Source: Author Resource Network: Li Hua Published: 2008.11.21 

　　Natural gas storage and transportation engineering is one of the key links and infrastructure of the entire natural gas industry. At present, the demand for natural gas in all countries in the world is continuously increasing, and natural gas accounts for an increasing proportion of primary energy consumption. Due to the constraints of their own resources, many countries are increasingly dependent on imported natural gas. To ensure energy security, many countries have taken active measures to solve the problems of natural gas storage and surplus and shortage adjustment. The construction of underground gas storage is a relatively advanced method to adjust the seasonal contradiction between supply and demand in the natural gas market. It has now become a very important part of the natural gas supply and marketing chain. At present, the basic situation of the development and construction of global underground natural gas storage is as follows: 

　　In 2000, the total working gas volume in the world reached 3100×108m3, and the daily peak shaving capacity reached 44.6×108m3. Western European countries have about 78 underground gas storages with a working gas volume of about 550 × 108 m3 and a daily peak shaving capacity of 10.9 × 108 m3. Eastern European and Central Asian countries have about 67 underground gas storages with a working gas volume of about 1310 × 108 m3. The daily peak shaving capacity can reach 10×108m3. Stimulated by changes in the natural gas market, the capacity of underground gas storage in the world has shown a rapid upward trend in recent years. As of 2004, the total number of underground gas storage in the world reached 610. Underground gas storage technology has received great attention from countries all over the world, and its related technologies have also been rapidly developed. my country's underground gas storage technology has already started. Up to now, six abandoned oil and gas reservoirs have been used to construct underground gas storage. There is still a certain gap between my country's underground gas storage technology and foreign countries. At present, foreign countries are working to develop the following new technologies. 

　　1. Looking for four-dimensional seismic exploration technology suitable for geological body construction 

　　Finding a geological body suitable for building a reservoir is different from exploring an oil and gas reservoir. The former is more complicated. An oil reservoir with suitable cap rocks is not necessarily able to store natural gas. The geological structure that can store natural gas must ensure that the stored natural gas will not leak. It must have the continuity of the cap rock and the tightness of the structure. Modern fine seismic exploration technology can show smaller structures, even the lateral variation of gas-liquid interface and stratigraphic facies. The four-dimensional seismic technology, which is in the research stage, is a relatively promising technology for prospecting suitable for underground gas storage structures. Four-dimensional seismic technology is based on a number of technologies, such as seismic sensors placed on the ground at even intervals or permanently placed in the well; multi-layered seismic technology, such as AVO (Amplitude VersusOffset), can better study the physical properties of reservoir rocks. Deepening the seismic exploration technology can reduce the uncertainty in the initial stage of underground gas storage construction, reduce the number of observation wells, and help to deploy the gas storage wells in favorable parts of the structure and reduce the number of wells. 

　　2. Bottom-up design technology 

　　In the investment cost of building an underground gas storage, the cost of bottom-up gas accounts for the largest proportion, generally accounting for 30%-40% of the total investment. If a certain gas can be used to replace natural gas as the bottom gas, this part of the investment cost will be significantly reduced. Foreign countries have tested inert gas or mixed gas as the bottom gas. At present, mixed gas is more widely used, and 7 gas storages have been tested. The application of mixed gas as a bottoming gas requires specialized techniques, models and measurement tools to accurately deal with gas miscibility. The test results show that the investment cost can be reduced by 20% after applying the bottom line. 

　　3. Large borehole and horizontal well technology 

　　The use of large borehole completions in underground gas storage can significantly increase the peak shaving capacity of natural gas. If there is no production of liquids (oil, water, condensate), large boreholes must be used abroad, which has become a design criterion. In addition, in order to reduce the pressure loss along the production string, most wells are designed with a single pipe diameter to reduce diameter reduction and avoid gas turbulence. The main purpose of drilling horizontal wells in underground gas storage is also to increase the peak-shaving gas volume of a single well. If the reservoir permeability is low, horizontal wells are more applicable than vertical wells. For the same reservoir, the peak-shaving gas volume of horizontal wells is 1.5-6 times higher than that of vertical wells, which mainly depends on the nature of the reservoir and the length of the horizontal section. During operation, horizontal wells can also restrain water coning. If the horizontal section is on the gas-water interface, during the gas production process, because the horizontal section builds a reservoir in an aquifer with extremely low permeability, these technologies are absolutely necessary. These large boreholes and horizontal wells are also applicable to the conversion of low-permeability depleted oil reservoirs to underground gas storage.

