Research on the Influencing Factors of the Void Volume of Insoluble Sediment in Salt Cavern Gas Storage

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


Introduction
In the initial phase, SCGS in China prioritized salt mines characterized by the high purity of salt rock, the considerable thickness of the salt layer, the minimal content of the insoluble matter within the strata, and sparse interlayers for construction purposes [1][2][3][4].However, driven by the imperatives of economic and societal progress, particularly in light of the gas storage reservoir construction area in key consumption zones across central and eastern China, the establishment of SCGS necessitated a shift towards developing sites featuring multiple interlayers and elevated levels of IS [5][6][7][8].Moreover, IS accumulates at the cavity bottom, occupying approximately half of the cavity space and resulting in diminished economic benefits [9].Consequently, the effective utilization of the void space of IS has emerged as a critical concern in SCGS.It has been demonstrated that IS exhibits a high void ratio and good connectivity, making it capable of large-scale gas storage [10][11][12][13].Therefore, achieving a breakthrough in low-grade salt mines holds substantial promise for expanding the site selection and effectively utilizing the underground space of salt caverns.
Processes 2024, 12, 636 2 of 13 During SCGS cavity creation, IS within the strata gradually accumulates at the bottom of the cavity following water-soluble flaking, sediment deposition, and compaction from overlying IS [14,15].Thus, ISs are the loose and porous accumulations.Researchers have conducted extensive studies on IS.
One research aspect is the investigation of the morphology and distribution of IS accumulations.For instance, Li et al. performed simulations on IS particles by mechanically crushing cores and studied the accumulation morphology through laboratory simulation experiments and mathematical methods [16,17].They developed morphology equations for two-dimensional axisymmetric salt cavities, providing valuable insights into the shape of accumulations.
Additionally, Yvan Charnavel et al. studied the impact of different morphologies on storable space [18].If the IS top surface morphology mirrors a "triangle" or "wave" shape, it will lead to an approximate 10% reduction in gas storage space during the present gas injection and brine discharge operations.
Based on the rock core of Jintan SCGS, Zhang et al. analyzed the physical characteristics of IS during the core water solution process and the flow law of IS in brine during gas injection and brine discharge [19].Another important aspect is the characteristic of the particle size distribution and porosity of IS, which Ren et al. studied through core water solution experiments [20,21].Their findings indicate a significant void ratio in IS, which is critical for using void space in IS.
Regarding the issue of the water swelling of IS, Chen Xiaoyuan et al. found that the bulking coefficient of IS is independent of the size of the IS and is inversely proportional to the IS surface area.The more edge-to-edge contacts, the smaller the bulking coefficient, while the more edge-to-corner contacts, the greater the bulking coefficient; the more uneven the particle size ratio, the greater the bulking coefficient [22].
In addition to investigating the particle size and water swelling of IS in relation to accumulation patterns, a number of studies have conducted physical simulation experiments to quantify the available gas storage space within IS.Liang et al. proposed a method based on liquid injection testing to derive the volume balance relationship of liquid injection testing to predict the effective volume of IS.The results showed a difference of 0.79% compared to on-site testing [23].Researchers, such as Li et al., have also analyzed the void network characteristics of IS using advanced imaging techniques like x-ray computed tomography.They found that IS permeability positively correlates with porosity, highlighting the importance of porosity in enhancing gas storage capacity [24,25].He et al. conducted an analysis of IS in the docking well and investigated the gas injection pressure and particle size influencing sediment space in the connecting channel through experiments of gas injection and brine discharge [26].
Moreover, based on these scientific investigations, researchers are developing innovative approaches to utilize void space effectively.Li et al. designed a gas injection and brine discharge simulation experimental setup to determine the void ratio of IS, which aids in optimizing gas storage operations [27].These findings provide critical insights into the behavior of IS and can guide the optimization of gas storage strategies.
Liu et al. provided a new idea for the reconstruction of SCGS with multiple interlayers and low salt purity in China.They utilized complex connected old cavities to shorten the construction period and increase economic benefits [28].This idea avoids various problems caused by IS.
Previous studies have focused on the composition, properties, and behavior of IS in SCGS.Various analytical techniques, such as X-ray diffraction, geophysical surveys, laboratory experiments, and numerical modeling, have been employed to characterize IS.But in the earliest studies of IS, they obtained IS for experiments by water solution or mechanically fractured cores.Due to the lack of production data for cavity wells in research, relevant prediction methods are not applicable to this field.
To align more closely with actual conditions and to guide production practice, this study, for the first time, used ISs salvaged from the X SCGS site as experimental materials and analyzed their particle size distribution.Additionally, this study analyzes X SCGS logging data, core lithology, interlayer conditions, and IS accumulation.The conclusion provides a basis for setting parameters such as IS particle size, mineral composition, and compaction pressure for subsequent experiments on measuring the VVISs.

