[H-RE31_2AM2] Climate control and geosciences (CCS,CO2-EOR,CCUS,geoengineering)

[HRE31-06] Seismic monitoring at the commercial-scale CO2 geological storage site, Cranfield, U.S (Part 3)

*Makiko TAKAGISHI¹, Tsutomu HASHIMOTO¹, Shigeo HORIKAWA², Kinichiro KUSUNOSE³, Ziqiu XUE¹ (1.RITE, 2.Suncoh Consultants Co., Ltd., 3.AIST)

Keywords:CO₂geological storage, Seismic monitoring, fluid injection

Public concerns about felt seismic events induced by fluid injection have been raised recently. These felt events have magnitudes of more than M_L3 and occur in the area where seismicity not active. The induced seismic events were triggered due to the pressure changes at the reservoir. CO₂ geological storage, a kind of the fluid injection activities, is regarded as a key potential technology to mitigate greenhouse gas emission. Since this technology involves long-term and large amount of CO₂ injection, some researchers warm that CO₂ geological storage would trigger felt seismicities at the site. Passive seismic monitoring is conducted at CCS sites around the world since 2000's. A few of the sites reported occurrences of seismic events related to CO₂ injection and these events were unfelt with small magnitudes. To ensure the safety against induced seismicity and obtaining public acceptance, seismic monitoring is necessary for operating CCS project, especially for countries with high seismicity such as Japan. RITE has performed a long-term seismic monitoring at the commercial-scale CO₂ injection site in the U.S. to elucidate the relation between CO₂ injection and occurrences of seismic events collaborating with Lawrence Berkeley National laboratory (LBNL) and Bureau of Economic Geology, University of Texas at Austin (BEG) since 2011. Seismic monitoring is conducted at the Cranfield oilfield, Mississippi. This oilfield is the CO₂-EOR field and a million tonnes of CO2 is annually injected into the Cretaceous sandstone reservoir (porosity 20 ~ 30%, permeability for 10 ~ 200mD) at the depth of about 3,100m. A total of more than four million tonnes of CO₂ have stored as of January 2013. We composed a circle seismic monitoring array deploying 6-3component of seismometers at the depth of 100m in a 3km radius. We have monitored seismicities more than two years now, but we have recorded no seismic events at the Cranfield site. The recorded of vertical components of waveforms were examined by semi-automated processing and visual judgments for the entire monitoring of period, and the triggered signals were all identified as artificial noises, noises due to weather changes such as lighting or strong wind, and distant earthquakes. In this presentation, we discuss why seismic events were not recorded at the Cranfield site. We estimated minimum detectable magnitudes of our monitoring array by means of theoretical calculations based on discrete wavenumber integration method which concerned geological properties from surface to the reservoir. We confirmed that the array could detect seismic events with more than Mw0.4 at the hypocentral distance of 3.2 km. We also discuss the question in terms of pressure changes at the reservoir and other studies conducted at the Cranfield site. Acknowledgements: This study was funded by Ministry of Economy, Trade and Industry (METI) as a part of the 'safety evaluation technology development projects carbon dioxide capture and storage'.