An elastic properties computation to predict 4D seismic effects for CO2 sequestration - a methodology

Abstract

Within Otway Basin CO2 sequestration program, a small amount of CO2 is currently being injected into a depleted Naylor gas field, onshore Victoria. The reservoir is relatively deep (2 km) and complex with area extent of approximately 0.5 km2 limiting the monitoring program to the application of seismic methods only. However the injection of CO2 into this heterogeneous reservoir, where residual gas saturation is present throughout most of the sand column, is expected to cause very subtle changes in elastic properties of the reservoir rock. Indeed initial approximate modelling of 4D seismic response showed that only 4-6 % change in the elastic parameters could be expected. Such small effect could be ?lost? even through approximate fluid substitution methodology. Considering inherently low repeatability of land seismic it becomes even more important to accurately predict 4D seismic at this site. For that purpose we have investigated various methodologies that could increase the accuracy of the predicted changes in elastic properties of the reservoir rock. We derived a methodology for accurate prediction of elastic properties of the reservoir rock through calibration of the log and petrophysical data with core sample. The result showed core saturated velocities and log measurement agree with each other when the ?effective? Kgrain is applied. It suggested that ?effective? Kgrain could be used to represent the average mineralogy of the grains if we do not know the exact mineral composition making up the rock. However, comparative analysis and calibration of log measurement with core sample proved that accurate fluid substitution methodology at this site is hard to achieve without having dense core sample test results within the reservoir interval. In this paper, we present a methodology to derive elastic properties of the reservoir rock through calibration of the log and petrophysical data with core sample measurement.