Transport in Porous Media最新文献

Natural fractures in subsurface reservoirs are frequently partially cemented with mineral precipitates, and it is unclear if fracture permeability models developed for rough barren fractures are applicable for fractures where roughness originates from cement linings. Here, we use a digital rock physics workflow to quantify the error in fracture permeability predicted by these models for five digitally synthesized rough fractures and four fractures imaged using three-dimensional X-ray computed microtomography. Samples include a rough, artificially-induced barren fracture in sandstone, a cement-lined natural fracture in limestone sampled from outcrop, and two cement-bridged natural fractures in tight-gas sandstones sampled from reservoir core. The images are then processed, segmented, characterized to determine statistical moments of the aperture distribution, and used in lattice Boltzmann model flow simulations. We address complications in measuring aperture distributions from images when the fracture pore space morphology deviates from the typical theoretical description of rough fractures and evaluate three different methods of measuring local aperture. The alternative cubic law using the nominal mean aperture is found to overestimate fracture permeability by upwards of one to two orders of magnitude, while the fracture permeability models using statistical moments of the aperture distribution are far more accurate for both rough barren and partially cemented fractures. We also define an empirical description of the upper and lower bounds of fracture permeability estimates as a function of relative roughness that is applicable to both rough barren and partially cemented fractures.

Much of the continental margins in the world oceans provide the necessary thermodynamic conditions to store CO(_2) as ice-like hydrates (CO(_2cdot)6 H(_2)O). While resistant to buoyant migration and leakage, the fundamental growth mechanisms that control the injection, capacity, and security of CO(_2) hydrates stored in the seafloor remain unresolved. Extensive field and laboratory testing give rise to conflicting views on the kinetics and growth configurations of hydrates, where mechanistic models reconciling the formation of hydrates observed in nature remain missing. This work elucidates a fundamental pore-scale reactive transport mechanism that underpins the rate and morphology of hydrate formation. We reveal a previously unrecognized mode of hydrate formation in porous seafloor sediments, hydrate film growth via reaction-imbibition, where superhydrophilic hydrate crystallites ((theta sim 0^circ)) formed at water–CO(_2) interfaces create a secondary microporous medium ((sim) 10 to 100 nm pores) within lithologic sediment pores ((sim) 10 to 100 (mu)m pores) to promote further hydrate growth. Unlike past diffusion-controlled models, we show that spontaneous water imbibition into the hydrate micropores establishes rapidly new water–CO(_2) interfaces (i.e., hydrate formation surfaces) via capillary-driven convection and is the dominant mechanism for supplying water to the hydrate formation interface.

The capillary pressure defines pressure difference between non-wetting and wetting fluids. The capillary pressure is part of the flow governing equations, and its definition can have a profound impact on the nature of fluids displacement in a multiphase flow environment. Conventionally, capillary pressure–saturation relationships are determined under equilibrium conditions which signify that all the fluid–fluid interfaces that exist at the pore scale maintain a static configuration at a certain instant in time. However, there exist experimental and numerical evidences that state that the dynamic nature of fluid flows indeed plays a prominent role in defining the trends of the capillary pressure–saturation relationships. In this paper, we develop a first of a kind semi-analytical model to predict the capillary pressure–saturation curves during drainage displacement by integrating the dynamics of fluid flow based on fundamental laws of fluid mechanics. The proposed semi-analytical model can potentially be incorporated into existing multiphase flow simulators to rapidly compute the capillary pressure at various saturations of the flow medium under dynamic flow conditions. The presented semi-analytical model has been validated against experimental and numerical data sets available in the literature at various flow conditions and considering different sets of fluid properties. We noticed a satisfactory match of the results predicted by the proposed semi-analytical model against the literature data. After performing a holistic sensitivity analysis, we notice that the properties of the porous medium, and the fluid–solid interactions play a significant role in defining the trends of the capillary pressure–saturation curves.

