CO2 injection into subsurface reservoirs leads to pressure and saturation changes. Furthermore, CO2-brine-minerals interaction could result in dissolution or reprecipitation of rock frame-forming minerals. Observed time-lapse seismic associated with CO2 injection into poorly consolidated sandstone at the Frio CO2 injection site (Texas, USA) could not be predicted using classical rock-physics models (i.e., models involving elastic changes in the rock frame due to saturations and/or pressures changes only, and assuming no changes in the rock microstructure). That, and the changes in the fluid chemistry after CO2 injection, suggests that the assumption of a constant rock microstructure might be violated. Using high-resolution time-lapse crosswell data, we have developed a methodology for estimating changes in the rock frame by quantifying the rock-frame drained moduli before and after CO2 injection. Based on rock microstructure diagnostics, we found that the changes in the drained frame elastic properties are due to the changes in the grain contact-cement percentage. The reduction in contact-cement percent is found to be variable throughout the reservoir, with a maximum near the injection well, down to 0.01% from the initial 0.1% contact cement; this results in more than 40% reduction in the drained frame shear and bulk moduli. CO2 saturation was estimated using this model for uniform and patchy saturation cases. Our rock-physics analysis may allow improved interpretation of time-lapse seismic for CO2 saturation in the context of other poorly consolidated sandstones with similar geomechanical properties. Having the P- and S-wave velocity time-lapse data is key to improve saturation estimates with this analysis method.

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