Quantitative CO2 saturation estimation from time lapse sonic logs by consideration of uniform and patchy saturation

An appropriate assumption of gas saturation state is very important to estimate CO2 saturation from P-wave velocity changes. By careful observation of velocity and porosity logs, we found that P-wave velocity decrease gradually with increasing CO2 saturation, although the P-wave velocity should decrease rapidly at very small CO2 saturation in the case of uniform saturation. Thus we expected that the CO2 saturation state in pore space is patchy or heterogeneous saturation. Furthermore existence of critical saturation was expected from the log data. In order to confirm our idea, we calculated theoretical curves of the relation between velocity and CO2 saturation for each saturation state, uniform, patchy, and the heterogeneous saturation expressed by Brie's equation using rock physics modelling. In this study, we use Friable-sand model (Dvorkin and Nur, 1996) for dry frame and grain bulk and shear modulus are calculated by Hill's average of quartz and clay. Additionally, we employed the critical saturation and modified each saturation equation using this critical saturation. Comparing the data and theoretical curves, we confirmed that the curves for patchy saturation or heterogeneous saturation agreed with the data, and the curve for uniform saturation didn't fit the data. In addition to the rock physics modelling study, we estimated CO2 saturation from P velocity changes from baseline log using Gassmann equation, effective fluid and patchy equation. Comparing CO2 saturation from P velocity changes with CO2 saturation from neutron logs, we found that uniform saturation gives much smaller CO2 saturation than the value calculated from neutron logs, but patchy saturation and Brie's equation give reasonable value and good agreement with CO2 saturation calculated from neutron log. Therefore we concluded that CO2 saturation state in pore space is considered as patchy saturation at Nagaoka. And also this conclusion is consistent with the fact that distribution of CO2 tends to become patchy, not uniform, by rapid injection in super critical state of CO2 with low viscosity. (C) 2011 Published by Elsevier Ltd.