Feasibility Analysis of the Utilization of Directional Butted-Well Salt Caverns with Large Height Difference for Underground Energy Storage

Abstract

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.

1. Introduction

With the increasing severity of global climate change, sustainable development has become a major challenge facing the world today. Low-carbon development to achieve carbon peak and carbon neutrality has become an international consensus for addressing climate change and promoting green and sustainable development [1,2,3]. Energy transition is a crucial part of the global response to climate change, with the core being the construction of a new green and low-carbon energy system [4]. The “Fostering Effective Energy Transition 2024” report released by the World Economic Forum uses the Energy Transition Index to evaluate the performance of the current energy systems of 120 countries. Europe continues to lead the global Energy Transition Index rankings, with the top 10 completely composed of European countries (Figure 1). This is mainly due to their substantial investment in the research and development of clean energy technologies and the widespread use of clean energy [5,6]. In recent years, China has also made significant progress in the energy transition. It has not only significantly increased the production capacity of renewable energy but has also further developed and invested in the manufacturing capabilities of clean energy technologies such as electric vehicle batteries, solar panels, and compressed-air energy storage [7]. Among various clean energy technologies, the vigorous development of underground energy storage facilities is considered crucial for China to achieve “carbon neutrality” and upgrade its energy structure [8,9].

Underground energy storage refers to the use of deep underground spaces to store energy or energy substances such as oil, natural gas, hydrogen, compressed air, and carbon dioxide, as well as strategic scarce materials like helium [10]. Salt caverns are considered the most ideal underground energy storage facilities and have attracted wide attention. Since China built its first salt-cavern gas storage facility in 2012, it has successively planned and designed more than ten salt-cavern gas storage facilities in cities like Pingdingshan, Huai’an, Chuzhou, Jianghan, and Zhangxing. In recent years, it has also planned the construction of multiple salt caverns for compressed-air energy storage, hydrogen storage, and helium storage in the cities of Yingcheng, Heze, Feicheng, Jintan, and Yexian, marking that the underground energy storage industry has entered a golden development period. Restricted by the geological conditions of complex layered salt strata, the construction process of salt-cavern underground energy storage facilities in China is slow, severely restricting the development of the energy storage industry [11]. Taking the Jintan salt-cavern gas storage facility as an example, the geological conditions of the Jintan salt mine are relatively good, with an average mineral content of 85%, a depth between 1000 and 1200 m, and a low interlayer content. The construction period of a 300,000-cubic-meter salt cavity can be four years at the shortest and as long as eight years, with a large time span. The salt mines for over 10 upcoming salt-cavern storage facilities in Pingdingshan, Huaian, Chuzhou, Zhangxing, and other locations exhibit characteristics of low salt rock quality, high residue content, and deep burial depths. Their geological conditions are worse than those of the Jintan salt mine. Referring to the construction model of the Jintan gas storage facility, the construction period will be longer. In addition, the proposed construction models for two-well and horizontal-well salt caverns and the utilization of residues are still in the exploration stage, and there is still a long way to go for on-site application [12].

Against this backdrop, the utilization of old salt caverns has been put on the agenda and has attracted wide attention [13]. Thanks to China’s long-standing salt mining history, salt enterprises have left behind a large number of old salt caverns from salt production. Some of these old caverns are still in production, while others are in a state of disuse. Statistics show that the underground space of the old caverns left by the salt mines in Pingdingshan and Huai’an exceeds 200 million cubic meters, and the volume is still increasing at a rate of 20 million cubic meters per year. Making full use of these old caverns to convert them into underground energy storage facilities can save a significant amount of time for cavern creation, greatly shorten the construction cycle, and quickly create a large underground storage capacity [14]. Currently, the pilot projects for converting old caverns into gas storage facilities for the planned salt-cavern gas storage projects in Pingdingshan and Zhangxing are being carried out to solve various problems in the construction of these old caverns. The old caverns of salt enterprises are mainly butted-well salt caverns (BWSCs). Among the checked old caverns, there are notable features such as large flat-topped caverns and those with significant height differences (as illustrated in Figure 2), which present numerous challenges for their conversion into energy storage facilities. At present, there are few studies on the conversion of old caverns into underground storage facilities. Domestic scholars have conducted preliminary exploratory research [15,16,17], but there are still many problems to be solved, such as how to utilize the old butted-well salt caverns with a large height difference. This paper focuses on the problems associated with converting old salt caverns for gas storage, specifically targeting the difficulties in converting the large-height-difference butted-well salt caverns. By integrating the production mode and structural characteristics of these old caverns, it examines the feasibility of conversion from key aspects such as gas injection and brine drainage (GIBD) methods, operation parameter design, and stability evaluation. The findings of this study offer reference and guidance for converting old butted-well caverns with large height differences into underground energy storage facilities and accelerating the construction of underground storage facilities.

