Reassessment of the depth of groundwater circulation in geothermal systems

doi:10.13745/j.esf.sf.2024.12.83

Abstract

Abstract:

The depth of geothermal water circulation is a crucial parameter for assessing geothermal resources and their renewability. Traditional methods estimate this depth based on the temperature change in the groundwater recharge section and the average geothermal gradient of the system. However, the temperature distribution differs between the recharge and discharge sections of a geothermal system, leading to an overestimation of circulation depth when based on the recharge section alone. In convective geothermal systems, temperature varies significantly in the recharge section but is relatively stable in the discharge section. This implies that the geothermal gradient in the recharge section should theoretically be steeper than in the discharge section. Contrary to this expectation, field observations-such as those from the Xinzhou geothermal field in Yangjiang, Guangdong, where the gradient is 3.04 ℃/100 m in the recharge section compared to 4.97 ℃/100 m in the discharge section-show the opposite. This discrepancy likely occurs because the temperature-depth profile in the recharge section does not strictly follow the field’s geothermal gradient. In contrast, the discharge section is close to the heat exchange zone, where deep heat sources and thermal convection maintain high water temperatures. The temperature drop here is primarily controlled by heat conduction to the surrounding rock (or cooling due to adiabatic expansion). Consequently, the temperature-depth relationship in the discharge section follows the local gradient, which is more representative of the background geothermal gradient. Therefore, the circulation depth calculated from the temperature change in the discharge section and its gradient represents the depth to the top of the convective zone in the heat exchange area. In contrast, the depth estimated from the recharge section reflects the maximum flow depth of groundwater in that section. A case study of the Xinzhou geothermal field illustrates this: the depth to the top of the convection zone estimated from the discharge section (0.75-1.49 km) is much shallower than the flow depth derived from the recharge section (3.25-4.34 km). The fault zone within this convective heat exchange zone (between 1.49 km and 4.34 km) represents an ideal target for geothermal development for water and heat extraction.

Key words: groundwater circulation depth, temperature, geothermal heating rate, the discharge section, Xinzhou geothermal field

CLC Number:

P314.3

MAO Xumei, LI Cuiming. Reassessment of the depth of groundwater circulation in geothermal systems[J]. Earth Science Frontiers, 2025, 32(5): 220-229.

Figures/Tables 6

Fig.1 The diagram of vertical temperature distribution features in wall rock

Fig.2 The diagram of groundwater temperature distribution in convective hydrothermal system

Fig.3 Hydrogeological map and sampling distribution of Xinzhou geothermal field

Table 1 The circulation depths of geothermal water in Xinzhou geothermal field estimated with the recharge section (traditional method) and with the discharge section (new insight proposed in this paper), respectively

样品	类型	热交换温度/℃	循环深度 (传统方法)/km	混合前地热水温度/℃	热交换区到混合区迁移距离/km	热交换区对流顶部深度 (新认识)/km
HW1	热井	140	3.88	124	0.32	1.02
HW2	热井	148	4.14	126	0.44	1.14
HW3	热井	154	4.34	131	0.46	1.15
HW4	热井	153	4.31	132	0.42	1.12
HW5	热井	144	4.01	116	0.56	1.26
HS1	热泉	138	3.81	116	0.45	1.15
HS2	热泉	132	3.62	128	0.09	0.79
HS3	热泉	123	3.32	120	0.06	0.75
HS4	热泉	125	3.38	86	0.79	1.49
HS5	热泉	134	3.68	97	0.74	1.44
HS6	热泉	121	3.25	112	0.18	0.88

Fig.4 Comparison of groundwater circulation depth evaluated by new and old methods in Xinzhou geothermal field

Fig.5 The flow-temperature model for fault-dominated convective geothermal systems

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