Integrating Rock Physics and Seismic Modeling for Time-Lapse Analysis in a CO₂ Enhanced Oil Recovery Project

Currently there are 128 enhanced oil recovery projects worldwide using CO₂ injection as a tertiary recovery method (Kootungal, 2010). When revitalizing mature oil fields using CO₂, it is important to monitor pore pressure and fluid saturation changes. Multicomponent time-lapse seismic imaging has been advocated as a modern tool to interpret reservoir changes. To understand how pressure and fluid saturations influence compressional wave (P) and shear wave (S) velocities, five core samples are taken from the Morrow A sandstone formation at Postle Field, Texas County, Oklahoma. These samples represent high permeability, low permeability and cemented zones. We measured P- and S-wave velocities in the laboratory as a function of confining pressure, pore pressure and fluid type. The testing sequence begins with measuring each sample in the dry rock state and is followed by flushing each sample with brine, live oil, live oil with a 0.334 mol fraction of CO₂ and pure CO₂. This includes CO₂ in the gas and supercritical phase. The objective of these laboratory experiments is to determine the effect of pore pressure changes on the different lithological zones. The samples taken from the high permeability zone exhibit maximum sensitivity to changes in pore pressure. The P-wave velocity shows a response that is sensitive to both fluid saturation and pore pressure, whereas S-wave velocity is mainly sensitive to pore pressure. In addition, the S-wave velocity shows a greater sensitivity to changes in pore pressure than P-wave velocity.

We use the fluid and stress response measured from the core samples to modify velocity well logs through a log facies model correlation. The modified well logs represent the brine and CO₂ saturated cases at minimum and maximum reservoir pressure. These well logs are used for full waveform seismic modeling. For both the brine and CO₂ saturated cases, the P-wave response shows a maximum time-lapse amplitude difference at near offsets. Whereas the S-wave response shows a maximum time-lapse amplitude difference at near offsets for the brine case and at far offsets for the CO₂ saturated case. This demonstrates the advantage of using offset limited stacks for time-lapse analysis as compared to full offset stacks. The seismic modeling results, verified and calibrated to the laboratory core measurements, demonstrate the importance of multicomponent time-lapse seismic data for reservoir monitoring.

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