Basic Overview of IS in X SCGS
This study is based on the characteristics of IS in X SCGS.The analysis of IS on-site provides a basis for the setting of subsequent experimental parameters, which will also make the experimental results more in line with production.

Characterization of IS in X SCGS
Based on a comprehensive analysis of the logging results of X SCGS, the top boundary of the salt rock is to be buried at a depth range of 1000~2000 m.The thickness of the salt-bearing strata ranges from approximately 300 to 750 m, with an average of 450 m.The cumulative thickness of the salt layer ranges from approximately 200 to 50 m, with the NaCl content generally ranging from approximately 80 to 90%.The interlayer consists mainly of mudstone, salt-bearing mudstone, and mudstone with anhydrite, with the thickness of the interlayer being generally less than 4 m.The content of anhydrite in the interlayer ranges from approximately 1 to 5%, with some cases reaching up to 12%.The clay substance content ranges from approximately 1 to 5%, with some cases reaching up to 10%.Illite is the dominant clay mineral, followed by small amounts of kaolinite and chlorite.Clastic rocks are not typically developed within the salt-bearing layer system, except for the presence of quartz and feldspar fragments found in mudstone.
By conducting interlayer statistics and core lithology analysis on cavity wells, the thickness of the interlayer in well H is observed to range from 2 to 13.2 m, with an average thickness of 4.1 m.The thinnest is 0.36 m, while the thickest is 13.2 m, as shown in Figure 1.The interlayer composition consists of 0.7~13.8%quartz, 4.2~17.7%calcite, 0.9~54.6%anhydrite, and a total clay content ranging approximately from 21 to 60.3%, as shown in Table 1.In well Z, the thickness of the interlayer ranges from 0.5 to 14 m, with an average thickness of 3.4 m.The thinnest is 0.11 m, while the thickest is 14 m.The interlayer composition in well Z consists of 2.3~20.5% quartz, 2.3~30.4% calcite, 13.2~82.4%anhydrite, and a total clay content ranging from approximately 4.9 to 38.3%.Further analysis of the lithology of other cores reveals that the interlayer consists mainly of mudstone and saltbearing mudstone.The interlayer components are primarily composed of clay minerals, anhydrite, calcite, and quartz.

Experimental Analysis of Particle Size of IS
We conducted screening experiments using a standard sample sieve with mesh numbers of 120, 100, 80, 50, 35, 20, 10, and 5, respectively, in accordance with GB/T 6003.1-2012[29].ISs were obtained through Venturi salvage tools from the on-site cavity at X SCGS.ISs were utilized to analyze the distribution pattern of particle size, as illustrated in Figure 2.
Before the experiment, in order to prevent the oil, salt, and other substances in the IS from affecting the experimental results [30], the samples were washed with pure water 3~5 times.Considering that the samples contained a large number of clay, anhydrite, and other minerals, they were dried in a controlled humidity oven with a relative humidity of 40% and a temperature of 60 °C.Subsequently, using a balance with an accuracy of 0.01 g weighed four groups of 1000 g IS samples.The above samples were sieved from large to small using a standard sample sieve, and the samples with different particle sizes were weighed and their volume was measured, as illustrated in Figure 3.

