Influence of the Flow Rate on Dissolution and Precipitation Features during Percolation of CO2-Rich Sulfate Solutions through Limestone Fractured Samples

Abstract:

A test site for a prospective CO₂ geological storage is situated in Hontomín (Burgos, northern Spain) with a reservoir rock that is composed of limestone (calcite). 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 conduits for flow of fluids. As a consequence, the structure 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 limestone fractured cores were performed under P = 150 bar and T = 60 ºC. Different flow rates ranging from 0.2 to 60 mL/h and sulfate-rich and sulfate-free solutions were used.

Variation in fracture volume induced by calcite dissolution and gypsum precipitation was measured by means of 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 calcite dissolution 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 forming dissolution pattern.