内嵌地质不连续体对共和干热岩水力裂缝扩展行为的影响

    The influence of embedded discontinuous rock on hydraulic fractures propagation behaviors of hot dry rock in Gonghe Basin

    • 摘要: 开发干热岩需要建立增强型地热系统(enhanced geothermal system, EGS),其核心是通过水力压裂建立具备一定规模的人工热储。与沉积岩不同,花岗岩型干热岩中除发育有常见的天然裂缝外,还广泛分布石英脉等内嵌地质不连续体,其特点是结构面两侧岩石力学性质存在差异,这使得水力裂缝扩展路径存在不确定性。针对青海共和盆地干热岩热储层岩心开展水力压裂实验及有限元数值模拟,研究内嵌地质不连续体对干热花岗岩水力裂缝扩展行为的影响机制。结果表明:① 以石英脉为代表的地质不连续体,受自身力学性质的影响,易诱导水力裂缝沿岩石基质体与不连续体交界面扩展;② 随着地应力的逐渐增大,水力裂缝穿透地质不连续体的难度逐渐增加,不连续体与岩石基质体抗拉强度的差异越大,水力裂缝越难穿透地质不连续体;③ 实际干热岩压裂过程中,水力裂缝与大尺寸的地质不连续体沟通后沿交界面扩展造成结构面滑移,并展现出井口压力缓慢上升、微地震事件数量减少且原始波形S/P波振幅比上升的特点,诱发超过ML1.5地震的可能性增大。研究可为花岗岩型干热岩控震压裂工艺设计提供理论与实验支撑。

       

      Abstract: The exploitation of hot dry rock (HDR) requires the enhanced geothermal system (EGS), whose core is to build a thermal reservoir by hydraulic fracturing. In addition to natural fractures, the embedded discontinuous rocks, such as quartz veins, are developed in granite (typical HDR). This characteristic demonstrates significant differences compared to sedimentary rocks, which makes it difficult to predict the propagation pattern of hydraulic fractures (HFs). Hydraulic fracturing experiments were conducted on the core specimens from the HDR reservoir in the Gonghe Basin. Moreover, the numerical simulation based on the extended finite element method (XFEM) was conducted to investigate the influence mechanism of embedded discontinuous rocks on the HF propagation behaviors of HDR. The results show that: (1) Due to the mechanical properties, the embedded discontinuous rock in granite, is prone to induce HFs to propagate along the interface between the rock matrix and discontinuous rock. (2) As the in-situ stresses gradually increase, the HFs encounter greater difficulty in penetrating the discontinuous rock. Furthermore, a larger tensile strength contrast between the embedded discontinuous rock and the rock matrix leads to the increased resistance to HF penetration through the discontinuity. (3) During the hydraulic fracturing of HDR in the field, HFs intersecting with large-scale geological discontinuities tend to propagate along the interfaces, inducing the slip of structural planes. This process is characterized by a gradual increase in wellhead pressure, a reduction in the number of microseismic events, and an elevated S/P-wave amplitude ratio in raw waveforms. Consequently, the likelihood of triggering seismic events exceeding ML1.5 is significantly enhanced. The research can provide theoretical and experimental support for the design of the fracturing technology for controlling induced-earthquakes during HDR exploitation.

       

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