The role of mineral heterogeneity on the hydrogeochemical response oftwo fractured reservoir rocks in contact with dissolved CO2

International audienceIn this study we compare the hydrogeochemical response of two fractured reservoir rocks (limestone composed of100 wt.% calcite and sandstone composed of 66 wt.% calcite, 28 wt.% quartz and 6 wt.% microcline) in contactwith CO2-rich sulfate solutions. Flow-through percolation experiments were performed using artificially fracturedlimestone and sandstone cores and injecting a CO2-rich sulfate solution under a constant volumetric flow rate(from 0.2 to 60 mL/h) at P = 150 bar and T = 60 ◦C. Measurements of the pressure difference between the inletand the outlet of the samples and of the aqueous chemistry enabled the determination of fracture permeabilitychanges and net reaction rates. Additionally, X-ray computed microtomography (XCMT) was used to characterizeand localized changes in fracture volume induced by dissolution and precipitation reactions.In all reacted cores an increase in fracture permeability and in fracture volume was always produced even whengypsum precipitation happened. The presence of inert silicate grains in sandstone samples favored the occurrenceof largely distributed dissolution structures in contrast to localized dissolution in limestone samples. Thisphenomenon promoted greater dissolution and smaller precipitation in sandstone than in limestone experiments.As a result, in sandstone reservoirs, the larger increase in fracture volume as well as the more extended distributionof the created volume would favor the CO2 storage capacity. The different distribution of created volume betweenlimestone and sandstone experiments led to a different variation in fracture permeability. The progressive steppedpermeability increase for sandstone would be preferred to the sharp permeability increase for limestone tominimize risks related to CO2 injection, favor capillary trapping and reduce energetic storage costs.2D reactive transport simulations that reproduce the variation in aqueous chemistry and the fracture geometry(dissolution pattern) were performed using CrunchFlow. The calcite reactive surface area had to be diminishedwith respect to the geometric surface area in order to account for the transport control of the calcite dissolutionreaction at pH < 5. The fitted reactive surface area was higher under faster flow conditions, reflecting a decrease intransport control and a more distributed reaction in sandstone compared to limestone