Tracking pressure, hydraulic and thermal fronts in porous media

Abstract

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.