Implications of the European gas market for the long‐term construction of underground gas storage in China

Underground gas storage (UGS) is an important part of the natural gas industry. Its peaking characteristics and storage capacity are critical to national energy supply and energy security. Compared with the mature gas market in Europe, the construction of UGSs in China faces many challenges. This study is expected to provide references for China UGS through a detailed analysis and calculations of the European natural gas market and underground storage. First, the current status and development of the European gas market were summarized. And the situation of primary energy structure, gas utilization, and UGSs construction was compared between the EU and China. Second, the working gas volume of UGSs in Europe was analyzed from the perspective of its relationship with total consumption and peak‐shaving volume. Finally, the simulation and prediction of the working curve of the energy storage facilities and the reasonable ratio of the energy storage to load were solved by the numerical fitting method. Meanwhile, the recommendations for China's natural gas market and UGSs were provided. It is appropriate to set the planning target of China UGS capacity to load at 5% of total consumption and the actual peaking gas volume will be 14.4 billion cubic meters by 2030.


| INTRODUCTION
In recent years, low carbon mode has been regarded as a key strategy for economic development in many countries. It is crucial to reduce carbon emissions from energy consumption. Natural gas has many advantages compared with other fossil fuels and renewable energy. 1 Since the main component of natural gas is methane, the CO 2 emissions are relatively lower than that of oil and coal. 1 Meanwhile, natural gas has the characteristics of lower cost, more mature technologies, and higher reliability compared with renewable energy sources. 1,2 Therefore, the related issues of natural gas production, transportation, consumption, and storage are the focal points of energy transformation and development.
For countries or regions, energy security is one of the essential factors to ensure social stability in the current complex and changing international situation. Global gas supply problems occur frequently due to geopolitical influences. Especially as the recent war conflict between Russia and Ukraine has disrupted the gas supply from the Nord Stream 1 and Nord Stream 2 systems, the European gas crisis has intensified. It is expected to suppress demand and significantly reduce inventories from imported pipelines. The natural gas market which is highly dependent on imported sources has been given the suggestions that it needs to pay more attention to the role of gas storage. Therefore, it is of great practical importance to perform data mining of historical data related to natural gas storage and natural gas markets, and thus summarize the statistical relationship between natural gas demand and storage size. The natural gas storage construction is one of the necessary means to reduce the external dependence of energy. 3 Meanwhile, it is of great strategic significance for the construction of the national production-supply marketing system and the improvement of the oil-gas industry. Among them, the underground gas storage (UGS) is a significant link in the gas industry chain and an important part of the pipeline transportation system, which is also a vigorously developed technology in recent years. However, its characteristics of seasonal peak-shaving and storage capacity are general concerns. 1 Especially for the development of different countries, the construction targets and capabilities of UGS are extremely distinct.
China is currently the country with the largest energy consumption and carbon emissions in the world. In the future economic development and energy transformation, natural gas will contribute significantly as a transition energy source from fossil to renewable energy sources. There is no doubt that it has a certain necessity for China to construct natural gas storage vigorously. However, the construction timeconsuming target should not be set too fast and the capacity target should not be set too large. Otherwise, the construction targets may not be achieved as scheduled and many other energy-economic-social problems will arise. The accurate judgment on the natural gas future development trend is essential to set reasonable targets for gas storage construction. If the capacity of the gas storage is too large, it will inevitably lead to problems, such as investment waste and facility abandonment. On the other hand, if the gas storage capacity is too small, the seasonal peak regulation and the balance of supply-demand cannot be achieved. Meanwhile, the construction of natural gas storage in China is relatively late, with insufficient experience and limited available data. Therefore, drawing on the experience of the mature natural gas market in the world will provide effective references for the construction of China in the future.
Europe, North America, and East Asia are the three major regions with mature natural gas markets in the world. Especially for the Europe gas market, it has high imports proportion and obvious seasonal fluctuations, which are similar to the Chinese natural gas market. It is critical to establish relevant natural gas storage facilities, especially the UGS has been built relatively well in the EU. Pan mentioned that 20% of the annual natural gas consumption volume in winter comes from UGS in the United States. 4 It is worth noticing that this percentage refers to the proportion in the wintertime, not to the whole year. Zhou et al. suggested that the working gas volume of gas storage in the United States accounts for 17.4% of the annual gas consumption volume and the reserve capacity of gas storage facilities in most European countries accounts for more than 15%. 5 Sanchez proposed a natural gas-underground facility optimal management scheme combined with water resource constraints of a specific region, and the storage time, storage quantity, and usage time of natural gas and water were determined. Recommendations for future demand under extreme weather conditions or for peak demand of both gas and electricity are given. 6 An internal automation module was developed by Juez-Larré to explore the final capacity of potential UGS in the Netherlands. And it also concluded that the UGS will play a significant role in the politics and economy of the future. 7 According to Ren et al., the seasonal peak-shaving volume of natural gas in the United States in 2019 was 63.9 billion cubic meters (bcm), which was defined as the accumulated difference between the consumption of each month and the average monthly consumption of the year. 8 On the basis of a study of natural gas supply and demand relationships, Höffler and Kübler predicted the natural gas storage requirements in Northern Europe under different scenarios from 2005 to 2030. 9 It can be seen that there are many common problems in the global natural gas storage construction, such as gas storage capacity, peak-shaving ratio, and demand forecast.
On the contrary, there are certain differences between the European gas market and the Chinese gas market. For the Asian premium in natural gas trade, Zhang et al. selected natural gas prices in Japan, the United States, and Germany as samples to analyze the impact of oil prices and market fundamentals. 10 The results suggest that Asian gas prices are more heavily determined by oil prices. The inability to liberally regulate gas prices based on market fundamentals is also the reason for the Asian gas premium. In recent years, there were many studies on the relationship between oil and gas prices in different regions. Zhang and Ji simulated oil and gas price relationships in the United States, Europe, and Asia. 11 The results show that the fundamentals of oil and gas in the United States were completely different, while oil and gas prices in Europe and Asia still tend to have an oil price indexation relationship. With the gradual formation of the European gas market hub in recent years, the mechanics of its market fundamentals were improved. However, for the Asian regions, the oil-led gas price mechanism still has a greater impact. The mechanisms and processes of decoupling natural gas and oil prices in China were examined by Miao et al. 12 The results show that there was currently insufficient evidence that oil and natural gas prices have been decoupled. For the natural gas market in China, the oil pricing mechanism still dominates. It can be concluded that the most important reason for the difference between the two is the pricing. Chinese natural gas is still dependent on oil pricing while the European natural gas market has become a hub. Meanwhile, there is no doubt that a liberalized and highly liquid hub is not only well adapted to market changes but also protected against extreme risks. 13 However, the European gas market is matured, while the Chinese gas market is still rapidly developing and changing. It is reasonable and necessary to consider the European gas market as the future direction and target of China's development. At least before the war conflict between Russia and Ukraine, the European gas market had reached a relatively stable state with many aspects to draw on. Therefore, it is also of great significance to study the characteristics of the European natural gas market, to set corresponding natural gas development plans in light of the development stage of the Chinese natural gas market.
