Simulation of carbon dioxide as a cushion gas in underground gas storage reservoirs

Abstract

Fossil fuels are a part of the primary source of energy, and today the majority of industrialized and developing nations use oil, coal, and natural gas as their primary fossil fuels. Natural gas, one of these fossil fuels, is a versatile, efficient, clean-burning fuel that is utilized in a range of applications. The gas industry encompasses various sub-sectors that contribute to the overall expansion and maintenance of a reliable gas supply. One of these vital components is underground gas storage, which plays a crucial role in ensuring consistency in gas supply. Underground gas storage involves the practice of storing natural gas in reservoirs that have significant capacities. This strategic approach allows for the management of high import volumes during periods of low demand, as well as the provision of an adequate supply of natural gas during periods of high demand. The primary purpose of underground gas storage is to balance the fluctuating demand and supply dynamics of the gas market. By storing natural gas during times when demand is low, such as during the summer season or periods of reduced industrial activity, the excess supply can be stored underground in reservoirs. This practice helps to avoid the wastage of gas resources and ensures that the gas supply is readily available when demand increases. Moreover, underground gas storage facilities contribute to the overall energy security of a region or country. By maintaining a sufficient inventory of stored natural gas, countries can reduce their dependence on external sources of gas supply during times of geopolitical uncertainties or disruptions in gas imports. This enhances energy resilience and provides a buffer against potential supply disruptions, thus ensuring the uninterrupted functioning of industries, power generation facilities, and residential heating systems. Overall, underground gas storage is a critical sub-sector within the gas industry. It provides a means to balance supply and demand, manage seasonal variations, and enhance energy security. By investing in the expansion and maintenance of underground gas storage facilities, countries can increase customers' access to a reliable gas supply and strengthen their overall energy infrastructure. In the process of storing and withdrawing gas from an underground storage reservoir, certain considerations need to be addressed to ensure smooth operation. When it comes to withdrawing gas, it is crucial to maintain the average reservoir pressure above a certain value to ensure the fluent extraction of the stored gas. This is where the concept of cushion gas or base gas comes into play. The cushion gas refers to the amount of gas that needs to stay in place to maintain the required pressure levels. Traditionally, natural gas has been used as cushion gas due to its compatibility with the stored gas and the reservoir conditions. However, as alternative storage methods and gas management strategies have been explored, other gases such as carbon dioxide (CO2) have gained attention as potential cushion gases. Carbon dioxide offers several advantages as a cushion gas. Firstly, it can be readily available as a byproduct of industrial processes, making it an attractive option for utilization. Additionally, carbon dioxide can exhibit favorable thermodynamic properties, allowing it to function effectively in maintaining the reservoir pressure within the desired range. The selection and amount of the cushion gas depends on various factors, including the specific reservoir characteristics, gas storage requirements, and environmental considerations. Each gas has its own unique properties, and the choice of cushion gas should be made based on a comprehensive assessment of these factors. By employing an appropriate cushion gas, the gas storage facility can ensure that the average reservoir pressure remains above the minimum level required for efficient gas extraction. This allows for a reliable and consistent supply of gas during periods of high demand, contributing to the overall stability and effectiveness of the gas storage and retrieval process. The main objective of this study is to investigate the feasibility of utilizing carbon dioxide (CO2) as a cushion gas in an underground storage reservoir. In addition, the behavior of the mixing zone is also investigated. The simulation process is conducted using the Generalized Equation of State Model Compositional Reservoir Simulator (GEM), a software developed by the Computer Modelling Group. In this study, several scenarios are modeled using the simulation program. Each scenario represents a specific combination of reservoir conditions, including reservoir temperature, average reservoir pressure, and the compositions of carbon dioxide and methane within the reservoir. The simulation aims to provide a comprehensive understanding of how the reservoir behaves under various conditions when carbon dioxide is used as a cushion gas. By inputting the specific reservoir properties and gas compositions into the GEM simulator, the researchers can assess the performance of the reservoir in each scenario. The simulation results include data depending on factors such as reservoir pressure, reservoir temperature, and the behavior of the carbon dioxide and methane within the reservoir. These results will help to evaluate the suitability of using carbon dioxide as a cushion gas and determine the potential benefits or limitations of such a storage approach. Overall, this study contributes to the field of underground reservoir storage by investigating the use of carbon dioxide as a cushion gas, providing valuable insights into the dynamics of such a system and its potential implications for carbon dioxide storage and management strategies. Due to the different compressibility behavior of carbon dioxide at certain temperatures and pressure conditions, it can be both an advantageous and disadvantageous gas when it is used as a base gas. The results showed that carbon dioxide usage as a cushion gas at reservoir temperatures of 313.15 K, 323.15 K, 333.15 K, and 343.15 K and pressure ranges below 120 bar is quite beneficial as it provides pressure support because of its higher compressibility values than that of methane at these reservoir conditions. However, carbon dioxide loses its advantage when the initial reservoir pressure is increased to 180 bar since its compressibility is lower than the compressibility of methane at higher reservoir pressures. Moreover, the concentration of methane and carbon dioxide has a huge impact on the average reservoir pressure decline rate. Furthermore, results illustrate that the mixing zone length formed between working and cushion gas tends to extend with time. The mixing zone is assumed to be the part in which tracer concentration is between 0.1 and 0.9 and the length of the mixing zone for early, mid, and late time is 270 m, 458 m, and 567 m respectively. It was also observed that mixing zone length is proportional to the square root of dimensionless time.