干热岩是一种绿色低碳，储量丰富的清洁能源，具有稳定性高、环境兼容性好、效率高等优点。由于干热岩储层具有低孔低渗且存在较多天然裂缝的特点，往往采用酸化压裂技术提高储层的连通性。干热岩酸化压裂过程多场耦合效应明显，储层温降对岩石力学性质的影响以及压裂裂缝的扩展延伸规律是人工热储建造过程中的关键问题。

针对以上问题，本文以青海共和盆地的花岗岩为研究对象，通过物模实验研究了不同温度热处理作用后花岗岩孔渗导热特征、表面微观结构以及力学性能的变化规律。同时通过数值模拟研究了酸化压裂裂缝的扩展延伸规律以及人工裂缝与天然裂缝的交互扩展规律。本文主要工作内容和成果如下:

（1）      对不同温度热处理的花岗岩进行了孔隙率、渗透率、导热系数测试以及表面微观结构观测实验。结果表明：随热处理温度升高，花岗岩的孔隙度、渗透率逐渐增大，当热处理温度超过300℃后，花岗岩的导热系数大幅下降；450℃热处理后花岗岩的表面微观结构劣化明显，并产生次生裂缝，这解释了花岗岩在某一温度阈值内孔渗导热以及力学特性的大幅变化。

（2）      对不同温度热处理以及酸化处理后的花岗岩进行单轴三轴加载实验。结果表明：随热处理温度的升高，花岗岩的岩石骨架劣化程度增大，三轴加载时出现更明显的粉碎性破坏特征，花岗岩的弹性模量以及抗压强度都在350℃-450℃内出现剧烈下降；酸化处理后花岗岩表面变得粗糙同时有些区域出现了微裂缝，导致了花岗岩力学性能的下降。

（3）      基于相场法构建了裂缝扩展控制方程，建立了干热岩压裂热流固化耦合数学模型，模拟了酸化压裂下裂缝扩展延伸的力学行为。结果表明：酸化压裂显著增加了裂缝的宽度，对起裂压力的影响不明显；射孔角度以及地应力差对裂缝起裂偏转角影响显著，二者增大时，裂缝起裂扩展需要克服更多阻力，起裂压力增大，裂缝的起裂扩展速度变慢；人工裂缝与天然裂缝交互扩展形态受地应力差以及天然裂缝与最大主应力夹角的影响，在较低地应力差条件下，天然裂缝更易开启使裂缝扩展转向。

（4）      通过对多级压裂的地热能储层进行热能开采模拟，研究了注入井压强、温度、主裂缝宽度以及副裂缝密度对储层输出响应的影响。结果表明：注入井温度和副裂缝密度的变化对生产井输出的影响大致呈线性变化；注入井压强和主裂缝宽度为影响储层输出响应的关键因素，过度增大主裂缝宽度会导致生产井热突破提前。

Dry hot rock is a kind of green low-carbon, abundant reserves of clean energy, with high stability, good environmental compatibility, high efficiency. Due to the characteristics of low porosity, low permeability and many natural fractures in hot and dry rock reservoirs, acidizing fracturing technology is often used to improve reservoir connectivity. The multi-field coupling effect is obvious in the acidizing and fracturing process of hot and dry rock. The influence of reservoir temperature drop on rock mechanical properties and the law of fracture expansion and extension are the key issues in the construction of artificial heat reservoir.

To solve the above problems, this paper takes the granite in Gonghe Basin of Qinghai province as the research object, and studies the pore permeability and thermal conductivity characteristics, surface microstructure and mechanical properties of the granite after heat treatment at different temperatures through physical model experiments. At the same time, the expansion and extension law of acidizing fracture and the interactive expansion law of artificial fracture and natural fracture are studied by numerical simulation. The main contents and achievements of this paper are as follows:

(1) The porosity, permeability, thermal conductivity and surface microstructure observation experiments of granite heat treated at different temperatures were carried out. The results show that the porosity and permeability of granite gradually increase with the increase of heat treatment temperature. When the heat treatment temperature exceeds 300℃, the thermal conductivity of granite decreases significantly. After heat treatment at 450℃, the surface microstructure of the granite is obviously deteriorated and secondary cracks are generated, which explains the substantial changes in the porosity and thermal conductivity and mechanical properties of the granite at a certain temperature threshold.

(2) Uniaxial triaxial loading experiments are carried out on the granite after heat treatment and acidification at different temperatures. The results show that: with the increase of thermal treatment temperature, the deterioration degree of the granite rock skeleton increases, and more obvious smashing failure characteristics appear during triaxial loading. The elastic modulus and compressive strength of the granite both decrease sharply within 350℃-450℃. After acidification, the surface of granite becomes rough and micro-cracks appear in some areas, which leads to the deterioration of the mechanical properties of granite.

(3) Based on the phase field method, the governing equation of fracture propagation is constructed, and the coupled mathematical model of heat flow solidification in dry hot rock fracturing is established to simulate the mechanical behavior of fracture propagation and extension under acidizing fracturing. The results show that the acidizing fracturing significantly increases the fracture width, but has no significant effect on the initiation pressure. The effects of perforation Angle and ground stress difference on crack initiation and deflection Angle are significant. When they increase, the crack initiation and propagation need to overcome more resistance, the crack initiation pressure increases, and the crack initiation and propagation speed slows down. The interactive propagation patterns of artificial fractures and natural fractures are affected by the difference of in-situ stress and the Angle between natural fractures and the maximum principal stress. Under the condition of lower in-situ stress difference, natural fractures are easier to open and turn the propagation of fractures.

(4) The effects of pressure, temperature, width of main fracture and density of secondary fracture on the output response of the reservoir were explored through the simulation of thermal energy production of the multistage fractured geothermal energy reservoir. The results show that the influence of inlet well temperature and secondary fracture density on production well output is roughly linear. Injection well pressure and main fracture width are the key factors affecting reservoir output response. Excessive increase of main fracture width will lead to premature thermal breakthrough in production well.