Comparison of enhanced geothermal system with water and CO2 as working fluid: A case study in Zhacanggou, Northeastern Tibet, China

In the study, we analyzed the hot dry rock geothermal field of the Guide Basin in Qinghai Province, China. We used T2Well software-a coupled wellbore-reservoir simulator-to build a "wellbore-reservoir" coupled model with a "three-spot" well pattern (one injection and two production wells). We simulated several fixed flow rate cases in which water or CO2 is injected. The objectives of our present work are (1) to investigate the fluid flow and thermal processes of water circulating at well bottoms, wellbores, and wellheads; (2) to identify the changing parameters at all physical fields; (3) to understand the influence of injection rates on heat extraction; and (4) to measure the maximum heat extraction capacity of the Guide area. Water extracts more heat than CO2 at the same flow rate. However, water consumes more pressure in reservoir, and its pressure decreases more quickly as the flow rate increases. In contrast, CO2 is in a sense a better working fluid. CO2 consumes less pressure when it flows and can circulate automatically due to the siphon phenomenon. In this way, a lower injection pressure is required in a higher CO2 flow rate case. The density of CO2 is sensitive to both temperature and pressure and vice versa. Inside a wellbore, such interactions are extremely complicated. When the fluid rate is slow, a system could operate for 30 years and remain stable, and there is only a small decrease in temperature. However, with higher flow rate scenarios-namely 50, 75, and 100 kg/s the reservoir will exhibit greater heat loss. The reservoir's production temperature and extraction efficiency will drop dramatically. Therefore, for the Guide area, if a "three-spot" well pattern is used for geothermal extraction either with water or CO2 as the working fluid, the most appropriate flow rate is 50 kg/s.