Exergy analysis of heat extraction from hot dry rock by enclosed Water recycling in a horizontal Well

In this paper, a novel mathematical model for the heat extraction process from hot dry rocks (HDRs) by enclosed water recycling in a horizontal well is established, on a basis of which a series of exergy analyses are conducted. The pressure and temperature distributions in the wellbore and the exergy extracted under different working conditions are calculated. Eight factors are specifically studied to evaluate their effects on the accumulated exergy of the produced hot water. It is found that the accumulated exergy first gradually increases but decreases subsequently with the water rising flow rate. The accumulated exergy is noticed to increase obviously with the increase of the length for the horizontal section, temperature of the HDR, thermal conductivity of the HDR, and diameter of the casing. The temperature distributions in the HDR around the wellbore are analyzed at different time. More specifically, the temperature drop of the HDR gradually spreads to far area of the wellbore with the continuous extraction of geothermal energy. The temperature of the rock around the wellbore decreases by increasing the injection rate. A higher HDR thermal conductivity leads to a quick heat transfer from the remote to near wellbore area. The exergy analyses in this study provide strong theoretical supports to utilize the HDR heat extraction.