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Laboratory seismic monitoring of supercritical CO 2 flooding in sandstone cores using the Split Hopkinson Resonant Bar technique with concurrent x‐ray Computed Tomography imaging
Author(s) -
Nakagawa Seiji,
Kneafsey Timothy J.,
Daley Thomas M.,
Freifeld Barry M.,
Rees Emily V.
Publication year - 2013
Publication title -
geophysical prospecting
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.735
H-Index - 79
eISSN - 1365-2478
pISSN - 0016-8025
DOI - 10.1111/1365-2478.12027
Subject(s) - geology , supercritical fluid , saturation (graph theory) , mineralogy , bulk modulus , igneous petrology , bar (unit) , core sample , engineering geology , seismology , core (optical fiber) , materials science , composite material , chemistry , oceanography , mathematics , organic chemistry , combinatorics , volcanism , tectonics
Accurate estimation of CO 2 saturation in a saline aquifer is essential for the monitoring of supercritical CO 2 injected for geological sequestration. Because of strong contrasts in density and elastic properties between brine and CO 2 at reservoir conditions, seismic methods are among the most commonly employed techniques for this purpose. However the relationship between seismic (P‐wave) velocity and CO 2 saturation is not unique because the velocity depends on both wave frequency and the CO 2 distribution in rock. In the laboratory, we conducted measurements of seismic properties of sandstones during supercritical CO 2 injection. Seismic responses of small sandstone cores were measured at frequencies near 1 kHz, using a modified resonant bar technique (Split Hopkinson Resonant Bar method). Concurrently, saturation and distribution of supercritical CO 2 in the rock cores were determined via x‐ray CT scans. Changes in the determined velocities generally agreed with the Gassmann model. However, both the velocity and attenuation of the extension wave (Young's modulus or ‘bar’ wave) for the same CO 2 saturation exhibited differences between the CO 2 injection test and the subsequent brine re‐injection test, which was consistent with the differences in the CO 2 distribution within the cores. Also, a comparison to ultrasonic velocity measurements on a bedded reservoir rock sample revealed that both compressional and shear velocities (and moduli) were strongly dispersive when the rock was saturated with brine. Further, large decreases in the velocities of saturated samples indicated strong sensitivity of the rock's frame stiffness to pore fluid.