 82 million CNY, which is significantly lower than the 450 million CNY required for onshore storage tanks. Compared with newly constructed single- and double-well salt caverns, it offers advantages in cycle time, cost, and revenue. Accordingly, this research can provide theoretical guidance for evaluating abandoned cavern reserves and conducting feasibility studies. Furthermore, it offers technical support for the large-scale, sustainable storage of carbon dioxide, hydrogen, compressed air, and other renew-able energy carriers in abandoned salt caverns.

1. Introduction

The global marine challenges have become increasingly complex due to long-standing environmental changes, and energy security has emerged as an issue of growing urgency [1]. Currently, the development of offshore oil and gas in China is advancing rapidly. However, the capacity of onshore storage tanks is clearly insufficient, revealing shortcomings in the strategic petroleum reserve system [2,3]. Thus, a large-scale energy storage system must be established to ensure the country’s long-term sustainable development. We investigated China’s natural gas production, import volume, and external dependence from 1997 to 2025 (Figure 1d). It was found that its total output gradually increased and reached 262.06 billion m3 in 2025. Its highest external dependence ratio was 44.93% in 2021, a figure likely attributable to the impact of the COVID-19 pandemic [4]. It is worth noting that Figure 1a shows the average annual electricity generation for each province in China over the past 29 years, with a national average of 458.4 TWh. Herein the annual average power generation of Guangdong, Jiangsu, and Shandong provinces exceeded 380 TWh (Figure 1b). Furthermore, the proportion of electricity generated from fossil fuels in China remains at approximately 76% as of now (Figure 1c) [5]. Therefore, how to utilize offshore oil and gas resources for large-scale strategic reserves has become a key issue.
As a novel energy storage facility, salt caverns offer advantages including low cost, flexible injection and withdrawal, good sealing, and large capacity, meeting the needs of energy transition in the context of the “dual carbon” goals [6,7,8]. Herein, the successful construction of the Huai’an salt cavern gas storage (SCGS) facility has promoted the development of energy reserve capacity in coastal regions [9]. However, the geological space suitable for constructing SCGS facilities in China is limited, primarily due to the low grade of bedded salt rock and the presence of numerous thick interlayers [10,11]. Previous studies have shown that the porosity of sediment in salt caverns can reach 40~50%, which is much higher than that of conventional gas reservoirs [12,13]. Here, Li et al. discovered through similar physical model experiments that, under the two injections and one discharge operation mode, the gas storage capacity of sediment voids increased by 46%, confirming the feasibility of gas storage in such voids [9]. Therefore, if the larger voids formed between coarse-grained sediments can be effectively utilized, they will help enhance China’s gas storage capacity in layered salt rock formations. In addition, China’s SCGS facilities are primarily located in inland regions. Although the capacity of each individual facility is substantial (105~106 m3), the transportation and storage costs associated with delivering gas from offshore fields to these inland facilities are extremely high. In contrast, the capacity of large onshore storage tanks is typically no more than 105 m3, and they require a considerable amount of ground space [14]. All these factors have economically constrained the advancement of the strategy to build a maritime power. From another perspective, approximately 85% of the abandoned salt caverns in China’s low-grade salt rock regions adopt the two-butted-well horizontal (TWH) connection type. Here, the three-dimensional (3D) view of the TWH-cavern is shown in Figure 2. If renovated and utilized, they could provide a gas storage capacity of about 2 × 107 m3. Those formed in coastal sedimentary strata can store approximately 6.4 × 106 m3 of natural gas [15]. To ensure that the strategic goal of safe operation for the coastal sedimentary SCGS facility is achieved, it is particularly crucial to systematically assess its long-term stability and overall economic viability.
The geological research on salt caverns in China has evolved over the past two decades. Through mechanical testing, physical and numerical simulations, and artificial intelligence techniques, it has provided both technical support and a theoretical foundation for addressing complex challenges in the construction and operation of salt caverns [16,17,18]. However, the cavern roof remains the key factor limiting the feasibility of low-grade coastal SCGS [19]. Zhao et al. analyzed the safety and reliability of the roof of gas storage caverns using the Monte Carlo method and suggested selecting high-quality salt rock layers with shallow burial depth and few interlayers for cavern construction [20]. Zhang et al. investigated the roof stability of a horizontal-cavern compressed air energy storage (CAES) reservoir via numerical simulation and found that the maximum internal pressure during the cycling process is the primary controlling factor [21]. In terms of the stability of salt caverns within sediment-filled void, Xie et al. proposed a new model for utilizing the void space in salt cavern sediments and analyzed the feasibility of converting existing solution-mined caverns into SCGS facilities from the perspective of roof stability [22]. Moreover, Liang et al. investigated the promoting effect of sediment on the stability of the TWH-cavern; however, they did not analyze how different operational cycles affect cavern stability [23]. Fu et al. quantitatively analyzed the influence of pore deformation induced by creep and plastic strain on permeability using a hydro-mechanical coupling model, and evaluated the gas-tightness and safety of salt caverns under various operating conditions [24]. Fu et al. combined an orthogonal experimental design (OED) with a backpropagation artificial neural network (BPANN) to investigate the influence of six operational parameters on salt cavern stability and found that burial depth and the minimum internal gas pressure ratio (IGPR) were the primary influencing factors [25].
Although the above-mentioned research has focused on the positive impact of sediment on the stability of SCGS, it remains incomplete or inconclusive. Nevertheless, in response to the actual demand for power peak shaving in coastal areas, the impact of sedimentary bodies on the efficient operation of key gas storage reservoirs during injection and production cycles—and the underlying mechanisms—remains to be systematically clarified. Furthermore, SCGS facilities are larger in scale than above-ground natural gas storage tanks; however, they still lack quantitative economic evaluations. Therefore, the innovation of this research lies in using Fast Lagrangian Analysis of Continua in 3 Dimensions (FLAC3D 7.0) and ANSYS Mechanical APDL numerical simulation software to model the operational scenarios of a low-grade salt mine in a coastal area under varying injection–production cycles. The safety of natural gas storage in coastal sedimentary salt caverns is confirmed by analyzing the cavern’s volume shrinkage rate, displacement, and plastic zone volume. In addition, based on practical engineering experience, the economic viability of four storage options is quantitatively evaluated: (1) abandoned salt caverns; (2) newly constructed single-well caverns; (3) newly constructed double-well caverns; and (4) onshore storage tanks. It highlights the advantages of coastal sediment-type abandoned salt caverns in terms of storage capacity, construction cost, and operational revenue. These findings are conducive to expanding the range of candidate sites and increasing the construction scale of coastal sedimentary-rock-based SCGS facilities. Meanwhile, it also enables large-scale, sustainable storage of carbon dioxide, hydrogen, compressed air, and other renewable energy carriers in abandoned salt caverns.

