Dissolution Hotspots in Fractures

• Published in: Geophysical research letters 2021-10, Vol.48 (20), p.n/a
• Main Authors: Hu, Ran, Wang, Ting, Yang, Zhibing, Xiao, Yang, Chen, Yi‐Feng, Zhou, Chuang‐Bing
• Format: Article
• Language: English
• Subjects: dissolution hotspots; dissolution patterns; gravity effect; radial flow; rough fractures; wormholes
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL094118

The injection of reactive fluid into fractured reservoirs is relevant to many subsurface processes. During injection, fracture dissolution forms various patterns. Despite its importance, the evolution of geometries in radial fractures remains unexplored experimentally. Here, by flow‐visualization experiments, we characterize dissolution patterns and show that the patterns shift from compact to wormhole to uniform as flow rate increases. For the first time, we observe dissolution hotspots with maximum local dissolution rate in radial fractures, occurring at a distance from the inlet. We elucidate the key role of gravity for the occurrence of hotspots. The locations of hotspots correspond to the state that the effects of transport in the horizontal and vertical directions are comparable. We establish a theoretical model for predicting locations of hotspots in good agreement with experiments. This work demonstrates transitions of dissolution patterns in fractures and reveals the underlying mechanism for dissolution hotspots previously unidentified. Plain Language Summary Rock fractures commonly serve as dominant pathways for the fluids flow in the Earth's crust. If the flowing fluid is reactive, such fluid injection into fractured reservoirs expands the fracture aperture and produces complex dissolution patterns. It then significantly affects flow pathways and is critical for many subsurface processes, including geological carbon sequestration, acid‐injection enhanced oil recovery, and hydraulic fracturing of the shale formation. Here we perform flow‐through dissolution experiments in radial fractures. We report that the dissolution morphology shifts from compact to wormhole to uniform patterns as the flow rate increases. For the first time, we observe dissolution hotspots occurring at a distance from the inlet. We elucidate that even for the horizontal fractures, gravity plays a key role in the occurrence of dissolution hotspots. We show that due to the gravity effect and the stratification, the locations of hotspots correspond to the state that the effects of transport in the horizontal and vertical directions are comparable. We present a theoretical model to capture this state, and the model can reasonably predict the locations of hotspots. Our work improves understanding on how the flow rate affects the dissolution patterns and further reveals the underlying mechanism for dissolution hotspots previously unidentified. Key Points We report the direct observ