A Study on the Dissolution Characteristics of Salt Rock Using an Extended Rapid Cavity Creation Device

Abstract

The efficiency and safety of salt cavern gas storage are critically dependent on the construction speed and structural integrity of the cavern. To tackle these issues, this paper presents a novel Extended Rapid Cavity Creation Device that employs water jet technology to effectively reduce the construction time and enhance control over the cavity structure. A simulation analysis of the device’s external flow field was conducted using FLUENT software. An experimental system was developed to investigate the effects of nozzle inclination and rotation speed on the dissolution of salt rock samples. The simulation and experimental results indicate that the intensity and shape of turbulence have a significant impact on the formation of the internal cavity within the salt rock. Specifically, a 45° nozzle inclination generates a conical turbulent flow that significantly enhances the mass transfer efficiency. As the rotation speed increases, the intensity and range of turbulence in the external flow field gradually extend towards the centre of the salt cavern cavity. This turbulence promotes the dissolution of salt rock, significantly reducing the ‘step’ structure at the bottom of the cavity. This study provides a valuable foundation for the further optimization of device design and a deeper understanding of the dissolution mechanism.

1. Introduction

Nowadays, energy security and environmental protection are two major conundrums facing human society [1,2]. However, energy supply faces numerous challenges, including uncertainties such as frequent natural disasters and social instability, which lead to continuous fluctuations and shocks in energy prices. These factors pose significant threats to global energy security. To address these challenges, developing safe, reliable, and economically feasible energy storage solutions is essential [3]. As the global pursuit of clean energy transitions continues, underground gas storage, as a mature energy storage technology, will play an increasingly important role in future energy systems due to its advantages of a large capacity, long-term storage capability, and high safety [4,5]. The volume of the caverns typically ranges from 300,000 to 500,000 cubic metres, with larger outliers reaching about 1 million cubic metres. Salt caverns provide rapid delivery capabilities for energy storage [6,7]. Among the various types of underground gas storage, salt cavern gas storage has garnered significant attention due to its unique advantages. Salt rock possesses excellent characteristics such as low permeability, low porosity, favourable rheological properties, and the ability to be extracted through water dissolution [8,9], making it widely recognized as an ideal medium for hydrogen storage [10,11], compressed air energy storage [12], and nuclear waste disposal [13,14,15,16]. However, the construction of salt cavern gas storage faces many challenges. The widely used single vertical well (SWV) solution mining method has significant limitations [17,18,19]. This method has a long cavity construction period, often requiring several years to complete a single salt cavern, which greatly increases construction costs and time [20,21].

To address the issue of the long construction period of salt caverns, several researchers have proposed various solutions. For example, Ban et al. [22] studied methods for rapid cavity creation in salt caverns and proposed rapid cavity creation technologies, such as optimizing cavity creation parameters, reaming, and using rapid devices to promote dissolution. Yuan et al. [23] developed a rapid cavity creation device suitable for the early stage of cavity construction, which can increase the cavity creation speed during the construction period. Yuan et al. [24] proposed a rapid dissolution scheme for the construction period using dissolution-promoting devices and developed nozzle-type, extended nozzle, and hose-type dissolution-promoting devices. Their field application results demonstrate that the salt cavern dissolution rate can be approximately doubled. Zhang et al. [25] proposed a horizontal multi-step dissolution technology, which utilized simultaneous fluid injection and extraction through horizontal and vertical wells. This approach effectively addressed the challenges of multi-layer penetration in thin salt formations, enabling the creation of horizontal caverns. This method significantly enhanced salt formation utilization while reducing cavern construction costs and energy consumption. Derakhshani et al. [26] applied an artificial intelligence (AI) framework to the site selection of underground hydrogen storage caverns in Poland. The study introduced eight AI algorithms and highlighted the superior performance of the Cat Boost algorithm in evaluating cavern suitability. This framework provided decision-making support for stakeholders, including government agencies, geological services, and renewable energy industries. However, the above rapid cavity creation methods have not revealed the relationship between the parameters of the dissolution-promoting devices and the structure of the constructed cavity, nor have they fully considered the impact of fluid turbulence on the dissolution of salt rock.