2. Methods

In order to clarify the engineering characteristics of coastal sedimentary SCGS and to provide real-world salt cavern data for subsequent numerical model development, this study investigated the insoluble sediment content ratio, formation depth, sediment void volume, and distance to the sea (defined as the shortest straight-line distance from the salt mine to the coastline, Figure 3) for 17 salt caverns in China, as shown in Table 1. Low-grade salt mines refer to those in which the content of insoluble matter exceeds 35% of the total volume of the cavern [26]. Accordingly, except for the Jintan, Zhaoji, Heze, Ningjin, Zigong, Zhangshu, and Yulin salt mines, all other salt mines listed in Table 1 are low-grade. In this study, salt mines located within 200 km of the coast are defined as coastal salt mines. Accordingly, the Huai’an and Sanshui salt mines are classified as coastal salt mines (low-grade), whereas the others are inland salt mines. Therefore, if the void space in the sediment of coastal salt mines can be fully utilized to store natural gas extracted from offshore sources, it will significantly enhance the scale and economic viability of strategic natural gas reserves.

2.1. Establishment of Numerical Model

In this study, taking a TWH-cavern containing sedimentary bodies in a coastal area of China as an example, the surrounding rock of the abandoned cavern was appropriately simplified for modeling convenience. Herein the thickness and height of the strata are primarily determined as the average of those measured in Wells H1 and H2. The ANSYS Mechanical APDL module was used to establish a geomechanical model for stability evaluation and to mesh it using the free meshing method. The meshing strategy aimed to balance computational accuracy and efficiency. Due to the large model size, only the region near the cavern was subjected to detailed analysis [27]. Hence, radial grids are used in the modeling process, and the element size increases with distance from the cavern. This method can effectively ensure both the accuracy and efficiency of the calculation [28,29].
As shown in Figure 4, a numerical model has been established to analyze the stability of the H1-H2 cavern under two gas storage scenarios: effective space gas storage and sediment void gas storage. The cavern consists of two connected rhomboidal bodies. For the convenience of numerical calculating, the left and right bodies are symmetrical. Taking the center of the bottom face as the origin (point o), the model has a length of 670 m, a width of 300 m, and a height of 750 m. Among them, the cavern has a length of 130 m, a width of 50 m, and a height of 60 m. The horizontal plane corresponds to the X-Y plane, and the vertical direction aligns with the positive Z-axis. The bottom and side surfaces of the model are fixed; thus, their boundary displacements are zero. In addition, the upper surface is not fixed and allows displacement in the Z-direction [30]. To simplify the calculation, it is assumed that the formation pressure distribution over the upper surface of the model is uniform. Based on the actual depth and average density of the rock stratum, the vertical stress acting on its upper surface is calculated as 30 MPa. This mathematical expression is interpreted as follows.
σ z = ∑ i = 1 n ρ i g h i
where σz represents the vertical stress applied to the upper surface of the model, MPa; n is the number of strata located above the upper surface of the simulation model; ρi is the density of the i-th overlying layer, kg/m3; g is the gravitational acceleration, taken as 9.8 N/kg; and hi is the thickness of the i-th overlying layer, m.
Subsequently, the established model will be imported into FLAC3D, and simulation calculations will be performed by executing control command scripts under various working conditions. In the stability assessment of the salt cavern, we treat the sediment body as a whole and use FLAC3D to accurately simulate the mechanical behavior of rock and soil materials under conditions approaching or reaching their strength or yield limits [26]. Upon completion of the calculations, the required data will be exported for secondary processing.