2. Overview of Old BWSCs with a Large Height Difference

Salt companies mostly adopt directional butted wells for brine extraction and salt production. A directional butted-well system consists of a vertical well and a directional well, with the distance between the two wells typically ranging from 250 to 400 m. Initially, a vertical well is drilled, and a single-well cavity building system is installed. This system is operated for a period to create a small cavity of a specific volume. Then, a directional well is drilled to enter into the small cavity. The production casing is directly lowered into the inclined wellbore section in the salt formation, cemented, and used as the cavity-building pipe. Following this, an alternating water-injection method between the two wells is used to extract brine. As salt production aims to achieve economic viability, the companies typically require that the extracted brine reaches a saturated concentration. Consequently, the water injection displacement is generally at a low level, averaging around 60 m³/h, and the production period can last from over a decade to several decades. Additionally, the salt mining companies usually use a solvent-free cavity-dissolution method to control the entire production process based on empirical methods and convert the salt extraction volume accordingly. During this process, the duration of alternating water injection significantly affects the development morphology of the cavities on both sides. On the side with a longer water injection time, the cavity expands more extensively. Due to the absence of solvent at the cavity roof, the dissolution rate of the cavity roof is faster than that of the cavity periphery, causing the salt cavity to continuously dissolve and expand upward. Additionally, on the brine-discharging side, the high brine concentration results in a lower dissolution rate. Ultimately, under this unbalanced brine-extraction method, large-height-difference butted-well salt caverns are formed. The longer the cumulative water injection time on one side, the greater the height difference between the two cavities. A schematic diagram illustrating the formation of a large-height-difference butted-well salt cavity is presented in Figure 3. As shown in Figure 2, the maximum height difference between the cavity roofs on both sides of the measured butted-well salt cavity approaches 200 m, representing a typical cavity with a large height difference.

3. Selection of Gas Injection and Brine Discharge Method

3.1. Proposition of Different GIBD Methods

Gas injection and brine drainage is the final step in transforming old salt caverns into energy storage facilities and is a pivotal determinant of their storage capacity. In China, the layered salt formations often contain insoluble interlayers. As a result, a substantial amount of insoluble residue accumulates at the bottom of salt caverns, notably diminishing their gas storage volume. In recent years, the idea of utilizing the residue space at the cavern bottom for gas storage has been proposed and explored [18,19]. This initiative aims to boost the gas storage capacity of salt caverns and fully capitalize on their potential. The utilization of the residue space for gas storage is closely related to the GIBD method. In foreign salt dome formation, the salt rock boasts high purity, and there is little residue at the bottom of the salt caverns. Therefore, the GIBD processes do not account for residue utilization. However, in China, especially in the measured old caverns formed by butted wells, the residue can take up as much as 90% of the cavern volume. Disregarding this residue volume would lead to a significant waste of gas storage space [20]. The implementation of the GIBD method hinges on the overall shape of the salt cavern. For interconnected-well salt caverns, the shape of the cavern occupied by the residue and the connecting channels cannot be measured by instruments, which restricts the transformation process. Currently, an estimation method (Equation (1)) based on brine extraction volume, well logging data, etc., has been proposed to calculate the volume of the bottom residue, providing key parameters for the economical utilization of old caverns. Moreover, based on the wellbore trajectory, the shape of the upper cavity, the residue volume, and practical drilling knowledge [21,22], the depth of the top interface of the bottom residue and the connecting channels can be roughly described, which provides a basis for making decisions regarding the GIBD method.

$$V_{se}={\displaystyle \frac{V_b·c·\alpha ·\beta }{{\rho }_s·\left(1-\alpha \right)}}$$

(1)

For the butted-well salt caverns, different GIBD methods result in different utilization rates of the residual space volume and involve different construction techniques. Based on this, three gas injection and brine drainage methods are proposed—“one injection and one discharge (OIOD)”, “two injection and one discharge (TIOD)”, and “two injection and two discharge (TITD)”—as illustrated in Figure 4. The OIOD method refers to a process where a brine drainage pipe string is lowered into only one well. Gas is injected through one well, while brine is drained from the other well. After gas injection on one side is completed, independent gas injection and brine drainage operations are carried out on the side of the brine-draining well. The TIOD method requires drilling a new well in the middle of the two existing wells to get into the connecting channel. Gas is simultaneously injected through the two side wells, and brine is drained from the new drilling well. The TITD method involves lowering brine drainage pipe strings into both wells down to the top surface of the residue, and each well undergoes independent GIBD operations. The advantages and disadvantages of the three GIBD methods are shown in

Table 1. Advantages and disadvantages of three types of GIBD.

Method	Advantage	Disadvantage	Applicable Objects
OIOD	A set of conventional simplified brine drainage equipment Capable of utilizing residual slag space at the bottom of one side cavity Requiring only one conventional snubbing operation	Long brine drainage time High pressure control requirements	The residual volume is comparable to the cavity capacity, or a significant height difference exists in residues between the two caverns.
TIOD	Large-diameter brine discharge pipes can be installed, ensuring rapid discharge rates Residues in both caverns can be utilized, achieving high residue utilization rate No snubbing operation required	New drilling required to access residue channels Increases new drilling costs Post-discharge treatment of brine discharge wells required after completion	High residue content at the bottom of both cavities with small cavern volume.
TITD	Conventional brine discharge equipment, easy to construct/install Dual-well simultaneous injection capability, high discharge capacity, short operational cycle Wide applicability	Two sets of brine drainage equipment need to be installed High pressure control requirements for both wells Unutilized residual space Two snubbing operations required	Low residue content at the bottom of both cavities with large cavern volume.

.

