Читать книгу Geophysical Monitoring for Geologic Carbon Storage - Группа авторов - Страница 76

The E,G, a _E , and a _G inverted from measured resonances are presented in Figure 5.4. We do not compute P‐wave and S‐wave velocities and attenuation from these results, because a fractured sample is inherently anisotropic. Generally, the moduli of the samples increased nonlinearly with the applied stress, while the attenuations decreased.

From Figure 5.4a (Carbon Tan #1 core), a mated fracture (Frac I) had only a small effect on the Young's and shear moduli changes compared with an intact sample. In contrast, both moduli of the samples with a sheared fracture (Frac Ib, Frac Ic) were reduced more significantly. The reductions in the Young’s modulus were rather unexpected. We suspect that a slight mismatch between the lengths of the sheared two halves of the core may have caused imperfect mechanical coupling between the sample and the metal resonant bars, in spite of the use of soft metal foils at all relevant interfaces. Attenuations were generally small (~0.5%) except for the sheared and shortened Frac Ic sample (Fig. 5.4c). During a postexperiment examination, we recognized a small intrusion of the plastic jacket into the fracture, caused by the high confining stress. Possible large local dynamic strain of the intruded jacket may have contributed to anomalously large energy dissipation.

For the fractured Carbon Tan #2 core, the reduction in the Young's modulus was more prominent than the Carbon Tan #1 core because of the large compliance of the fracture perpendicular to the core axis (Fig. 5.4b). (Note that the larger reductions in the shear moduli of the samples Frac Ib and Frac Ic compared with Frac IIb and Frac IIc are attributed to the decreases in the torsional rigidity of the sample by the core‐parallel fracture, and maybe to imperfect sample interfaces.) Attenuation for this core showed a similar trend as Carbon Tan #1, decreasing monotonically with increasing confining stress (Fig. 5.4d).

Figure 5.4 Young's modulus E and shear modulus G and their attenuations determined from SHRB tests during initial dry loading tests on Carbon Tan sandstone cores. Note that dry measurements for Frac Id case are not shown. Also note that several cycles were performed for each sample, resulting in small hysteresis between the loading and unloading cycles of the tests (not indicated in the figures). (a) Carbon Tan #1 elastic moduli; (b) Carbon Tan #2 elastic moduli; (c) Carbon Tan #1 attenuations; (d) Carbon Tan #2 attenuations.

Figure 5.5 Young's modulus and related attenuations determined from SHRB tests during scCO₂ injection experiments on Carbon Tan sandstone cores: (a) Carbon Tan #1 elastic moduli; (b) Carbon Tan #2 elastic moduli; (c) Carbon Tan #1 attenuations; (d) Carbon Tan #2 attenuations.