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During our scCO₂ injection tests, measured resonance frequencies were mostly in the range of 1.4–1.5 kHz for the longitudinal (Young's modulus) mode, and 780–930 Hz for the torsion (Shear modulus) mode. Young's modulus and related attenuation during scCO₂ injection determined from the resonances are shown in Figure 5.5. For both core‐parallel (Carbon Tan #1 core) and core‐perpendicular (Carbon Tan #2 core) fracture cases, intact cores (Intact I and II) and the core with the mated fracture (Frac Ia) showed similar trends: monotonically decreasing Young's modulus with increasing CO₂ saturation, and concomitant increasing attenuation with a rather poorly defined maximum. Core #2 showed less overall changes than Core #1, however, with smaller final scCO₂ saturation of the pore space (Core #1: 25%–26%; Core #2: ~18%). X‐ray CT images of scCO₂ invasion into the core (presented in section 5.3.2.2) indicate this may be attributed to different porosity distribution and bedding structure in the samples.

Frac Ib, Ic, and Id samples containing core‐parallel, sheared fractures (average aperture 0.54 mm, determined from X‐ray CT images) showed much smaller Young's and shear moduli than the intact core, which, interestingly, did not change during scCO₂ injection. In contrast, attenuation increased monotonically until scCO₂ broke through the core. The final scCO₂ saturation was much smaller (7%–9.5%) than for the intact core, because of the fast‐passing effect of the open, permeable fracture.

For the samples Frac IIa and Frac IIb containing core‐perpendicular, sheared fractures (average aperture 0.26 mm), Young's modulus showed similar behavior as the intact sample, decreasing monotonically as scCO₂ saturation increased. (Note that Frac IIa shows much smaller final scCO₂ saturation compared with the core‐parallel cases and the Frac IIb, because the scCO₂ fast passed along the Mylar layer.) However, upon close examination, the changes in the modulus exhibited more complexity. Unlike the intact sample's Young's modulus, which showed the most rapid changes at the beginning of the scCO₂ injection (saturation <~10%), the Young's modulus of the fractured samples dropped most quickly at intermediate scCO₂ saturations. Such differences in the behavior were even more prominent for the attenuation: in contrast to the smooth and more‐or‐less monotonic changes for the intact sample, the fractured samples showed a sharp peak in the attenuation followed by an abrupt decrease, and subsequent gradual increases.

Compared with the changes in the Young's modulus during the scCO₂ injection, shear modulus and its related attenuation were largely unchanged during the experiment (Fig. 5.6).

This indicates that the observed changes in the Young's modulus are attributed to the mechanical, poroelastic effect, rather than changes in the rock's mineral properties caused by chemical interactions with the injected scCO₂.