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.

中文翻译：

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加利福尼亚州萨尔顿海地热田，监测储层对地震和流体提取的反应。

连续监测原位储层对应力瞬变的响应，为深入了解地热储层的演化提供了见识。通过利用地震速度变化的应力依赖性，我们研究了加利福尼亚州萨尔顿海地热田（SSGF）的储层应力状态的时空演化。我们发现SSGF经历了许多突然的速度降低（〜0.035至0.25％），这很可能是由于局部和区域地震引起的动态应力瞬变（低至0.08 MPa至0.45 MPa）导致的裂缝开裂引起的。 。速度变化的深度估计约为0.5至1.5 km，与注入井和生产井的深度相似。通过将速度变化与动应力相关联，我们得出了地震速度变化的经验性原位应力敏感性。我们还观察到了2009年11月中旬和2010年12月下旬地震群期间的系统速度降低（0.04至0.05％）。根据体积静态和动态应力变化，最大地震的预期速度降低为3级这些群中的4至4小于0.02％，这表明这些地震可能与在群中观察到的速度变化无关。取而代之的是，我们认为速度的降低可能是由于地震变形引起的裂缝的多孔弹性开裂引起的。我们还观察到长期速度增加（〜0.04％/年），这很可能是由于地热生产引起的孔隙弹性收缩所致。我们的观察结果表明，地震干涉测量技术可以为现场储层对应力变化的响应提供洞察力。