Carbon dioxide capture and storage (CCS) has been recognized as a promising option for dealing with emissions from fossil fuel combustion to decrease the release of CO
2 into the atmosphere. In this context, the Spanish Foundation “Ciudad de la Energía” (CIUDEN) is developing a CO
2 Geological Storage Program in saline aquifers in Spain.
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Distribution of the 30 seismic stations of the Hontomín Seismic Network (HO). Black triangles correspond to the 20 SS-45 sensors whereas the LE-3D seismometers are plotted with white triangles. The cross symbol marks the location of the possible CO2 injection point. |
CO
2 monitoring will play an essential role to control the possible leakage of the injected fluids to the atmosphere or other geologic formations and will help to track them in the subsurface over time. Passive seismic monitoring is a complementary technique that has potential as long-term, cost-effective method for continuous monitoring of CO
2 injection activities. In this context, we have designed a passive seismic network consisting of 30 surface seismic stations and a borehole seismic array that has been deployed before the injection activities start. The Hontomín seismic network (code HO) is composed of 20 three-component SARA SS-45 sensors with natural frequency 4.5 Hz, and 10 Lennartz LE-3D seismometers with natural period 20 s, which have been deployed at about 2-meters in depth. The stations have been distributed, within logistical constraints, in concentric ellipses around the planned CO
2 injection well. The interstation distances range from 1 to 2 km, thus covering an area of about 9 km x 8 km. Seismic data are acquired by means of SARA SL-06 digitizers at a sample rate of 200 Hz and they are continuously recorded on-site. The implementation of a digital radio link transmission to a recording data centre is in progress. The surface network will be supplemented in the near future with an array of 12 triaxial 8-12 Hz geophones in a well specifically designed for geophysical monitoring.
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| Images of station HO08. A detail of the instrumentation is also shown. |
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| (a) Seismogram of a regional earthquake occurred in Lorca (southeastern Spain) at about 568 km from the site; (b) teleseismic recording of the Tohoku (Japan) earthquake occurred at a geodetic distance of about 10,410 km from Hontomín. |
The analysis of the first available seismic data has allowed to estimate the ambient noise levels at the station sites and to validate the performance of the network to locate small magnitude induced earthquakes. The quantification of the station short-term noise baselines has served to characterize the seismic noise conditions at each site, as well as to provide us with a tool to assess the quality of the data. Further processing, such as the computation of earthquake locations, will rely on the quality of the seismic data acquired.
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| Three-component Power Spectral Density (PSD) for stations HO01 and HO27 (Lennartz 20 s seismometer, top) and stations HO20 and HO29 (SARA 4.5 Hz seismometer, bottom). The median and the 5% and 95% percentiles of the PSD distribution are plotted. The gray lines represent the New High Noise Model (NHNM) and New Low Noise Model (NLNM) of Peterson (1993) |
Together with the identification of geomechanical deformations through the location of microseismic events, we plan to take advantage of the seismological technique Passive Image Interferometry (PII) to monitor small temporal changes of seismic velocities in the reservoir area. Preliminary correlation analyses performed with seismic noise data from HO network show that the source receiver co-located Green’s functions as well as for larger station distances can be computed for frequencies between 0.5 and 8 Hz with a temporal resolution of 1 day.
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| Temporal stability of the vertical component autocorrelation functions at station HO06 for the frequency range 0.5-1 Hz. |
We can conclude that the CIUDEN Project at Hontomín test site constitutes a good opportunity to validate, under favorable conditions, the applicability of passive seismology techniques to monitor CO
2 storage in deep saline formations.
