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The use of repeated pulsed neutron (PN) logging has proved effective in numerous CO2 storage monitoring portfolios around the world. It is regarded as the go-to technique for interpreting saturation changes at the well-bore and has been routinely used for monitoring saturation changes in hydrocarbon reservoirs since tools appeared in the 1960s. The output from this type of tool that is almost always used for saturation monitoring is the so-called ‘Sigma’ curve, which can be used to distinguish formation fluids on the basis of how fast thermal neutrons are captured. Through the AQUISTORE Project in Saskatchewan, Canada, the Petroleum Technology Research Centre (PTRC) is demonstrating time-lapse pulsed neutron logging is an appropriate method to monitor near well CO2 saturation changes in a highly saline reservoir due to a large Sigma contrast between the saline formation waters and the injected CO2. PTRC has conducted time‐lapsed PN logging at the injection well to observe the saturation in the perforated interval and monitor any above zone migration. In addition, a total of 20 PN logging runs were also undertaken to detect the arrival of CO2 at the project's observation well, which is located approximately 150 m from the injection well. Here we present an interpretation of a secondary measurement produced by the tools, which is a measure of the hydrogen index obtained from the total count rates of secondary gamma-rays. We use the Ratio output to determine the suitability of this curve for monitoring CO2 movement and to see any additional benefits compared to the ‘Sigma’ curve. We find that the Ratio shows slightly better repeatability than the Sigma curve and could certainly be used in its place to monitor saturation changes. Furthermore, at the Aquistore site the wells were drilled with oil based mud. The Ratio measurement is almost unaffected by the presence of oil, and responds strongly to carbon dioxide thus is more reliable for interpreting saturation. |