3.2. GIBD Method for BWSCs with a Large Height Difference

For large-height-difference butted-well salt caverns, both the OIOD and TIOD methods are applicable from the perspective of residue utilization. The large-height-difference salt cavern in Figure 2 is simplified into the cavern model in Figure 5. The two side caverns are cylindrical with semi-ellipsoidal tops, and some height-difference parameters remain the same. The height of the cavities on both sides and the diameter of the cylinders are kept consistent. By using the single-variable method and fixing the total height of each of the two caverns, the variation law of the percentage increase in the residue utilization volume of the TIOD method compared to the OIOD method in large-height-difference BWSCs with various parameters is analyzed, as shown in Figure 5. It can be seen from the figure that the percentage increase in the residue volume decreases with the increase in the height of the connecting channel and the height of the brine cavity, while it increases as the height of the residue in the deep cavern increases. It basically does not change with the synchronous change in the diameters of the two caverns. Compared to the OIOD method, the TIOD method primarily enhances the residue volume on the side of the deep cavern. When the height of the residue in the deep cavern is small or the height difference between the residues in the two caverns is large, the increase in the residue volume is also limited. Furthermore, the gas storage capacity of the residue depends on the gas storage efficiency of the void space within the residue. Some scholars’ research indicates that the gas storage efficiency of the void space in the residue formed by the dissolution of typical layered salt rocks ranges from 8.2% to 13.6% [23,24]. If calculated at 10%, an increase of 300,000 cubic meters in the residue volume by the TIOD method compared to the OIOD method only corresponds to an increase of about 30,000 cubic meters in the gas storage volume. From a technological and economic perspective, the TIOD method necessitates drilling a new well, which elevates the drilling cost compared to the OIOD method. Additionally, it is challenging to drill into the residue channel without a full understanding of the height of the connecting channel and the shape of the residue. For the purpose of facilitating understanding, taking the example of a butted-well salt cavern, a comparative list of the cost and residue volume utilization rate differences among the three gas injection and brine discharge methods is shown in

Table 2. Comparison of costs and residue volume utilization rates among the three GIBD methods.

Method	Gas Injection and Brine Discharge Operation Engineering Quantity and Cost Differences							Residue Volume Utilization Rate
	Brine Discharge and Control Device/Sets No.	Cost 104/CNY	Snubbing Operation Times	Cost 104/CNY	Newly Drilled Brine Discharge Wells No.	Cost 104/CNY	Total Cost 104/CNY
OIOD	1	850	1	120	/	/	970	50–70%
TIOD	1	850	/	/	1	1200	2050	90%
TITD	2	1700	2	240	/	/	1940	0

. As can be seen from the table, apart from some common expenses, the cost of the TIOD method is much higher than that of OIOD method. After a comprehensive analysis of economic and technological feasibility, the OIOD method is recommended as the optimal GIBD method for large-height-difference BWSCs.

3.3. Parameter Determination for BWSCs with a Large Height Difference

In the case of large-height-difference BWSCs, through an analysis of their construction mechanism, it is found that there is a positive correlation between the cavern height and the height of the residue accumulated at the cavern bottom. Under the optimized OIOD mode, only the residue volume of one side cavern can be used for gas storage. The entire GIBD process can be divided into two distinct stages, as illustrated in Figure 6. In the first stage, gas injection is preferably carried out from the side of the cavern with a high residue level, while brine is discharged from the side of the cavern with a low residue level. The process continues until the wellhead of the salt cavern with a low residue height indicates an increase in pressure and the presence of gas, which was observed in the experiment described in Reference [25]. At this point, the gas injection valve at the wellhead of the cavern with a high residue level is closed. In the second stage, gas injection is switched to the annulus of the side of the cavern with a low residue level, and brine is discharged through its own brine discharge pipe until the gas-–liquid interface moves to an appropriate position above the residue top surface. Throughout the GIBD process, the precise control of technology parameters is of crucial importance. Therefore, a set of calculation methods for GIBD parameters has been established.

The entire OIOD system is analogous to a U-shaped tube. The parameters of gas injection and brine drainage can be determined by establishing system equations based on the U-shaped tube principle. During the GIBD process, the following pressure balance equation exists at any gas–liquid interface within the cavity:

$$P_i={\rho }_{b}gh+P_{ft}+P_{fs}+P_b$$

(2)

The flow of brine in the discharge pipe column generates flow frictional resistance, which can be calculated using the frictional resistance formula (Equation (3)). The magnitude of this resistance depends on fluid properties, pipe roughness, and flow regime. The flow regime can be determined by the Reynolds number (Equation (4)), while the pipe roughness satisfies Equation (5). The hydraulic friction coefficient is closely related to the flow regime, with empirical formulas provided in

Table 3. Empirical formula of hydraulic fraction.

Fluid Regime	Reynolds Number	Common Empirical Formulas
Laminar flow	$\mathrm{R}\mathrm{e}<2000$	$\lambda >{\displaystyle \frac{64}{\mathrm{R}\mathrm{e}}}$
Turbulent flow	$2000<\mathrm{R}\mathrm{e}\le {\displaystyle \frac{59.7}{{\epsilon }^{8/7}}}$	$\lambda ={\displaystyle \frac{0.3164}{{\mathrm{R}\mathrm{e}}^{0.25}}}$
	${\displaystyle \frac{59.7}{{\epsilon }^{8/7}}}<\mathrm{R}\mathrm{e}\le {\displaystyle \frac{665-765\mathrm{l}\mathrm{g}\epsilon }{\epsilon }}$	${\displaystyle \frac{1}{\sqrt{\lambda }}}=-1.8\cdot \mathrm{l}\mathrm{g}\left[{\displaystyle \frac{6.8}{\mathrm{R}\mathrm{e}}}+{\left({\displaystyle \frac{\mathsf{\Delta }}{3.7d}}\right)}^{1.11}\right]$
	$\mathrm{R}\mathrm{e}>{\displaystyle \frac{665-765\mathrm{l}\mathrm{g}\epsilon }{\epsilon }}$	$\lambda ={\displaystyle \frac{1}{\left(2\mathrm{l}\mathrm{g}\frac{3.7d}{\mathsf{\Delta }}\right)}}$

