Estimation of rock frame weakening using time-lapse crosswell: The Frio brine pilot project

Abstract

[Formula: see text] injection into subsurface reservoirs leads to pressure and saturation changes. Furthermore, [Formula: see text]-brine-minerals interaction could result in dissolution or reprecipitation of rock frame-forming minerals. Observed time-lapse seismic associated with [Formula: see text] injection into poorly consolidated sandstone at the Frio [Formula: see text] 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 [Formula: see text] 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 [Formula: see text] 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. [Formula: see text] 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 [Formula: see text] 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.