New analytical model for underground storage of natural gas with carbon dioxide as cushion gas and for sequestration of carbon dioxide

Abstract

Natural gas is a strategically important, valuable fuel used in heating, industry and transportation. Natural gas is the smallest member of hydrocarbon paraffins. While some countries produce and export surplus natural gas, some countries are dependent on import of natural gas. Turkey is a country in need of imports for natural gas. For this reason, some of the imported natural gas is used, while the unused portion is stored for use when needed. One of the natural gas storage methods is to store natural gas underground. Depleted natural gas and oil reservoirs and salt domes can be used for underground storage. Storage of natural gas is very important for countries due to seasonally changing gas demand, fluctuations in gas prices and strategic reasons. Although the stored gas is generally methane, not all of the stored natural gas can be produced due to the pressure difference between the reservoir and the surface. Some of the stored natural gas is left in the reservoir as base gas to create pressure support. This leads to economic loss. Using carbon dioxide instead of methane as cushion gas provides significant economic, environmental, and operational benefits. In this study, the effects of using carbon dioxide as cushion gas were investigated. The physical properties of carbon dioxide and methane such as density, compressibility and compressibility factor were investigated. Although the denser of the two gases with different densities in the tank sinks to the bottom and the other one is at the top of the tank, the area between the two gases where these two gases form a homogeneous solution is called the mixing zone of these two gases. Since there will be a region consisting of a mixture of these two gases as a transition zone in a reservoir containing carbon dioxide and methane, the compressibility factor of the mixture region containing different percentages of carbon dioxide and methane was calculated using Peng Robinson Equation of State. Since looking at the physical properties, the compressibility of carbon dioxide at temperatures between 60-120 bar and 50-70 °C is higher than that of methane, it is concluded that using the same amount of carbon dioxide as cushion gas by volume gives significantly beneficial results in terms of pressure optimization and increases the amount of methane produced. Since carbon dioxide is cheaper than methane, its use as cushion gas may give satisfactory results both economically and environmentally in natural gas storage reservoirs. It is seen that the use of carbon dioxide as an enhanced gas recovery method as cushion gas in methane storage and production is more efficient in terms of reservoir management and economy. In this study, how the pressure changes during methane production in gas reservoirs containing carbon dioxide and methane as cushion gas for different production scenarios is observed by the use of the new material balance equation presented by Tureyen et al., (2023). Thermodynamically, how methane and carbon dioxide affect the reservoir properties and how they change with different initial reservoir pressures, molar percentages of methane and carbon dioxide, temperatures, and production scenarios are investigated. As a result, it has been observed that the use of carbon dioxide as cushion gas in the temperature and pressure range of 50-70 °C and 60-120 bar increases the methane storage and production efficiency, which is, the working gas capacity. Considering the compressibility behavior of methane and carbon dioxide, it has been observed that the mixing zone containing the same volumetric ratio of methane and carbon dioxide shows a compressibility factor behavior closer to methane. For this reason, a new analytical equation was introduced by taking the mixing zone into account. CO2 is injected into a reservoir containing methane initially, followingly only methane is produced from the reservoir and average reservoir pressure change is observed during the injection and production stage with analytical models where one of the analytical models does not include the mixing zone into consideration and the other one does. CMG (Computer Modelling Group) is used to verify the results. It is seen that the analytical model which includes a mixing zone gives better results than the analytical model assumes no mixing zone in the reservoir. Finally, assuming that carbon dioxide will be located in the lower part of the reservoir and methane in the upper part of the reservoir due to the density difference, it is important to observe how the transition zone of methane and carbon dioxide changes with methane production. Since only methane production is targeted, it is important to follow the transition zone height in order to prevent carbon dioxide production. For this reason, the change in the height of the transition zone between carbon dioxide and methane with methane production in the reservoir containing 50% carbon dioxide and 50% methane for different reservoir shapes such as cylindrical, trapezoidal and hemispherical was investigated.