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Fracture, fluid and saturation effects on the seismic attributes of rocks from the Southern Negros geothermal field, Philippines

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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.
Original languageEnglish
Article number107099
JournalJournal of Volcanology and Geothermal Research
Volume408
Early online date29 Oct 2020
DOIs
Publication statusEarly online - 29 Oct 2020

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  • Fracture Fluid and Saturation Effects

    Accepted author manuscript (Post-print), 5.43 MB, PDF document

    Due to publisher’s copyright restrictions, this document is not freely available to download from this website until: 29/10/21

    Licence: CC BY-NC-ND

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