Abstract:

In enhanced geothermal systems, fluids flow through rough rock fractures to obtain heat. A practical rock sample from Qiabuqia area in Gonghe Basin, Qinghai, China was used to obtain the real three-dimensional morphology in single fracture using high-precise scanning. This single fracture was used to propose a three-dimensional simulation model to numerically calculate convective heat transfer of water and CO₂. The effects of inlet fluid temperature, fluid flow rate, rock initial temperature, fracture openness and injection-production pressure difference on the heat extraction performance were analyzed. Increasing temperature difference between the fluid and the rock increased the heat transfer coefficient, and decreased the fluid outlet temperature. With increasing fluid injection flow, fracture openness and injection-production pressure difference, the heat transfer coefficient increases and the fluid outlet temperature decreased. The effective strength on heat transfer of fluid flow rate was the highest, followed by injection-production pressure difference and fluid inlet temperature. The fluid flow rate was increased from 10mL/min to 80mL/min, and the heat transfer quantity with water was increased from 109W to 351.2W, and the heat transfer coefficient was increased by 140.61W/(m²·K) for each increase of 1mL/min fluid flow rate.The heat transfer quantity with carbon dioxide was increased from 36.9W to 126.6W, and the heat transfer coefficient was increased by 19.84W/(m²·K) for each increase of 1mL/min fluid flow rate. H₂O with higher specific heat capacity and thermal conductivity carried out more heat than CO₂.

Key words: geothermal energy, single fracture, carbon dioxide, heat transfer, model

摘要：

向高温地质裂缝内注入冷流体进行流动取热是目前地热开发常用的一种方式。增强型地热系统中的流体多在粗糙不规律的岩石裂隙中流动换热。为研究流体的传热量，基于青海共和盆地恰卜恰干热岩储层，通过扫描获得真实的单裂隙三维形貌。采用有限元三维仿真模拟建立真实准确的单裂隙流动传热数值模型。对比分析了注入温度、注入流量、初始岩样温度、裂隙开度和注采压差对水和二氧化碳传热效果的影响。研究结果表明：流体与岩样之间的温差越大，换热效果越好，流体的出口温度越低；流体注入流量、裂隙开度及注采压差越大，换热效果越好，流体的出口温度越低；运行参数对换热效果的影响程度由高到低依次是注入流量、注采压差、注入温度。将注入流量从10mL/min提高至80mL/min，以水作为取热工质的取热量由109W提高至351.2W，注入流量每提高1mL/min传热系数提高140.61W/(m²·K)；以二氧化碳作为取热工质的取热量由36.9W提高至126.6W，注入流量每提高1mL/min传热系数提高19.84W/(m²·K)，具有较高比热容和热导率的水会携带更多的热量，换热效果更强。

关键词: 地热能, 单裂隙, 二氧化碳, 传热, 模型