Experimental Analysis of Particle Size of IS
We conducted screening experiments using a standard sample sieve with mesh numbers of 120, 100, 80, 50, 35, 20, 10, and 5, respectively, in accordance with GB/T 6003.1-2012[29].ISs were obtained through Venturi salvage tools from the on-site cavity at X SCGS.ISs were utilized to analyze the distribution pattern of particle size, as illustrated in Figure 2.
bers of 120, 100, 80, 50, 35, 20, 10, and 5, respectively, in accordance with GB/T 6003.1-2012[29].ISs were obtained through Venturi salvage tools from the on-site cavity at X SCGS.ISs were utilized to analyze the distribution pattern of particle size, as illustrated in Figure 2.
Before the experiment, in order to prevent the oil, salt, and other substances in the IS from affecting the experimental results [30], the samples were washed with pure water 3~5 times.Considering that the samples contained a large number of clay, anhydrite, and other minerals, they were dried in a controlled humidity oven with a relative humidity of 40% and a temperature of 60 °C.Subsequently, using a balance with an accuracy of 0.01 g weighed four groups of 1000 g IS samples.The above samples were sieved from large to small using a standard sample sieve, and the samples with different particle sizes were weighed and their volume was measured, as illustrated in Figure 3.For the four groups of IS samples, it was observed that particles larger than 0.125 mm were predominant, constituting over 50 percent of the total mass and volume of the particles analyzed, as illustrated in Figure 4.Moreover, the mass and volume proportion of 2.5, 5, 10, and 20 meshes in different samples exceeded 10%.And the experimental results were utilized to establish the parameters of the VVISs measured in the experiment, ensuring a closer alignment of the experimental findings with practical production.Before the experiment, in order to prevent the oil, salt, and other substances in the IS from affecting the experimental results [30], the samples were washed with pure water 3~5 times.Considering that the samples contained a large number of clay, anhydrite, and other minerals, they were dried in a controlled humidity oven with a relative humidity of 40% and a temperature of 60 • C. Subsequently, using a balance with an accuracy of 0.01 g weighed four groups of 1000 g IS samples.The above samples were sieved from large to small using a standard sample sieve, and the samples with different particle sizes were weighed and their volume was measured, as illustrated in Figure 3.For the four groups of IS samples, it was observed that particles larger than 0.125 mm were predominant, constituting over 50 percent of the total mass and volume of the particles analyzed, as illustrated in Figure 4.Moreover, the mass and volume proportion of 2.5, 5, 10, and 20 meshes in different samples exceeded 10%.And the experimental results were utilized to establish the parameters of the VVISs measured in the experiment, ensuring a closer alignment of the experimental findings with practical production.

IS Accumulation in Different Cavity Wells
At X SCGS, multiple cavity wells were surveyed using sonar-based cavity inversion techniques to assess the accumulation of IS at the bottom of the cavities [31].It was discovered that IS reached heights of approximately one hundred meters and occupied

IS Accumulation in Different Cavity Wells
At X SCGS, multiple cavity wells were surveyed using sonar-based cavity inversion techniques to assess the accumulation of IS at the bottom of the cavities [31].It was discovered that IS reached heights of approximately one hundred meters and occupied around 60% of the cavity space, as shown in Table 2.This condition significantly reduces the efficiency of the cavity creation of SCGS.Assuming that the IS density is 2600 kg/m 3 , the one hundred meters of IS exerts significant compaction pressure on the lowermost IS, considering only the influence of self-weight.And the lowermost IS experiences compaction pressure ranging from 0.5 to 2.5 MPa.Therefore, when measuring the VVISs, it is necessary to take into account the impact of compaction.
Based on the previous analysis of the lithology, particle size, and accumulation of IS in the X SCGS cavity, this paper selects quartz, albite, illite, chlorite, and anhydrite as experimental materials.The focus is on studying coarse-grained particles and considering the influence of different compaction pressures.