. In 1944, American engineer Moody developed the well-known Moody diagram based on a vast amount of experimental data on pipe friction [26]. Starting from the Darcy–Weisbach equation, this diagram quantitatively correlates fluid property parameters and pipe parameters, with the equivalent roughness serving as a crucial variable.

$$P_{ft}={\displaystyle \frac{{\rho }_b\lambda h{v}^2}{2d}}$$

(3)

$$Re={\displaystyle \frac{{vd\rho }_b}{\mu }}$$

(4)

$$\epsilon ={\displaystyle \frac{2∆}{d}}$$

(5)

In addition, the flow friction resistance generated during brine flow within the residue is correlated with the brine flow rate, flow path length, and fluid flow regime. This flow friction resistance can be determined through parameter fitting utilizing a modified dual-well operation approach—one well for brine injection and another for brine extraction [27].

$$P_{fs}=F\left(Q_S,L_S,{\lambda }_S\right)$$

(6)

Gas is injected into the cavity through the wellbore, following the gas pipe flow theory. The gas pressures in the cavity and at the wellhead exhibit the following conversion relationship [28]:

$$P_p=P_{g}exp\left({\displaystyle \frac{0.03415{\gamma }_{0}h}{\overline{Z}\overline{T}}}\right)$$

(7)

Additionally, according to the gas state equation, the following relationship exists when converting the gas injected into the cavity to ground standard conditions.

$${\displaystyle \frac{P_0{V}_0}{Z_0{T}_0}}={\displaystyle \frac{PV}{\overline{Z}\overline{T}}}$$

(8)

To facilitate the analysis of the parameters’ variation rules, based on the real parameters of the BWSC in Figure 2 and the simplified model shape parameters in Figure 5 (which form Data

Table 4. Basic parameters for gas injection and brine discharge in the BWSC with large height difference.

Item	Value	Item	Value
Roof depth of shallow cavern, m	1526	Natural gas density, g/cm3	0.56
Bottom depth of shallow cavern, m	1586	Gas average compression factor in well	0.80
Roof depth of deep cavern, m	1721	Average temperature in the cavity, ℃	60
Bottom depth of deep cavern, m	1781	Volume of shallow cavern, m3	200,960
Brine discharge pipe depth, m	1773	Volume of deep cavern, m3	200,960
Brine discharge pipe size, mm	114.3	Pipe roughness, mm	0.20
Brine discharge wellbore size, mm	215.9	Brine density, g/cm3	1.20

), the above formulas are applied to calculate the relationship chart between the gas injection rate, gas injection pressure, casing shoe pressure, and brine discharge rate in the first stage of Figure 6, as shown in Figure 7. It can be seen from the figure that as the brine discharge rate increases, the gas pressures both at the wellhead and at the casing shoe increase. Under the given upper-limit allowable pressure of the casing shoe, the maximum brine discharge rate and the wellhead gas-injection rate can be determined according to this chart. Meanwhile, it can also provide parameter guidance for the phased gas injection and brine discharge design. The utilization volume of the residue can be calculated by equivalent calculation of the volume of a cylinder with an equal diameter, combining the wellbore trajectory, the bottom shape of the cavity as measured by sonar, and the volume calculated by Equation (1). The first stage of the OIOD method was tested in a pair of reconstructed BWSCs in a domestic salt mine. Figure 8 presents the gas injection and brine drainage parameters of this test well group along with the fitting results obtained using the established model. It can be observed that the errors between the two are minimal, demonstrating that the constructed model exhibits strong adaptability.

4. Stability Evaluation

After gas injection and brine discharge, the salt cavern undergoes its official injection–production operation. For a large-height-difference BWSCs, the notable depth variation leads to considerable stress disparities in the formation strata. Under interconnected pressure systems between caverns, designing an injection–production scheme that ensures long-term cavern stability and maximizes gas storage capacity poses a critical challenge. To tackle this issue, this section conducts numerical simulation analysis based on stability assessment rule for a large-height-difference BWSC, investigating its mechanical feasibility from a geomechanical perspective.

