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Analysis of Patchy CO2 Saturation from Time-lapse Sonic Logs Using Rock Physics Modelling
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 73rd EAGE Conference and Exhibition incorporating SPE EUROPEC 2011, May 2011, cp-238-00015
- ISBN: 978-90-73834-12-5
Abstract
We compare time-lapse sonic and neutron porosity logs of the Nagaoka CO2 sequestration experiment against the uniform and patchy saturation models, which represent two end-members of the P-wave velocity and CO2 saturation relationship. Most of the data points fall between the two limits, suggesting that the relationship is somewhere between uniform and patchy saturation. The behaviour between these limits can be explained by the mechanism of wave-induced-fluid on mesoscopic fluid heterogeneities (porescale << mesoscale << P-wavelength), which causes wave attenuation and velocity dispersion. We model these fluid effects using the 1D and 3D continuous random media model (CRM). The log data approximately follow the predictions of the CRM models for fluid patch sizes of 1 to 5 mm. This heterogeneity scale is much larger than the porescale features of a reservoir thin section, indicating that the mechanism of wave-induced fluid flow on the mesoscale can occur in sonic log data and therefore controls the velocity-saturation relation.