fracture cannot be explained by the Gassmann model which assumes uniform pressure within the core. For this sample, because the length of the sample (and the separation distance between the fracture and the sample interfaces) far exceeded the pressure diffusion length, the effect of the fracture on the fluid substitution was seen only when the scCO2 arrived at the fracture.
The laboratory‐observed changes in seismic velocity and attenuation during scCO2 injection were strongly dependent upon the orientation of the fractures. Particularly, there is an indication that preferential saturation of a fracture by scCO2, oriented perpendicular to the compressional wave direction, can result in sudden decreases in the seismic velocity and attenuation. Because fracture orientation has a dominant effect on the migration of scCO2 and its saturation in reservoir rock, the observed changes can be used for improved assessment of the scCO2's behavior from seismic measurements. Caution must be used in their applications, however, because the pressure diffusion length in reservoir rock is often very short even at the surface seismic exploration frequencies, limiting some of the laboratory‐observed fluid‐substitution‐induced changes in the rock properties (seismic properties) to a small volume around the fractures. Additionally, observed changes in seismic waves in the field are averaged over the effects from multiple fractures with different mechanical properties and orientations.
ACKNOWLEDGMENTS
Initial experimental work in this research was supported by the Assistant Secretary for Fossil Energy, Office of Natural Gas and Petroleum Technology, CSRP Program, through the National Energy Technology Laboratory. Additional experiments (Frac IIb test) and the rock‐physics analyses were supported by the Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences of the U.S. Department of Energy, under the U.S. DOE Contract No. DE‐AC02‐05CH11231.
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