Observations of carbon dioxide saturation distribution and residual trapping using core analysis and repeat pulsed-neutron logging at the CO2CRC Otway site

Abstract

Time-lapse pulsed-neutron well logging has been applied at the CO2CRC Otway site to measure changes in carbon dioxide saturation profiles across an injection interval. Three stages of contrasting saturation were logged: when the formation was fully water saturated; after CO2 was injected; and after water was injected to drive the CO2 to residual saturation. This allowed for a unique opportunity to observe changing fluid saturation responding to relative permeability hysteresis at the field scale as part of a controlled experiment. The high vertical resolution of the logs (<0.2m) provided detailed fluid saturation profiles of the near-well region. These data were used to interpret the thickness and variability of saturation from the injected carbon dioxide plume. The interpreted saturation profiles from the Otway site show an average residual saturation of 0.20, with an overall range of 0.07 to 0.32. A consistent correlation was observed between the saturation values measured before and after water injection. Higher values for residual CO2 saturation were obtained in the upper portion of the 7m thick injection interval where higher initial CO2 saturations were reached. In a comparison study with core-scale fluid saturation measurements from the same interval, it was found that the correlation between initial and final saturation from the field measurements gives a similar fit to a Land coefficient derived from the laboratory measurements. Observations of the spatial variation in the trapped gas from both core and logs show that residual trapping is a function of the initial saturation achieved and is sensitive to geological heterogeneity.