According to the previous section, it is no doubt the construction of UGS will be crucial to the development of many countries. The mature natural gas markets in countries and regions such as the United States and Europe are a strong reference and guide for, for example, the developing natural gas market in China. However, economic development, infrastructure development, natural gas supply, and demand vary greatly from country to country and region to region. Comparative analysis of the gas markets characteristics in specific countries and benchmark countries is also needed, so that the reasonable and feasible recommendations will be obtained. For China, as the second largest economy in the world, its economic aggregate is similar to that of the EU. It is also comparable to Europe in terms of land area. Moreover, China and Europe are in the same latitudinal region, with similar climatic conditions and seasonal fluctuations in demand for natural gas due to winter heating needs. Therefore, it is relevant to utilize the regulations of the mature European gas market to resolve the problems related to the Chinese gas market. In particular, it is of great significance to analyze and study the China gas storage peak-shaving issue of the relationship between actual peak-shaving gas volume and the maximum working gas volume as well as the annual consumption of the gas storage.
In 2021, the natural gas consumption in China was 372.6 bcm, accounting for 9% of primary energy consumption. Meanwhile, the external dependence of natural gas has reached more than 44%. By the end of 2021, the peak-shaving capacity of UGS in China was about 16.41 bcm, accounting for about 4.4% of natural gas consumption. With considering the gas storage capacity of storage tanks in coastal liquefied natural gas (LNG) terminals, the comprehensive peak-shaving capacity of China accounts for about 6.4% of its total consumption, which is far from 15% to 20% of that in Europe and the United States. 14 Relevant research showed that the proportion of natural gas in primary energy will reach 15% in 2035, and the consumption will exceed 600 bcm. With the growth of natural gas demand, the problem of natural gas supply is becoming more and more prominent. In addition, the construction and operation of UGSs in China generally takes 5-8 years to reach the designed working gas volume 1 and is difficult to significantly improve the storage capacity in a short period. Therefore, it is necessary to accurately estimate the targets of future natural gas capacity and peakshaving volume as far as possible from the beginning of natural gas storage construction, Nowadays, the development of China's natural gas market and related industries is critical for both international and domestic purposes. 15 Shaikh et al. attempted to devise a foreign natural gas import scheme for China by constructing a mathematical programming model. 16 Chen et al. used the facility location model to discuss the location selection of UGS facilities in Sino-Russian natural gas pipeline, which is related to China. 17 Nandi et al. introduced the influence of major natural gas producers, such as China on India's natural gas policy choices. 18 Confort and Mothe used linear regression analysis to study the effect of proven reserves, production, consumption, infrastructure, total natural gas imports and exports, and percentage of total national energy consumption on the storage capacity of 38 countries with underground natural gas storage. 19 Meanwhile, some experts have given analyses and suggestions on the construction and operation of gas storage in China. Liu et al. analyzed the consumption fluctuations of China's natural gas market in 2017 and pointed out that the seasonal peak-shaving gas volume was 15.18 bcm. The gas volume and share of UGS, LNG receiving station, and gas field were 7.2 (47.4%), 4.68 (30.8%), and 3.3 (7%) bcm. 20 The resilience of the China natural gas system was examined and measured combined with system dynamics models by Ding et al. 21 Zhang et al. studied the demand for peak shaving of China UGSs and predicted the current situation and trend of peak-shaving development. Therefore, it has been paid much attention to the related issues of Chinese UGS, including gas storage capacity, seasonal peak regulation, and dynamic planning. However, most of the existing studies are based on data analysis in the current stage or qualitative judgments about future development. The independent research on China's natural gas market suffers from inadequate quantitative forecasting methods and insufficient historical data.
According to the literature review, it is apparent that the maximum working gas volume and the actual peaking gas volume are the most critical indexes for the construction of the Chinese natural gas market and UGS. On the one hand, if storage capacity targets are set too high, it could result in unnecessary additional construction investment for the development of some storage projects with inappropriate geological conditions. On the other hand, if storage targets are set too low, the natural gas supply system will be extremely unstable due to a lack of reserves and an inadequate ability to respond to supply disruptions or seasonal fluctuations. Moreover, the actual peaking gas volume will be related to the efficiency of the gas storage. In previous studies, the working gas volume or actual peaking gas volume was calculated independently or analyzed qualitatively with limited historical data. On the above premises, the working gas volume and the actual peaking volume of UGS are fitted into a quantitative relationship by the sine function fitting method in this work. In other words, when the future gas demand in a given gas market is known, the regression analysis can determine the working gas volume of the underground storage, and thus the actual peaking volume of the storage can be further determined.
In general, it is of great significance to reasonably plan the construction of UGSs in China to meet energy supply and demand, match seasonal peaking, and synchronize socioeconomic development. Meanwhile, the storage capacity and peaking volume targets of the mature gas market in the EU are of reference and guidance for China. However, first, it may be inappropriate to suggest the proportion of working gas volume in the consumption volume of China simply based on that in a certain year of other regions or countries. Second, there are also controversies about the evaluation indexes of UGS, such as working gas volume, peak-shaving gas volume, and their ratio to consumption volume. Last but not least, some of the historical data have not been fully explored. Therefore, it is necessary to conduct statistical analysis and prediction on the peak-shaving gas volume and storage capacity in a long term. In addition, a correlation study was carried out on the relationship between the maximum working gas volume as well as the peak-shaving gas volume and consumption.
On the above premises, the main purposes of this study are as follows: (1) To find the quantitative relationship between the consumption of natural gas and the proportion of working gas volume using the numerical fitting method. (2) To build a mathematical law represented by limited constants using tens of thousands of data points from past historical years. (3) To provide a novel way to draw more convincing conclusions and provide more reasonable suggestions for the storage capacity and peak-shaving targets of China UGS.

