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Abstract
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 (S) logging (Sakurai et al, 2005). However when the targeted reservoir’s water salinity is not sufficiently saline uncertainty in the S 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 S based monitoring. To overcome this apparent complication, S 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.

Introduction
CO2 sequestration involves the long term storage of CO2 into permeable saline aquifers, depleted oil and gas reservoirs and/or the deep ocean for global warming mitigation. For CO2 injected into saline aquifers the reservoir parameters of most interest are; i.) injectivity, ii.) containment and iii.) capacity. Injectivity is a measure of the reservoir’s ability to absorb the injected CO2, containment is the reservoir’s ability to seal or trap CO2, and capacity is the amount of CO2 that a reservoir can store.  Reservoir permeability has implications for injection. Higher permeabilities mean fewer wells need to be drilled, but it could also imply that the injected CO2 will have greater mobility and hence will have a larger areal coverage with an increased risk to intersect leak points such as faults and/or fractures. Capacity is linked to reservoir porosity; in a clean, sandstone, saline reservoir it is equivalent to total reservoir porosity minus the irreducible water volume.

The Research Institute of Innovative Technology for the Earth (RITE) was responsible for a five-year project entitled, “Research and Development of Geological Sequestration Technology for Carbon Dioxide" (RITE), (Xue et al, 2006). The project aims were to establish a technology that provides stable, safe and long-term geological sequestration of carbon dioxide emitted from large-scale sources in Japan.