Capillary-Driven Backflow During Salt Precipitation in a Rough Fracture

Salt precipitation is a crucial process occurring during CO2 injection into saline aquifers. It significantly alters the porous space, leading to reduced permeability and impaired injectivity. While the dynamics of precipitation have been studied within porous media, our understanding of precipitation patterns and permeability evolution within rough fractures remains inadequate. Here, we conduct flow-visualization experiments on salt precipitation, wherein dry air invades brine-filled rough fractures under various flow rate conditions. Our observations reveal that the precipitation pattern shifts from ex situ precipitation to homogeneous form as the flow rate (capillary number Ca) increases. Through real-time imaging of the salt precipitation process, we determine that ex situ precipitation is due to capillary-driven backflow. This backflow phenomenon occurs when previously precipitated salt, acting as a hydrophilic porous medium, attracts the brine flow backward. As a result, precipitation occurs at a location different from the original site. We further show that the impact of capillary-driven backflow is significant at low flow rates and is gradually suppressed as the flow rate increases. We provide a theoretical estimation for the critical Ca for the occurrence of capillary-driven backflow. As Ca is smaller than this critical value, backflow-precipitation positive feedback causes fracture voids to become completely clogged, thereby leading to a more substantial permeability reduction. In contrast, a homogeneous precipitation pattern tends to only partially clog the fracture voids, causing a relatively smaller permeability reduction. This study enhances our understanding of the role of capillary-driven backflow in controlling salt precipitation and permeability reduction in fractures. Injecting CO2 into underground water layers (saline aquifers) is one way to tackle climate change by storing it away from the air. However, this process can lead to salt formation within the rock fractures, especially near the injection well, which can block the flow pathways and make it more challenging to inject additional CO2. Our research focuses on how salt forms within the rock fractures when we introduce dry air into areas filled with salty water, at different flow rates. We discover that at slower flow rate, the salt forms in patches due to a process where the salt already formed pulls more water toward it, leading to blockages. At higher flow rates, this doesn't happen, and the salt is distributed more uniformly, causing less blockage. We identify a specific flow rate at which the transition between these two types of salt formation occurs. Understanding this can help us better manage CO2 injection strategies and make it more effective by minimizing the risk of blockages. This work is important for enhancing how we store CO2 underground, an important strategy in reducing its levels in the atmosphere and fighting global warming. We show that precipitation pattern shifts from ex situ to homogeneous form and ex situ precipitation is due to capillary-driven backflow We verify that capillary-driven backflow occurs when previously precipitated salt, as a hydrophilic porous medium, draws brine flow back We quantify that capillary-driven backflow causes voids to be completely clogged, leading to a more substantial permeability reductions