Study On Heat Extraction Performances Of Hot Dry Rock

The study of heat transfer in the wellbore of enhanced geothermal systems is mostly done by calculating the wellbore part separately from the reservoir part and integrating the calculation results;the study of heat extraction of enhanced geothermal systems is mostly done by using a single fluid,and the study of the heat extraction performance of mixed fluids as working fluids when mixed in different proportions is rare.In this paper,we address these issues.A three-dimensional discrete network fracture model was constructed according to the Monte Carlo method and directly coupled with the wellbore model to study the effects of injection temperature,mass flow rate,thermal conductivity of the rock matrix,specific heat capacity,thermal conductivity of the wellbore and ground temperature gradient on the heat extraction performance of the enhanced geothermal system as well as the temperature distribution inside the wellbore and the heat transfer through the wellbore wall.The results show that:（1）increasing ground temperature gradient,mass flow rate,and reducing thermal conductivity of the outer wall of the wellbore are beneficial to the production temperature;reducing injection temperature,wellbore thermal conductivity,and increasing mass flow rate and temperature gradient can increase the heat extraction amount.（2）The thermal conductivity of the outer wall of the wellbore has the greatest influence on the percentage of temperature rise in the wellbore.Decreasing the injection temperature,temperature gradient and mass flow rate and increasing the wellbore thermal conductivity,rock matrix specific heat capacity and rock matrix thermal conductivity will result in an increase in the percentage of temperature rise in the wellbore.（3）Both production temperature and heat extraction increase to some extent with increasing system operating time.A parameterized surface fracture model is constructed in present work.The temperature variation,heat extraction performance and flow pump work consumption of nine different working fluids including SCCO₂-He mixture,He,and SCCO₂ in the fracture channel are numerically probed.To take into account the heat transfer and flow resistance of the working fluid in the fissure channel,the ratio of heat extraction to flow pump work consumption is used to evaluate the comprehensive performance of the working fluid,the comparison and analysis are performed among considered working fluids.The results show that under the same condition of inlet flow rate and temperature,when the mass fraction of He is 10%and 20%in mixed-working fluids,the production temperature of the mixed-working fluids is higher than that of the SCCO₂,but the heat extraction performance is lower than that of SCCO₂.The outlet temperature of rest mixed-working fluids decreases with the augment of He content in mixed-working fluids,while the heat extraction performance is improved.Considering the heat transfer performance and flow resistance of the working fluid comprehensively,the performance of SCCO₂ is optimal.