| GAS INDUSTRY CHARACTERISTICS OF THE EU AND CHINA
It is necessary to first analyze and compare the characteristics of the European and Chinese natural gas markets before drawing on the European actual construction of its underground storage reservoirs to inform the development targets for the Chinese natural gas industry. It will illustrate why analogies can be applied and clarify the gaps in the natural gas markets development of China and Europe. The performance of the European natural gas market and UGS from 2011 to 2020 were summarized and analyzed in this study. During this decade, the demand for natural gas was continuously increasing in European countries, with an annual growth rate of about 2%-4%. Although the growth rate varies from country to country, the dependence on imported gas was consistently high, reaching 80% by 2020. 22 The transportation of natural gas has always been dominated by pipeline, LNG receiving terminals and UGS in Europe have been gradually built with the development of the global natural gas market. In particular, the construction of UGS in Europe is also being accelerated to improve natural gas peaking and emergency response capabilities. The natural gas industry has gone through the development process from regulation to liberalization. The gas storage has gradually become an independent link in the industry chain from an auxiliary facility of pipelines to commercial operation. The price of natural gas has also evolved from no independent pricing mechanism to separate government pricing. In recent years, geopolitical factors such as the war conflict between Russia and Ukraine have impacted the European gas market. For the energy policies of European countries, the consumption of natural gas will be gradually reduced and the transition to clean energy will be accelerated.
However, natural gas will continue to serve an important role for China as a transitional energy source between traditional energy and clean energy for a relatively long time. In the future, natural gas is also a companion to renewable energy. The energy policy of vigorously developing natural gas will not change, and the demand for UGS will also continue to increase in China. In this context, it is relevant to study the construction of UGS in China with reference to the mature European gas market. In this section, the natural gas market comparison and natural gas-underground storage situation of Europe and China will be discussed.

