随机裂隙网络储层与井筒热流耦合数值模拟

增强型地热系统(Enhanced Geothermal System,EGS)的渗流传热通道主要由注采井井筒与随机裂隙网络储层两大部分组成,过去对裂隙网络储层进行热流耦合模拟研究大多数都忽视了井筒壁的换热,导致模拟结果的准确性欠佳。为了更准确地评价EGS出力、寿命等性能指标,开展了井筒—随机裂隙网络储层热流耦合数值模拟研究,基于商业有限元软件COMSOL Multiphysics对井筒与储层的渗流场、温度场进行耦合求解,分析了影响EGS采出温度与热开采速率的各项要素。研究结果表明:(1)注、采井的开孔长度(L_0)对EGS的产能与寿命具有重要影响,400 m为最佳开孔长度,其EGS具有最佳出力与寿命;(2)在井筒壁上加保温材料可以有效提高开采初、前期的采出温度,减少热损失,提高开采速率;(3)随着开采的进行,注入井周围出现明显的低温区,并沿裂隙通道向采出井方向推移,这将导致系统达到开采寿命而衰竭,应停止开采一段时间后再进行热能开采;(4)裂隙渗透率、裂隙宽度等参数对开采速率的影响都呈现正相关性,参数值增大会提高开采速率、缩短开采年限。结论认为,井筒壁的换热对于EGS出力与寿命的综合评价具有重要的意义,考虑井筒壁热损失的井筒—热储耦合模拟能够实现对EGS的完整性评价。

Numerical simulation of the thermal–hydraulic coupling in wellbore and random fracture network reservoirs

The seepage and heat transfer channel of enhanced geothermal system (EGS) is mainly composed of the wellbore of injection and production wells and the random fracture network reservoirs. The previous thermal–hydraulic coupling simulation studies only focused on fracture network reservoirs, but ignored the heat transfer of wellbore wall, so the simulation results are less accurate. For more accurate evaluation of the performance indexes of EGS (e.g. output and life), the thermal–hydraulic coupling of wellbore and random fracture network reservoir was numerically simulated in this paper. Then, based on the commercial finite element software COMSOL Multiphysics, the coupling solution of seepage field and temperature field of wellbore and reservoir was conducted, and the factors affecting the recovery temperature and thermal mining rate of EGS were analyzed. And the following research results were obtained. First, the opening length (L0) of injection/production wells has an important effect on the productivity and life of EGS. The optimum opening length is 400 m, and its corresponding EGS has the optimum output and life. Second, installing thermal insulation materials on the wellbore wall can effectively increase the recovery temperature at the beginning and early stage of mining, reduce the heat loss and improve the mining rate. Third, as the mining goes, obvious low temperature zones occur around the injection well and advance to the production well along the fracture channel, and consequently the system will reach the recovery life and become exhausted. In this case, the thermal energy recovery shall be continued after stopping for a period of time. Fourth, the influence of fracture permeability and thickness on the thermal mining rate is in positive correlation. The increase of parameter values will lead to the increase of thermal mining rate but the reduction of mining life. It is concluded that the heat transfer of wellbore wall is of great significance to the comprehensive evaluation on EGS output and life, and the integrity evaluation on EGS can be realized by virtue of the wellbore–geothermal reservoir coupling simulation considering wellbore wall heat transfer.