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| Titel |
Laboratory calibration of the seismo-acoustic response of CO2 saturated sandstones |
| VerfasserIn |
A. F. Siggins, M. Lwin, P. Wisman |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250031034
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| Zusammenfassung |
| Geological sequestration can be regarded as one of the promising mitigation strategies against
the negative effects of atmospheric carbon dioxide on global climate change. Injection of
CO2into depleted natural gas reservoirs in particular, sandstone formations at depth with
suitable porosity and seals, seems to be a promising scenario for on-land storage. In fact, a
demonstration project is currently underway in the Otway Basin in South Eastern Australia
under the auspices of the Australian CO2CRC.
One of the most useful geophysical remote sensing tools for monitoring sub surface CO2
injection is seismic imaging. Interpretation of seismic data for the quantitative measurement
of the distribution and saturations of CO2 in the subsurface requires a knowledge of the
effects of CO2as a pore fluid on the seismo-acoustic response of the reservoir rocks. This
report describes some recent experiments that we have conducted to investigate this aspect
under controlled laboratory conditions at pressures representative of in-situ reservoir
conditions.
Prior to the availability of core from the actual Otway injection site, two synthetic
sandstones were tested ultrasonically in a computer controlled triaxial testing rig
under a range of confining pressures and pore pressures representative of in-situ
reservoir pressures. These sandstones comprised; (1) a synthetic material with calcite
intergranular cement (CIPS) and (2), a synthetic sandstone with silica intergranular cement.
Porosities of the sandstones were respectively, 32%,and 33%. Initial testing was carried
on the cores at room temperature-dried condition with confining pressures up to
65MPa in steps of 5 MPa. Cores were then flooded with CO2, initially at 6MPa, 22
degrees C, then with liquid phase CO2at pressures from 7MPa to 17 MPa in steps of 5
MPa. Â Confining pressures varied from 10 MPa to 65 MPa. A limited number of
experiments were also conducted in an additional rig at 50oC with supercritical phase
CO2.
Ultrasonic waveforms, both P- and S-wave were recorded at each effective pressure
increment at nominal pulse centre frequencies of 250 kHz. Velocity–effective pressure
responses were calculated from the experimental data for both P- and S-waves. Attenuations
(1/Qp) were calculated from the waveform data using spectral ratio methods. Fluid
substitution calculations for velocity-effective pressure for each sandstone with various
saturants were made using Gassmann effective medium theory in combination with data from
CO2 phase diagrams. Once core from the Otway injection site (the CRC1 injection well) was
available these experiments were repeated but with an intervening brine saturation step.
The testing sequence for the CRC1 core materials was as follows: The dry core
were tested at increments of effective pressure as above. This was followed by
brine saturation, then gas saturation. The brine salinity used was 20,000 ppm of
NaCl which is representative of the field conditions however the injected gas is
predicted to replace existing CH4 which has re-pressurised the depleted well to
some extent. The CO2was replaced with a CO2/CH4 gas mixture of 80% CO2, 20%
CH4. A total of 6 core samples were prepared from the CR1 well but only 3 were
successfully tested using the above procedures due to the friability of the materials. The
core depths ranged from 2056 to 2077m within the Otway, Waarre C sandstone
formation.
In general the velocities determined in the ultra-sonic experiments on the CRC1 core
agreed well with the well-log acoustic data. It was found that flooding the cores with gaseous
phase CO2 or CO2/CH4 produced very small changes in velocity-effective stress response
compared to the dry state (air saturated) for all cores including field and synthetic
samples. Flooding with liquid phase CO2at various pore pressures lowered the P-wave
velocity-effective response by approximately 8% on average compared to the air saturated
state. The S-wave velocities lowered from the dry state by 10% with liquid CO2 saturation.
Similarly, flooding the dry core samples with brine increased the P-wave velocity–effective
pressure response by approximately 3% but lowered the S-wave velocity response by
5%.
Attenuations increased with liquid-phase CO2 flooding compared to the air-saturated
case. Surprisingly, experimental data agreed well with the Gassmann fluid substitution
calculations within experimental error for all saturants at higher effective pressures despite
the theory being strictly only applicable to low-frequencies. Â The value of effective pressure,
when this agreement occurred, varied with sandstone type. Discrepancies are thought to be
due to differing micro crack populations equivalent to “soft porosity” in the microstructure of
each sandstone type. The effective pressure at which the experimental data agreed with
Gassmann occurred around 30 MPa. This is close to the effective pressure which will
be present when injection is complete. Agreement with the Gassmann model at
effective pressures is significant and gives some confidence in predicting seismic
behaviour under similar field conditions from laboratory data when CO2 is injected. |
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