4.1. Geomechanical Model Establishment

Based on the geological conditions of the Haolong area in the Pingdingshan Salt Mine, Henan province, the strata information is analyzed. The Pingdingshan Salt Mine features 21 sets of salt groups developed sequentially from top to bottom, with the uppermost being the No. 0 salt group and the lowermost the No. 20 salt group. According to the statistical data from drilled wells in the Haolong area, the final drilling formations of all the wells operated by the salt mining companies are the first member of the Hetaoyuan Formation. The salt-mining intervals span from the No. 14 to the No. 18 salt groups, and there are multiple mudstone interlayers of varying thicknesses within the salt layers. The salt cavern in Figure 2 reparents an old cavern mined by butted wells in the Haolong area. According to the sonar cavity-measuring data, the shallow cavity has a top depth of 1527 m and a bottom depth of 1585 m, while the deep cavity has a top depth of 1722 m and a bottom depth of 1780 m. The total volume of the two cavities is approximately 243,000 cubic meters. To investigate the influence of height difference on the deformation and damage of the BWSC, a regular salt cavern model was used to substitute the real one, and the shape of the residue was simplified, taking into account the height-difference data of the salt cavern in Figure 2, wellbore trajectory data, and stratigraphic conditions. A three-dimensional geomechanical model containing a large-height-difference BWSC was then established, as depicted in Figure 9. The model has dimensions of 740 m in length, 300 m in width, and 1000 m in height. The spacing of the two wells of the large-height-difference BWSC is 260 m. Each cavity has a height of 60 m and a diameter of 80 m. The top height difference between the two cavities is 195 m, and the height of the residue channel is 15 m. To simplify the calculation, an overlying pressure with a value of 25.46 MPa is applied on the top surface of the model, calculated based on the actual depth and the average density of the overburden strata (ρ = 2300 kg/m³). Normal displacement constraints are applied to the four lateral surfaces of the model, meaning they are just allowed to produce displacement in the vertical direction. Considering the stress release in a long-term rheological environment, the in situ stress of the rock salt formation is applied using equal principal stresses in three directions, with an in situ stress gradient of 0.023 MPa/m, and its redistribution is firstly calculated before creep analysis during the numerical simulation. The gas pressure is applied on the inner surface of the cavity. The entire model is meshed with tetrahedral elements. The mesh quality meets the accuracy requirements, consisting of a total of 755,780 meshes and 1,029,697 nodes. The areas around the BWSC model are set with encrypted meshes, and the meshes gradually become sparser as the distance from the salt-cavern model increases. The Flac3D v6.0 software is used to build the whole model and carry out numerical simulation analysis. The mechanical constitutive model uses the Cpower model in the software, which integrates the Mohr–Coulomb elastoplastic model and the Norton Power creep model. The rock mechanical parameters are shown in

Table 5. Mechanical parameters of salt formation [29].

Lithology	Elastic Modulus (GPa)	Poisson’s Ratio	Cohesion (MPa)	Friction Angle (°)	Tensile Strength (MPa)	A	n
Upper and lower mudstone	11.2	0.26	4.24	26.5	3.24	/	/
Rock salt	7.71	0.3	3.77	34	1.34	1.08 × 10−7	3.17
Mudstone interlayer	11.2	0.24	4.24	26.5	3.24	2.4 × 10−8	1.7

, and the mechanical parameters of the residue are set as 10% of those of the interlayers. The simulation results are post-processed using the Tecplot 360 v2015 R1 software. The maximum unbalanced force was monitored to check the convergence of the model during the calculation.

4.2. Operating Pressure Scheme and Evaluation Method

4.2.1. Operating Pressure Plan

Operating pressure plays a pivotal role in determining the storage capacity and ensuring the cavity’s safety. According to Canadian standards for underground gas storage, the maximum operating pressure should not exceed 80% of the cap-rock fracture pressure [30]. In China, safety regulations stipulate that the upper-limit pressure of salt-cavern gas storage must not be higher than 80% of the formation fracture pressure at the production casing shoe, nor can it exceed 80% of the overlying formation pressure at the casing shoe [31]. The minimum operating pressure must ensure that no significant sidewall spalling, cavity roof collapse, or ground subsidence occurs in the cavity-surrounding rock during long-term injection and production operations. The established Jintan gas storage facility uses 0.007 MPa/m as the lower-limit pressure. Some studies also take 20–30% of the overlying rock pressure at the casing shoe as the minimum allowable pressure [32,33]. Here, 75% of the overlying rock pressure at the casing shoe is adopted as the upper-limit pressure and 30% as the lower-limit pressure. Due to the special characteristics of the large-height-difference BWSCs, the design of their operating pressure needs to consider the safety and stability of both the shallow and deep cavities. According to the aforementioned pressure-design method, the casing shoe is located 50 m above the cavity roof. For the deep-cavity casing shoe, the designed upper-limit pressure is 29 MPa, and the lower-limit pressure is 12 MPa. For the shallow-cavity casing shoe, the designed upper-limit pressure is 25.5 MPa, and the lower-limit pressure is 10.5 MPa. To explore the deformation characteristics of the surrounding rock, a constant-pressure rheological simulation analysis is conducted for these four pressures.

4.2.2. Assessment Criterion

Numerous studies have indicated that for salt-cavern gas storage facilities to maintain safety and stability, the caverns need to meet the requirements of sealing performance, stability, and usability, which necessitates a reasonable safety evaluation method [34,35]. After long-term exploration, a set of stability evaluation indicators has been introduced to assess the stability of the cavern’s surrounding rock during injection and extraction operations, including displacement, plastic zone, volume shrinkage rate, expansion safety factor, and equivalent strain [36,37]. Regarding plastic damage, it is essential that there are no large connected plastic regions in the surrounding rock of the cavern wall. In terms of volume deformation, the design requirements for salt-cavern gas storage vary by country. In Germany, the cavern volume shrinkage rate should not exceed 20% after 30 years of operation, while in France, the limit is set at 30% over the same period [38]. In China, considering the creep inhibition effect of mudstone interlayers, the volume shrinkage rate of salt-cavern gas storage is evaluated based on the French volume deformation requirements. For the expansion safety factor, local damage is considered to have occurred when it drops below 1.5. A value lower than 1.0 indicates destruction, and a value below 0.6 implies collapse [39]. The equivalent strain, used to evaluate the rock mass’s creep damage, should not exceed 3% after 30 years of operation [40,41].