2.2. Constitutive Model and Parameters

Mechanical parameters serve as the foundation for the stability analysis of rock and soil structures. The elastoplastic analysis is based on the classical Mohr–Coulomb theory [31,32]. It is generally believed that this theory is essentially a shear-stress-based strength theory that comprehensively reflects the strength characteristics of rocks. It is applicable not only to shear failure in plastic rocks but also to that in brittle salt rocks. In addition, it captures the characteristic that rock tensile strength is much lower than its compressive strength. Among them, the creep model and its associated mechanical parameters were determined based on field-measured data [33]. Currently, this numerical model has been successfully applied to multiple domestic SCGS projects in locations including Jintan and Huai’an in Jiangsu Province and Pingdingshan in Henan Province, and has passed rigorous testing [13,17]. Accordingly, the established numerical model demonstrates reliable accuracy and broad applicability. The mechanical parameters of bedded salt rock are summarized in Table 2.
In this study, assuming compressive stress to be negative, the yield function for the Mohr–Coulomb failure criterion is as follows:
f s = σ 1 − σ 3 N ∅ + 2 c N ∅
where Nϕ = (1 + sin ϕ)/(1 − sin ϕ), c represents cohesion, and ϕ represents the angle of internal friction.
The tensile yield function is as follows:
f t = σ 3 − σ t
where σt represents the tensile strength of salt rock.
The maximum tensile strength should not exceed σ m a x t , and its value is calculated using Equation (4).
σ m a x t = c t a n   ∅
The shear failure potential function adopts a non-associated flow rule [34].
g s = σ 1 − σ 3 N Ψ
where Nψ = (1 + sin ψ)/(1 − sin ψ), and ψ represents the expansion angle, °.
Furthermore, the tensile failure potential function adopts the associated flow rule [35].
g t = − σ 3
Finally, based on the above criteria, a C-power exponential creep model was proposed through systematic creep tests on domestic bedded salt rock [36]. The model expression is shown below.
ε ˙ t = A ( σ 1 − σ 3 σ * ) n
where ε ˙ t represents the steady-state creep rate of the salt rock; σ1 and σ3 are the maximum and minimum principal stresses, respectively, MPa; σ* is the unit stress, typically taken as 1 MPa; and A, and n are material parameters related to the rock’s mechanical properties.
Through previous creep mechanics tests on rock salt and parameter optimization, the creep parameters for this study were determined [9,13]. In this research, the creep parameters are set as follows: for salt rock, the values of A and n are 3.25 × 10−6 a−1 and 3.52 MPa−1, respectively; for the interlayer, the values of A and n are 7.20 × 10−7 a−1 and 3.52 MPa−1, respectively; for the top and bottom mudstone, the values of A and n are 3.20 × 10−7 a−1 and 3.5 MPa−1, respectively.

2.3. Operating Parameters of Gas Storage Facilities

According to the construction requirements for SCGS facilities, the development process consists of four sequential stages: cavern formation, gas injection and brine discharge, gas injection and production, and cavern abandonment [37]. During the gas injection and production operational phase, SCGS facilities operate cyclically—typically with one to two injection–production cycles per year. As depicted in Figure 5a, within a single injection–production cycle, the process proceeds through four stages: the “high-pressure operating stage” (Stage I, 233 days), the “gas production depressurization stage” (Stage II, 36 days), the “low-pressure operating stage” (Stage III, 36 days), and the “gas injection boost stage” (Stage IV, 60 days). For other operational cycles (occurring n times per year, where n = 2, 3, or 4), the duration of each stage can be determined simply by dividing the annual cycle duration by n. This is similar to the operating cycle of the Jintan SCGS facility. Herein the pressure during the “high-pressure operation stage” is defined as the maximum working gas pressure, denoted by Pmax; and the pressure during the “low-pressure operation stage” is defined as the minimum working gas pressure, denoted by Pmin. The design service life of an SCGS facility is 30 years.
There are many factors that affect the stability of SCGS, such as cavern geometry, operating pressure, interbedded layers, casing shoe height, and cavern spacing [38]. However, when converting an abandoned cavern into a SCGS facility, the stability of the resulting facility is directly related to its operating pressure. Therefore, when conducting numerical simulations to assess stability, priority should be given to the operating pressure of the gas storage reservoir. Meanwhile, an evaluation system should be established, using the volume shrinkage rate, plastic zone volume, and displacement of key points as evaluation indicators. According to the safety technical regulations for SCGS, the internal pressure of the cavern shall not exceed 80% of the overburden pressure (upper limit) nor fall below 30% of that pressure (lower limit) [39]. The formation density is taken as 2400 kg/m3, the depth of the cavern roof is 1275 m, and the corresponding vertical stress σz = 30 MPa. In this research, the lower-limit pressures were set to 0.3 σz, 0.35 σz, and 0.4 σz, respectively. The corresponding calculated internal pressure ranges were 9–24 MPa, 10.5–24 MPa, and 12–24 MPa, respectively.
Due to the seasonal peak shaving characteristics of natural gas, it is necessary to appropriately adjust the injection and production cycle of the salt cavern. To discuss the influence of different injection–production cycles on the stability of the abandoned salt cavern, numerical simulations are conducted under four distinct operational scenarios. The injection–production cycles considered are 12 months, 6 months, 4 months, and 3 months per cycle. The minimum and maximum internal operating pressures of 10.5 MPa and 24 MPa were selected for all four periodic operating conditions. The internal pressure–time curves of the cavern are shown in Figure 5.

