Seismic Signature of CO₂ Saturated Sedimentary Rocks

Seismic technology offers promise to monitor the CO₂ flood front in the reservoirs undergoing enhanced oil recovery and CO₂ sequestration. This laboratory study is focused on understanding seismic signature of CO₂ flooding in rocks. Compressional (Vp) and shear wave (Vs) velocities were measured on several samples (fabricated glass bead, Berea sandstone and Tuscaloosa sandstone) as a function of effective pressure and pore fluids. In these experiments, phase change of CO2 was achieved by varying pore pressure. Realistic subsurface conditions were simulated by having mixed phase (brine, hydrocarbon and CO2) in the rock samples. It is observed that both Vp and Vs decrease when CO₂ changes from gas to liquid, contrary to the general expectations. This behavior can be explained due to smaller change in fluid compressibility (89%) as compared to fluid density change (800%) with phase change.

Shear velocity measurements as function of effective pressure showed Biot effective pressure coefficient (n) is approximately equal to 1.0 and independent of the fluid phase. For compressional wave velocity, n is approximately 1.0 for gaseous CO₂ and much less than 1.0 for liquid CO₂.The experimental work allows feasibility study of mapping CO₂ front from surface seismic survey.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California