Fluid substitution and seismic modelling in a sandstone aquifer

Virginia Cecilia Vera, Donald C. Lawton

Fluid substitution and seismic modelling were applied in order to evaluate the Paskapoo Formation as a potential CO₂ geological storage unit. The first stage of this project deals with the application of the Gassmann substitution model using well data and evaluation of property variations due to CO₂ saturation changes. Seismic modelling was undertaken in the area of interest simulating pre, during and post CO₂ injection scenarios. From Gassmann calculations it was found that the P-wave velocity drops between 0 to 20% CO₂ saturation and starts a subtle rise at 30% whereas the S-wave velocity increases directly proportional to CO₂ saturation. The P-wave velocity decreases approximately 7%, the S-wave velocity increases 0.8 %, Vp/Vs decreases an average value of 8% and the basal reflector presents a time delay of 1.6 msec. From seismic modelling it was found that the injection zone can be delineated in the CDP stack section through an amplitude change the top reflector and a time delay for the basal reflector. The reflectivity coefficient was evaluated using the Shuey approximation and qualitative observations of the sections, showing a decrease in the reflectivity with increasing CO₂ saturation, with a major drop in the first 10% and a further amplitude decrease with offset (higher angles). These parameters allow us to estimate the conditions that would help to interpret the real data in further phases of this study.