Abstract
Two-phase flow interfacial dynamics in rough fractures is fundamental to understanding fluid transport in fractured media. The Haines jump of non-Darcy flow in porous media has been investigated at pore scales, but its fundamental processes in rough fractures remain unclear. In this study, the micron-scale Haines jump of the air-water interface in rough fractures was investigated under drainage conditions, with the air-water interface tracked using dyed water and an imaging system. The results indicate that the interfacial velocities represent significant Haines jumps when the meniscus passes from a narrow “throat” to a wide “body”, with jump velocities as high as five times the bulk drainage velocity. Locally, each velocity jump corresponds to a fracture aperture variation; statistically, the velocity variations follow an exponential function of the aperture variations at a length scale of ~100 µm to ~100 mm. This spatial-scale-invariant correlation may indicate that the high-speed local velocities during the Haines jump would not average out spatially for a bulk system. The results may help in understanding the origin of interface instabilities and the resulting non-uniform phase distribution, as well as the micron-scale essence of the spatial and temporal instability of two-phase flow in fractured media at the macroscopic scale.
Highlights
Two-phase fluid flow and transport in naturally and artificially fractured media are of interest for many engineering applications, such as (1) tight reservoir exploitation for oil and methane recovery with environmentally conscious hydraulic fracturing in recent decades[1, 2]; (2) the geological sequestration and leakage evaluation of nuclear waste and anthropogenic CO23, 4; (3) geotechnical applications for associated engineering disasters, including tunnel deformation and coal-water/gas outbursts during deep mining[5, 6]; and (4) geothermal and enhanced geothermal systems[7]
These observations may have the following implications for multiphase fluid flow in naturally and artificially developed fracture media: (1) an interfacial Haines jump may develop within a fracture segment with large aperture variations locally, with the highest jump velocity observed ~5 times the bulk drainage velocity; (2) the dynamic interface velocity results in more residual water in narrow “throats” than in wide “bodies”, which may explain the origin of non-uniform phase distribution and considerable variability in the lumped relative permeability
In a Haines jump, the elastic energy initially contained in the liquid–liquid menisci at “throat” by higher capillary force is converted into kinetic energy, with substantial inertial contributions when the menisci move to the “body” with sharply reduced capillary force[46, 55], and dissipated
Summary
Two-phase fluid flow and transport in naturally and artificially fractured media are of interest for many engineering applications, such as (1) tight reservoir exploitation for oil and methane recovery with environmentally conscious hydraulic fracturing in recent decades[1, 2]; (2) the geological sequestration and leakage evaluation of nuclear waste and anthropogenic CO23, 4; (3) geotechnical applications for associated engineering disasters, including tunnel deformation and coal-water/gas outbursts during deep mining[5, 6]; and (4) geothermal and enhanced geothermal systems[7]. The spatial instability of two-phase flow in single fractures and fracture networks has been extensively investigated, experimentally 10, 37–40 and numerically[8, 41,42,43,44], and attributed to the heterogeneity of fracture permeability and aperture Despite these studies, the interfacial dynamics in rough fractures remain poorly understood because the widely used macroscopic mathematical models, based on solving the continuum Darcy’s law, neglect non-equilibrium and locally dynamic effects when the bulk flow is under laminar conditions. Whether the local Haines jump at the pore scale would average out for a bulk system remains debatable; some reports on porous media have shown its significant influences on the macroscale behaviour[46, 52,53,54] In fractured media, this interfacial phenomenon has not been investigated. The interfacial velocity is calculated based on the distance travelled and corresponding time between two sequential images, and its dependency on the local aperture variations is discussed
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