Carbon capture and storage projects targeting quartz-rich sandstone reservoirs generally assume that geochemical changes associated with carbon dioxide (CO2) injection are slow and limited over operational timescales. Recent field pilot projects and laboratory measurements report non-negligible mechanical and elastic responses in feldspar-rich sandstones during CO2 injection. In this study, CO2-rich brine was injected into labradorite sands under high pore and confining pressures, and axial strain and ultrasonic velocities were monitored with high temporal resolution. We observe that CO2-rich brine injection into feldspar sands triggers rapid axial compaction within minutes. Cyclic changes occur preferentially in S-wave velocity then develop over hourly timescales, with increases during CO2-brine exposure and partial decreases following subsequent fresh injections. These responses occur significantly faster than previously assumed. Our results provide new insight into coupled geochemical, geomechanical, and elastic responses occurring over short operational timescales during CO2 injection into feldspar-rich sandstone reservoirs.