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| Titel |
Micro-seismicity and hydrologic compartmentalization in the Coso geothermal reservoir, Inyo County, California |
| VerfasserIn |
J. Ole Kaven, Stephen H. Hickman, Nicholas C. Davatzes |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250055917
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| Zusammenfassung |
| High precision seismic locations and subsurface velocity structure provide potential insights
into fracture system geometry, fluid conduits/compartmentalization, and reservoir mechanics
critical to geothermal exploration and reservoir management. We analyze 16 years of
seismicity to improve hypocentral locations of micro-seismicity and simultaneously invert for
seismic velocity structure within the Coso geothermal reservoir. The Coso geothermal
reservoir has been continuously producing since the 1980’s and is separated into two main
producing compartments: the main field and the east flank. We use differential travel
times of adjacent micro-seismic events to remove path-dependent errors in relative
hypocentral locations and simultaneously invert for the three-dimensional velocity
structure within the reservoir. We find that relocated seismicity in the main field is
shallower than in the east flank and occurs at the same depths as the injection and
production wells, while the east flank seismicity extends about a kilometer below the
injection and production wells. In the east flank, many of the earthquakes appear to
align along planar features, suggesting through-going, pre-existing faults that may
act as conduits for fluid and heat transport. The seismic velocity structure reveals
heterogeneous distributions of compressional (Vp) and shear (Vs) wave speed, with Vp
generally lower in the main field when compared to the east flank and Vs varying more
significantly in the shallow portions of the reservoir. The Vp/Vs ratio appears to
outline the two main compartments of the reservoir at depths of ~ -0.5 to 1.5 km,
with a narrow zone of relatively high Vp/Vs separating the main field from the
east flank. In the deeper portion of the reservoir this zone becomes less prominent.
Comparison of the distribution of Vp/Vs ratios with a temperature model generated from
well logs reveals a significant correlation between regions of low Vp/Vs and high
temperature. However, since variables such as fracture geometry/density and fluid
saturation can also effect Vp/Vs, the physical cause of these variations cannot be
uniquely determined. Nevertheless, these findings suggest that low Vp/Vs ratios may
correlate with regions of high geothermal production at Coso. In summary, our
results suggest that simultaneous, high-precision earthquake relocations and velocity
inversions in active geothermal reservoirs can serve two distinct and important purposes:
1) help identify faults that serve as fluid and heat conduits, and 2) delineate the
nature and extent of hydrothermal compartmentalization within these reservoirs. |
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