Experimental Setup and Methods
For this study, the IS gas drive experimental device developed by the China Petroleum Exploration and Development Research Institute [5] was utilized as the basis.The following setup is used to measure the porosity of IS: firstly, place the mechanically crushed core samples into the intermediate container; then, add water until it covers the surface; lastly, displace the water with gas at a certain pressure.It was further updated and transformed into a VVIS measurement experimental device.By studying the change in the patterns of the VVISs under individual factors and determining the magnitude of influence for different factors, this research provides valuable insights for the practical utilization of IS in the field.

Experiments on VVISs Measurement under the Influence of Particle Size and Mineral Composition
The experimental setup for investigating the effects of particle size and mineral composition on the VVISs is illustrated in Figure 5a.It mainly consists of an intermediate container, gas cylinder, measuring cylinder, six-way valve, pressure gauge, and other components.The intermediate container is filled with rock particles saturated with water.The gas cylinder is used to inject gas into the intermediate container, driving the water from the voids within the rock particles.The displaced water is collected and measured in a measuring cylinder.Valves and pressure gauges are employed to control and monitor the displacement pressure in real time.Once the experimental setup is installed and connections are made, the air tightness is checked, and if no issues are found, the experiment begins.Valve 2 is adjusted to control the inlet gas pressure, valve 3 is opened to inject gas into the intermediate container, and valve 1 is opened to allow the displaced water to flow into the measuring cylinder.At the end of the experiment, the volume is measured, which approximately represents the VVISs.connections are made, the air tightness is checked, and if no issues are found, the experiment begins.Valve 2 is adjusted to control the inlet gas pressure, valve 3 is opened to inject gas into the intermediate container, and valve 1 is opened to allow the displaced water to flow into the measuring cylinder.At the end of the experiment, the volume is measured, which approximately represents the VVISs.In this study, groups of different particle sizes of quartz, chlorite, albite, illite, and anhydrite were placed into the intermediate container, and the VVISs was measured.The volume of water that remained unchanged in the measuring cylinder for a duration of 10 min was recorded as the final result.

Experiments on Void Volume Measurement under the Influence of Particle Size and Mineral Composition
The experimental setup for studying the effect of compaction on the VVISs is shown in Figure 5b.The same experimental setup as earlier was used, it will not be repeated here.This setup primarily divides the intermediate container into two separate sealed spaces using a connecting rod piston.Valve 2 is opened to reach the set pressure, subsequently opening valve 5 to inject gas into the space above the piston, pushing the piston to compact the rock particles below.After the compaction is under the set pressure, valve 5 is closed, valve 3 is opened, and gas is injected into the interior of the rock particles through the piston rod, displacing the water in the voids for measurement.Upon completion of a set of experiments, valves 2 and 3 are closed, and valve 4 is opened to connect the pump, allowing water to be injected into the rock particles at a constant flow rate through the piston rod until the outflow rate matches, and this indicates that the rock particles have reached saturation again.The pressure transducer can detect changes in pressure and flow rate within the pump.
Different pressures should be sequentially to compact rock particles, the above operations are repeated, and the variation in the VVISs under different compaction pressures is obtained.At the same time, except for observing the volume of displacement water inside the measuring cylinder, it is also necessary to observe the movement of the piston rod.
The advantages of this experiment are as follows: Firstly, to ensure the accuracy of the experimental results, each set of identical experiments was repeated three times, with unreasonable outcomes eliminated and the average value taken.Secondly, to make the experimental results more convincing, under the consideration of three factors, the study also compared the VVISs changes in different accumulation volumes, as well as various mineral components of IS with different particle sizes.Moreover, the setting of the parameters is based on actual field conditions.For example, the experiment measured the VVIS changes under compaction pressure from 0.2 to 2 MPa, which equivalently represents the VVISs at different depths.
As for areas requiring improvement in this study, on the one hand, because of the limitations of the experimental setups, the experiments conducted involved accumulation volumes of no more than 2000 mL.So, it remains to be seen whether the patterns discovered would differ with an increase in accumulation volume.On the other hand, the experimental setups involve the aspect of gas displacement, necessitating the airtight testing of all valves and pipeline connections.Leakage during the experimental process would significantly impact the results [32].

