Wellbore–reservoir coupled simulation to study thermal and fluid processes in a CO2-based geothermal system: identifying favorable and unfavorable conditions in comparison with water

Abstract

Using CO2 as a heat transmission fluid to extract geothermal energy is currently considered as a way to achieve CO2 resource utilization and geological sequestration. As a novel heat transmission fluid, the thermophysical properties of CO2 are quite different from those of water. CO2 has many advantages, such as larger mobility and buoyancy resulted from the lower density and viscosity. This will reduce the consumption of pressure driving the circulation, and save the energy of external equipment. The cycle even can be achieved by siphon phenomenon under a negative circulating pressure difference. However, there are still some disadvantages for CO2 as the heat transmission fluid, such as small heat capacity, leading to a less heat at the same mass flow rate. At the same time, because of the lager expansion and compression coefficient for CO2, changes in temperature and pressure may cause a more complex flow and thermodynamic processes. The lager compressibility makes it possible to get high temperature at the bottom of the injection well, whereas the lager expansion coefficient makes the temperature drop rapidly along the production well. Therefore, how to scientifically control the production pressure to guarantee sufficient high temperatures at the head of production well and, thereby, improve the efficiency of heat extraction are the key issues needed to be further addressed. The geological and geothermal conditions correspond to the central depression of the Songliao Basin located in the Northest of China. This depression has a high geothermal gradient and heat flow. In this article, a classic idealized “five-spot” reservoir model coupled with wellbores is used for simulations and analyses. The objectives of the present work are: (1) to investigate the fluid flow and thermal processes of supercritical CO2 along the wellbore and in the reservoir, (2) to understand the heat-extracting mechanism, (3) to identify advantages and disadvantages of using CO2 as the heat transmission fluid, and (4) to provide a theoretical basis for the selection of heat transmission fluid.