4.3. Result Analysis

4.3.1. Deformation of the Surrounding Rock of the Cavity

Figure 10 shows the cloud diagrams depicting the total displacement of the salt cavern’s surrounding rock under different internal pressures. To facilitate the observation of the deformation of the salt cavern’s surrounding rock in the area covered by sediment, the sediment layer is concealed in the figure. As the internal pressure rises, the deformation displacement of the salt cavern’s surrounding rock gradually diminishes, indicating an improvement in the cavern’s stability. In the large-height-difference BWSC, it is evident from the figure that the deep cavity experiences greater deformation displacement than does the shallow cavity. Under low-pressure conditions, the maximum difference between them reaches approximately 2 m, with the maximum displacement occurring at the waist of the deep cavity. Notably, at a pressure of 10.5 MPa, the maximum displacement of the deep cavity reaches 8.3 m, exceeding 10% of the salt cavern’s maximum diameter. According to the allowable range mentioned in Reference [42], it is clear that a pressure of 10.5 MPa no longer meets the requirements. By contrast, at a pressure of 12 MPa, the maximum displacement is 7.3 m, which falls within the allowable range. Additionally, the deformation displacement of the cavity covered by sediment is smaller than that of the empty cavity, highlighting the sediment’s inhibitory effect on the deformation of the cavity wall’s surrounding rock. A similar phenomenon was also reported in Reference [43]. Figure 11 shows the curves representing the deformation displacement of the salt cavern’s roof over time under different internal pressures. Evidently, as time progresses, the settlement displacement of the cavern roof gradually increases, albeit with a decreasing rate of increase. The figure also clearly demonstrates that, under the same pressure, the deep cavity’s roof experiences greater deformation displacement than the shallow cavity’s roof. It is worth emphasizing that the displacement of the deep cavity’s roof at a pressure of 12 MPa even surpasses that of the shallow cavity’s roof at a pressure of 10.5 MPa, which underscores the unique structural characteristics of the large-height-difference BWSC.

4.3.2. Distribution of Plastic Zone

The plastic zone serves as a crucial indicator for assessing the damage and failure of the salt cavern’s surrounding rock. Figure 12 presents the distribution patterns of the plastic zones in the salt cavern’s surrounding rock under various internal pressures. Evidently, when the internal pressure is low, the plastic zone at the cavern wall exhibits a widespread distribution. In the case of the large-height-difference BWSC, the plastic zone at the top of the deep cavern is more extensive than that of the shallow cavern. This suggests that the top of the deep cavern is more vulnerable to damage. Such findings hold significant implications for determining the setting depth of the casing shoe during the subsequent well-drilling process for reconstruction. Under low-pressure conditions, there is no plastic connection between the deep and shallow caverns. Additionally, the plastic zone of the cavern covered with residue is relatively small, which indicates that the residue can inhibit the deformation and damage of the cavern wall’s surrounding rock. Figure 13 shows the curves of the volume ratio of the plastic zone of the salt cavern’s surrounding rock varying with time under different internal pressures. The volume ratio of the plastic zone is defined as the ratio of the total volume of the cavern’s surrounding rock that enters the plastic state to the initial volume of the two cavities. From Figure 13, the volume of the plastic zone of the surrounding rock gradually increases with the operation time, but the increase rate gradually decreases, reflecting the characteristic that the unbalanced stress difference between the salt cavern and the surrounding rock gradually decreases and the whole system tends to be in equilibrium. Notably, the volume ratio of the plastic zone under an internal pressure of 10.5 MPa is 63% higher than that under 12 MPa. This clearly shows that the surrounding rock of the salt cavern is more likely to undergo plastic damage at lower pressures. These insights offer valuable guidance for designing the operating pressure of the salt cavern.

4.3.3. Volume Loss Rate

The volume shrinkage rate is an important indicator for measuring the volume deformation of salt caverns. Figure 14 shows the curves of the volume shrinkage rates of deep and shallow salt caverns varying with time under different internal pressures. Seen from the figure, as the operation time increases, the volume shrinkage rate gradually increases, which indicates that the deformation of the salt cavern becomes larger and the volume loss becomes more severe. With the increase in the operating pressure, the volume shrinkage rate of the salt cavern gradually decreases, and the cavern tends to be more stable. In addition, under the same pressure, the volume shrinkage rate of the deep cavern is significantly larger than that of the shallow cavern, with a difference of about 3%, which also reflects the characteristic of large deformation in the deep cavern. The figure also shows that after 30 years of operation at an internal pressure of 10.5 MPa, the volume shrinkage rates of the shallow and deep caverns are 33.7% and 36.9%, respectively, both exceeding the allowable limit of 30%. This indicates that both salt caverns face a risk of deformation instability under this pressure. However, after 30 years of operation at an internal pressure of 12 MPa, the volume shrinkage rates of the shallow and deep caverns are 27.9% and 31.0%, respectively, with an average of 29.5%. Compared with the internal pressure of 10.5 MPa, the volume shrinkage rate is significantly reduced, and the stability of the salt caverns is greatly improved.

4.3.4. Dilatancy Safety Factor

The expansion safety factor mainly reflects the damage situation of the salt cavern’s surrounding rock. Figure 15 presents the distribution map of the expansion safety factor of the salt cavern’s surrounding rock under different internal pressures. Seen from the figure, it is evident that areas with a safety factor below 6 are predominantly situated around the cavity. By contrast, the safety factor of the salt cavern covered with residue is generally above 6, which demonstrates the residue’s inhibitory effect on the damage and deformation of the salt cavern. At low pressure, the figure reveals that the safety factor of the interlayer at the shoulder of the deep cavity drops below 1, which indicates that this interlayer is damaged during operation, and the interlayer is the weak point of damage in the deep cavity. Moreover, under the same pressure, the area with a low safety factor at the top of the deep cavity is significantly larger than that at the top of the shallow cavity, which reflects that the stability of the deep cavity is relatively weaker than that of the shallow one. As the pressure increases, for example, from 12 MPa to 25.5 MPa, the expansion safety factor of the cavity’s surrounding rock increases significantly, thereby enhancing the cavity’s stability.