3. Results

3.1. Volume Shrinkage Rate

The cavern’s volume shrinkage rate is a key parameter for evaluating the stability and feasibility of SCGS. The volume shrinkage rate is defined as the ratio of the volume reduction in the underground SCGS during operation to its initial volume. It effectively reflects the safety and economic performance of the underground SCGS [40]. The greater the volume shrinkage rate, the more likely the cavern is to undergo significant deformation, which may compromise its stability. Accordingly, throughout the entire service life of the SCGS facility, it is necessary to avoid an excessively high volumetric shrinkage rate.
Figure 6 demonstrates the relationship between the cavern volume shrinkage rate and creep time under the effective space gas storage mode and the sediment space gas storage mode at different cyclic periods. According to relevant research and regulations, the overall volumetric shrinkage rate of SCGS facilities over a 30-year operational period should not exceed 30%, and the average annual shrinkage rate should not exceed 1% [41]. From this perspective, only the conditions of one cycle every four months and one cycle every three months can basically meet the requirements when gas storage relies solely on effective cavern volume (Figure 6a), whereas, when the sediment space is also used for gas storage, all four conditions satisfy the requirements (Figure 6b).
As can be seen from Figure 6, the volume shrinkage rate of the abandoned salt cavern exhibits periodic fluctuations over time. However, the cavern volume shrinkage rate has been continuously increasing. Even under different cycle periods, the volumetric contraction rate of gas storage using the sediment space is lower than that of gas storage using only the effective space. Accordingly, we can consider that the sediment inhibits creep in salt rock. In addition, as the number of cycles per year increases, the cavern’s volume shrinkage rate decreases accordingly. The occurrence of this situation is primarily attributed to the shortened duration of continuous operation at low pressure (Stage III). Among these four different cycle periods, the durations of low-pressure operation are 60 days, 30 days, 20 days, and 15 days, respectively. Thus, to extend the service life of SCGS, the injection and production cycles can be appropriately lengthened while keeping the total duration of low-pressure operation constant. In this simulation, the volume shrinkage rates of the effective space and the sediment space after 30 years of operation under the three-month, one-cycle working condition were 21.39% and 6.94%, respectively. This case exhibits the lowest volume shrinkage rate among the four working conditions.

3.2. Variation in Displacement

Overall, the change in displacement more accurately reflects the degree of deformation of the surrounding rock around the cavern. The roof and waist positions are both key indicators for assessing the stability of the cavern wall [42]. Excessive displacement in the upper portion of the cavern may lead to serious accidents such as roof collapse and damage to the casing shoe, whereas excessive displacement in the midsection may cause pillar instability. Previous studies have found that the roof plate and the waist are the most likely locations to become unstable [26]. Therefore, three key monitoring points were selected for displacement monitoring during operation: the roof of the cavern (point A), the waist of the cavern (point C), and the connection position (point B) (Figure 7).
Figure 8 and Figure 9 show the displacement variations at points A, B, and C of the cavern after 30 years of creep under four different cyclic loading periods, for both the effective space gas storage mode and the sediment space gas storage mode. It can be seen that, under the two gas storage modes, the cavern exhibited varying degrees of subsidence and contraction at points A, B, and C. Under the gas storage mode that utilizes only the effective storage space, the maximum settlement at point A under the four operating conditions is 1.06 m (12-month, one-cycle), 0.87 m (6-month, one-cycle), 0.71 m (4-month, one-cycle), and 0.58 m (3-month, one-cycle), respectively (Figure 8a). Similarly, the maximum settlement at point B is 1.42 m, 1.18 m, 0.96 m, and 0.79 m (Figure 8b); and the maximum shrinkage at point C is 0.53 m, 0.47 m, 0.45 m, and 0.43 m (Figure 8c). Under the gas storage mode that utilizes the sediment space, the maximum settlements at point A under the four working conditions are 0.83 m, 0.81 m, 0.72 m, and 0.63 m, respectively (Figure 9a). In addition, the maximum settlements at point B are 1.12 m, 1.09 m, 0.98 m, and 0.85 m, respectively (Figure 9b); and the maximum shrinkage at point C are 0.73 m, 0.61 m, 0.53 m, and 0.41 m, respectively (Figure 9c). Both of these modes have shown a gradual downward trend. This fully demonstrates that extending the cavern’s operational cycle helps mitigate sediment accumulation and cavern shrinkage.
Figure 10 and Figure 11 demonstrate the overall displacement distribution of the surrounding rock in the axial (Y-Z) plane after 30 years of cavern operation under four cyclic loading frequencies, corresponding to the effective space gas storage mode and the sediment space gas storage mode. In the cloud map, the white areas represent the gas storage capacity of the cavern. It can be seen that, under both gas storage modes, the overall cavern displacement decreases as the circulation frequency increases; the settlement reduction in the roof plate is particularly significant. At the same time, it can be observed that the sediment inside the cavern undergoes relatively large displacement, as it consists of loose particles. During cavern contraction, the cavern wall pushes sediment inward, causing it to undergo compressive deformation and form an arch-shaped structure in the upper part of the sediment deposit [43]. Hence, the maximum displacement occurs in the upper-middle portion of the sediment within the cavern. Nevertheless, as the circulation frequency increases, the maximum displacement of the sediment decreases accordingly, indicating that the contraction rate of the cavern is also decreasing and its stability is improving.