| Comparison of primary energy structure between Europe and China
According to the world energy review of 2020, the primary energy structures of Europe, the EU, and China are compared in Figure 1. It can be seen that oil is the main source of energy consumption by European countries followed by natural gas. Without considering the special impact of COVID-19 in 2019, natural gas accounted for 23.8% of primary energy in Europe and 24.6% in the EU. 23 However, coal is the backbone of China's energy consumption, which accounted for the largest proportion of primary energy. Unlike the mature natural gas consumption market in Europe, natural gas consumption in China accounted for only 7.8%. It can be concluded that it has a considerable development potential for the Chinese natural gas market and related industries.
In 2019, the proportion of natural gas among the six major natural gas-consuming countries in Europe is shown in Figure 2, which is 16.2% in France, 24.3% in Germany, 40.0% in Italy, 37.8% in Spain, 37.8% in the Netherlands, and 36.2% in the UK. Except for France, the proportion of natural gas was slightly lower, the other five countries all exceed 20%, and four of them are close to 40%. On the other hand, the proportion of natural gas consumption in China was far less than that of the six countries. However, in the context of the current CO 2 , the Chinese government has made clear commitments to reduce the contribution of coal in the total energy sources. Therefore, the natural gas development targets will be formulated and a future energy transformation basis will be provided by taking the mature natural gas market in Europe as a reference and combining the actual situation of China.

| Comparison of natural gas supply and demand between Europe and China
In this study, the method of normalized principal triangle diagram is used to compare the gas sources structure changes between Europe and China over the years. The triangular diagram is used to represent the proportional relationship between the three components. After the data are normalized, the data coordinate value of each point is A + B + C = 1. In this paper, A, B, and C represent the proportion of domestic production, imported LNG, and imported pipeline gas in the current year, respectively. As the results are shown in Figures 3  and 4, the data points of the European gas sources map are relatively concentrated, which can be considered to be a relatively mature natural gas market. Meanwhile, The primary energy structures in Europe, the EU, and China.  the data points of the China gas sources map show that the proportion of imported LNG was increasing year by year. The proportion of pipeline gas imports is at a plateau, but it is approaching a relatively stable status. It also can be indicated that there is a delay in the growth of imported pipeline gas to some extent. In general, the gas sources structure in Europe is relatively stable, while the natural gas supply in China is still in the process of relatively rapid change.
The natural gas consumption by sectors in Europe and China are shown in Figures 5 and 6, respectively. 26,27 It can be seen from the figures that the natural gas of residential (city gas) consumption is the largest for both Europe and China. The second natural gas consumer is industrial consumption, which is followed by power generation. From the perspective of natural gas demand by sectors, there is a certain similarity between Europe and China.

| Europe natural gas UGS facilities and characteristics
There are many factors affecting the fluctuation of the natural gas market, such as climate, economy, and even international situation. This study correlates 2016 EU gas storage data and 2016 Beijing daily average temperature data to find a correlation between the two mentioned above. As shown in Figure 7, there are many similarities between the climates of EU countries and China, including dimensions, winter heating cycles, and so forth. Combined with the analysis of the features between China and Europe, an analogous approach is considered to analyze the Chinese gas market development using data from mature European gas markets. Meanwhile, according to World Energy Statistics, 23 the natural gas consumption volume of the EU in 2018 and 2019 were 457.2 and 469.6 bcm. The natural gas consumption volume in 2019 was similar to that in 2011, with little overall growth as shown in Figure 8.
According to a Global UGS report in 2019 by International Natural Gas Association, 28 there were 141 UGS facilities in operation in the EU by the end of 2018, including 48 salt caverns, 73 depleted fields, and 20 aquifers. It is also found that the maximum working gas volume of UGS in the EU was 99.8 bcm in 2019, which was 1.64 times of 60.8 bcm in 2011. The maximum gas in storage was 97.6 bcm, the minimum gas in storage was 39.4 bcm, and the actual peak-shaving gas volume was 58.2 bcm. The peak-shaving gas volume and the maximum working gas volume accounted for 12% and 21% of the annual consumption volume, respectively. The annual utilization rate of the gas storage capacity was 58%.

| China natural gas UGS facilities and characteristics
In 2021, China natural gas consumption reached 372.6 bcm. According to the National Development and Reform Commission and the National Bureau of Statistics, the natural gas consumption and production by provinces (municipalities) of China in 2021 are shown in Figure 9. From the consumption perspective, the provinces that account for more than 5% of the national consumption are Guangdong (10.3%), Jiangsu (8.8%), Sichuan (8.1%), Shandong (6.7%), Hebei (6.4%), Beijing (5.5%), and Zhejiang (5.2%). Except for Sichuan, which is a major natural gas production province, other major F I G U R E 7 Correlation of European gas storage data with daily average temperature in Beijing. UGS, underground gas storage. Data sources: European Gas Infrastructure website and BP World Energy Statistics 2020. | 2143 consumption provinces are economically developed provinces located on the eastern coast. Economic growth is the main driving factor for natural gas consumption. From the perspective of production, China's natural gas production was 205.5 bcm in 2021. As shown in Figure 9B, the provinces that account for more than 5% of national production are Sichuan (25.7%), Xinjiang (18.9%), Shaanxi (14.3%), Inner Mongolia (12.7%), Guangdong (6.4%), and Shanxi (6.0%). The comparison shows that Chinese natural gas production is mainly concentrated in a few provinces of the central and western regions. Meanwhile, production and consumption are separated. Most provinces have extremely unbalanced production and consumption. Coupled with seasonal factors, natural gas storage plays an important role in the balance of supply and demand.
Meanwhile, the designed work capacity distribution and the current actual peak-shaving capacity distribution are shown in Figure 10. It can be seen that the main UGSs are concentrated in northeastern China and the middle to the north of the Yellow River. On the basis of public data from relevant databases, government departments, and companies, 42 gas storages are listed which were built or are under construction in China by 2021. As shown in Table 1, it had a designed storage capacity of 109.06 bcm, which was equivalent to 29.3% of the national consumption in 2021. The designed working gas volume was expected to be 50.76 bcm (equivalent to 13.6% of the national consumption in 2021), and the current peak-shaving capacity was 16.405 bcm/a (equivalent to 4.4% of the national consumption in 2021).
At present, there is still a huge gap between the existing actual peak-shaving capacity (4.4%) and the designed working capacity (13.6%). Moreover, with the continuous increase of natural gas consumption, Chinese natural gas consumption is expected to reach 550-600 bcm in 2030. However, the geological resources for UGS construction in 2025-2030 will be exhausted, and the peak-shaving gas volume to consumption ratio may be difficult to reach the level of Europe and the US (15%-20%). Therefore, it is necessary to deeply analyze the rationality of the overall Chinese gas storage setting target.