Experimental Results
The experiment approximates the volume of discharged water Vout as the VVISs.Based on the analysis of the experimental results, the influence between particle size (mesh N), mineral composition (M), compaction pressure (P), and changes in the VVISs is obtained.

1.
The influence of particle size on the VVISs is significant.With the change in particle size, the range of the VVISs will change from 5% to 30%, as illustrated in Figure 6.
As particle size decreases, the mutual accumulation between rock particles becomes tighter, leading to a continuous decrease in the VVISs [33].When comparing the reduction in the VVISs between 30 and 50 mesh and 40 and 80 mesh, as well as between 40 and 80 mesh and 80 and 100 mesh, it is evident that a more mixed particle size distribution results in a slower rate of the VVISs reduction.Moreover, with an increase in accumulation volume, the VVISs also tends to increase for rock particles of different sizes, although this effect is less pronounced for small particles.Overall, it is assumed that the experiment involved uniformly distributed rock particles of varying sizes.For every 10-mesh increase in particle size, the VVISs in 1 m 3 is estimated to decrease by approximately 1000-2500 mL.

2.
Different mineral compositions can result in a difference of 6-23% in the VVISs, as illustrated in Figure 7.Under the same particle size, the internal void volume of minerals such as quartz, albite, and illite is relatively large; mineral components such as anhydrite are prone to water absorption and expansion [34,35].They decompose into small particles or mix with water to form mud-like deposits at the bottom or adhere to the surface of rock particles, which is not conducive to the expulsion of void water; therefore, the VVISs is also low.Considering mineral compositions with different particle sizes, the trend of a variation in void volume is relatively consistent.Similarly, when the particle size decreases to a certain extent, the difference in the VVISs also becomes inapparent.Overall, when other factors are consistent, the more mineral components such as quartz, albite, and illite are present in IS, and the larger the VVISs will be.
ered would differ with an increase in accumulation volume.On the other hand, the experimental setups involve the aspect of gas displacement, necessitating the airtight testing of all valves and pipeline connections.Leakage during the experimental process would significantly impact the results [32].

Experimental Results
The experiment approximates the volume of discharged water Vout as the VVISs.Based on the analysis of the experimental results, the influence between particle size (mesh N), mineral composition (M), compaction pressure (P), and changes in the VVISs is obtained.1.The influence of particle size on the VVISs is significant.With the change in particle size, the range of the VVISs will change from 5% to 30%, as illustrated in Figure 6.As particle size decreases, the mutual accumulation between rock particles becomes tighter, leading to a continuous decrease in the VVISs [33].When comparing the reduction in the VVISs between 30 and 50 mesh and 40 and 80 mesh, as well as between 40 and 80 mesh and 80 and 100 mesh, it is evident that a more mixed particle size distribution results in a slower rate of the VVISs reduction.Moreover, with an increase in accumulation volume, the VVISs also tends to increase for rock particles of different sizes, although this effect is less pronounced for small particles.Overall, it is assumed that the experiment involved uniformly distributed rock particles of varying sizes.For every 10-mesh increase in particle size, the VVISs in 1 m 3 is estimated to decrease by approximately 1000-2500 mL. 2. Different mineral compositions can result in a difference of 6-23% in the VVISs, as illustrated in Figure 7.Under the same particle size, the internal void volume of minerals such as quartz, albite, and illite is relatively large; mineral components such as anhydrite are prone to water absorption and expansion [34][35].They decompose into small particles or mix with water to form mud-like deposits at the bottom or adhere to the surface of rock particles, which is not conducive to the expulsion of void water; therefore, the VVISs is also low.Considering mineral compositions with different particle sizes, the trend of a variation in void volume is relatively consistent.Similarly, when the particle size decreases to a certain extent, the difference in the VVISs also becomes inapparent.Overall, when other factors are consistent, the more mineral components such as quartz, albite, and illite are present in IS, and the larger the VVISs will be.The variation in the VVISs with different particle sizes was fitted to obtain the fitting relationship, as shown in Table 3.