Previous studies have shown that turbulence can significantly enhance the dissolution rate of salt rock. For instance, Jiang et al. [27] found through experiments that the dissolution rate of rock salt increases with the increase in flow rate, and this change follows an exponential relationship. Durie and Jessen [28] discovered through experiments that the salt removal rate in turbulent flow is 10 to 20 times that in laminar flow. Zhang et al. [29] found through experiments that the low-pressure jet erosion efficiency is 10 times higher than indoor dissolution methods, and increasing the flow rate and nozzle diameter can significantly enhance the overall performance of erosion. Stiller et al. [30] measured the corrosion of salt rock in diluted brine and found that the dissolution rate is related to the initial salinity of the solution, and in experiments with stirring, the dissolution rate is twice that without stirring. Wang [31] considered the effects of turbulence and established a multi-physical coupling model, finding that turbulence stimulates the formation of vortices, which in turn affect the distribution of the brine concentration. However, current research rarely systematically explores the impact of fluid turbulence on the dissolution of salt rock, especially in practical engineering applications, where understanding how to utilize turbulence to accelerate the dissolution process of salt rock remains an urgent issue to be addressed.

To effectively address the above issues, this paper presents a novel Rapid Cavity Creation Device. This device can significantly shorten the construction period and improve construction efficiency, and can effectively utilize turbulence to accelerate the dissolution process of salt rock. As the cavity volume increases, the effectiveness of the device gradually decreases, but its initial acceleration effect is significant, greatly shortening the construction period. This paper utilizes FLUENT software to conduct a simulation analysis of the external flow field of the device. Through the construction of an experimental system for dissolving salt rock samples, this study analyses the impact of two different nozzle inclination angles and rotation speeds on the actual cavity shape of the dissolved salt rock, particularly the impact of fluid turbulence on the dissolution rate and cavity structure of the salt rock. Finally, this paper verifies the performance of the device in practical applications and explores the mechanism of turbulence in the dissolution process of salt rock. This work provides a valuable foundation for the further optimization of device design and a deeper understanding of the dissolution mechanism, and offers new insights for the efficient construction and structural control of future salt cavern gas storage facilities.

2. Salt Cavern Initial Construction and Device Design

Initial Cavity Construction

The SWV method for leaching cavities consists of two main processes: the cavity construction period and the cavity formation period [32]. The Extended Rapid Cavity Creation Device is applied during the cavity construction period to accelerate the formation of the cavity. Based on the research on the application background of this device, and considering the characteristics of layered salt rock, such as its thin salt layers, high clay content, and the requirement for a high concentration of extracted brine, a process scheme based on the Extended Rapid Cavity Creation Device has been determined (as shown in Figure 1). The specific process steps are as follows:

(1)

After running the technical casing and cementing, a conventional drill bit is used to drill down to a position 10 m above the bottom of the salt layer, ensuring that the salt layer is not penetrated. After open-hole drilling is completed, an under-reamer is used with the original drill string to enlarge the open hole to the maximum possible size.

(2)

An inner tube and an outer tube are placed inside the wellbore. Corrosion inhibitors (usually oil) are injected between the production casing and the outer tube, and the salt cavern is leached using the direct circulation method until the cavity diameter reaches 0.5 m. According to the design principles of the cavity creation string and the device size for salt cavern gas storage, the cavity creation string combination selected is “cavity creation outer tube Φ177.8 mm + cavity creation inner tube Φ114.3 mm”, which is mainly used for the construction of underground gas storage in 1500 m deep salt layers [33].

(3)

The inner tube is removed, and the drill pipe is connected to the device. The device is lowered to the bottom of the cavity, and space is reserved for sediment settlement. Once the device is stabilized at the bottom of the cavity, high-pressure fluid is injected. Under hydraulic pressure, the high-pressure steel pipe begins to extend. When the extension is complete, the drill pipe drives the device to rotate, and the rotation speed of the device is gradually increased to 3 r/s, causing the cavity volume to gradually increase. When the diameter of the salt cavern cavity reaches 4~5 m, the concentration of the brine returned at the wellhead basically reaches saturation. At this stage, the drill pipe is removed, the inner and outer tubes are installed, and the process transitions to the conventional cavity dissolution operation stage.

Figure 1. Schematic diagram of the rapid cavity creation process.

Figure 1. Schematic diagram of the rapid cavity creation process.

In the above-mentioned initial cavity construction process of the salt cavern gas storage, the Extended Rapid Cavity Creation Device plays a crucial role. To better understand and apply this innovative device, this paper will provide a detailed explanation of its structural characteristics and working principles.