| ANALYSIS OF UGSS WORKING GAS VOLUME IN EUROPE
To illustrate the characteristics of UGS and facilitate the comparison of natural gas markets between China and Europe, the following concepts are defined in this study first as shown in Table 2. Meanwhile, to analyze the characteristics of the stable natural gas market for the reference of Chinese energy market researchers and energy storage engineers, the impacts of COVID-19 are excluded. The relatively stable market historical data used in this study are from 2011 to 2019. To explain each concept more intuitively, the relationship among maximum working gas volume, peak-shaving gas volume, maximum gas storage volume, and minimum gas storage volume are shown in Figure 11.

| Proportion of working gas volume in the natural gas consumption of EU
To study the performance of the proportion over working gas volume in the EU, the relationship between working gas volume and total consumption of EU was calculated,  T A B L E 2 Description of natural gas storage concepts.

Concept Definition
Maximum working gas volume The maximum storage capacity of the gas storage, not the amount stored in the gas storage Maximum gas in storage The peak (i.e., maximum) volume of gas stored in the gas reservoir within 1 year

Minimum gas in storage
The valley (i.e., minimum) volume of gas stored in the gas reservoir within 1 year Peak-shaving gas volume The maximum gas in storage minus the minimum gas in storage within 1 year, which represents the working gas volume utilized by the gas storage in one injection and production cycle Filling rate The percentage of storage gas to the maximum working gas volume, the inventory rate Annual utilization rate The ratio of the peak-shaving gas volume to the maximum working gas volume in 1 year

Proportion of maximum working gas volume
The ratio of maximum working gas volume to annual consumption volume Proportion of peak-shaving gas volume The ratio of peak-shaving gas volume to annual consumption volume as shown in Figure 12. It can be seen that there is a strong correlation between the working gas volume and natural gas consumption volume. According to the total amount of gas consumption, the natural gas markets in the EU are divided into three types: small market, medium market, and large market, as shown in Table 3. 29 Although the working gas volume and its proportion of total consumption in the small market show a relatively obvious linear relationship. However, it may not be of great reference for the large natural gas market in China. Therefore, this study focuses on the relationship between working gas volume and consumption in European medium and large gas markets, as shown in Figure 13. It can be seen from Figure 13 that there is a nonlinear relationship between working gas volume and consumption in the European medium and large natural gas market. With the increase of working gas volume, the proportion of working gas volume first increases rapidly, and then the increase rate slows down. This is due to the relatively slower growth in the proportion of working gas volume than that in working gas volume with the natural gas market expanding. Meanwhile, the working gas volume proportion of the six major natural gasconsuming countries accounted for an average of 21.2%.

| Working gas volume and peakshaving gas volume of UGSs in the EU
According to the historical data analysis of the EU as shown in Table 4 F I G U R E 12 Relationship between working gas volume and consumption volume in the European natural gas markets.
T A B L E 3 Three typical types of natural gas market in the EU.

Gas market types Definition Typical representative countries
Large market Total natural gas consumption larger than 20 bcm France, the United Kingdom, Germany, and so forth Medium market Total natural gas consumption between 6 and 20 bcm Austria, Romania, Poland, and so forth Small market Total natural gas consumption between 2 and 6 bcm Bulgaria, Denmark, Portugal, and so forth F I G U R E 13 Relationship between working gas volume and consumption volume in the gas markets of typical countries in Europe.
peak-shaving gas volume to the annual consumption were further analyzed, and the scatter diagram was obtained as shown in Figure 14.
It can be seen that the annual utilization rate of UGSs in the EU between 2011 and 2019 was between 49% (2014) and 69% (2018), with an average of 58%, a median of 57% (2015), and the typical annual value is 56% (2016). The average, median, and typical annual values were basically close, which also shows that the "typical year" determined by the least squares method in this study has a certain representative significance and can be used as a reference value for other years. Similarly, the issues caused by extreme values or simple averages will be avoided by analyzing the proportion of maximum working gas volume and peak-shaving gas volume to annual consumption based on statistical principles. Among them, the proportion of working gas volume in annual consumption volume ranges from 13% (2011) to 23% (2015), with an average of 20%, a median of 21% (2017), and a typical year of 22% (2016). Meanwhile, the proportion of peakshaving gas volume in annual consumption volume ranges from 7% (2011) to 15% (2018), with an average of 12%, a median of 12% (2017), and a typical year of 13% (2016).
In summary, from 2011 to 2019, the annual utilization rate of EU UGS was close to 60%, the proportion of the maximum working gas volume is close to 20%, and the proportion of peak-shaving gas volume is about 12%. And compared with the proportion of the maximum working gas volume, the proportion of peak-shaving gas T A B L E 4 Classification of European natural gas markets and the related gas volume parameters. volume needs to be paid more attention as it can better reflect the actual output of the gas storage.