3.3. Plastic Zone of Surrounding Rock

During the gas injection and production process in the cavern, the stress and strain distributions in the surrounding rock exhibit spatial anisotropy and non-uniformity [44]. The interlayers and salt rocks may deform, potentially compromising the stability of the cavern.
Hence, it is necessary to study the mechanical behavior of the surrounding rock. The distribution of the plastic zone is an effective means of characterizing the interaction between the interlayer and the salt rock, helping to identify potential hazard locations and thereby enabling corresponding safety assessments and operational adjustments. Here, the plastic zone is modeled using the Mohr–Coulomb criterion, and its mathematical expression is given below [45].
τ = c + σ t a n   ∅
where τ represents the shear strength of the shear failure surface, MPa; and σ is the normal stress acting on that surface, MPa.
When the shear stress on a certain plane within the rock mass reaches that plane’s shear strength, the material enters a plastic state, and the corresponding region is termed the plastic zone [46,47]. The volume of the plastic zone here is defined as the ratio of the volume of the plastic zone in the surrounding rock to the initial volume of the cavern.
The relationship between the volume of the plastic zone and elapsed time under the four loading cycles is shown in Figure 12. As the number of cycles increases, the size of the plastic zone decreases. This indicates that the cyclic loading frequency influences the plastic zone, although the effect is not significant. The volume of the plastic zone increases with fluctuations in both the upper and lower regions. Within each cycle, the plastic zone exhibits a gentle trough followed by a sharp peak (Figure 12a). The growth trends of the four cyclic frequencies are similar as running time increases. Among the four cyclical frequencies, the 12-month one-cycle exhibits the largest fluctuation amplitude. Here, the plastic zone primarily expands during the low-pressure operating stage. Thus, the duration of the low-pressure operation phase should be strictly controlled. Taking the 6-month, one-cycle operation condition as an example, after 30 years of creep deformation, the volume of the plastic zone under the effective space gas storage mode is 1345% of the initial volume, whereas that under the sediment space gas storage mode is 87% of the initial volume. Obviously, the volume of the plastic zone in the cavern containing sediment is much smaller than that in the cavern without sediment.
Figure 13 shows the distribution of the plastic zone in the cavern after 30 years of operation under different cyclic loading frequencies. In Figure 13a, the red region—labeled “shear-n/shear-p/tension-p” in the average regional state—indicates that, during the simulation, the rock mass within this region first underwent tensile failure (plastic state) in an earlier loading cycle and subsequently experienced shear failure (plastic state) in both that same cycle and the current cycle. The numerical simulation results show that, under four different working conditions, the plastic zones in the salt cavern exhibit similar spatial distribution patterns. Moreover, shear damage occurs in the surrounding rock at the cavern roof, near the interlayer, and in the sedimentary layer at the cavern bottom (Figure 13b). Among them, as the cyclic loading frequency increases, the plastic zone on the roof of the cavern is significantly reduced, whereas the changes in the plastic zones around the cavern and at its bottom remain negligible. This is mainly because sediment accumulation around and at the bottom of the cavern is relatively thick, resulting in a relatively large contact area with the surrounding rock. The supporting effect of the sediment on the surrounding rock of the salt cavern remains unchanged despite fluctuations in gas injection and production frequency, which cause variations in the gas storage pressure. Here, the roof of the cavern is supported solely by the pressure of the stored gas. During gas extraction and the subsequent continuous low-pressure phase, the surrounding rock mass around the cavern undergoes significant deformation. Thus, the distribution of the plastic zone in the roof plate of the cavern is significantly influenced by the frequency of injection and production.
According to the numerical simulation results for the volume shrinkage rate of the abandoned cavern after 30 years of operation, under high injection–production cycling conditions, the sediment body exerts a significant inhibitory effect on creep deformation of the cavern roof. The displacements at the roof (point A), waist (point C), and connect-ion (point B), as well as the development of the plastic zone in the surrounding rock, support this conclusion. Herein, the specific manifestation is that the deformation of the cavern’s roof plate gradually decreases with increasing cycle number, whereas the deformations of the middle section and the cavern connections remain essentially unchanged across cycles. Hence, from the perspective of operational safety in caverns, it is feasible to use coastal sedimentary salt caverns for natural gas storage.