3.
It is observed that the VVISs is inversely proportional to compaction pressure, with VVISs decreasing by 5% to 80% as the compaction pressure increases, as illustrated in Figure 8. Larger IS particles exhibit a more rapid decrease in the VVISs compared to smaller particles under increasing pressure due to the initial VVISs between the particles.However, as the compaction pressure continues to rise, the rate of the VVISs' reduction gradually slows down [36,37].The mutual support force between rock particles can explain why the VVISs does not decrease initially during mechanical compression.Additionally, mineral components like chlorite and illite are more susceptible to compression [38][39][40], leading to a more pronounced reduction in the VVISs as the compaction pressure increases.Ultimately, when the compaction pressure reaches 2 MPa, the void volume is approximately 100 mL.Considering the principle of similarity analysis, in practical scenarios, IS of the deepest can provide a little space for gas storage.
erals such as quartz, albite, and illite is relatively large; mineral components such as anhydrite are prone to water absorption and expansion [34][35].They decompose into small particles or mix with water to form mud-like deposits at the bottom or adhere to the surface of rock particles, which is not conducive to the expulsion of void water; therefore, the VVISs is also low.Considering mineral compositions with different particle sizes, the trend of a variation in void volume is relatively consistent.Similarly, when the particle size decreases to a certain extent, the difference in the VVISs also becomes inapparent.Overall, when other factors are consistent, the more mineral components such as quartz, albite, and illite are present in IS, and the larger the VVISs will be.The variation in the VVISs with different particle sizes was fitted to obtain the fitting relationship, as shown in Table 3.  8. Larger IS particles exhibit a more rapid decrease in the VVISs compared to smaller particles under increasing pressure due to the initial VVISs between the particles.However, as the compaction pressure continues to rise, the rate of the VVISs' reduction gradually slows down [36][37].The mutual support force between rock particles can explain why the VVISs does not decrease initially during mechanical compression.Additionally, mineral components like chlorite and illite are more susceptible to compression [38][39][40], leading to a more pronounced reduction in the VVISs as the compaction pressure increases.Ultimately, when the compaction pressure reaches 2 MPa, the void volume is approximately 100 mL.Considering the principle of similarity analysis, in practical scenarios, IS of the deepest can provide a little space for gas storage.For different mineral compositions, the fitting relationship between compaction pressure and the VVISs can be obtained, as shown in Table 4.

Figure 1 .
Figure 1.Thickness of mudstone and salt rock in the 19 cavity wells.

Figure 1 .
Figure 1.Thickness of mudstone and salt rock in the 19 cavity wells.

Figure 5 .Figure 5 .
Figure 5. Experimental device for measuring VVISs.(a) The experimental setup for investigating the effects of particle size and mineral composition on VVISs; (b) the experimental setup for studying the effect of compaction on VVISs.In this study, groups of different particle sizes of quartz, chlorite, albite, illite, and anhydrite were placed into the intermediate container, and the VVISs was measured.The volume of water that remained unchanged in the measuring cylinder for a duration of 10 min was recorded as the final result.3.1.2.Experiments on Void Volume Measurement under the Influence of Particle Size and Mineral Composition

Table 1 .
Analysis of rock core lithology.

Table 2 .
Accumulation of IS in the cavity wells.