　　4. Salt cavern gas storage technology 

　　Reservoir-type gas storage and aquifer gas storage both store natural gas in natural rock pores, while salt cavern gas storage is different in that it stores natural gas in a cavity formed by artificially dissolving salt. The main method to reduce the construction cost of salt cavern gas storage is to apply modern salt-dissolving technology to increase the salt cavern volume; reduce the minimum operating pressure and increase the maximum operating pressure. The use of salt caverns as gas storage began in the 1960s. The first salt cavern gas storage in the United States was built in Marysville, Michigan in 1961; the first salt cavern gas storage in Canada was built in Melville, Saskatchewan in 1964; the first salt cavern in Armenia The gas storage was built in Abovian in 1964; the first salt cavern gas storage in France was built in Tersanne in 1968; the first salt cave gas storage in Germany was built in Kiel in 1969. In the early stage of development of salt cavern gas storage, its size and capacity were relatively small, only (3-10)×108m3/d. With the development of engineering technology, by the end of the 20th century, the total capacity of European salt cavern gas storage has reached (400-600) × 1012m3/d. The salt cavern capacity is mainly limited by the pressure loss of the gas injection and production wells and the safety requirements during equipment operation. Lowering the minimum operating pressure can increase the effective working air volume and reduce the investment of cushioning air. Every 1MPa decrease in the minimum operating pressure can save about 10%-15% of the total investment. The maximum gas storage capacity of a single cavity is related to the maximum operating pressure. The maximum operating pressure is increased by 10%, and the effective working gas volume can be increased by approximately 15%. However, the maximum operating pressure is restricted by the gas tightness of the salt layer. Generally speaking, the maximum operating pressure is less than the weight of the overlying layer of the salt layer, and the gradient is about 0.23MPa/m; in some special blocks, the gradient is less than 0.2MPa/m. 

　　The cost of dissolving salt to build the cavity accounts for about 25% to 35% of the total investment, and it requires a long-term process. Every 1 m3 of salt needs 7-9m3 of water. The salt water concentration formed by the dissolved salt is about 300kg/m3. Usually used by industry to produce sodium chloride, or reinjected into the ground, or even discharged into the sea. Two pipe strings are required to build a cavity by dissolving salt, one for injecting water and the other for returning brine. At the same time, various tests are required to ensure that the cavity shape conforms to the design after the salt is dissolved. The depth of the pipe string shoe needs to be adjusted gradually according to the dissolved salt situation, but it is necessary to use liquefied petroleum gas, crude oil, nitrogen, or even natural gas to form a protective layer on the top to protect the upper roof from being dissolved. At present, many countries in the world are committed to the research of salt-dissolving optimization technology (such as SMRI or SALTXX of INVDIR and SURMOVINER of Gaz de France). It is said that the use of new research results can at least reduce the number of process steps and acoustic detection by 50%, and reduce investment by more than 10%. In the process of dissolving salt, pressurized injection of gas (natural gas or inert gas), diesel, liquefied petroleum gas, etc. can protect the roof. After the cavity is dissolved, the well needs to be re-completion to discharge the brine and perform the first gas injection. Natural gas is injected from the annular space between the brine discharge pipe string and the production tubing (or casing), and brine is produced through the brine discharge pipe string. The overall time for building a database is related to the size of the salt dome (salt-containing structure), some take more than a year, and some take several years. Salt dissolution and gas storage are a new technology for salt cavern construction. It has been successfully applied in the Moss Salt Mine in Texas and Egan Salt Mine in Louisiana, which has significantly reduced investment. 

