干热岩孔隙率影响的 CO₂-EGS 耦合模型研究

Study on the CO ₂-EGS coupling model affected by porosity of hot dry rock

摘要

为了更真实、准确地模拟超临界 CO₂ 在增强型地热系统（enhanced geothermal system，EGS）中的对流换热特性以及热提取性能，考虑干热岩储层孔隙率、渗透率在热、流、固三场共同作用下的变化特性，提出了考虑干热岩储层孔隙率动态演化的 THM 耦合 CO₂-EGS 模型，并通过数值模拟方法对其进行实现。将数值计算结果与解析解进行对比，验证了该模型的有效性。对模型渗流场、温度场、应力场以及超临界 CO₂ 的热物性参数进行了研究与分析，阐明了超临界 CO₂ 与干热岩储层之间的相互作用机理。结果表明:干热岩孔隙率和渗透率随系统运行年限增加产生显著变化，且超临界 CO₂ 在 EGS 中能够达到较好的对流换热效果及热提取性能。随着超临界 CO₂ 在裂隙与储层基质中迅速扩散，基质温度不断降低，出现了“高温波峰现象”；基质孔隙压力变化和热收缩导致 EGS 储层基质产生位移，渗透率和孔隙率不断增加，超临界 CO₂ 在基质和主裂隙中的流速不断增大；储层应力不断重新分布，在系统运行 120 d 左右达到平衡；随着储层压力和温度的变化，超临界 CO₂ 的热物性参数产生较大变化，呈现非线性增长趋势。

Abstract

In order to achieve a more realistic and accurate simulation of the convective heat transfer characteristics and thermal extraction performance of supercritical CO₂ (SCCO₂) in the enhanced geothermal system (EGS)，this study investigates the dynamic evolution of porosity and permeability in hot dry rock reservoirs under the combined influence of heat，flow，and solid fields. A THM-coupled CO₂-EGS model that accounts for the dynamic evolution of porosity in hot dry rock reservoirs is proposed，and numerical simulation methods are employed to implement it. The effectiveness of this model is validated by comparing numerical results with analytical solutions. This study delves into and analyzes the internal connection between the multi-physics field and the thermal properties parameters of SCCO₂ to elucidate the interaction mechanism between SCCO₂ and hot dry rock reservoirs. The results show that SCCO₂ can achieve good convective heat transfer effect and thermal extraction performance in EGS. As SCCO₂ rapidly diffuses in fractures and matrix of the reservoir，the matrix temperature decreases continuously，thus leading to high-temperature peak phenomenon. Changes in matrix pore pressure and thermal shrinkage result in displacement of EGS reservoir matrix，increasing permeability and porosity continuously while the flow velocity of SCCO₂ increases in both matrix and main fractures. Reservoir stress redistributes continuously until reaching an equilibrium after approximately 120 days of system operation. With changes in reservoir pressure and temperature，significant variations occur in thermal properties parameters of SCCO₂，which show a non-linear growth trend.

关键词

干热岩 / 孔隙率 / 多场耦合 / EGS / 超临界 CO₂ / 对流换热

Key words

hot dry rock / porosity / multi-field coupling / EGS / SCCO₂ / convective heat transfer

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基金资助

国家自然科学基金资助项目(51974186)

国家自然科学基金资助项目(52274203)

国家自然科学基金资助项目(52204218)