Characterization of rock for constraining reservoir scale tomography at the Geysers geothermal field

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A suite of laboratory measurements are being conducted on Geysers graywacke recovered from a drilled depth of 2599 meters in NEGU-17. The tests are being conducted to characterize the effect of pressure and fluid saturation on the seismic properties of the graywacke matrix. The measurements indicate that the graywacke is an unusual rock in many respects. Both compressional and shear velocities exhibit relatively little change with pressure. Water saturation causes a slight increase in the compressional velocity, quantitatively consistent with predictions from the Biot-Gassmann equations. Shear velocity decreases with water saturation by an amount greater than that predicted by the ... continued below

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231-236

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Boitnott, G.N. & Bonner, B.P. January 20, 1994.

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Description

A suite of laboratory measurements are being conducted on Geysers graywacke recovered from a drilled depth of 2599 meters in NEGU-17. The tests are being conducted to characterize the effect of pressure and fluid saturation on the seismic properties of the graywacke matrix. The measurements indicate that the graywacke is an unusual rock in many respects. Both compressional and shear velocities exhibit relatively little change with pressure. Water saturation causes a slight increase in the compressional velocity, quantitatively consistent with predictions from the Biot-Gassmann equations. Shear velocity decreases with water saturation by an amount greater than that predicted by the Biot-Gassmann equations. This decrease is attributed to chemomechanical weakening caused by the presence of water. Measurements of Q, from torsion experiments on room dry samples at seismic frequencies indicate unusually high Q, (~500). Water saturation decreases the shear modulus by 12 percent, again indicative of chemomechanical weakening. Q, is lower for the water saturated condition, but still relatively high for rock at low stress. Results of ultrasonic pulse propagation experiments on partially saturated samples are typical of low porosity rocks, being characterized by a monotonic decrease in compressional and shear velocity with decrease in saturation. An increase in shear velocity and low frequency shear modulus after vacuum drying indicates the presence of chemo-mechanical weakening resulting from the presence of small amounts of water.

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231-236

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  • Proceedings, nineteenth workshop on geothermal reservoir engineering, Stanford University, Stanford, CA, January 18-20, 1994

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  • Report No.: SGP-TR-147-33
  • Grant Number: AC07-76ID01570
  • Office of Scientific & Technical Information Report Number: 889216
  • Archival Resource Key: ark:/67531/metadc873554

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Office of Scientific & Technical Information Technical Reports

Reports, articles and other documents harvested from the Office of Scientific and Technical Information.

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  • January 20, 1994

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  • Sept. 21, 2016, 2:29 a.m.

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  • Nov. 22, 2016, 10:18 p.m.

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Boitnott, G.N. & Bonner, B.P. Characterization of rock for constraining reservoir scale tomography at the Geysers geothermal field, article, January 20, 1994; Livermore, California. (digital.library.unt.edu/ark:/67531/metadc873554/: accessed October 18, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.