　　The initial part of the salt cavity is formed by the traditional water-soluble salt method. First, the upper salt layer is dissolved to the designed size, while the lower part has not yet dissolved. This technology requires special equipment, such as wellheads and salt-dissolving tubing strings. While ensuring gas storage in the upper cavity, the lower portion can continue to dissolve salt and build the cavity; while the upper salt cavity is draining brine and storing gas, the lower salt layer begins Dissolve and build cavity. The gas stored in the upper part can be used as the protective layer of the roof when the lower salt layer is dissolved. In the process of dissolving the salt, the interface between the gas and the brine is strictly controlled and basically kept in the middle of the salt cavity. Once the lower salt layer is dissolved to the same diameter as the upper cavity, continue to use the above completion methods and wellhead equipment to optimize the operation of the gas storage and expand the gas storage capacity. At this time, the interface between air and salt water is no longer maintained at the original position, but often moves up and down. At this time, it is necessary to strictly control the gas-salt water interface to protect the roof and avoid natural gas leakage. The salt cavern gas storage is generally built in a salt layer with a thickness of about 150-400m. A thin salt layer with a thickness of 60-100m can also be used to build underground gas storage, but the gas storage capacity is small, between (5-10) × 104m3/d. However, in some blocks, mainly at the edges of sedimentary basins, the thickness of the salt layer is less than 60m. Obviously, it is impossible to build underground gas storage on this salt layer with conventional technology. The main problem is the control of the dissolved salt channel. The thin salt layer must be doped with insoluble layers. Unlike ordinary large-sized salt chambers, gravity cannot play a key separation role in this salt layer. According to geomechanics and the three-dimensional model study of the salt cave channel shape on the cross-sectional area of ​​1000-3000m2, it is appropriate to build an underground gas storage with a horizontal section of several hundred meters and a capacity of (10-100)×104m3 for this kind of salt layer. . The investment of this kind of gas storage is 15%-20% higher than that of conventional salt cavern gas storage. But in Europe and North America, this technology is gradually being widely used. Russia is also planning to apply this technology. 

　　5. Linear rock formation cave construction technology 

　　Linear rock formation cavern construction technology provides an option for countries that do not have suitable geological structures to build underground gas storage. The main principle of this technology is to store gas in a relatively shallow (100-200m) linear rock cave at high pressure (15-25MPa). The pressure is transmitted by pipelines fixed in the cement layer, and the role of the rock is to withstand high pressure. The limitation of this kind of gas storage lies in the rock properties of the formation. In Grangesberg, Sweden, successfully applied this technology to build a 130m3 cavity 50m underground. In 1997, a demonstration gas storage facility was built in Skallen, southern Sweden, with a storage capacity of 40,000 m3. The pilot test and related tests for this kind of gas storage were completed in 2003. The construction cost of the gas storage is about 2-4 times that of the ordinary gas storage, but the maintenance and operation cost is far lower than the operation cost of the LPG storage equipment. Due to the large amount of gas transported, the LPG storage equipment can be recycled several times a year, so its service cost is equivalent to the operating cost of other gas storage equipment under the same conditions. 

　　6. Optimized operation technology of gas storage 

　　Currently, many new technologies are used to help managers manage underground gas storage more effectively. The more important ones are: gravel packing and sand control technology for non-cemented pore reservoirs; polymer profile control and water control technology; non-cemented pore reservoir consolidation technology; mathematical models and software for improved design, including the ability to provide reservoir geometry and Three-dimensional simulation software for rock properties; equipment maintenance and operation technology for underground gas storage; new logging tools, such as nuclear magnetic resonance, improved three-dimensional seismic sensors, multiple fluid and sand detectors; permanent sensors and/or in-well fiber optic energy Provide reservoir parameters (especially salt caverns) at any time, etc. In addition to technological improvement and new technology research and development, the owners and operators of gas storage are also committed to solving technical and economic optimization problems. At present, more attention is paid to: First, more complex judgment tools, such as risk management, stochastic simulation, etc.; Second, how to connect the gas storage model into a simplified gas network model; Third, the daily management and control of each equipment ; The fourth is real economic data, such as marginal cost, operating cost, changes in the amount of gas stored by each device, and the cost of relying on imports to maintain gas.