| Correlation analysis of the proportion of working gas volume in the EU
There are many factors that may be related to the proportion of working gas volume. Combined with the actual characteristics of Chinese natural gas market and to provide the references for the China UGSs construction, the following factors are selected: import gas volume from Russia, ratio of import volume from Russia to consumption volume, peak and valley ratio (high monthly consumption/ low monthly consumption), total pipeline gas imports, and total LNG imports. The three major natural gas sources are onshore pipeline gas import, offshore LNG terminal import, and domestic gas. Meanwhile, the onshore pipeline natural gas imports of Europe mainly come from Russia. It can be said that the Russian natural gas to Europe is equivalent to the role of pipeline natural gas imports to China's natural gas market. According to multiple regression analysis and stepby-step regression, insignificant independent variables were eliminated one by one, including peak-to-valley ratio (high monthly consumption/low monthly consumption), total pipeline gas import volume, and total LNG import volume, as shown in Table 5. The results show that the import volume from Russia and its proportion has the greatest impact on the proportion of working gas. In general, the ratio of imports gas from Russia is more closely related to the proportion of working gas, which should be paid more attention to. The correlation analysis in this study shows that when formulating the general plan of UGS and setting the construction target in China, it is necessary to concern the influence of natural gas pipeline import volume and its proportion in consumption volume.

| Revelation of working gas volume for natural gas storage in China
To quantitatively determine the correlation between working gas volume and natural gas consumption, the regression equation of the two parameters is fitted, as in the following equation: where Q represents the consumption volume (bcm) and P is the proportion of working gas volume (%).

| 2149
Equation (1) reflects the relationship between natural gas consumption and working gas volume obtained by fitting data from 2011 to 2019 for the European natural gas market. The quantified parameters of the above three formulas are incorporated into the formulations.
According to the regression results shown in Table 6, the coefficient of determination R 2 = 0.71. The analysis of variance F = 10.54, which indicates the regression equation is highly significant. It reflects that Q has a highly significant nonlinear effect on P. As natural gas consumption increases, the proportion of working gas in UGS decreases, which is a nonlinear inverse relationship. Combining a large amount of historical data, a statistical method was fitted to obtain an inverse relationship between natural gas working volume of UGSs and consumption volume. The formula can be applied to explain that storage cannot grow indefinitely with the growth of natural gas demand. This is also one of the key innovations of this study, which can be used for trend forecasting of China UGSs working gas volume.
The comparison of consumption and the proportion of working gas volume between actual value and forecast value are shown in Figure 15. When China's natural gas consumption in 2021 is 360 bcm, the theoretical working volume of UGS required 19.5 bcm according to Equation (1), and the proportion of gas storage working volume to consumption is 5.4%. By comparing the theoretical value with the actual value, this theoretical value is in the same order of magnitude and closer to the actual peaking capacity of 16.41 bcm of UGS in China. It indicates that Equation (1) has the value of the practical application. Meanwhile, the equation is applied to predict the working gas volume in 2030. As shown in Figure 16, the natural gas consumption in China will reach 540 bcm, 30 the working volume of UGS will be 27.0 bcm, and the related proportion of working gas in storage to consumption will be 5.0%.

| Filling rate and annual utilization rate of gas storage in the EU
The filling rate of gas storage is an important indicator to reflect the status of the natural gas market. According to the filling rate historical data of the EU, 31 the minimum and maximum filling rates were 18% (2018) and 98% (2019), respectively. During the 9 years, the average value of the maximum and minimum filling rates were 91% and 34%, respectively. The difference of maximum value and minimum value indicates that the average annual T A B L E 6 Regression analysis of natural gas consumption and the proportion of working gas in UGSs. utilization rate of the gas storage was 57% as depicted in Figure 17.

