Numerical simulation of heat extraction in single-well enhanced geothermal system based on thermal-hydraulic-chemical coupling model

doi:10.16085/j.issn.1000-6613.2022-0754

Abstract

Abstract:

The minerals dissolution and precipitation in geothermal water has a significant impact on geothermal resource development. Based on the control equation of water-SiO₂ reaction rate, a thermal-hydraulic-chemical (THC) coupling numerical model of single-well enhanced geothermal system (SEGS) was established. The accuracy of flow heat transfer model and solute transport model was validated by the analytical solution. The coupling relationships between reservoir temperature, SiO₂ concentration, reaction rate and fracture width were simulated and analyzed. Based on the discrete fracture model, the effects of injection mineral concentration and injection flow rate on the heat transfer performance of SEGS were investigated. The results showed that, ① mineral precipitation caused by over-saturated injection and mineral dissolution caused by under-saturated injection occured at a certain distance away from the injection section, the evolution of fracture aperture was distributed as a band at a fracture plane, and the peak value of reaction rate moved to the periphery with time; ② the maximum differences between injection pressures and production temperatures obtained from the coupling model of TH and THC were 2.12MPa and 4.66℃, respectively. Under-saturated injection reduced the injection pressure and accelerated the attenuation of production temperature, while over-saturated injection was the opposite; ③ when the SiO₂ concentration increased by 0.005mol/kg, the average injection pressure increased by 0.5MPa and the average recovery temperature increased by 0.4℃. When the injection flow increased from 8kg/s to 12kg/s, for over-saturated injection, the injection pressure increased by 7.2MPa and the production temperature decreased by 8.58℃ after 30 years; for under-saturated injection, the injection pressure increased by 5.14MPa and the production temperature decreased by 10.43℃.

Key words: geothermal energy, thermal-hydraulic-chemical coupling, porous media, permeability, mineral concentration

摘要：

地热流体中的矿物溶解沉淀对地热资源开发具有重要影响。本文基于水-SiO₂反应速率控制方程，建立了单井增强地热系统（SEGS）热-流-化（THC）耦合数值模型，利用解析解验证了流动换热模型和溶质运移模型的准确性，模拟分析了储层温度、SiO₂浓度、反应速率和裂隙宽度之间的耦合关系。基于离散裂隙模型，分析了注入矿物浓度和注入流量对SEGS取热性能的影响。结果表明：① 过饱和注入引起的矿物沉淀与欠饱和注入引起的矿物溶解均发生在远离注入段的一定距离处，裂隙宽度的改变呈带状分布，反应速率峰值随时间推移向外围移动；② 由热-流（TH）与热-流-化（THC）耦合模型得出的注入压力和采出温度最大相差2.12MPa和4.66℃。欠饱和注入降低了注入压力，加快了采出温度的衰减，过饱和注入则与之相反；③ 注入流体中的SiO₂浓度每增加0.005mol/kg，30年平均注入压力增加0.5MPa，平均采出温度增加0.4℃。将注入流量从8kg/s提高至12kg/s，对于过饱和注入，系统运行30年后的注入压力提高7.2MPa，采出温度下降8.58℃；对于欠饱和注入，注入压力提高5.14MPa，采出温度下降10.43℃。

关键词: 地热能, 热-流-化耦合, 多孔介质, 渗透率, 矿物浓度

CLC Number:

TK529

GUO Zhipeng, BU Xianbiao, LI Huashan, GONG Yulie, WANG Lingbao. Numerical simulation of heat extraction in single-well enhanced geothermal system based on thermal-hydraulic-chemical coupling model[J]. Chemical Industry and Engineering Progress, 2023, 42(2): 711-721.

郭志鹏, 卜宪标, 李华山, 龚宇烈, 王令宝. 基于热-流-化耦合作用的单井增强地热系统性能分析[J]. 化工进展, 2023, 42(2): 711-721.

Figures/Tables 22

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参数	数值
注入浓度/mol·kg	1
地下水流速/ m·d^-1	1
孔隙率	1
扩散系数/m²·s^-1	10^-9
纵向弥散系数/m	100
横向弥散系数/m	10
注入区域半径/m	44

参数	数值
注入浓度/mol·kg	1
地下水流速/ m·d^-1	1
孔隙率	1
扩散系数/m²·s^-1	10^-9
纵向弥散系数/m	100
横向弥散系数/m	10
注入区域半径/m	44

参数	数值
岩石密度/kg·m^-3	2700
岩石比热容/J·kg^-1·℃^-1	1000
岩石热导率/W·m^-1·℃^-1	2.8
裂隙宽度/m	0.001
注射速度/m·s^-1	0.01
初始温度/℃	80
注入温度/℃	30

参数	数值
岩石密度/kg·m^-3	2700
岩石比热容/J·kg^-1·℃^-1	1000
岩石热导率/W·m^-1·℃^-1	2.8
裂隙宽度/m	0.001
注射速度/m·s^-1	0.01
初始温度/℃	80
注入温度/℃	30

参数	压裂储层	裂隙
密度/kg·m^-3	2600	2000
比热容/J·kg^-1·℃^-1	1000	800
热导率/W·m^-1·℃^-1	3	2.8
渗透率/m²	3×10^-15	5×10^-11
孔隙度	0.15	1
单位质量岩石SiO₂摩尔数/mol·kg^-1	10	10
溶质扩散系数/m²·s^-1	10^-9	10^-9