Modeling the behaviour of carbon dioxide content in geothermal reservoirs using multiple tank lumped parameter models

Energy is an essential subject since nothing can be done without energy. For the supply of energy, there is a search for alternative energy sources which are clean, sustainable and renewable. One of the alternative sources is geothermal energy. Geothermal energy is the heat stored underground, that is used for direct uses or electricity generation. The heat is deposited in rock pores and water. Accurate geophysical and geochemical data with observations show whether a field is eligible for geothermal or not. Geothermal energy usage is increasing nowadays for being a clean, sustainable and renewable energy source. Three types of geothermal power plants are considered. Dry steam power plants, flash steam power plants and binary power plants. Turkey is one of the countries in which geothermal energy exits. In, Aydın, Denizli, Manisa regions which are located in the west of Turkey, there are geothermal power plants. Turkey has vast sources, and the amount of energy demand will increase in years with the development of geothermal energy. In order to use geothermal energy efficiently; good reservoir engineering must be practiced. In a geothermal reservoir, noncondensable gases such as carbon dioxide exist. All geothermal fields in Turkey contain different amounts of carbon dioxide. Carbon dioxide has considerable effect in geothermal reservoirs. Even small portions of carbon dioxide increase the flashing point of the geothermal water. The carbon dioxide content can change based on the production/reinjection operations in a geothermal field. So the change must be modeled well in order to make predictions for future performance. In this study, a mathematical model is developed for modeling the changes in the carbon dioxide content. The developed mathematical model which gives the change of carbon dioxide as a function of time is a result of a simple mass balance that can be used over any control volume and is only for liquid-dominated geothermal reservoirs. In this study, a geothermal reservoir is modeled using tanks in which mass balance and energy balance equations are used to model pressure, temperature, and carbondioxide content. The physical parameters of the tanks are bulk volume, porosity, the density of the water and compressibility, recharge mass rate, reinjection mass rate and mass production rate. The model is first validated with existing solutions in the literature. Then the model is used for investigating how the carbondioxide content would change in a reservoir under various production/reinjection scenarios. In all the modeling conducted in this study, the reservoir is represented by two tanks. One tank to represent regions close to the recharge source and the other tank to represent the regions further away from the recharge source. The specific schemes that have been considered in this study are; the effects of the recharge carbondioxide content, the location of the production and reinjection areas. In all cases considered in this study the reinjection water is assumed to contain no carbondioxide. Results show that the change of carbondioxide content in the reservoir is highly dependent on where the production and reinjection operations are performed. Furthermore, the recharge carbondioxide content also has significant effect.