Influence of the flow rate on dissolution and precipitation features during percolation of CO2-rich sulfate solutions through fractured limestone samples

Abstract

Calcite dissolution and gypsum precipitation are expected to occur when injecting CO2 in a limestone reservoir with sulfate-rich resident brine. If the reservoir is fractured, these reactions will take place mainly in the fractures, which serve as preferential paths for fluid flow. As a consequence, the geometry of the fractures will vary leading to changes in their hydraulic and transport properties. In this study, a set of percolation experiments which consisted of injecting CO2-rich solutions through fractured limestone cores was performed under P=150bar and T=60°C. Flow rates ranging from 0.2 to 60mL/h and sulfate-rich and sulfate-free solutions were used.Variation in fracture volume induced by calcite dissolution and gypsum precipitation was measured by X-ray computed microtomography (XCMT) and aqueous chemistry. An increase in flow rate led to an increase in volume of dissolved limestone per unit of time, which indicated that the calcite dissolution rate in the fracture was transport controlled. Moreover, the dissolution pattern varied from face dissolution to wormhole formation and uniform dissolution by increasing the flow rate (i.e., Pe from 1 to 346). Fracture permeability always increased and depended on the type of dissolution pattern.