Feasibility of CO2 plume detection using 4D seismic: CO2CRC Otway Project case study — Part 1: Rock-physics modeling

ABSTRACT A key objective of stage 2 of the Cooperative Research Centre for Greenhouse Gas Technologies Otway Project is to evaluate the seismic detection limit of greenhouse gas injected into a saline aquifer. For this purpose, injection of a small amount of CO2-rich gas into the Paaratte Formation, a saline aquifer located at a depth of approximately 1.5 km, is planned. Before the injection experiment is undertaken, we assessed the detectability of injected gas with seismic methods in a modeling study. A key objective of this study was to model changes in elastic properties caused by CO2-saturation effects using predictions of reservoir simulations. To this end, we established an elastic property/porosity relation to link the reservoir flow model and the elastic properties of the subsurface. Predicting changes in elastic properties requires suitable velocity-saturation relations. To choose an appropriate velocity-saturation relation, we analyzed the effect of fluid distribution on the time-lapse seismic response by performing 1.5D poroelastic and elastic modeling based on reservoir simulations. The modeling results emphasized the importance of taking the variability of rock properties into account and to carefully estimate dry bulk moduli to adequately represent the sensitivity of rock properties to fluid changes. Furthermore, we determined that the Gassmann-Wood relation was an appropriate velocity-saturation relation at seismic frequencies for the Paaratte Formation. However, changes in acoustic contrasts caused by CO2 saturation between layers below the seismic resolution had to be considered. In this sense, an appropriate velocity-saturation relation also depends on the scale at which we model the seismic response.