冷虹,胡隽. 2024. 水力压裂延迟活化断层的多场耦合数值模拟研究—以加拿大福克斯溪地区诱发地震为例. 地震学报,46(3):1−19. doi: 10.11939/jass.20230070
引用本文: 冷虹,胡隽. 2024. 水力压裂延迟活化断层的多场耦合数值模拟研究—以加拿大福克斯溪地区诱发地震为例. 地震学报,46(3):1−19. doi: 10.11939/jass.20230070
Leng H,Hu J. 2024. Multi-field coupling numerical simulation study on delayed reactivation of hydraulic fracturing induced faults:A case study of induced earthquakes in the Fox Creek area of Canada. Acta Seismologica Sinica46(3):1−19. doi: 10.11939/jass.20230070
Citation: Leng H,Hu J. 2024. Multi-field coupling numerical simulation study on delayed reactivation of hydraulic fracturing induced faults:A case study of induced earthquakes in the Fox Creek area of Canada. Acta Seismologica Sinica46(3):1−19. doi: 10.11939/jass.20230070

水力压裂延迟活化断层的多场耦合数值模拟研究—以加拿大福克斯溪地区诱发地震为例

Multi-field coupling numerical simulation study on delayed reactivation of hydraulic fracturing induced faults:A case study of induced earthquakes in the Fox Creek area of Canada

  • 摘要: 加拿大西部盆地的福克斯溪(Fox Creek)页岩气开采区自压裂开采以来,地震频度急剧增加,引发工业界和科学界的广泛关注。目前一些典型诱发地震案例的断层活化动力学机制尚未完全厘清。本文以2014年初Fox Creek地区Duvernay地层附近发生的地震群及构造为研究对象,开展地下断层受流体扰动而活化的多场耦合数值模拟研究。文中重点讨论的1号平台附近发生的诱发地震主要沿着两个较为清晰的发震断层发生,最大震级MW3.9事件发生于压裂井下方约1 km处的结晶基底内,并具有典型的延迟触发特点。本文就上述断层的滞后活化现象开展数值模拟研究。首先,基于PKN裂缝扩展模型计算并验证注入流体的应力扰动输入项,根据地震数据识别出断层的具体位置,结合地层和构造信息建立二维地质模型;然后,耦合固体力学、流体渗流定律和断层活化理论搭建多孔弹性介质内的断层活化数值仿真模型;最后,采用有限元方法数值模拟水力压裂活化断层的全过程,通过计算库仑应力改变量(ΔCFS)的值来观测断层活化前后的流固耦合场和应力应变场的演化特征。结果表明,经过5天注水和15天扩散的流体作用,西断层附近的ΔCFS持续增加,验证西断层的延迟活化的主要成因是流体逐渐扩散累积到结晶基底并改变了其应力状态。此外,模拟结果表明东断层的存在使得西断层更容易活化。断层位错产生的正ΔCFS区域与诱发地震发生位置高度吻合。本文的数值模拟研究重现了水力压裂活化断层的物理过程,相关机制的正演分析若能事先开展,将可能为地震危害性预测提供科学依据。

     

