CO2 rock physics as part of the Weyburn-Midale geological storage project

Abstract

To develop confidence in the seismic techniques that are used to (i) qualitatively locate and track the movement of the CO2 plume, and (ii) quantitatively determine the amount of CO2 in place in the Weyburn pool, a good understanding of the effects of CO2 on seismic waves is critical. For this purpose, an extensive series of ultrasonic measurements were performed on over twenty core samples from the Weyburn-Midale carbonates (Marly and Vuggy units) and the overlying and underlying formations. Care was taken to separate pore fluid effects from pore pressure build-up effects during the experiments. This allowed for the sampling of the effects of the CO2's varying phase states (gas–liquid–supercritical fluid) on the overall rock seismic response. The current paper provides a subset of measurements conducted on four samples from the Marly and Vuggy units. Of the observations arising from the measurements, there are two that are of particular note. First, both the P- and S-wave speeds decrease substantially as the CO2 transforms from gas to either liquid or supercritical phase. This observation is consistent with the increase of CO2 fluid density across these phase boundaries. Second, across the gas–liquid phase transition both wave speeds drop abruptly as would be expected for the change in the physical properties of the CO2 across this first order phase boundary. In contrast, across the gas–supercritical phase boundary the velocities change more gradually. This suggests that it may be difficult to distinguish the gas–supercritical boundary using seismic reflection techniques. Illustrative modelling of seismic reflectivities within a hypothetical geological formation with physical properties equal to that of one of the measured samples, however, suggests that a CO2 liquid–water contact is a good seismic reflector.