Double-Diffusive Gravity Currents in Porous Media – Implications for Applying Density Compensation in Aquifer Thermal Energy Storage Systems

To improve the efficiency of aquifer thermal energy storage (ATES) systems, it is desirable to minimize the spreading of the buoyant injectedfluid. One proposed approach is density compensation, whereby asolute like salt is added to the injectate to increase its density andreduce buoyancy-driven spreading. This study investigates the physicsof double-diffusive porous media gravity currents and thereby assesses,from a small-scale fluid mechanical point of view, the benefits anddrawbacks of such density compensation. Accordingly, we performdirect numerical simulations of hot and salty into cold and freshlock-exchange flows in the Boussinesq limit. Numerical output showsthat the dynamics of double-diffusive gravity currents differ markedlyfrom their single-diffusive counterparts. For instance, where convective instabilities, i.e, fingers, occur, they have the effect of increasingthe speed of propagation and therefore increasing rates of mixing andspreading. Whereas similar observations have been made in the context of double-diffusive gravity currents in free media (c.f. Konoplivand Meiburg, J. Fluid Mech., 797, 729-764, 2016), the inclusion of the porous medium introduces fundamentally new effects because, for example, the medium can entrap heat but not salt. Moreover, porosity is a dynamically-significant variable because instability onset is retarded in tight media. Overall, the findings suggest that although density compensation slows gravity current spread in an ATES context, its effectiveness depends on suppressing fingering instabilities; hence, slightly salting the injectate to ensure a density greater than that of the native aquifer fluid is recommended.