Laboratory Study of Ultrasonic Velocity Variations During CO2 Flooding in Tuscaloosa Sandstone

Abstract

Summary Surface seismic technology offers a promising technique for monitoring carbon dioxide (CO2) flood fronts during the enhanced-oil-recovery (EOR) process. Changes in the seismic signature have been observed with CO2 flooding, but its quantification with respect to subsurface saturation is still in its infancy. This study is focused on quantifying variation in the seismic parameters (velocity and impedance) as a function of subsurface fluid type and saturation. We present results of a laboratory study in which velocity and density were monitored as the pore fluids (formation brine and oil, and CO2) were replaced sequentially. All the experiments were performed at in-situ pressure conditions on core plugs (Tuscaloosa Sandstone) recovered from a well in a field currently undergoing CO2 flooding. These plugs are characterized as fluvial (quartz ≈87%, clay ≈10%) and distributary channels (quartz ≈75%, clay ≈17%). When dry samples were flooded with brine, a decrease in compressional-wave (P-wave) velocity (≈2%) was observed until 95% saturation of brine was achieved. For the remaining 5% of saturation, the velocity increased by 7 to 12.5%. After attaining 100% brine saturation, oil was pumped to displace the brine until irreducible water saturation was achieved. A linear drop of 3 to 4% in velocity to oil saturation was observed during this step. Thereafter, liquid CO2 was injected to displace the oil/brine system and a drop of 5 to 10% in P-wave velocity was observed. Biot-Gassmann modeling shows good agreement with experimental results for the gas/brine and oil/brine systems, but not for liquid-CO2 flooding. An empirical relationship was derived from the experimental results, and was applied to sonic logs and used for sensitivity analysis of 4D-seismic data. Post-flooded sonic data were compared to a theoretical sonic curve estimated from an empirical and ‘patchy’ model. Also, a significant increase in seismic amplitude difference was observed when CO2 saturation was greater than 50%.