Taira Taka'aki, Nayak Avinash, Brenguier Florent, Manga Michael
Berkeley Seismological Laboratory, University of California, Berkeley, Berkeley, CA 94720, USA.
Institut des Sciences de la Terre, Université Grenoble Alpes, CNRS, Grenoble, France.
Sci Adv. 2018 Jan 10;4(1):e1701536. doi: 10.1126/sciadv.1701536. eCollection 2018 Jan.
Continuous monitoring of in situ reservoir responses to stress transients provides insights into the evolution of geothermal reservoirs. By exploiting the stress dependence of seismic velocity changes, we investigate the temporal evolution of the reservoir stress state of the Salton Sea geothermal field (SSGF), California. We find that the SSGF experienced a number of sudden velocity reductions (0.035 to 0.25%) that are most likely caused by openings of fractures due to dynamic stress transients (as small as 0.08 MPa and up to 0.45 MPa) from local and regional earthquakes. Depths of velocity changes are estimated to be about 0.5 to 1.5 km, similar to the depths of the injection and production wells. We derive an empirical in situ stress sensitivity of seismic velocity changes by relating velocity changes to dynamic stresses. We also observe systematic velocity reductions (0.04 to 0.05%) during earthquake swarms in mid-November 2009 and late-December 2010. On the basis of volumetric static and dynamic stress changes, the expected velocity reductions from the largest earthquakes with magnitude ranging from 3 to 4 in these swarms are less than 0.02%, which suggests that these earthquakes are likely not responsible for the velocity changes observed during the swarms. Instead, we argue that velocity reductions may have been induced by poroelastic opening of fractures due to aseismic deformation. We also observe a long-term velocity increase (0.04%/year) that is most likely due to poroelastic contraction caused by the geothermal production. Our observations demonstrate that seismic interferometry provides insights into in situ reservoir response to stress changes.
对原位储层对应力瞬变的响应进行连续监测，有助于深入了解地热储层的演化。通过利用地震速度变化对应力的依赖性，我们研究了加利福尼亚州索尔顿海地热田（SSGF）储层应力状态的时间演化。我们发现，SSGF经历了多次突然的速度降低（约0.035%至0.25%），这很可能是由局部和区域地震产生的动态应力瞬变（小至0.08MPa，大至0.45MPa）导致裂缝张开引起的。速度变化的深度估计约为0.5至1.5千米，与注入井和生产井的深度相似。通过将速度变化与动态应力联系起来，我们得出了地震速度变化的经验原位应力敏感性。我们还观察到在2009年11月中旬和2010年12月下旬的地震群期间出现了系统性的速度降低（0.04%至0.05%）。根据体积静态和动态应力变化，这些地震群中震级为3至4级的最大地震预期的速度降低小于0.02%，这表明这些地震可能不是导致地震群期间观测到的速度变化的原因。相反，我们认为速度降低可能是由非地震变形导致的孔隙弹性裂缝张开引起的。我们还观察到长期的速度增加（约0.04%/年），这很可能是由地热生产引起的孔隙弹性收缩导致的。我们的观测结果表明，地震干涉测量法有助于深入了解原位储层对应力变化的响应。
