An Experimental Study of CO2 Flooding the Reservoir-caprock System: Implication for the Stability of Caprock during CO2 Intrusion

The study of caprock assumes paramount significance, particularly in elucidating alterations in the sealing conditions of petroleum reservoirs. However, it is imperative that corresponding simulation experiments transcend a singular focus on CO₂ and caprock, extending to a comprehensive study of the reservoir-caprock system. Experimental protocols were implemented, entailing the sequential flow of CO₂-rich fluid first through the reservoir and subsequently through the caprock. The chosen samples and conditions were drawn from potential CO₂ utilization and storage blocks in the Subei Basin, China, featuring sandstone reservoirs and mudstone caprocks. The experimental paradigm simulated the interaction when CO₂ migrating along the sandstone and then reaching the mudstone caprock, spanning a duration of 37 days.

Results underscore that the introduction of CO₂-rich fluid predominantly instigates the dissolution of sandstone reservoirs, with notable dissolutions observed in feldspar and clay minerals, while secondary mineral precipitation remains negligible. Upon the fluids traversal through the mudstone caprock, initial dissolution occurs in carbonate minerals, accompanied by continuous precipitation of secondary clay minerals. These secondary minerals not only occupy calcite dissolution pores but also precipitate on the surfaces of rock particles, asserting dominance in the water-rock reaction within the mudstone.

Supported by geological observations and numerical simulations, these experiments illuminate the material adjustment processes ensuing from the introduction of CO₂-rich fluid into the reservoir-caprock system. The infusion of CO₂-rich fluid induces mineral dissolution, augmenting pore space within the reservoir, with ion products subsequently transported to the mudstone caprock. Under conditions characterized by a slower flow rate and a more extensive water-rock reaction surface area in the mudstone caprock, the water-rock interaction accelerates the dissolution and reprecipitation of calcite and other minerals. The reprecipitated minerals effectively occupy caprock pores and fractures, thereby enhancing the caprock's sealing capacity.