干热花岗岩剪切裂隙损伤机理及渗流规律演化研究

Shear fractures are of good self-stability and fluid heat exchange efficiency during fluid seepage, and therefore are becoming important in hot dry rock development. This project plans to conduct a systematic study on the damaging of granitic shear fractures and the evolution of seepage during the fluid flow of circulation water in granitic hot dry rock reservoir with experimental, theoretical and numerical methods. First, shear fractures will be obtained by large size rock shear testing, then fracture seepage experiments will be conducted under different temperatures, stresses and hydraulic conditions, and at the same time the fracture damage time and location will be monitored using acoustic emission technology, and micro scale differences of fracture surface will be detected using SEM scanning technology. Based on that, the fracture damaging mechanism can be disclosed and the effect of different factors on the fracture damaging can be analyzed. Then, based on the fracture surface laser-scanning results, the three dimensional fractal dimensions and fracture average apertures before and after the seepage experiments can be calculated. The modifier formula between fractal dimensions and fracture average aperture will be established, and then calibrated with measurements of water volume and water pressure drop. The relation between fracture damaging and seepage law evolution can be found out using fracture damaging conditions and the change of seepage characteristics. Finally, numerical modeling of fracture seepage experiments will be conducted with particle flow code, in order to optimize the fracture aperture modifier formula and the relation between fracture damaging and seepage law evolution. The research results can provide advice for the exploration and development of hot dry rock, and is of significant theoretical and practical values.

剪切裂隙因其具有良好的自稳性及渗流换热效果而在干热岩开发中得到重视。本项目针对干热岩储层中循环水在花岗岩剪切裂隙渗流中的裂隙损伤及渗流演化问题，拟采用试验、理论与数值模拟相结合的方法进行系统研究。首先通过大尺寸剪切实验获得剪切裂隙，再对剪切裂隙进行不同温度-应力-水力耦合条件下的渗流试验，通过声发射监测损伤发生的时间和位置，采用SEM扫描找出裂隙面矿物颗粒微观变化，揭示裂隙损伤机理并分析各耦合因素对裂隙损伤的影响程度。然后基于裂隙面激光扫描结果，计算试验前后裂隙三维形貌的分形维数及裂隙平均开度，并建立裂隙开度修正公式。通过试验得到的流量和压降结果对获得的修正公式进行验证。进而通过裂隙损伤情况及渗流特征的变化，找出裂隙损伤-渗流演化关系。最后用颗粒流法对裂隙渗流试验进行数值模拟，完善裂隙开度修正公式及裂隙损伤-渗流演化关系。研究成果可为干热岩勘探开发提供参考，具有一定的理论价值和实际意义。

通过增强型地热系统从干热岩中提取的热能是一种绿色可持续的清洁能源，能够在改善我国能源结构，缓解环境污染问题具中发挥重要作用。干热花岗岩裂隙在水的渗流换热过程中可能发生花岗岩裂隙面的损伤，从而影响裂隙渗透性能。本项目针对干热岩储层中循环水在花岗岩裂隙渗流中的裂隙损伤及渗流演化问题，采取了试验、理论与数值模拟相结合的方法进行了系统研究。首先开展高温花岗岩遇水冷却后物理力学试验研究，采用SEM观察和巴西劈裂试验等，揭示其物理力学性能的变化规律。然后采用自主研发的试验设备，开展高温高压条件下花岗岩裂隙的渗流试验，研究在长时间渗流条件下，以及不同温度和压力条件下花岗岩裂隙的渗透性变化情况。最后采用PFC颗粒流数值模拟软件建模，基于花岗岩不同矿物成分的粒径尺寸分布，建立了花岗岩宏观力学性能与数值模型微观参数之间的联系。研究表明当温度大于200℃时，花岗岩遇水冷却的热冲击使其物理力学性能衰减较明显，通过岩石纵波波速可以很好地反映岩石内部受损情况，以波速的变化作为衡量损伤的指标可建立不同温度冷却处理下试样的劈裂抗拉强度与损伤因子的关系。高温下的力学与化学损伤是影响干热花岗岩裂隙渗透性的主要因素。围压引起的岩石力学变形与破坏在渗流初期是一个主要的影响因素。高围压会使岩石裂隙闭合更加紧密，同时可能破坏裂隙面接触点，导致裂隙水力开度下降，渗透率降低。长期来看，化学溶蚀和沉降随渗流时间增长而逐渐成为影响渗透性的主要因素。