液氮作用下干热岩储层缝网形成机理及数值模拟研究

Hot dry rock geothermal energy is a clean and renewable energy source, which is of great significance to optimizing China’s energy structure. Due the low permeability, hot dry rock reservoir can be only developed economically by fracturing treatment to form a space fracture networks with large heat transfer area. The conventional fracturing technologies are difficult to meet the requirement, and therefore, it is urgent to explore new fracturing methods that can improve the effective heat transfer area of hot dry rock reservoir. Using liquid-nitrogen as fracturing fluid, it may have positive effects on the formation of fracture networks in the following aspects: ① hot dry rocks are quenched, which can induce a large number of micro-cracks, and then to increase the potential for forming complex fracture networks; ② gasification of liquid-nitrogen can increase the fluid pressure within fractures, which may promote the shear slip of natural fractures and accelerate the propagation of micro-cracks; ③ the cooling shrinkage of rock mass reduces the effective normal stress acting on the natural fracture surfaces, which may also promote the shear slip of natural fractures. Based on the interaction between liquid-nitrogen and hot dry rock, we propose to develop a 3D, fully coupled thermo-hydro-mechanical finite element model for accurately modeling the initiation and propagation of hydraulic fractures in naturally fractured hot dry rock reservoirs. The mechanical mechanism of promoting fracture network formation and increasing effective heat transfer area under the effects of liquid-nitrogen are revealed, which will provide theoretical basis for improving the efficiency of hydraulic fracturing treatment in hot dry rock reservoirs.

干热岩地热能是一种清洁的可再生能源，其成功开发对优化我国的能源结构具有重要意义。由于渗透率极低，干热岩储层必须经过压裂改造形成具有高有效换热面积的空间缝网才能实现商业开采，而常规的压裂技术尚难以完全满足要求，因此亟待探索能够提高干热岩热储有效换热面积的压裂新方法。若尝试采用液氮作为压裂液对干热岩储层进行改造，可能在以下方面对缝网的形成产生积极作用：①高温岩石骤冷，形成大量微裂缝，可提高复杂缝网形成的可能性；②液氮受热气化膨胀，提高缝内流体压力，可促进天然裂缝剪切滑移，并加速微裂缝扩展；③岩体冷却收缩，使天然裂缝面上的有效正应力降低，也可促进天然裂缝剪切滑移。因此，本研究拟通过实验掌握液氮-干热岩的相互作用规律，发展模拟裂缝性干热岩储层液氮压裂水力裂缝起裂与扩展的流-固-热耦合有限元方法，揭示液氮促进干热岩储层缝网形成、提高热储有效换热面积的力学机理，为提高干热岩储层改造效率提供理论依据。

干热岩地热能是一种清洁的可再生能源，其成功开发对优化我国的能源结构具有重要意义。由于渗透率极低，干热岩储层必须经过压裂改造形成具有高有效换热面积的空间缝网才能实现商业开采，而常规的压裂技术尚难以完全满足要求，探索能够提高干热岩热储有效换热面积的压裂新方法是亟待解决的关键问题之一。项目提出了采用低温液氮作为压裂液的新思路，压裂过程中高温干热岩储层岩石发生冷却收缩，能够促进天然裂缝发生剪切滑移，提高压裂改造效果。首先，开展了干热岩力学实验，获得了岩石基础弹性参数和力学参数，为数值模拟分析提供了必要参数。其次，建立了低温流体作用下天然裂缝剪切滑移有限元分析模型，揭示了液氮作用下干热岩储层天然裂缝剪切滑移的机理和影响规律。最后，构建了一套适用于裂缝性干热岩储层液氮压裂水力裂缝扩展的流-固-热耦合有限元方法，分析了干热岩储层中水力裂缝网络的起裂与扩展规律，揭示了地层应力差、摩擦系数和注入速率等因素对干热岩复杂水力裂缝网络的影响。本项目研究结果对干热岩储层改造方案设计具有一定的参考意义。