    Abstract:
    The Western Canadian Sedimentary Basin is one of the most active regions in the world for hydraulic fracturing-induced earthquakes (Atkinson et al, 2016; Schultz et al, 2016). Bao and Eaton (2016) elaborated on the spatiotemporal correlation between hydraulic fracturing operations and seismic activity in the Fox Creek area of the Western Canadian Sedimentary Basin, and found that the largest event (MW3.9) occurred on a fault that appeared to extend from the injection zone to the crystalline basement is typically delayed triggering earthquake. Gao et al2021) believe that the triggering of the forementioned MW3.9 earthquake was due to the existence of complex fluid migration pathways, along which injected fluids spreading accompanied by the Duvernay formation, the eastern fault, and the horizontal pathway of the crystalline basement.
    Based on the observed seismic catalog and fault information in the Fox Creek area of Canada, we conduct a numerical simulation study on delayed fault activation. This simulation will combine the tectonic background, seismic distribution, relevant engineering, and lithological parameters of the Fox Creek area, and based on the hydraulic fracturing principle, fracture seepage theory, fault instability criterion, and fluid-solid coupling theory, it will analyze in detail the mechanism and dynamic process of hydraulic fracturing delayed activation fault.
    First, the PKN fracture extension model is used to calculate the stress perturbation input term of the injected fluid, identify the specific location of the fault based on the seismic data, and establish a 2D geological model by combining the stratigraphic and tectonic information.
    Then, a numerical simulation model of delayed fault activation in porous elastic media is constructed by coupling solid mechanics, fluid seepage law, and fault activation theory.
    Finally, the full process of hydraulic fracturing-induced fault activation was numerically simulated using the finite element method, and the evolution characteristics of the fluid-solid coupling field and stress-strain field before and after fault activation were observed by calculating the value of Coulomb stress change (∆CFS). The simulation was carried out according to the actual construction plan of the fracturing well section and fault activation, and was divided into the following three main stages: the first stage was the water injection stage, where the well section closest to the target fault was fractured and was injected with water for 5 days; the second stage was the stop injection stage, during which the fluid continued to spread and lasted for 15 days; the third stage was the fault activation and subsequent stage, and the activation time was designed to be at the end of the second stage. The fault continued to be calculated for 20 days after activation, and the total simulation time was 40 days.
    The results showed that after 5 days of water injection and 15 days of diffusion, the ∆CFS near the western fault continued to increase, verifying that the delayed activation of the western fault was mainly due to the continuous diffusion and accumulation of fluid to the crystalline basement, resulting in changes in the stress state. Based on the calculation results of the PKN model and the actual fracturing parameters (Bao, Eaton, 2016), the average fluid injection volume of the Duvernay shale layer was 2 015 m3/d, and after conversion, the injection rate was about 23.32 kg/s. We sliced the pore pressure, Y-direction displacement, and ∆CFS at 6 key time points during the entire simulation process, as shown in Fig.1. Once fluid injection began (Day 1, Fig. 1a, 1g, 1m), high-intensity pore pressure diffusion occurred near the reservoir injection points, as well as upward and downward Y-direction displacement and corresponding ∆CFS. After 5 days of injection, the pore pressure had spread to deeper areas, and there was obvious fluid accumulation below the surrounding rock layer and above the crystalline basement boundary (Day 5, light-colored area in Fig. 1b). Corresponding Y-direction displacement and ∆CFS also further increased (Fig. 1h, 1n). Moreover, due to the downward diffusion of the fluid, there was an obvious high-value area of pore pressure near the lower part and endpoint of the fault inserted into the crystalline basement (Fig.1n). At this time, the fault in the crystalline basement already showed had an activation trend, but the shear stress on the fault plane had not reached the critical value for sliding and was in a state of creeping or slow sliding. Although the injection source was lost, the fluid still diffused mainly downward under the action of gravity. Because the permeability of the crystalline basement was lower, the fluid accumulated at the upper boundary of the crystalline basement (Fig.1c, 1d). However, the fault extending to the basement had a relatively large permeability, so the fluid continued to inject into the lower end of the fault, and the corresponding ∆CFS gradually increased (Fig.1o, 1p). When reached the critical value, the fault was activated. The continuous diffusion of fluid and accumulation along the lower end of the fault described above was the key reason for the delayed activation of the western fault.
    In the case when only the presence of the western fault is considered, after 5 days of water injection and 15 days of fluid diffusion, a high ∆CFS value area with 2.56 MPa appeared at the endpoint of the western fault, and a high ∆CFS value area with 1.62 MPa appeared near the MW3.9 earthquake rupture point. In the case of two faults coexisting, the above ∆CFS high-value areas increased to 2.98 MPa and 2.11 MPa, respectively. This indicates that the ∆CFS concentration area of the eastern fault in the crystalline basement has made the Coulomb stress of the western fault increased to some extent, leading to the western fault more prone to activation. It should be noted that a ∆CFS high-value area connection zone has formed between the eastern and western faults in the crystalline basement, which is the result of mutual stress disturbance between adjacent faults (Fig.1p). When the stress of a certain fault reaches its critical value, it will be activated first.
    Numerical simulation calculation results showed that the Coulomb stress increased area generated by the activation of the western fault controlled the MW3.9 earthquake and its aftershocks, indicating that the actual spatial distribution of earthquakes is consistent with the fault setting and stress evolution results in the model.
    In summary, it is very important to simulate the physical mechanism of water injection-induced earthquakes, and if the forward analysis of the relevant mechanism can be carried out in advance, it will provide a scientific basis for predicting the seismic hazard.

     

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