The structure of the Extended Rapid Cavity Creation Device is shown in Figure 2. This device mainly consists of the power structure, the linkage structure, and the supporting structure. The power structure provides rotational and extension power for the device and is equipped with a safety release mechanism. The linkage structure achieves the extension of the high-pressure steel pipe through the connection of hinges. The supporting structure provides structural support to stabilize the device inside the cavity.

The upper end of the device is connected to the drill pipe, and the rotational power of the power structure comes from the rotation of the drill pipe. The lower end of the device is fixed in the settled water-insoluble residue through the guide shoe in the supporting structure. When the device is in operation, on one hand, the high-pressure water injected into the drill pipe flows partly through the piston sleeve into the high-pressure hose and then into the high-pressure steel pipe. On the other hand, it provides the pressure to overcome the spring force, pushing the piston rod downward. The piston rod extends the high-pressure steel pipe radially through the hinge. Meanwhile, the power structure uses the engagement of the upper and lower face ratchets to transfer the rotational power of the suspension joint to the linkage structure. The presence of the support shaft in the supporting structure enables relative rotation between the supporting structure and the linkage structure. Therefore, when the device is in operation, the power structure and the linkage structure are in a state of rotation relative to the supporting structure, and the rotation of the power structure drives the rotation of the linkage and supporting structures.

3. Numerical Simulation

3.1. Model and Meshing Scheme

Based on the working principles described in Section 2, high-pressure fluid enters the device through the suspension joint and is ultimately ejected into the salt cavern cavity via the nozzle. The evaluation of the device’s dissolution performance primarily relies on observing the extent of the turbulent flow generated by the device. Therefore, conducting a simulation analysis of the external flow field is a key approach to assessing the dissolution performance of the device. The following assumptions are made for the external flow field model:

• The internal flow field of the device is neglected;
• It is assumed that the flow rates of the four nozzles are the same;
• The device is assumed to be a cylindrical shell, and the influence of irregular shapes on the external flow field is neglected;
• The influence of cavity changes on the external flow field is neglected, and only the turbulence range generated by the device in a pure water environment is considered.

Based on these assumptions, the computational fluid domain for the operation of the Extended Rapid Cavity Creation Device is established, as shown in Figure 3. The entire computational fluid domain has a height of 5 m and a diameter of 3 m. In this simulation, due to the use of the sliding mesh method to simulate the tool rotation and the complexity of the simplified tool model, a hybrid mesh generation scheme was employed. In the complex regions, tetrahedral meshes with a grid size of 1 mm were utilized, while in the regular regions, hexahedral meshes with a grid size of 10 mm were applied. The overall number of mesh elements was 891,883, and the average quality of the mesh elements was 0.838.

3.2. Governing Equations and Turbulence Models

As the tool rotates, the initial submerged jet is significantly influenced by centrifugal forces, causing the fluid motion to become more complex and altering the flow characteristics. At this point, the jet not only retains its initial ejection momentum but also generates strong secondary flows due to the centrifugal forces, leading to a pronounced anisotropy in the flow. This characteristic makes the turbulent structures in the flow field more complex, and the traditional standard k-ε model struggles to accurately capture these changes. However, the RNG k-ε model, which incorporates an additional R term in the ε transport equation, is better equipped to handle the anisotropic phenomena arising from such highly rotational flows. Scholars have utilized this model to simulate the flow field in rotating jet nozzles. For instance, Rehbinder et al. [34] (Atanov et al. [35]; Wang et al. [36]; Song et al. [37]; Niu et al. [38]; Fu et al. [39]) have all employed the RNG k-ε model to predict the flow field of rotating jets with considerable accuracy, and their computational results are in good agreement with the experimental data. The RNG k-ε model has been successfully used to precisely simulate the flow within high-swirl, high-curvature rotating jet nozzles, with the governing equations given by:

$${\displaystyle \frac{\partial \left(\rho k\right)}{\partial t}}+{\displaystyle \frac{\partial \left(\rho k{u}_i\right)}{\partial x_i}}={\displaystyle \frac{\partial \left({\alpha }_k{u}_{eff}\frac{\partial k}{\partial x_j}\right)}{\partial x_j}}+G_k+\rho \epsilon$$

