热储层水力压裂低温诱导热应力致裂机理研究

Thermal stress cracking induced by low temperature is the key factor affecting the exploitation performance of Enhanced Geothermal Systems (EGS). At present, the mechanism of low-temperature-induced thermal stress cracking in hydraulic fracturing for thermal reservoirs is still ambiguous, resulting in the invalidation to carry out effective hydraulic fracturing schemes design and accurate heat mining performance evaluation. In this subject, employing high-temperature specimens to carry on core permeability tests, meso-structure damage and mechanical parameters tests under the low-temperature-induced condition intend to reveal the mechanism of meso-structure damage and the evolution of macroscopic physical and mechanical properties caused by low-temperature-induced thermal stress cracking under different conditions. Then physical simulation experiments of hydraulic fracturing with large size and high-temperature specimens will be carried out. Combined with the high-energy CT scanning technique, the influence of different factors on the scale and scope of low-temperature-induced thermal stress cracking in hydraulic fracturing process will be investigated. Finally, the thermo-hydro-mechanical-damage (THMD) model will be established based on the experimental data and theoretical research, the fracture propagation numerical simulation of hydraulic fracturing in EGS will be carried out, and the fracture propagation mechanism of hydraulic fracturing in thermal reservoir will be revealed. Combining the laboratory experiments, theoretic analyses and numerical simulations, the mechanism of thermal stress cracking induced by low temperature in process of EGS will be revealed. The simulation technology of hydraulic fracturing in thermal reservoirs will be further developed to provide more reliable theoretical basis and technical support for the design of EGS and the calculation of heat mining performance, and also provide new ideas for improving the theory of hydraulic fracture network creation.

低温诱导热应力致裂是影响增强型地热系统开发效果的关键因素，但因机理不明确，导致难以有效地开展水力压裂效果预测和施工方案优化设计。课题拟通过高温试件低温诱导条件下渗透率、细观结构损伤及岩石力学参数实验测试，揭示不同条件下低温诱导热应力致裂行为引发的岩石细观结构损伤机制及宏观物理力学性质演变特征；通过开展大尺寸高温试件水力压裂物理模拟实验，借助高能CT扫描技术，探究水力压裂过程中不同因素对于低温诱导热应力致裂尺度与规模的影响；基于实验分析和理论研究，构建温度-渗流-应力-损伤（THMD）耦合模型，开展热储层水力压裂裂缝扩展数值模拟研究，阐明热储层水力压裂裂缝扩展机制。结合实验测试、理论分析和数值模拟研究，揭示热储层水力压裂低温诱导热应力致裂机理，进一步发展热储层水力压裂改造模拟技术，为增强型地热系统的开发方案设计及效果预测提供理论依据和技术支持，为完善压裂缝网形成理论提供新视角。

低温诱导热应力致裂是影响增强型地热系统开发效果的关键因素，但因机理不明确，导致难以有效地开展水力压裂效果预测和施工方案优化设计。课题通过高温试件低温诱导条件下渗透率、微观孔喉结构及岩石力学参数实验测试，明确了不同处理条件下低温诱导热应力致裂行为对岩石物理力学参数的影响程度及其增渗机制；基于细观损伤力学，构建了温度-渗流-应力-损伤（THM-D）耦合模型，并开展了高温试件水力压裂物模实验对所建立的耦合模型及其求解方法进行验证；采用已验证正确性的模型，首先对热应力在水力压裂过程中的作用机理开展研究，明确了低温诱导热应力与注入水压力的相互作用关系。随后，开展了热应力影响下的高温岩石水力压裂数值模拟研究，分析各影响因素对裂缝萌生与扩展形态的影响规律。研究结果表明：由于低温诱导热应力的作用，急剧冷却处理温度升高，岩石损伤加剧，渗流能力增强、力学性能降低。岩石矿物组成非均质性越强、矿物颗粒几何形状越不规则、岩石杨氏模量越大都有利于岩石发生热应力致裂。在高温岩石水力压裂实验中，基岩温度升高岩石破裂压力降低。以不加载轴压条件为例，基岩温度为50℃时，破裂压力为16.2MPa，当温度升高至200℃时，破裂压力降低为10.5MPa。高温岩石水力压裂过程中，不断增加的注入压力与低温诱导热应力均以拉应力作用在基岩上，在两者的共同作用下裂缝开始起裂。基岩温度的升高、换热系数的增加、杨氏模量的增大、非均质性的增强都可导致更复杂的裂缝网络。对于含裂隙高温岩石的压裂，形成的次生裂缝垂直于天然裂缝。裂缝导流能力和裂缝闭合压力对次生裂缝的形成具有重要影响。Kf=360×10-14m2时产生的损伤破坏数较Kf=120×10-14m2时提升67.9%。结合实验测试、理论分析和数值模拟研究，揭示热储层水力压裂低温诱导热应力致裂机理，进一步发展热储层水力压裂改造模拟技术，为增强型地热系统的开发方案设计及效果预测提供理论依据和技术支持，为完善压裂缝网形成理论提供新视角。