LEE- Petrol ve Doğal Gaz Mühendisliği-Yüksek Lisans
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ÖgeTracking pressure, hydraulic and thermal fronts in porous media(Graduate School, 2022-01-21)Geothermal energy is the heat energy stored in the subsurface. It is a clean, renewable, and sustainable energy source. Therefore, geothermal energy is a popular energy resource in the world. There are two types of utilization of geothermal energy which are direct use and indirect use. Geothermal energy is used directly for space heating, greenhouse heating, tourism, etc. However, heat energy is converted to another type of energy for indirect utilization. The main purpose of indirect utilization is electricity production. Geothermal power plants are used to convert heat energy to electricity. There are three types of geothermal power plants which are dry steam power plants, flash steam power plants, and binary power plants. For sustainable management of a geothermal resource, future performance predictions must be made. This requires good reservoir engineering practices and good reservoir characterization. One of the ways of characterizing the reservoir is by way of using tracers. Generally, tracers are made up of material that does not exist in the geothermal reservoir. Almost all of the geothermal fields in Turkey contain some amount of carbon dioxide. The carbon dioxide is usually dissolved in the geothermal water in various mass fractions. Depending on the amount, carbon dioxide can have a significant effect on production performance. Because of reinjection operations (where water with either little or no carbon dioxide is reinjected), the amount of carbon dioxide in the reservoir decreases. Depending on the reinjection amount, the produced carbon dioxide from wells also decreases once reinjected water reaches the production wells. This provides the opportunity to treat the carbon dioxide data as tracer data. Analyzing the decline of carbon dioxide at the production wells would provide a better characterization of the reservoir. Hence a model is necessary to model the decline of the carbon dioxide level. When reinjection operations are carried out, usually there are three fronts involved: the pressure front, hydraulic front, and thermal front. In this study, a model is developed to analyze how the fronts propagate in the reservoir. In the mathematical model, mass balance on the water, mass balance on carbon dioxide, and overall energy balance are applied to model pressure, temperature, and mass fraction of carbon dioxide in the geothermal reservoir. The model developed is a numerical model where the reservoir is split into grid blocks and mass and energy equations are solved simultaneously. To track pressure, thermal, and hydraulic fronts, the geothermal reservoir is divided into 175 homogenous grid blocks. These grid blocks are hydraulically connected with each other. In this study, the effects of injection operation and some petrophysical properties on the displaced pressure, thermal, and hydraulic fronts are studied. It is important to note that there are several assumptions. First, the geothermal reservoir is assumed to be a liquid dominated geothermal reservoir. Second, it is assumed that there is a 1D linear flow. Furthermore, it is important to note that injection is operated with a constant mass flow rate. Finally, the impact of carbon dioxide diffusion is ignored. Analytical equations of the breakthrough time of both thermal and hydraulic fronts are provided. Comparison of numerical and analytical solutions of these fronts are also provided.
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ÖgeNew analytical model for underground storage of natural gas with carbon dioxide as cushion gas and for sequestration of carbon dioxide(Graduate School, 2023-10-18)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.
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ÖgeSimulation of carbon dioxide as a cushion gas in underground gas storage reservoirs(Graduate School, 2023-10-18)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.