4. Discussion

In Section 3, the stability of the coastal sediment-type SCGS was evaluated, and it was verified that sediment presence can suppress creep in the horizontal connecting salt cavern between the two wells, thereby ensuring safe operation of the cavern. Theoretically, utilizing the existing void space in sediments for natural gas storage can reduce construction costs and time, thereby yielding economic benefits. Nevertheless, no quantitative comparison has yet been made between natural gas storage in abandoned caverns of coastal sedimentary salt mines and that in newly constructed salt caverns, making it impossible to provide intuitive references or guidance for relevant decision-makers and engineers. Accordingly, in this section, we discuss the economic benefits of using abandoned salt caverns compared with those of onshore storage tanks, newly constructed single-well caverns, and newly constructed double-well caverns.
(1) Gas storage in abandoned caverns at the coastal sedimentary salt mine.
The horizontal interconnection of the abandoned sedimentary-type gas storage facility requires sequential execution of three stages: geological and geophysical exploration; well remediation and drilling of new brine discharge wells; and gas injection coupled with brine discharge. Based on engineering experience, it is reasonable to allocate three months, three months, and six months to these three stages, respectively. In other words, if the sediment-filled space in an abandoned salt cavern is used for gas storage, all operations—from geophysical exploration and drilling to brine discharge—can be completed within one year.
(2) Gas storage in newly constructed single-well caverns at the coastal salt mine.
The construction of a new single-well gas storage facility involves four sequential processes: geological and geophysical exploration, drilling and well completion, solution mining (to create a water-soluble cavern), and gas injection accompanied by concurrent brine discharge. Among them, the time required to form a water-soluble cavern is the longest. For instance, it takes 3 to 4 years to construct a new single-well cavern with a volume of 250,000~300,000 m3. In this research, the formation time is estimated to be 3.5 years. The time settings for the remaining three stages are the same as those for sediment gas storage in the abandoned cavern. Thus, the total construction period for a new single-well gas storage facility is approximately 4.5 years.
(3) Gas storage in newly constructed double-well caverns at the coastal salt mine.
The construction of a new double-well gas storage facility also involves four key processes: geological and geophysical exploration, drilling and well completion, solution mining (to form a water-soluble cavern), and gas injection with brine discharge. Herein the time required for water-soluble cavern formation is the longest. Nevertheless, due to the high flow rate and large concentration gradient in horizontal double-well cavern formation, the cavern construction rate is significantly higher than that of a single-well configuration. It takes approximately two years to construct a 500,000 m3 single-well solution mining cavern. The time settings for the remaining three phases are the same as those of the previously abandoned cavern-sediment gas storage facility and the single-well gas storage facility. Therefore, the total construction period for the new double-well cavern gas storage facility will also be at least three years.
(4) Onshore natural gas storage tanks.
Onshore liquefied natural gas (LNG) storage tanks have individual capacities ranging from several thousand to tens of thousands of cubic meters. Moreover, its storage conditions are stringent (atmospheric pressure, low temperature of −162 °C), occupying a large area and having limited storage capacity [48,49,50]. The cost of onshore storage tanks ranges from 900 to 1200 CNY/m3, which is relatively high [51]. Assuming a unit cost of 900 CNY/m3, constructing a storage facility with a capacity of 500,000 m3 would cost 450 million CNY—excluding maintenance and operational costs during the storage period. In addition, if such storage tanks leak and explode, the consequences could be catastrophic [52].
Based on the above analysis, the construction and operating costs of the abandoned salt cavern project for single-well and double-well configurations are compared and presented in Table 3. In this study, the cost of cavern formation is calculated as 300 CNY/m3 for a single well, whereas that for a horizontal double-well configuration is slightly lower—240 CNY/m3. Due to the available sediment storage space in the abandoned cavern, the two-butted horizontal well must be constructed first; as a result, it enables earlier gas storage initiation compared to the single-well dissolution cavern and generates corresponding economic benefits. Currently, the profit from SCGS in China is approximately 0.3~0.4 CNY/m3; this study adopts a conservative estimate of 0.3 CNY/m3.
According to a previous study, the time required to construct a new single-well cavern, a new double-well cavern, and a connection to an existing abandoned cavern (with a cavern volume of 500,000 m3 in all cases) is 57 months, 39 months, and 15 months, respectively [13]. Accordingly, based on this timeline, a preliminary calculation shows that the abandoned cavern gas storage model for coastal sedimentary salt mines can store natural gas 42 months earlier than the new single-well model. Based on a 3-month one-cycle, 14 injection and production cycles can be completed, with a total investment of 82 million CNY—far less than the 450 million CNY needed for onshore storage tanks. In addition, apart from requiring lower total investment, when the cavern construction-to-operation period is 57 months, this approach can generate a profit of 112.71 million CNY compared to newly constructed single-well and double-well gas storage modes, amounting to a net profit of 30.71 million CNY.
On the whole, it is feasible to store natural gas extracted from offshore fields in salt caverns located in coastal sedimentary areas, with respect to both cavern safety during construction and operation, and it is highly economically advantageous. On the other hand, the favorable conditions offered by large-scale deep underground storage (located 800~1000 m below the ground surface) can be leveraged to avoid environmental pollution caused by leakage from surface storage tanks [55,56]. Accordingly, this natural gas storage strategy can serve as a reference for the strategic transition from inland to coastal regions in the future. At the same time, it also enables large-scale, sustainable storage of carbon dioxide, hydrogen, compressed air, and other renewable energy carriers in abandoned salt caverns.

