Hydraulic Fracturing Characteristics and Fracture Mechanism of Granite under High Temperature and Pressure
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摘要:
地热能作为一种分布广泛的可再生资源，已成为人类未来能源安全的重要保障。干热岩型地热能因其储量巨大和清洁、高效、可再生的特点具有重大的开采价值。目前，干热岩开采通常需要通过水力压裂来制造渗流换热通道，因此，储层水力压裂特征和破裂机制研究对于干热岩资源的高效开采具有重要意义。笔者以常见的干热岩储层——花岗岩为研究对象，研究了不同围压、温度下水力压裂特征；分析了压裂过程中声发射特征演化规律；阐明了裂隙扩展规律及水力压裂影响因素；揭示了水力压裂破裂机制和裂隙扩展准则。实验结果表明：①常温和高温下花岗岩的水力压裂曲线均可以分为管孔充水、管孔增压、水力压裂、裂隙扩展4个阶段。②压裂后的裂隙均沿着管孔的深度方向扩展，随着围压的增加，裂隙长度增大，裂隙路径更为复杂，裂隙扩展类型包括沿晶断裂和穿晶断裂。③水力压裂特征受围压和温度的影响明显，温度一定时，围压越大，破裂压力越大，二者的关系近似呈线性；围压一定时，温度越高破裂压力越大，但这种相关性并非线性。④花岗岩水力压裂主导破裂机制为张拉破裂，但裂隙仍存在剪切破裂特征；破裂压力受围压、温度和岩石材料性质的多重影响。研究结果可为花岗岩热储层的压裂设计提供支撑。
Abstract:Geothermal energy, as a widely distributed renewable resource, has become a crucial safeguard for humanity's future energy security. Enhanced geothermal systems (EGS), particularly those involving hot dry rock (HDR), hold significant exploitation value due to their vast reserves and characteristics of cleanliness, high efficiency, and renewability. Currently, the extraction of HDR typically requires hydraulic fracturing to create permeable flow channels for heat exchange. Therefore, research on the hydraulic fracturing characteristics and fracture mechanisms of reservoirs is of great importance for the efficient exploitation of HDR resources. This study focuses on granite, a common HDR reservoir, to investigate the hydraulic fracturing characteristics under varying confining pressures and temperatures. The evolution of acoustic emission (AE) characteristics during the fracturing process was analyzed, the fracture propagation patterns were elucidated, the influencing factors of hydraulic fracturing were clarified, and the fracture mechanisms and crack propagation criteria of hydraulic fracturing were revealed. The experimental results demonstrate that: (1) The hydraulic fracturing curves of granite under both room temperature and high temperature can be divided into four stages: borehole water filling, borehole pressure increase, hydraulic fracturing, and fracture propagation. (2) Post-fracturing cracks predominantly propagate along the borehole's depth direction. As the confining pressure increases, the crack length extends, and the fracture path becomes more complex. The fracture propagation modes include intergranular and transgranular fractures. (3) Hydraulic fracturing characteristics are significantly influenced by confining pressure and temperature. At a constant temperature, the breakdown pressure increases with higher confining pressure, exhibiting an approximately linear relationship. Under constant confining pressure, the breakdown pressure rises with increasing temperature, though this correlation is nonlinear. (4) The dominant fracture mechanism in granite hydraulic fracturing is tensile failure, though shear failure characteristics are also observed. The breakdown pressure is influenced by multiple factors, including confining pressure, temperature, and the material properties of the rock. The findings of this study can provide theoretical support for the fracturing design of granite-based geothermal reservoirs.
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Key words:
- hot dry rock (HDR) /
- hydraulic fracturing /
- fracture propagation /
- fracture mechanism
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