Effect of pore geometry and dissolved CO2 on ultrasonic transmission during pore pressure changes

It is common practice to use changes in fluid transmission velocity to determine whether a fluid is in gas or liquid phase during pore pressure changes, but transmission amplitude also changes when fluid phase changes during pore pressure reduction. We used two sizes of glass beads in a low-pressure cell to simulate porous rock and conducted experiments with pore spaces filled with distilled water, and with distilled water in which 0.3 mol of CO2 was dissolved. Ultrasonic transmission tests above and below bubble point showed that transmission frequencies and amplitudes were higher for distilled water and the smaller beads. There was a greater reduction in frequencies and amplitudes when gas was liberated by scattering from the small gas bubbles associated with the small beads. The water-CO2 mixture produced higher transmission amplitudes than distilled wateralone, which is consistent with increasing fluid density in the pores. Although Henry's law was appropriate for predicting the onset of bubbles, the ultrasonic response sensed bubble nucleation before the pressure predicted by Henry's law was reached. We also found that transmission amplitudes and frequencies changed more quickly than transmission velocity, which changed little by comparison. Our study suggests that for time-lapse monitoring of CO2 sequestration operations, changes of transmission amplitude and frequency may provide a quantitative assessment of the amount of dissolved CO2 in connate water. Observations of ultrasonic transmission amplitude and frequency are more important in this regard than velocity observations. This knowledge can be applied where CO2 migrates or changes phase after sequestration, be it at depth, or as a result of near-surface leakage. Walk-away VSP data can provide a suitable monitoring tool for this purpose.