5. Conclusions

In this study, a low-grade TWH-cavern located in a coastal area of China is used as an example. A numerical cavern model is established using ANSYS Mechanical APDL, and FLAC3D software is subsequently employed to investigate the influence of different injection–production cycles on cavern operational stability, and to evaluate the feasibility of utilizing sediment voids for gas storage. Furthermore, a comparative analysis of the economic performance of the following systems during construction and operation is conducted: abandoned salt caverns, newly constructed single-well caverns, newly con-structed double-well caverns, and onshore storage tanks. The following conclusions are as bellow.
(1) Through numerical simulations of TWH-caverns with and without sediment de-posits, the safety of natural gas storage in coastal sediment-type salt caverns is evaluated. The simulation results show that, under the operating conditions of a 3-month one-cycle and a 30-year service life, the effective space gas storage mode exhibits a volume shrink-age rate of 21.39%, and the volume of the plastic zone increases to 1345% of its initial value. In contrast, the sediment space gas storage mode shows a volume shrinkage rate of only 6.94%, and the volume of the plastic zone expands to 87% of its initial value. This fully demonstrates that the presence of sediment can significantly inhibit salt rock creep and ensure the long-term stability of the salt cavern.
(2) Under different cyclic conditions, increasing the number of cycles per unit time can significantly enhance cavern stability while maintaining the same total operating time at both high and low pressures. The greater the number of cycles per unit time, the smaller the volume contraction rate of the cavern, the settlement of the cavern roof, and the contraction at the cavern waist. At the same time, the extent of the plastic zone at the salt cavern roof decreases, whereas those around and inside the cavern remain largely unchanged.
(3) Compared with newly constructed single-well salt caverns, newly constructed double-well salt caverns, and coastal onshore storage tanks, coastal sediment-hosted salt caverns offer significant economic advantages in terms of construction duration, construction cost, and operational revenue. In detail, its estimated construction cost is approximately 82 million CNY, which is far lower than the 450 million CNY required for onshore storage tanks. This energy storage method not only delivers outstanding economic benefits but also aligns with China’s current strategic orientation for marine development and addresses the urgent need for energy storage and transportation infrastructure amid the rapid growth of the marine economy.
This research is primarily based on numerical simulations and presents a preliminary feasibility analysis of using coastal TWH-caverns for sediment-based gas storage from both safety and economic perspectives. However, this discovery still lacks support from actual engineering operation data, and the research has not addressed sediment permeability or hydrate-related risks. In the future, it will be verified through on-site monitoring or physical model testing. In addition, we will use the discrete-element PFC software to simulate the impact of sediment particles on cavern stability and to investigate the cavern’s sealing performance using a fluid–solid coupling approach.

Author Contributions

Conceptualization, Z.L. and Q.L.; methodology, Z.L. and Q.L.; software, Q.L.; writing—original draft preparation, Z.L. and Q.L.; writing—review and editing, Q.L.; formal analysis, Q.L.; investigation, Z.L.; resources, Z.L.; data curation, Z.L. and Q.L.; supervision, Q.L.; project administration, Q.L.; funding acquisition, Q.L. All authors have read and agreed to the published version of the manuscript.

