Correction of the circulation depth of geothermal water based on temperature variation in the discharge section of geothermal system

Abstract

The circulation depth of geothermal water providing the geothermal system framework significantly dominates the evaluation of renewal capacity and geothermal resources. The traditional evaluation of circulation depth is based on the groundwater temperature variation in the recharge section and the average geothermal heating rate of geothermal system. However, misunderstanding groundwater temperature distribution in geothermal systems will lead to overestimating groundwater circulation depth based on the recharge section. Temperature measurement in a 1000 m geothermal scientific borehole from the Xinzhou geothermal field of south China is discussed as a case study to reassess the circulation depth of geothermal water. For Xinzhou geothermal system, the recharge and discharge temperatures are from 26.2 °C to 32.6 °C and from 67.0 °C to 98.0 °C, respectively. And the heat exchange temperature at the deepest point is from 121 ℃ to 154 ℃. This indicates that the temperature gradient in the recharge section should be greater than that in the discharge section. But the actual observation is opposite that the temperature gradient in the recharge section and in the discharge section is 3.04 ℃/100 m and 4.97 ℃/100 m, respectively. We proposed that the depth of geothermal water circulation evaluated by the temperature change and the geothermal heating rate in the discharge section represents the top depth of convection in the heat exchange zone, and the depth evaluated by the recharge section represents the advection depth of groundwater in the recharge section. The top depth of convection in the heat exchange zone estimated by the discharge Sect. (0.75–1.49 km) is much shallower than the advection depth of groundwater in the recharge Sect. (3.25–4.34 km). In the convective heat exchange zone (between 4.34 km and 1.49 km), the fault zone at a certain depth is the ideal location for geothermal development to extract water and heat.