(1)

$${\displaystyle \frac{\partial \left(\rho \epsilon \right)}{\partial t}}+{\displaystyle \frac{\partial \left(\rho \epsilon u_i\right)}{\partial x_i}}={\displaystyle \frac{\partial \left({\alpha }_{\epsilon }{u}_{eff}\frac{\partial \epsilon }{\partial x_j}\right)}{\partial x_j}}+{\displaystyle \frac{{\mathrm{C}}_{1\mathsf{\epsilon }}^{\ast }}{K}}{G}_k-{\displaystyle \frac{{\mathrm{C}}_{2\mathsf{\epsilon }}\rho \epsilon }{k}}$$

(2)

where $k$ is turbulent kinetic energy; $\epsilon$ is the turbulent dissipation rate; $u_i$ is the average speed; $\rho$ is the fluid density; and $G_k$ is the generation term of the turbulent kinetic energy $k$ caused by the average velocity gradient. The values of the coefficients are ${\alpha }_k$ = ${\alpha }_{\epsilon }$ = 1.42, $C_2$ = 1.68, and the values of coefficients ${\mathrm{C}}_{1\mathsf{\epsilon }}^{\ast }$ can be calculated as follows:

$$C_{1\epsilon }^{\ast }=C_{1\epsilon }-{\displaystyle \frac{\eta \left(1-\frac{\eta }{{\eta }_0}\right)}{1+\beta {\eta }^3}}$$

(3)

where ${\eta }_0$= 4.377, $\beta$ = 0.012, and the parameter $\eta$ can be calculated as:

$$\eta ={\left(2E_{ij}{E}_{ij}\right)}^{{\displaystyle \frac{1}{2}}}{\displaystyle \frac{k}{\epsilon }}$$

(4)

3.3. Device External Velocity Flow Field

Figure 4 shows the flow field domain generated by the device at a rotation speed of 180 RPM and an inlet velocity of 16 m/s. Considering the significant impact of the nozzle jet on the entire flow field, Section A (the cross-section of the entire flow field along the direction of the nozzle jet) and Section B (the longitudinal section of the entire flow field along the direction of the nozzle jet) are selected to describe the impact of the device on the overall external flow field during operation. The shape of the device’s shell causes disturbances in the submerged flow during rotation, and during the rotation of the device, the turbulent circular jet forms vortices in the submerged flow.

4. Experimental Process

4.1. Specimen Preparation

The salt rock samples used in this experiment were obtained from a Himalayan salt mine in Pakistan. The main component was NaCl, with a content of approximately 98%, and the Ca²⁺ content was about 0.04%, while the Mg²⁺ content was approximately 0.01%. The initial size of the square standard salt rock samples was 130 mm × 130 mm × 130 mm. Erosion tests were conducted on these samples. According to the purpose of the experiment, the salt blocks were further drilled and processed, as shown in Figure 5a. The basic physical and mechanical properties of the salt rock specimens were obtained, as shown in

Table 1. Physical and mechanical properties of rock salt specimens.

Density (g/cm3)	Wave Velocity (km/s)	Compressive Strength (MPa)	Tensile Strength (MPa)	Elastic Modulus (MPa)
2.23	4.33	39.71	1.82	3689.72

[30].

4.2. Experimental Device and Procedure

Environmental factors are significant influences on the dissolution rate of rock salt. However, in this experiment, environmental factors such as temperature and humidity are controlled as constants. This study exclusively investigates the relationship between nozzle inclination and rotational speed on the dissolution rate of rock salt.

This experiment utilizes a self-designed and assembled experimental system (Figure 6). By controlling two different nozzle inclination angles and rotation speeds, the mass loss of the salt rock samples is measured, and the internal cavity morphology formed by the dissolution of the salt rock samples is observed. The experimental system consists of a water supply device, a dissolution device, and a power device. The water supply device includes a water supply tank, an adjustable-speed water pump, a fixed support frame, transparent water pipes, a flowmeter, and a flow display. The maximum power of the adjustable-speed water pump is 84 W, with a rated pressure of 0.147 MPa and a maximum flow rate of 30 L/min. The dissolution device consists of a rotary joint, a rotating shaft, and a salt rock sample. The rotating shaft is divided into two types based on the nozzle inclination angle of 0° or 45° (Figure 7). The power device includes a stepper motor, a motor controller, and a synchronous belt. The motor controller can control the rotation speed of the stepper motor.