Funding

The research work was funded by the High-level Talent Research Start-up Fund of Changzhou Institute of Technology (Grant No. YN25107).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Urban electricity consumption and energy share in China, 1997~2025. (a) Distribution of annual electricity consumption by city; (b) annual average electricity consumption data for each city; (c) the proportion of renewable energy to fossil energy; and (d) the output, import volume, and degree of external dependence on natural gas (Note: 1 to 34, respectively, represent Beijing Municipality, Tianjin Municipality, Hebei Province, Shanxi Province, Inner Mongolia Autonomous Region, Liaoning Province, Jilin Province, Heilongjiang Province, Shanghai Municipality, Jiangsu Province, Zhejiang Province, Anhui Province, Fujian Province, Jiangxi Province, Shandong Province, Henan Province, Hubei Province, Hunan Province, Guangdong Province, Guangxi Zhuang Autonomous Region, Hainan Province, Chongqing Municipality, Sichuan Province, Guizhou Province, Yunnan Province, Xizang Autonomous Region, Shaanxi Province, Gansu Province, Qinghai Province, Ningxia Hui Autonomous Region, Xinjiang Uygur Autonomous Region, Taiwan Province, Hong Kong Special Administrative Region, and Macao Special Administrative Region).
Figure 1. Urban electricity consumption and energy share in China, 1997~2025. (a) Distribution of annual electricity consumption by city; (b) annual average electricity consumption data for each city; (c) the proportion of renewable energy to fossil energy; and (d) the output, import volume, and degree of external dependence on natural gas (Note: 1 to 34, respectively, represent Beijing Municipality, Tianjin Municipality, Hebei Province, Shanxi Province, Inner Mongolia Autonomous Region, Liaoning Province, Jilin Province, Heilongjiang Province, Shanghai Municipality, Jiangsu Province, Zhejiang Province, Anhui Province, Fujian Province, Jiangxi Province, Shandong Province, Henan Province, Hubei Province, Hunan Province, Guangdong Province, Guangxi Zhuang Autonomous Region, Hainan Province, Chongqing Municipality, Sichuan Province, Guizhou Province, Yunnan Province, Xizang Autonomous Region, Shaanxi Province, Gansu Province, Qinghai Province, Ningxia Hui Autonomous Region, Xinjiang Uygur Autonomous Region, Taiwan Province, Hong Kong Special Administrative Region, and Macao Special Administrative Region).
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Figure 2. A 3D view of the TWH-cavern.
Figure 2. A 3D view of the TWH-cavern.
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Figure 3. The distances from 17 salt caverns in China to the coast (N-W, N, N-E, E, M-S, and S-W respectively represent the northwestern, northern, northeastern, eastern, central-southern and southwestern parts of China).
Figure 3. The distances from 17 salt caverns in China to the coast (N-W, N, N-E, E, M-S, and S-W respectively represent the northwestern, northern, northeastern, eastern, central-southern and southwestern parts of China).
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Figure 4. H1-H2 three-dimensional geological model.
Figure 4. H1-H2 three-dimensional geological model.
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Figure 5. Diagram of different injection–production cycles. (a) 12-month, one-cycle; (b) 6-month, one-cycle; (c) 4-month, one-cycle; and (d) 3-month, one-cycle.
Figure 5. Diagram of different injection–production cycles. (a) 12-month, one-cycle; (b) 6-month, one-cycle; (c) 4-month, one-cycle; and (d) 3-month, one-cycle.
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Figure 6. The volume shrinkage rate change in the cavern. (a) Effective space gas storage; and (b) sediment space gas storage.
Figure 6. The volume shrinkage rate change in the cavern. (a) Effective space gas storage; and (b) sediment space gas storage.
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Figure 7. Displacement monitoring points of the cavern.
Figure 7. Displacement monitoring points of the cavern.
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Figure 8. Key displacement points of the effective space gas storage. (a) Point A; (b) point B; and (c) point C.
Figure 8. Key displacement points of the effective space gas storage. (a) Point A; (b) point B; and (c) point C.
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Figure 9. Key displacement points of the sediment space gas storage. (a) Point A; (b) point B; and (c) point C.
Figure 9. Key displacement points of the sediment space gas storage. (a) Point A; (b) point B; and (c) point C.
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Figure 10. The overall displacement of the effective space gas storage cavern under different injection–production cycles.
Figure 10. The overall displacement of the effective space gas storage cavern under different injection–production cycles.
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Figure 11. The overall displacement of the sediment space gas storage cavern under different injection–production cycles.
Figure 11. The overall displacement of the sediment space gas storage cavern under different injection–production cycles.
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Figure 12. The volume of the plastic zone in the cavern. (a) Effective space gas storage; and (b) sediment space gas storage.
Figure 12. The volume of the plastic zone in the cavern. (a) Effective space gas storage; and (b) sediment space gas storage.
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Figure 13. The distribution of plastic zones in the H1-H2 cavern under different injection–production cycles. (a) Effective space gas storage; and (b) sediment space gas storage.
Figure 13. The distribution of plastic zones in the H1-H2 cavern under different injection–production cycles. (a) Effective space gas storage; and (b) sediment space gas storage.
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Table 1. Investigation on the bedded salt rock grade and coastal distance of 17 salt caverns in China.
Table 1. Investigation on the bedded salt rock grade and coastal distance of 17 salt caverns in China.
NumberName of Salt CavernSalt Grade/%Sediment Content/%Void Space of Sediment/%Stratum Depth/mDistance to the Coast/km
1Jintan84.115.97.1800~1300250
2Huai’an26.473.632.71300~2200160
3Zhaoji77.522.510.01350~2010220
4Yexian31.768.330.41030~1480750
5Qianjiang61.438.617.22000930
6Yunying51.548.521.6500~1000840
7Jianghan39.260.827.02000850
8Feicheng11.488.639.4960~1200320
9Tai’an20.679.435.3800~1000300
10Heze74.026.011.61452~1613370
11Ningjin86.513.56.02900340
12Zigong93.36.73.0840~10541200
13Anning58.941.118.3600~900460
14Xiangli55.744.319.72501150
15Sanshui34.865.229.01200~1400110
16Zhangshu69.630.413.5250580
17Yulin90.010.04.442200~2850860
Table 2. Mechanical parameters of bedded salt rock.
Table 2. Mechanical parameters of bedded salt rock.
Rock TypeDensity/
kg·m3		Bulk Modulus/GPaShear Modulus/GPaInternal Friction Angle/°Cohesion/MPaTensile Strength/MPa
Mudstone250026.713.1309.94.7
Salt22006.73.137.521.7
Interlayer250016.78.6456.11.9
Sediment19220.8670.395100.40.4
Table 3. Construction and operation costs of different types of SCGS facilities [53,54].
Table 3. Construction and operation costs of different types of SCGS facilities [53,54].
Cost (Unit: Thousand CNY)Geophysical Exploration CostDrilling and Completion CostWater-Solution Cavern Formation CostGas Injection and Brine Discharge CostProfit from Gas StorageNet Earning
New single-well cavern10,00040,000150,00010,0000−210,000
New horizontal well cavern10,00040,500120,00010,00040,830−130,670
Existing caverns (sediment-type)10,20060,000010,000110,27130,071
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Li, Z.; Li, Q. Assessing the Feasibility of Using Coastal Salt Caverns for Strategic Energy Storage from Safety and Economic Perspectives. Sustainability 2026, 18, 4949. https://doi.org/10.3390/su18104949

AMA Style

Li Z, Li Q. Assessing the Feasibility of Using Coastal Salt Caverns for Strategic Energy Storage from Safety and Economic Perspectives. Sustainability. 2026; 18(10):4949. https://doi.org/10.3390/su18104949

Chicago/Turabian Style

Li, Zuer, and Qihang Li. 2026. "Assessing the Feasibility of Using Coastal Salt Caverns for Strategic Energy Storage from Safety and Economic Perspectives" Sustainability 18, no. 10: 4949. https://doi.org/10.3390/su18104949

APA Style

Li, Z., & Li, Q. (2026). Assessing the Feasibility of Using Coastal Salt Caverns for Strategic Energy Storage from Safety and Economic Perspectives. Sustainability, 18(10), 4949. https://doi.org/10.3390/su18104949

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