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This paper reports on time lapse VSP study in a naturally fractured reservoir used for underground gas storage. Four 9-component VSPS were acquired ilong with a walkaway and then repeated after the reservoh properties changed. The initial survey was conducted with the reservoir mostly gas saturated and the follow-up survey was conducted with the reservoir mostly water saturated. The near-offset 9-c VSP was investigated for indications of shear-wave splitting. No clear evidence of S-wave splitting or other anisotropic wave propagation could be attributed to the reservoir horizon. Ray trace modeling was performed using the VSP velocity results which showed that very little horizontal propagation was within the reservoir for the ofEsetVSP sites. Because of the ray-path limitations, the offset VSP site data could not constrain time-lapse changes to the reservoir horizon. The P-wave walkaway did show a small time-lapse change within the reservoir horizon. This time-lapse change in velocity was interpreted in terms of a crack based equivalent media model to give a trade-off curve between crack density and saturation. A porous media approach was used to estimate porosity which shows time-lapse changes. Other analysis of the VSP including reflectivity and frequency vs. time analysis showed no clear time-lapse changes (plots in Appendix D). Surface seismic data near the VSP well was analyzed and modeled and we conclude that AVO analysis is not applicable to this site.

Monitoring underground gas storage in a fractured reservoir using time lapse vsp