Before the experiment begins, the salt rock samples are pre-processed. To ensure that the dissolution process occurs only inside the salt rock sample and does not affect its outer surface, a layer of transparent sealing glue is uniformly applied to the outer surface of the salt rock sample, and it is sealed with plastic wrap (Figure 5b). After the sealing glue solidifies, it forms a protective layer that ensures the outer surface is not eroded by the dissolution solution. Next, the water supply tank is filled with water, and the salt rock sample is secured at the centre of the experimental platform.

By rotating the speed control knob of the adjustable-speed water pump, the flow rate in the transparent water pipe is controlled to ensure that the fluid is in a turbulent state during the experiment. During the experiment, the water pump operates at maximum power. Initially, a 3 mm diameter nozzle and a 45° nozzle inclination angle are used for the dissolution experiment. The flow rate displayed on the flow display is recorded, along with the time. Every 2 min, the salt rock sample is removed, and the excess water on its surface is quickly wiped off. Following the drying process, the mass of the salt rock sample is measured and recorded. This process continues for 20 min. The used salt rock sample is then replaced with an unused one, and two different nozzle inclination angles and rotation speeds are applied. The experiment continues following the above procedure. The specific experimental variable settings are shown in

Table 2. Test set-up variables.

No. Parameter	1	2	3	4	5
Nozzle Inclination Angle	45°	45°	45°	45°	0°
Rotation Speed	1 r/s	2 r/s	3 r/s	0 r/s	2 r/s

.

5. Results and Discussion

5.1. Impact of Two Different Nozzle Inclination Angles on Dissolution Rate and Cavity Structure

In the experimental section, the salt rock samples were subjected to dissolution experiments at a rotation speed of 2 r/s and a nozzle diameter of 3 mm using two different nozzle inclination angles: 0° and 45°. Figure 8 shows the mass loss of the salt rock samples under two different nozzle inclination angles. Over time, the mass loss of the salt rock samples under both nozzle inclination angles show an increasing trend. However, the 45° nozzle inclination consistently shows a higher mass loss rate at all time points. This phenomenon can be attributed to the fact that the 45° inclination promotes the mixing of brine inside the cavity, enhancing the mass transfer efficiency and reducing the saturated area, thereby allowing low-concentration water to continuously contact the salt rock surface and accelerating the dissolution process. In addition, Figure 9 further illustrates the cross-sectional morphology of the internal cavities of the salt rock samples after 20 min of dissolution under two different nozzle inclination angles. The cavity shape dissolved by the 0° nozzle inclination is approximately “elliptical”, with a maximum width of 12.5 cm and a distance of 5.5 cm from the bottom of the salt rock sample. In contrast, the cavity shape dissolved by the 45° nozzle inclination is approximately “conical”, with a maximum width of 11.6 cm, a “rectangular” step at the bottom with a width of 5 cm, and a distance of 4 cm from the bottom.

In the numerical simulation section, the simulation conditions are consistent with the actual working conditions. The rotation speed of the tool is 2 r/s, the inlet velocity is 16 m/s, and the nozzle inclination angles are also 0° and 45°. Figure 10 reveals the velocity contour distributions under two different nozzle inclination angles, with the selected section being Section B in Figure 4. At a 45° nozzle inclination, the turbulence shape exhibits a conical form that is smaller at the top and larger at the bottom. In contrast, at a 0° nozzle inclination, the turbulence shape is elliptical. The shape of the turbulence influences the final size and shape of the internal cavity formed in the salt cavern.

Firstly, the comparison of cavity shapes shows that the 0° nozzle inclination forms an elliptical cavity in both the experiment and the simulation, while the 45° nozzle inclination forms a conical cavity. This indicates that turbulence contributes to the formation of a conical cavity. Secondly, the comparison of the mass loss shows that the 45° nozzle inclination exhibits a higher mass transfer efficiency and a faster dissolution rate in both the experiment and the simulation. This is closely related to the conical turbulence shape, as the conical turbulence can more effectively enhance the mass transfer efficiency and accelerate the dissolution process of salt rock.

