Roughness Control on Multiphase Flow in Rock Fractures

• Published in: Geophysical research letters 2019-11, Vol.46 (21), p.12002-12011
• Main Authors: Hu, Ran, Zhou, Chen‐Xing, Wu, Dong‐Sheng, Yang, Zhibing, Chen, Yi‐Feng
• Format: Article
• Language: English
• Subjects: Displacement; displacement pattern; Energy dissipation; Energy exchange; Fractures; Fronts; Methods; Multiphase flow; phase diagram; Phase diagrams; Porous media; rock fractures; Rocks; Roughness
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2019GL084762

The roughness of rock fractures induces irregular flow passages and significantly affects the displacement front. Previous studies focus on displacement instabilities in porous media, but how roughness controls displacement patterns in rock fractures remains unclear. Here, we derive a theoretical model that describes the two transitions of drainage displacement patterns from capillary fingering to the crossover to viscous fingering as functions of roughness. The phase diagram predicted by this model exhibits excellent agreement with our experimental results, showing that increasing roughness index λb destabilizes displacement fronts. We further find that in capillary fingering regime the percentage of dissipated energy to the total external work increases from 39% to 61% as λb increases from 0.082 to 0.245. Our work elucidates the mechanism of roughness control on multiphase flow in fractures and provides a basis for developing a rigorous upscaling methodology that relates Darcy‐scale flow behavior to the local fluid displacements via energy dissipation. Plain Language Summary Multiphase flow in fractured media is an important process involved in many natural processes and subsurface engineering applications. Geological fractures are inherently rough to various degrees. The roughness of rock fracture, inducing irregular flow passages, plays a fundamental role in the displacement of one fluid by another immiscible one. Despite its importance, how the roughness controls the displacement patterns remains poorly understood. Here, we propose a theoretical model to quantify the impact of displacement patterns, which describes the two transitions of displacement patterns from capillary fingering to the crossover to viscous fingering as functions of roughness. The phase diagram predicted by the theoretical model is in excellent agreement with our experimental results. We also find that higher roughness index λb causes more dissipated energy. The dissipation ratio, representing the percentage of dissipated energy to the external work, increases from 39% to 61% as λb increases from 0.082 to 0.245 in the capillary fingering regime. Our work proposes a phase diagram to elucidate the impact of roughness on multiphase flow that is previously not reported; it also provides a basis for an upscaling methodology that relates Darcy‐scale flow behavior to the local fluid displacements via energy dissipation. Key Points A theoretical model is proposed to describe the two trans