CO2 sequestration includes the injection of CO2 into permeable saline reservoirs. It has been shown that when water salinity is sufficiently high, in-situ monitoring of the CO2 front can be accurately described via traditional cased-hole time-lapsed Sigma (Σ) logging (Sakurai et al, 2005). However when the targeted reservoir's water salinity is not sufficiently saline uncertainty in the Σ approach increases and it becomes more qualitative than quantitative.

In this paper we summarize a CO2 monitoring project in Japan where formation water salinities were less than ideal for the pulsed neutron Σ based monitoring. To overcome this apparent complication, Σ and alternative measurements such as inelastic ratios and neutron porosity were acquired and combined with the open-hole resistivity, neutron-density and magnetic resonance logs to derive a robust interpretation.

The Hydrogen Index (HI) of CO2 is zero, hence neutron porosity in the presence of CO2 is low. In the presence of CO2 the carbon oxygen ratio (COR) is high due to changes in the carbon and oxygen yields.

The operational sequence of events involved the following; i.) resistivity, neutron-density and magnetic resonance logs were acquired in open-hole; ii.) well was completed, perforated and CO2 injected; iii.) pulsed neutron log data was acquired; iv.) saline water was injected and; v.) pulsed neutron log data was re-acquired.

The combined use of open-hole resistivity, neutron-density, magnetic resonance and cased-hole pulsed-neutron logs demonstrated that accurate CO2 front monitoring is viable in this relatively low salinity environment.

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