Sources of variation
The minimum filling rate of UGS in the EU from 2011 to 2019 was between 18% (2018) and 47% (2014), and the maximum filling rate was between 84% (2015) and 98% (2019), showing a wide range of variations. A least square method was used to determine the median value with statistical significance over the studied 9 years. The least square method was based on the principle of the minimum sum of squares of errors. 32 In this study, the median determined based on the least square method is recorded as "typical year." The "typical year" can be determined by the following: where r i represents the difference between the filling rate on a specific day in a specific year and that of the base year, K i represents the filling rate of a certain day in a year, k i represents the filling rate of a certain day in the base year, i − 1, 2, 3, …, 365, represents a day of the year; and j − 1, 2, 3, …, n, the number of the points. The specific calculation process and results for the "typical year" are as follows. First, one of the last 9 years was taken as the base year randomly. Second, the squared error of the filling rate of this research object and the other 8 years were calculated. Finally, the "typical year" was determined by comparing the results. The minimum sum of squares of the difference between the daily curves of the base year (consisting of 365 specific values) and other years was calculated by the corresponding daily values. The results show that 2016 is a typical year, as shown in Figure 18. In 2016, the minimum filling rate was 34% (on April 6) and the maximum was 90% (on October 19), with a difference of 56%. It indicates that the annual utilization rate of the gas storage in a typical year was 56%.
In conclusion, the typical year of EU gas storage determined by the least square method is 2016, and its annual utilization rate is 56%. This method is more representative and statistically significant than the 5-year average method. The final extracted data can be corresponded to specific year and time point by this method. It is convenient for comparing the changes of specific influencing factors in prediction or analogy analysis.

| Analysis of the filling rate law by sine function
It is proposed to carry out a sinusoidal fitting to the annual filling rate curve. Let filling rate be a function f(t) of time t, 0 ≤ f(t) ≤ 100, let y = f(t), the general expression for the sine formula is y x x t x x = sin( + ) + .
To calculate the formula (2) with a series of n points (t i , y i ), i = 0, 1, …, n − 1, it has to minimize S value: To minimize S,   S X / = 0 k should be satisfied, and  x 0 0 . The fitting formula is obtained: x t x x y The coefficient x x x x , , , 0 1 2 3 can be obtained by solving the four-element. The formula (3) is converted to formula (6).
Among them, t represents the time parameter, y 0 , B, t c , and w are all constants. B stands for amplitude, which is one of the important indicators of the fluctuation range. In this study, the sine curve fitting of the actual data of the filling rate of the UGS in a typical year (2016) was carried out, as shown in formula (7).
The R 2 reached 0.995, indicating a good fitting effect. And the curves of the filling rate of EU UGSs from 2011 to 2019 were fitted as shown in Table 7. The maximum value of amplitude B was 31.5 and the minimum value was 21.8, indicating that the filling rate from 2011 to 2019 was between 63.0% and 43.6%. The difference between each year can reach about 20%.
When   f t t ( )/ = 0, Equation (7) is obtained for predicting the actual peaking gas volume of UGSs (Q u ) corresponding to the maximum value of f(t) max = 90.1 or the minimum value of f(t) min = 36.9: During 2011-2019, the European gas market was not affected by the war conflict between Russia and Ukraine and the UGS developed steadily. The model is reliable and has a strong reference value for the development of UGS in China. Therefore, the quantitative parameters in Equations (7) and (8) can be considered as part of the formula.
As shown in Figure 19, the significance of formula (7) is to compress the 3285 data points of 9 years into a mathematical law represented by four constants, which is a high degree of generalization of the historical fluctuation regulation. According to the results and analysis in the previous section, China's natural gas consumption in 2030 will be 540 bcm, when working gas volume will be 27.0 bcm and the proportion of working gas volume in total consumption will be 5.0%. On the basis of Equation (8), the actual peaking gas volume of China in 2030 will be 14.4 bcm.
During the "13th Five-Year Plan" period (2016-2020), several policies were proposed by the Chinese government to speed up the construction of gas storage. In 2018, the opinions of accelerating the construction of gas storage facilities and improving the gas storage market mechanism and peak-shaving auxiliary services were promulgated. By the end of 2020, it was explicitly requested that the gas supply enterprises should have a storage capacity of no less than 10% of their annual contract sales. For city gas construction, it should form a storage capacity of no less than 5% of their annual gas consumption. And local governments storage capacity should also satisfy the demand for 3 days at least. 33 If the above goals are achieved, a gas storage capacity with a ratio of gas storage to consumption greater than 15% will be formed. However, as the results shown in this study that Chinese UGS capacity should be 5% of total consumption by 2030. The construction targets set by government are far more exceed than the actual demand of UGS. The reason for the unreasonable government target is due to the previous experience judging that the working gas volume of UGS is linearly related to the demand for natural gas. In contrast, this study summarizes the relationship between the working gas volume share of UGS and the total natural gas demand as a negative one power through regression analysis and data fitting.
The analysis of the empirical results of this study shows that the overly ambitious targets are difficult to achieve. It will also lead to wasteful investment flows into geological conditions that would otherwise be unsuitable for development as gas storage. It can be seen that the construction of natural gas storage is closely related to the future development of the Chinese natural gas market, and the relevant targets need to be set with comprehensive consideration of the market development.

