Hysteretic elastic wave velocity and attenuation in partially saturated sandstone by CO2 and brine: evidenced by an experimental study with X-ray CT

We present the results of an experiment for the measurement of P-wave velocity and attenuation in a sandstone saturated by supercritical CO2 (scCO2) and brine at ultrasonic frequency. In this study, we acquired simultaneously time-lapse records of ultrasonic waveforms and X-ray CT images of fluid saturation. Therefore, the quantitative relationships of P wave velocity and attenuation versus CO2 saturation in local wave pass zone were obtained. The results show obvious hysteresis between drainage and imbibition for these relationships. The imbibition exhibits lower velocity and high attenuation than drainage at the same CO2 saturation. For example, the velocity has a 6% reduction during drainage, while a 13% reduction during imbibition at 0.3 CO2 saturation. Moreover, the data during transition between drainage and imbibition shows retardation phenomenon, e.g. the velocity continues decreasing while CO2 saturation starts decreasing. We explain the hysteresis using the White's model, which suggest the patch size and patch saturation change from drainage to imbibition. (C) 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).