The Complexity of Nonlinear Flow and non‐Fickian Transport in Fractures Driven by Three‐Dimensional Recirculation Zones. Issue 9 (26th August 2020)

• Record Type: Journal Article
• Authors: Wang, Lichun; Cardenas, M. Bayani; Zhou, Jia‐Qing; Ketcham, Richard A.
• Is Part Of: Journal of geophysical research. Volume 125:Issue 9(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-08-26
• Subjects: recirculation zone -- fracture flow -- nonlinear flow -- non‐Fickian transport -- bimodal breakthrough curves -- Forchheimer equation; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2020JB020028
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Although nonlinear fracture flow and non‐Fickian transport are common in natural settings, the mechanisms driving and controlling these intertwined phenomena are seldom scrutinized together. Here we investigated the critical role of recirculation zones (RZs) in controlling both nonlinear flow and non‐Fickian transport through numerical simulation experiments within three‐dimensional real fractures. RZs were quantitatively mapped from fully resolved flow fields directly simulated across increasing Reynolds number ( Re ). The development and growth of RZs, which are related to aperture expansion and contraction, determine the degree of flow nonlinearity. Moreover, expanding RZs have more capacity to capture and later release solutes back to the main flow. This always results in non‐Fickian transport with bimodal and sometimes multimodal breakthrough curves (BTCs). The time interval between the BTC modes is related via a power law. The Re and RZ volume are sufficient for characterizing the bimodal BTC. Key Points: Recirculation zone development and growth in natural 3‐D fractures were mapped The degree of flow nonlinearity depends on the development of 3‐D recirculation zones, which in turn is connected with aperture expansion or contraction Three‐dimensional recirculation zones capture and retain solutes leading to non‐Fickian transport characterized by bimodal and multimodal breakthrough curves Plain Language Summary: Subsurface fluid flow and solute transport are typically described by first‐order or linear rate laws. However, deviations from the first‐order rate laws (or anomalous behavior) are typical and lead to nonlinear flow and non‐Fickian transport phenomena. The shared underlying mechanisms for these "anomalous" flow and transport phenomena, which are actually typical of fractures, are seldom analyzed. We investigated the co‐occurrence of nonlinear flow and non‐Fickian transport through numerical simulation experiments of pore‐scale flow and solute transport processes through three‐dimensional natural fractures. In these fractures, 3‐D recirculation zones (RZs) developed and grew with increasing flow rates. Flow nonlinearity resulted from the RZ development and growth, which shrank the main flow channel. Moreover, the same RZs also captured solutes from the main flow channel and released them back later. This retardation resulted in non‐Fickian transport. Thus, nonlinear flow and non‐Fickian transport are intertwined via their shared dependence on RZs.