4.3.5. Equivalent Strain

Figure 16 shows the equivalent strain distribution maps of the cavity’s surrounding rock under different internal pressures. The red color indicates that the equivalent strain value is higher than 3%, which suggests the existence of creep damage. From the figure, it can be seen that under low pressure, there are more red-colored areas, which are mainly located near the cavity. By contrast, the equivalent strain values of the salt cavity covered by residues are small, indicating that these areas are stable in terms of the equivalent strain index, which also reflects the inhibitory effect of the residues. Under the same pressure condition, the range of the red area at the top of the deep cavity is larger than that of the shallow cavity, while the range at the waist of the deep cavity is smaller than that of the shallow cavity. This reflects that the top of the deep cavity and the waist of the shallow cavity are the weak areas more prone to creep damage under low pressure. As the internal pressure rises, there is a gradual reduction in the red-colored areas. Notably, under high-pressure scenarios, the red areas almost vanish. This phenomenon suggests that the creep damage of the salt cavity is diminishing and its stability is being enhanced.

4.3.6. Tensile Zone

Figure 17 shows the maximum principal stress nephograms of the salt cavern’s surrounding rock under different internal pressures. A negative principal stress implies that the surrounding rock is under compressive stress, whereas a positive value indicates a tensile stress state. The figure shows that the maximum principal stress of the surrounding rock around the cavern under the four internal pressures is basically in a compressive stress state, without any tensile stress. As the internal pressure increases, the absolute value of the maximum principal stress of the surrounding rock at the cavern wall gradually increases. Under the same pressure, the maximum principal stress of the surrounding rock of the deep cavern is slightly less than that of the shallow cavern. The minimum absolute value of the maximum principal stress (−5.2 MPa) is mainly located in the interlayer at the waist of the deep cavern, which indicates that the stability of the deep cavern is weaker than that of the shallow cavern. Moreover, the absolute values of the maximum principal stress of the surrounding rock of the salt cavern covered with residue are all relatively large. This observation indicates that the residue provides support to the surrounding rock of the cavern wall, thereby mitigating the deformation and damage of the surrounding rock.

5. Discussion

5.1. Technical Analysis of the GIBD of BWSCs

Gas injection and brine discharge is a critical step in converting brine salt caverns into gas storage facilities. Unlike traditional single-well salt cavities, the GIBD method for butted wells in old salt caverns is closely linked to the utilization of residue. According to the analysis in Section 3.1, without considering the utilization of the void space in the residue, the TITD method is similar to the GIBD of single-well salt cavities and represents the most mature technology. However, this method is obviously difficult to adapt to the current situation of high residue content in old domestic BWSCs. Given the background of in-depth research on the gas storage utilization of residue void space and the demand for increased storage capacity, the other two GIBD methods are more suitable for the present reality. The OIOD and TIOD methods have advantages in residue space utilization. Depending on the residue content and the shape of the salt cavern, these two methods are the key technical means for the integrated and comprehensive utilization of the brine cavity and residue space of butted wells [44]. Although there is still an insufficient understanding of the gas storage efficiency in residue voids at present, the technical feasibility of these two GIBD methods has been preliminarily verified in the field. In China, ongoing pilot tests for the reconstruction and utilization of BWSCs and residue include the Zhangxing gas storage and Yunying compressed-air energy storage projects. For the Zhangxing gas storage project, the reconstruction work of old BWSCs has been carried out. The first-phase gas injection operation was completed using the OIOD method, with a gas injection volume of approximately 70 million cubic meters. This verifies the feasibility of reconstructing old BWSCs into gas storage facilities and the fluidity of brine in the connecting channels and residues. The Yunying compressed-air energy storage facility is also being built by reconstructing a pair of old BWSCs. The GIBD operation is being carried out using the TIOD method, and the first-phase project has been put into operation, which verifies the dual feasibility of the TIOD method for old BWSCs and the utilization of residue space [45]. The large-height-difference BWSC studied in this paper has significant structural differences compared with the conventional equal-height BWSC, but it still meets the requirements for the above two GIBD methods. The pilot project tests provide a practical basis for the gas injection and brine discharge methods for the old large-height-difference butted-well salt caverns in this study.

