Heat extraction mechanisms of CO2-water mixed-phase flow in a single fracture of hot dry rock

Abstract

Deep geothermal energy, recognized for its cleanliness and lack of pollution, has often overlooked the heat extraction dynamics of CO₂-water two-phase flow within fractures. This study investigates the influence of H₂O mixing ratios on the output thermal power(OTP) in CO₂-Enhanced Geothermal Systems, particularly in regions where CO₂ and water coexist. To explore this, a geometric model of a single fracture within dry hot rock (φ50 × 100 mm) was first developed using a 3D self-affine fractal function. Subsequently, a numerical model was constructed to account for the thermophysical property variations of the CO₂-water mixture within a pressure range of 30 MPa and temperatures between 200 °C and 330 °C. Key findings include: (1) An increase in fracture width results in non-linear variations in the temperature gradient between the inlet and outlet. Notably, the mixture containing 30 % H₂O and 70 % CO₂ exhibited the most significant reduction in temperature difference, with a maximum decrease of 0.9 °C. (2) Fracture width has a profound impact on heat extraction efficiency, with the 60 % H₂O and 40 % CO₂ mixture showing the highest increase in output thermal power. At a fracture width of 2 mm, coupled with higher flow velocities, this mixture achieved an output thermal power of 149.69 W. These results underscore the critical influence of H₂O mixing ratios on heat extraction power in CO₂-water coexisting regions, offering valuable insights into heat production processes within actual geothermal reservoirs.