FLOW RATE EFFECT ON CLAY DISSOLUTION IN THE MT. SIMON SANDSTONE ROCK UNDER GEOLOGIC CO2 SEQUESTRATION CONDITIONS

The Lower Mt. Simon formation rock is mainly composed of quartz and potassium feldspar, with minor calcite, chlorite, illite, kaolinite, and iron sulfides. This formation has been identified for CO₂ storage due to its high porosity and permeability. CO₂ injection acidifies resident brine, promoting and enhancing the rates of water/rock interaction in the reservoir. Dissolution and precipitation processes may affect reservoir permeability as a result of changes to porosity and rock structure. Nevertheless, the effect of reactive clay minerals on the evolution of fluid networks and flow paths through connected pore space is still unknown. Hence, we conducted flow-through experiments to evaluate changes to clay minerals and the response of the Lower Mt. Simon rock to CO₂-acidified brine exposure at three different flow rate conditions, and we correlate porosity observations before and after reaction with changes to permeability (k).

Three flow-through experiments have been conducted using a CO₂-saturated 2M NaCl solution introduced to core samples at a range of flow rates (0.05, 0.1 and 0.5 mL/min) under in situ reservoir conditions (P_confining = 24.8 MPa, pCO₂ = 5.7MPa and T = 50 ºC). The resulting effluent chemistry suggests the rapid dissolution of Si- bearing minerals, primarily potassium feldspar, and the extent of this dissolution correlates with increasing flow rate. Iron is released into solution primarily as a result of enhanced clay Fe-rich dissolution and presumably to a lesser extent due to pyrite reactivity. Electron microscopy and computed tomography images of the reacted cores confirm that alteration extends further into each sample at faster flow rates. Chemical analyses suggest that trace metal release from the Lower Mt. Simon is highly dependent on flow rate. The average experimental permeability (k) of these samples remains constant under slow and intermediate flow rates, whereas permeability is observed to increase at faster flow rate conditions, presumably as a consequence of clay dissolution.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.