On the Role of Gravity in Dissolving Horizontal Fractures.

Fractures are ubiquitous in geological systems. As reactive fluid flow through a fracture, dissolution of the fracture walls may occur, thus altering the fracture aperture and increasing permeability. It has been recognized that gravity plays an important role in dissolving vertical fractures due to buoyancy‐driven convection. However, the role of gravity in dissolving horizontal fractures is not well understood. Here, we combine microfluidics/Hele‐Shaw experiments and a numerical method to study how the interplay of buoyancy‐driven convection and forced convection controls dissolution dynamics and permeability increase in horizontal fractures. We first develop the micro‐continuum approach by incorporating the gravitational effects, and then, we perform experiments to validate our method, confirming that the method can well capture gravitational effects on dissolution. Through 3D simulations, we find that buoyancy‐driven convection breaks the symmetry of dissolution on the upper and lower surfaces. We employ a symmetry index to identify three dissolution regimes. As the importance of gravity increases (Ri increases), the dissolution regime shifts from forced convection to mixed convection and to natural (or buoyancy‐driven) convection. We establish a link between these dissolution regimes and permeability evolution. In the forced convection or natural convection regimes, the permeability nearly remains unchanged for various Ri. However, in the mixed convection regime, permeability increase is suppressed by the gravitational effects; the underlying mechanism is that the solid phase tends to dissolve near the fracture inlet due to gravitational instability. This work improves our understanding of the gravitational effects on dissolution regimes and permeability evolution in horizontal fractures. Plain Language Summary: Fractures are widely present in the shallow crust of the Earth. The dissolution of fractures is critical for many subsurface processes, including geological carbon sequestration and acid‐injection enhanced oil recovery. Gravity plays an important role in dissolving fractures because of the buoyancy‐driven convection, which is a heavy fluid layer due to the release of the solid phase resulting in convection cells with mushroomlike plumes. However, it remains unexplored how this buoyancy‐driven convection controls dissolution in horizontal fractures. Here, we combine flow‐visualization experiments and a numerical method to investigate gravitational effects. We find that buoyancy‐driven convection breaks the symmetry of dissolution on the upper and lower surfaces. Using a symmetry index, we identify three dissolution regimes, namely forced convection regime, natural convection regime, and mixed convection regime. We show that in the natural or forced convection regimes, the permeability nearly remains unchanged, while in the mixed regime, the permeability decreases with the increase of the importance of gravity. This work highlights the importance of the gravity‐driven dissolution even the relative density difference is down to 0.1% at slow flow rate. Gravitational effects would be amplified for lower flow rates and should be considered under reservoir conditions. Key Points: We develop a pore‐scale numerical method for flow‐dissolution processes, verified by our microfluidics and Hele‐Shaw experimentsWe identify three dissolution regimes to elucidate the role of gravity and establish a link between regimes and permeability evolutionIn mixed convection regime, permeability increase is suppressed by gravity because dissolution occurs near the inlet by convection cells [ABSTRACT FROM AUTHOR]