We performed a laboratory experiment to measure compressional (P)-wave velocity and its attenuation in low-permeability striped sandstone at ultrasonic frequency during drainage (CO2 injection) and imbibition (brine injection). X-ray computed tomography (CT) images of the rock sample were simultaneously recorded during the injections. On the basis of the CT images, we calculated the CO2 saturation inside a prolate-spheroid-shaped volume (the first Fresnel zone) that enclosed the wave-propagation path. The relationship of P-wave velocity and that of attenuation to the CO2 fraction were then obtained. Both relationships showed nonuniqueness with remarkable hysteresis in a drainage-imbibition cycle. To explain the nonunique relationship of P-wave velocity to the CO2 fraction, we investigated the continuous random patchy saturation model by using exponent-type autocorrelation. The nonunique relationship of velocity to the CO2 fraction was explained by the model with a correlation length from an order of tens of millimeters in drainage to an order of hundreds of microns in imbibition. We also examined the potential influence of the fractal dimension of CO2 distribution by using the fractal patchy saturation model and a von-Kármán-type autocorrelation function. The fractal dimension of pore-space modulus as related to CO2 distribution was estimated on the basis of the CT images. The fractal dimension during imbibition found a larger value than that during drainage. The change in fractal dimension had a limited effect on velocity and attenuation changes. These nonunique relationships might have a significant impact on the use of seismic methods for estimation of CO2 volume in an inhomogeneous storage reservoir.

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