地热系统中地下水循环深度的重新评估

doi:10.13745/j.esf.sf.2024.12.83

摘要/Abstract

摘要：

地热水循环深度是地热研究中的重要参数,在评估地热资源及其更新能力中起到重要作用。传统方法评估地热水循环深度基于补给段地下水的温度变化和地热系统的平均地热增温率。然而,由于地热系统中地下水补给段和排泄段的温度分布并不一致,基于补给段的地下水循环深度被高估。对流型地热系统的地下水温度分布特征表明:补给段的温度变化大,排泄段的温度变化小。这意味着补给段的地热增温率理论上应该大于排泄段的地热增温率。但实际观察结果恰恰相反,例如广东阳江新洲地热田补给段的地热增温率为3.04 ℃/100 m,排泄段的地热增温率为4.97 ℃/100 m。可能的原因是,补给段地下水温度随深度的变化并没有遵循地热田的地热增温率。排泄段靠近热交换区,深部热源和热对流使地热水保持高温,地下水温度主要受围岩热传导(或绝热冷却)的影响而降温,其变化遵循排泄段的地热增温率。因此,用排泄段地热水的温度变化和地热增温率评估的地热水循环深度代表热交换区对流顶部深度,用补给段评估的深度代表补给段地下水平流深度。在广东阳江新洲地热田研究实例中刻画了地下水各种深度的含义,用排泄段估算的热交换区对流顶部深度(0.75~1.49 km)比补给段地下水平流深度(3.25~4.34 km)要浅得多,对流热交换区(4.34~1.49 km)中某一深度的断裂带才是地热开发取水取热的理想位置。

关键词: 地下水循环深度, 温度, 地热增温率, 排泄段, 新洲地热田

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

中图分类号:

P314.3

毛绪美, 李翠明. 地热系统中地下水循环深度的重新评估[J]. 地学前缘, 2025, 32(5): 220-229.

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

图/表 6

图1 围岩内温度垂直分布特征示意图 A—地下水下降;B—地下水上升;C—无水运动。

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

图2 对流型水热系统中地下水温度分布示意图

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

图3 新洲地热田水文地质图及采样分布

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

表1 分别采用补给段(传统方法)和排泄段(本文提出的新认识)估算的新州地热田地热水循环深度

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

图4 新洲地热田地下水循环深度对比图

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

图5 断裂控制的对流型地热系统水流-温度模型

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

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