Numerical simulation of influence of reservoir characteristics on heating process of enhanced geothermal system of horizontal well multi fractures
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摘要:
将石油天然气行业发展很成熟的水平井多裂隙开发技术用于增强型地热系统(EGS)可显著提高EGS的经济效益。本研究建立了三维EGS水平井多裂隙物理模型, 采用CFX模拟分析了在井间距以及裂隙间距等不同储层特征条件下EGS的运行性能, 揭示了不同储层特征对于EGS储层采热过程的影响机理。研究结果表明: ①裂隙间距是影响EGS工程运行寿命和开采率的关键因素, 在相同注水流量下, 较大的裂隙间距不易形成热穿透, 系统运行寿命更长, 但降低了储层开采率; 过小的裂隙间距易形成热穿透, 系统运行寿命短, 但开采率高。②井间距对裂隙中的流体流速影响显著, 随着井间距增加, 在相同开采时间下, 产流流体的温度不断升高, 系统的寿命也会随着井间距的增加而增加, 井间距的增大也意味着储层的体积也就更大, 从而有更多的热量可供开采, 因而提高了系统的运行寿命。研究结果可以为EGS储层的建造提供理论指导, 为实现商业化开采地热能做好理论准备。
Abstract:The horizontal well multi-fracture development technology, which is well-developed in the oil and gas industry, can significantly improve the economics of enhanced geothermal systems (EGS). In this paper, a three-dimensional horizontal-well multi-fracture physical model is established for EGS. The performances of EGS under different reservoir characteristics of well spacing and fracture spacing are analyzed by CFX simulation, and the influence mechanism of different reservoir characteristics on the heat recovery process of the EGS reservoir is revealed. The results show that: ①The fracture spacing is the key factor affecting the operating life and mining rate of EGS engineering. Under the same water injection flow, the larger the fracture spacing, the smaller the possibility of forming thermal permeability, the longer the system operation life, but the lower the reservoir recovery rate. The smaller the fracture spacing, the higher the thermal permeability, the shorter the system life, and the higher the reservoir recovery.②The well spacing has a significant effect on the fluid velocity in the fracture. As the well spacing increases, the temperature of the flow-producing fluid increases continuously during the same mining time, and the life of the system also increases. The increase in well spacing also means that the reservoir volume becomes large, so there is more geothermal energy for exploitation and longer life for the system for operating. The research can provide theoretical guidance for the construction of EGS reservoirs and provide theoretical preparation for the commercial exploitation of geothermal energy.
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图 2 算例1，2与竖井模式下解析解TD随时间的变化
Figure 2. Evolution of TD for cases 1, 2 and analytical solution under vertical well mode
图 3 不同裂隙间距情况下截止温度为423.15 K时热储层等温度体积图(333.15~423.15 K)
Figure 3. Isothermal (333.15-423.15 K) volume of thermal reservoir under different fracture spacing for the cut-off temperature is 423.15 K
图 4 不同裂隙间距产流温度随时间的变化曲线
Figure 4. Evolution of production temperature for different fracture spacing
图 5 产流截止温度为393.15 K时不同裂隙间距对储层开采率的影响
Figure 5. Influence of different fracture spacing on reservoir production rate when the cut-off temperature of production temperature is 393.15 K
图 6 算例1，3，4在寿命期内EGS发电功率的变化情况
Figure 6. Evolution of power generation of EGS during the life of the case 1, 3, 4
图 7 算例4，5，6设置下裂隙中z=0处剖面(x-y平面)的速度分布
Figure 7. Velocity distribution of the z=0 cross section (x-y plane) in the fracture of cases 4, 5, 6
图 8 不同井间距(裂隙尺寸)下产流温度随时间的变化
Figure 8. Variation in the production temperature with time under different well spacings (fracture sizes)
表 1 流体与岩石的物理参数
Table 1. Physical properties of fluid and rock
参数 名称 数值 ρs 岩石密度/(kg·m-3) 2 650 λs 岩石热导率/(W·m-1·K-1) 3.49 Cs 岩石比热容/(J·kg-1·K-1) 920 ρf 水的密度/(kg·m-3) 900 λf 水的热导率/(W·m-1·K-1) 0.606 9 Cf 水的比热容/(J·kg-1·K-1) 4 181.7 μ 高温下水的动力黏度/(mPa·s) 0.3 k 裂隙渗透率/m2 3×10-9 γ 裂隙孔隙度 1 W 裂隙开度/m 0.002 a 代表性的粗糙高度/m 0.014 L1 裂隙高度/m 400 L2 裂隙长度/m 400 Tinj 注水温度/K 333.15 D 岩体单元厚度/m 100 
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表 2 算例设置
Table 2. Design of cases
算例 储层温度分布：平均温度473.15 K 流量q/(kg·s-1) 裂隙间距/m 井间距/m 上表面温度/K 下表面温度/K 温度梯度/(K·100 m-1) 1 461.15 485.15 6 8 100 495.0 2 473.15 473.15 无 8 100 495.0 3 461.15 485.15 6 8 80 495.0 4 461.15 485.15 6 8 50 495.0 5 461.15 485.15 6 8 50 636.4 6 461.15 485.15 6 8 50 777.8
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