5.2. Operation Pressure Determination of BWSC with Large Height Difference

The above stability analysis results indicate that under low pressure, there are significant differences in deformation and damage between the deep and shallow cavities of the large-height-difference BWSC, which is significantly different from the small-height-difference case in Reference [46]. This implies that the application of this type of gas storage requires special attention to the design of the operating pressure to ensure its safe operation. For the large-height-difference BWSC, considering an average in situ stress gradient of 0.023 MPa/m, and following the pressure determination method in Section 4.2.1, the upper-limit pressure difference and lower-limit pressure difference under different depth differences between the tops of the two salt caverns are calculated, and the variation curves are plotted as shown in Figure 18. The figure shows that as the depth difference between the tops of the two caverns increases, both the upper-limit pressure difference and the lower-limit pressure difference increase linearly, and the change in the upper-limit pressure difference is more significant than that in the lower-limit pressure difference. When the depth difference reaches 200 m, the upper-limit pressure difference is approximately 3.5 MPa, and the lower-limit pressure difference is approximately 1.4 MPa. This means that the upper-limit pressure of the shallow cavity is about 3.5 MPa lower than that of the deep cavity, and the lower-limit pressure is about 1.4 MPa lower, indicating a significant operation pressure difference. Under this difference, there is a problem that the operating pressure may meet the requirements of the deep cavity but not those of the shallow cavity. For example, the upper-limit pressure of the deep cavity may be higher than that of the shallow cavity. If the upper-limit pressure of the BWSC is designed according to the casing shoe depth of the deep cavity, there is a risk that the upper-limit pressure of the shallow cavity is too high, which may damage the cavity’s sealing. Given the significant structural differences in large-height-difference BWSCs, when the two cavities are connected and share the same injection–production pressure system, the operating pressure design must simultaneously meet the safety and stability requirements of both the deep and shallow cavities. Therefore, the conventional design mode of the upper and lower operating pressures for single-cavity salt caverns is no longer applicable.

In addition, when the large-height-difference BWSCs are filled with natural gas using the OIOD method, the wellhead pressure is adjusted to be equal through pressure balance. The gas pressure difference at the casing shoes at the tops of the two caverns can be calculated using Equation (7). For example, with an upper-limit pressure of 25.5 MPa and in combination with the parameters in Table 3, the results of the gas pressure difference at the tops of the two caverns under different height differences are shown in Figure 19. As shown in the figure, the gas pressure difference at the tops of the two caverns increases gradually as the height difference between them grows. When the height difference between the two caverns reaches 200 m, the calculated gas pressure difference at the casing shoes of the cavern tops is approximately 0.4 MPa, which reflects the characteristic of a small gas pressure difference at the casing shoes of the two caverns in large-height-difference BWSCs. This is significantly different from the pressure limit difference at the casing shoes of the two caverns determined by the formation stress gradient. After the large-height-difference BWSCs are filled with gas and the pressure is balanced, although the top depths of the two caverns differ greatly, the gas pressures at the casing shoes are almost the same and can basically be considered equal. This understanding is of great significance for determining the operating pressure range of large-height-difference BWSCs.

Based on the above analysis, a pressure range design model is proposed: adopting the upper-limit pressure for shallow caverns and the lower-limit pressure for deep caverns to establish the injection–production operating pressure for BWSCs with a large height difference. This approach provides safety assurances for the transformation and utilization of such caverns. For the given case study, the recommended operating pressure range is 12–25.5 MPa.

6. Conclusions

The numerous old BWSCs in China serve as crucial carriers for accelerating the large-scale construction of underground gas storage facilities, spurring the rapid development of new energy fields like energy storage and hydrogen storage, and achieving the “dual carbon” goals. Among them, the old large-height-difference BWSCs encounter significant challenges regarding the feasibility of their transformation and utilization. Relying on the geological conditions and data of BWSCs in Pingdingshan, Henan province, this paper takes the old large-height-difference BWSCs as the research object. It establishes relevant theoretical and geological models; focuses on issues such as the gas injection and brine discharge methods, technical parameters, and operating pressures of large-height-difference BWSCs; and explores the technical feasibility and safety stability of the transformation and utilization of this type of salt cavity. The main conclusions are as follows:

(1)

The gas injection and brine drainage method for the butted-well salt cavity depends on its residual content and cavity structure. A lower residual content makes the TITD method more suitable. When the residual content is high and the height difference between the residual top surfaces of the two-sided salt cavities is large, the OIOD method is preferable. If the residual content is high but the height difference of the residual top surfaces is small, the TIOD method is a better choice. For butted-well salt cavities with a large height difference, the residuals inside the two-sided salt cavities also have a significant height difference due to their mining mode, clearly indicating that the OIOD method is the most appropriate.

(2)

A set of operation parameter models for brine injection and discharge in salt caverns using an OIOD method has been established. These models have been verified through on-site testing and demonstrate good adaptability. They can provide predictive templates for wellhead gas injection pressure, gas injection rate, and casing shoe gas pressure during the GIBD process in butted-well salt caverns, showcasing the technical feasibility of the OIOD method for BWSCs with height differences.

(3)

The special structural characteristics of large-height-difference butted-well salt caverns lead to significant deformation and damage differences between the two connected caverns. The deeper cavern exhibits greater deformation damage compared to the shallower one, making it the weak point in such systems. Differing from conventional operating pressure determination methods for single-well single-cavity systems, a design model is proposed specifically for the large-height-difference interconnected cavern: adopting the lower limit of operating pressure for the deeper cavern and the upper limit for the shallower cavern. Stability evaluation has confirmed the feasibility of this model, effectively ensuring the operational safety of salt cavern systems.

(4)

The transformation and utilization of butted-well salt caverns with large height differences plays a significant role in rapidly expanding the scale of underground energy storage. This study demonstrates the feasibility of such transformations and utilizations from aspects of gas injection and brine discharge methods, operational parameters, and stability. However, research in this area is still in its early stages, with challenges that urgently need to be addressed, including the morphological characterization of salt cavities covered by residues, the distribution patterns and temporal effects of real residues within the salt cavities, the flow patterns of gas-driven brine displacement within the residues, and the efficiency of gas storage. These issues represent key areas for future research focus.