The Rock Physicochemical Basis for Time-Lapse Seismic Reservoir Monitoring of CO₂ Injection

This paper springs from a series of laboratory experiments and high-resolution imaging techniques assessing the changes in microstructure, transport, and seismic properties of brine-saturated sandstones and carbonates when injected with CO₂. Results show that the injection of CO₂ into a brine-rock system induces chemo-mechanical mechanisms, which permanently change the rock frame. The injection of CO₂ into brine-saturated-sandstones induces salt precipitation primarily at grain contacts and/or within small pore throats. Salt precipitation decreases permeability and increases P- and S- wave velocities particularly in sandstones characterized by porosity lower than 10%. On the other hand, the injection of CO₂-rich brine into carbonates induces dissolution of the microcrystalline matrix (i.e., micrite) leading to porosity enhancement. Dissolution counteracts and overwhelms a pressure-dependent, chemo-mechanical creeping of the rock leading to compaction. The overall result is the decrease of the elastic moduli of the dry rock frame. These findings demonstrate that monitoring the time-lapse seismic response of chemically stimulated systems is far from being a pure fluid-substitution problem.