5.2. Impact of Different Rotation Speeds on Dissolution Rate and Cavity Structure

In the experimental section, Figure 11 shows the mass loss of salt rock samples over time at different rotation speeds (0 r/s, 1 r/s, 2 r/s, and 3 r/s), with a pump power of 84 W, a nozzle inclination angle of 45°, and a nozzle diameter of 3 mm. The experimental results indicate that the relationship between rotation speed and mass loss is not a simple linear one. Specifically, the dissolution rates at 0 r/s and 1 r/s were basically consistent within the first 12 min, suggesting that rotation has an insignificant effect on the dissolution rate at low speeds. However, as the rotation speed further increases, the dissolution rate initially increases and then decreases. In particular, at a speed of 3 r/s, the dissolution rate was lower than that at 0 r/s, indicating that excessively high rotation speeds may inhibit the dissolution process. Figure 12 further illustrates the cross-sectional morphology of the internal cavities of the salt rock samples at different rotation speeds. The salt rock sample at 0 r/s was dissolved for only 12 min, and its cross-sectional shape was approximately “circular”, with a maximum width of 3.5 cm, and the circular cavity was located 4 cm from the bottom of the salt rock sample. For the samples at 1 r/s, 2 r/s, and 3 r/s, the dissolution time was 20 min, and the internal cavity shapes were all approximately “conical”, but with differences in their specific shapes. The “conical” cavity at 1 r/s had a maximum width of 12.6 cm, with a “rectangular” step at the bottom with a width of 7 cm; the “conical” cavity at 2 r/s had a maximum width of 11.6 cm, with a step width of 5 cm; and the “conical” cavity at 3 r/s had a maximum width of 11.4 cm, with a bottom protrusion width of 3.5 cm. As the rotation speed increased, the width and height of the “conical” cavity’s bottom step gradually decreased, and the maximum width also decreased. Particularly in the rotation speed range of 1 r/s to 2 r/s, the mass loss and cavity shape changes in the salt rock samples were most significant.

In the numerical simulation section, the simulation conditions were set with an inlet velocity of 16 m/s, and the selected section was Section A in Figure 4. Figure 13 shows that as the rotation speed of the tool increases, the high-speed rotating jet significantly expands the range of the turbulence. The turbulence range inside the flow field exhibits the following trends: both the maximum and minimum diameters gradually decrease, and the overall size of the turbulence range is in a state of dynamic equilibrium; the turbulence range transitions from a sheet-like discrete state to a cluster-like continuous state; and the turbulence range in the central region gradually increases.

In summary, both the experimental and numerical simulation results indicate that rotation speed has a significant impact on the dissolution rate and cavity shape of salt rock. In the numerical simulation, rotation speed affects the turbulence range in the central region. As the turbulence range in the central region increases, the width and height of the “conical” cavity’s bottom step in the experiments gradually decrease, and the dissolution rate initially increases and then decreases.

6. Conclusions

This paper presents an Extended Rapid Cavity Creation Device, which innovatively combines water jet technology with a rotating mechanical structure. The external flow field of this device was numerically simulated using version 2022R1 of the FLUENT software, and the dissolution impact of the device on salt rock was further validated through a salt rock sample dissolution system. Additionally, the relationship between the turbulence shape in the simulated velocity field and the resulting cavity shape within the salt rock samples was analyzed. The main conclusions are as follows:

(1)

The Extended Rapid Cavity Creation Device designed in this paper utilizes water jet technology and device rotation to expand the turbulence range inside the salt cavern cavity, thereby increasing the mass transfer efficiency in the fluid.

(2)

Over time, the mass loss of the salt rock samples under both nozzle inclination angles exhibits an increasing trend. However, the 45° nozzle inclination consistently demonstrates a higher mass loss rate at all time intervals. The turbulence shape observed in the simulation influences the final size and morphology of the internal cavity within the salt cavern. Specifically, the 0° nozzle inclination in the simulation produces an elliptical turbulence pattern, which contributes to the formation of an elliptical cavity within the salt cavern. In contrast, the 45° nozzle inclination generates a conical turbulence pattern, which facilitates the formation of a conical cavity within the salt cavern.

(3)

The relationship between rotation speed and mass loss is not simply linear. As the rotation speed increases, the dissolution rate initially increases and then decreases. However, excessively high rotation speeds may inhibit the dissolution process. Rotation speed can effectively address the issue of rapid lifting at the bottom of the cavity. With increasing rotation speed, the turbulence intensity and range in the external flow field gradually extend towards the central region of the salt cavern. This turbulence promotes the dissolution of the salt rock, significantly reducing the step width and height at the base of the “conical” cavity observed in the experiments.