| Prospect on the application of numerical fitting methods in China's natural gas market construction
The sinusoidal functions have a certain application value. Meanwhile, the principle of sinusoidal fluctuation can be applied to the prediction of the filling rate curve of Chinese gas storage and the analysis of other seasonal energy storage issues: (1) Apply the superposition principle of the sine function.
In theory, any sine function can be generated by the superposition of multiple other sine functions, so the sine function of the filling rate curve of the EU gas storage is theoretically the superposition curves of EU countries. Taking the EU as an analogy to China, it can solve the problem that the current data related to the working curve of UGS in China has not been published and the fluctuation curves are unknown.
Referring to the characteristics of the sine function curve, first simulate the sine curves assuming the operation of each gas storage in China, and then superimpose the curves to predict the overall working curve. (2) Explore the characteristics of the sine function of the working curve of seasonal energy storage facilities. The relevant theory of the sine function is very mature and has been widely used in physical motion, optics, telecommunications, and other fields. Therefore, the existing interdisciplinary sine function physics theory can be used for reference in the future, through the fragmentary data of the existing time series or the key data of the fluctuation range (amplitude) in China to forecast the energy storage data for the whole year. This prediction method can be used as a reference for China to solve more seasonal energy storage output regulation simulation problems.

| CONCLUSIONS AND RECOMMENDATIONS
Global energy problems occur frequently due to geopolitical influences. Countries and regions are committed to addressing energy security, especially natural gas supply. The natural gas-underground storage is critical to maintaining the stability and security of regional energy sources. Thus, its significance has been emphasized by the recent complex changes in the world situation and the huge impact of the war conflict between Russia and Ukraine on the European gas market. In this study, the required underground natural gas storage capacity and peak-shaving regulation in China are estimated with the references of gas industry characteristics in the EU countries. It provides reasonable quantitative targets for the medium and long-term construction of natural gasunderground storage in China. The main conclusions and suggestions are presented below: (1) To find a stable and mature natural gas market reference system for China, the characteristics of the European and Chinese natural gas markets are analyzed and compared. The EU natural gas market is similar to the future Chinese natural gas market from the perspective of energy structure, natural gas utilization, and UGS construction. The working gas volume and the peak-shaving gas volume of the EU UGS consumption accounted for nearly 20% and nearly 12%, respectively, while the annual utilization rate of the gas storage was 60%. Through numerical fitting method and trend analysis, a nonlinear inverse relationship between the proportion of working gas in underground storage to total gas consumption and national natural gas consumption is found in this study. The working volume of China UGSs will be 27.0 bcm in 2030. Meanwhile, it is found the early target of more than 10% (or 15%) set by government departments and researchers may be too large. It is recommended that China's UGS capacity should be 5% of total consumption by 2030. (2) The correlation analysis of UGS planning affecting factors on the six largest natural gas-consuming countries in the EU is carried out by the regression analysis method. The import-to-consumption ratio from Russia and imports from Russia have the most significant impact on EU UGS planning. By comparing the natural gas markets of the EU and China, it is found that there is a correlation between the ratio of pipeline natural gas imports to consumption and the ratio of gas storage peak-shaving gas to consumption. When planning the goal of UGS construction in China, it is necessary to focus on the affecting factor of the proportion of pipeline gas imports. (3) The sine function fitting is used to study the gas storage filling rate in the EU from 2011 to 2019. It is revealed that the gas storage filling rate curve has a significant sine function law. The numerical characteristics related to fluctuations such as amplitude and peak value can be well analyzed by sine function fitting. According to the function, a large amount of data can be compressed into a mathematical law represented by a limited number of constants, which is a high degree of generalization of the historical fluctuation law. The function is applied to predict the actual peaking gas volume of China, which will be 14.4 bcm in 2030. It can be said that the numerical fitting method provides a novel way to solve the issues of unpublished data about the working curve and unknown fluctuation regulations of China UGS. (4) The predictions based on numerical simulations in this study can provide reasonable and feasible references and suggestions for the long-term construction of natural gas-underground storage in China. Compared with the target set by the Chinese government for the construction of underground natural gas storage), the actual demand for its future development is far below the policy target. As can be seen, the construction of natural gas storage will be mainly influenced by the future development of the Chinese natural gas market. The relevant targets for gas storage need to be set with comprehensive consideration of market development. The results of the empirical study can provide support and reference for relevant government departments to formulate other energy policy objectives.
The method for determining construction targets of China used in this work by referring to the regulations of mature natural gas markets such as Europe can be applied to other developing natural gas markets. However, the analysis results obtained in this study are only applicable to the UGS construction of China due to the data limited only from Chinese natural gas market. There are still two issues that need to be considered for the applications to other regions. First, whether the economic development of the region, the characteristics of the natural gas market and the level of infrastructure development can develop UGS. Second, whether the region and the European gas market have similar commonalities or characteristics, such as high dependence on natural gas imports and seasonal influence on gas demand. In future work, more countries and regions will be focused on. The actual characteristics of each natural gas market will be considered to establish the corresponding theories and models.
AUTHOR CONTRIBUTIONS Xueping Du: Conceptualization, methodology, and formal analysis. Menglin Liu: Original draft and data curation. Shuo Qiu: Writing and editing, and paper proofreading. Jiangtao Wu: Supervision and project administration. Xianyang Meng: Investigation and validation.