Abstract Seismic based geophysical methods are seeing increased usage in evaluating geothermal resources in order to maximize resource potential. However, interpreting geophysical data (such as velocities and dynamic modulus and fracture density/alignment) generated from geothermal reservoirs remains difficult. Here we present the results of a new laboratory study measuring seismic attributes of fresh and hydrothermally altered rocks from a Philippine geothermal field (Southern Negros Geothermal Project - SNGP) during triaxial deformation. Two types of rocks were obtained by sub-coring samples of low porosity (~1%) andesite and higher porosity (~10%) volcaniclastic samples from the SNGP. Samples were prepared with two offset drill holes to allow a natural fracture to permit fluid flow along the fracture. An embedded array of Acoustic Emission (AE) sensors allows elastic wave and induced microseismic data to be collected. We measure a significant reduction in elastic wave velocities and moduli, with the exception of Poisson's ratio, after shear fracture development. An initially pre-fractured permeability of approximately 10−17 m2 is measured. We find that the permeability decreases from 2.0 × 10−14 m2 to lower than 7.4 × 10−15 m2 as the confining pressure is increased from 5 MPa to 30 MPa. A concomitant increase in P and S-wave velocities, dynamic bulk and Young's moduli are also measured. Finally, we simulate a geothermal ‘venting’ situation by intentionally releasing the high pore fluid (water) pressure from 10 to 50 MPa to ambient pressure, generating a swarm of AE that increases in duration with higher pore pressure. We postulate that this is due to fluid phase change (liquid to gas) and movement along the natural fracture plane and damage zone.

中文翻译：

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断裂、流体和饱和度对菲律宾南部内格罗斯地热田岩石地震属性的影响

摘要 基于地震的地球物理方法在地热资源评估中的使用越来越多，以最大限度地发挥资源潜力。然而，解释地热储层产生的地球物理数据（例如速度和动态模量和裂缝密度/排列）仍然很困难。在这里，我们展示了一项新实验室研究的结果，该研究测量了菲律宾地热田（Southern Negros Geothermal Project - SNGP）在三轴变形期间的新鲜和热液蚀变岩石的地震属性。通过对来自 SNGP 的低孔隙度 (~1%) 安山岩和高孔隙度 (~10%) 火山碎屑岩样品进行取芯，获得了两种类型的岩石。样品制备有两个偏置钻孔，以允许天然裂缝允许流体沿裂缝流动。嵌入式声发射 (AE) 传感器阵列允许收集弹性波和感应微震数据。我们测量了剪切断裂发展后弹性波速度和模量的显着降低，泊松比除外。测得的初始预压裂渗透率约为 10-17 平方米。我们发现，随着围压从 5 MPa 增加到 30 MPa，渗透率从 2.0 × 10−14 m2 下降到低于 7.4 × 10−15 m2。还测量了 P 波和 S 波速度、动态体积和杨氏模量的伴随增加。最后，我们通过有意将 10 到 50 MPa 的高孔隙流体（水）压力释放到环境压力来模拟地热“排放”情况，产生一组 AE，其持续时间随着孔隙压力的升高而增加。