Numerical simulation can provide detailed understanding of mass transport within complex structures. For this purpose, numerical tools are required that can resolve the complex morphology and consider the contribution of both convection and diffusion. Solving the Navier–Stokes equations alone, however, neglects self-diffusion. This influences the simulated displacement distribution of flow especially in porous media at low Péclet numbers (Pe < 16) and in near-wall regions where diffusion is the dominant mechanism. To address this problem, this study uses μCT-based computational fluid dynamics (CFD) simulations in OpenFOAM coupled with the random-walk particle tracking (PT) module disTrackFoam and cross-validated experimentally using pulsed-field gradient (PFG) nuclear magnetic resonance (NMR) measurements of gas flow within open-cell foams (OCFs). The results of the multi-scale simulations—with a resolution of 130–190 µm—and experimental PFG NMR data are compared in terms of diffusion propagators, which are microscopic displacement distributions of gas flows in OCFs during certain observation times. Four different flow rates with Péclet numbers in the range of 0.7–16 are studied in the laminar flow regime within 10 and 20 PPI OCFs, and axial dispersion coefficients were calculated. Cross-validation of PFG NMR measurements and CFD-PT simulations revealed a very good matching with integral differences below 0.04%, underpinning the capability of both complementary methods for multi-scale transport analysis.

Imaging of fluid flow at the pore scale in permeable media requires high spatial resolution to observe the topology of fluid in the pore system, along with high temporal resolution to study dynamic processes. The two most popular imaging techniques in modern experiments are microfluidic device imaging and X-ray micro-computed tomography, both having significant limitations as applied to the micro-level. In particular, microfluidic experiments examine flow in quasi-2D system of pores instead of natural 3D geometry of permeable media, whereas X-ray computed tomography (reconstruction of a 3D object representation from a set of 2D projections collected at different rotation angles) is considerably slow when studying fast pore-scale events. In this work, we present a novel approach to examination of local fluid dynamics by combining traditional fast X-ray microtomography and radiographic analysis of successive projections. After initial tomographic imaging of the 3D pore structure, we perform projection-wise analysis comparing differences between two successive projections. As a result, we obtain flow visualization with time resolution determined by the projection time, which is orders of magnitude faster than standard microtomographic scan time. To confirm the effectiveness of this approach, we investigate the pore-scale mechanisms of unstable water migration that occurs during gas-hydrate formation in coal media. We first show that the displacement of brine by methane gas due to cryogenic suction can lead to multiple snap-off events of brine flow in pores. Second, we study a fast local drainage process accompanied by the formation of the gradually swelling gas bubble in the center of the pore. The measured maximum interfacial velocity in our experiments varies from 1.3 to 5.2 mm/s. We also simulate this outflow process accompanied by the bubble expansion and estimate the average brine flow rate during brine-methane displacement.

Electroosmotic flow through porous media is a crucial contemporary research field that finds its application in the areas of various engineering, geological, and biological settings. Obeying Darcy’s law for electroosmotic flow through porous media in similar lines to that of pressure-driven flow yields a very important physical property of electro-permeability. This work aims to examine the influence of wall zeta potential, Debye length, the solid particle shape, and preferential orientation on the electro-permeability tensor using multiscale homogenization methodology for a single-phase fluid flow. For determining the range of possible particle shapes from prolate-oblate ellipsoid to sphere, the parameter of aspect ratio is employed. Additionally, anisotropy ratio and tortuosity have been explored. The governing equations for this study comprise a mass continuity equation, an advection–diffusion equation, a Poisson–Boltzmann equation for electric double layer, and a Laplace equation for solving the electric field in a fully coupled manner. A two-scale computational homogenization technique is employed to model the fluid-saturated periodic media subjected to external electric effects. The finite element approach is adopted to solve the multiscale and multi-physics problem in a coupled manner. The results indicate that the electro-permeability is significantly affected by wall zeta potential, aspect ratio, and orientation of solid particles. Also, one of the major findings is that the EDL thickness has a vital effect on the electro-permeability, anisotropy ratio, and tortuosity of the porous media.

We study the formation of oil droplets from an initially trapped large oil ganglion under surfactant flooding, using a microfluidic device consisting of a two-dimensional array of regularly spaced square posts. We observe that above a critical capillary number for oil mobilization, breakage of the ganglion results in the formation of either trapped patches spanning multiple pores or numerous mobile droplets that exit the device at a velocity comparable to the average flooding fluid velocity. These mobile droplets, however, are only observed when above a secondary capillary number threshold. The formation of these droplets is found to involve the simultaneous occurrence of three different passive droplet generation mechanisms where a droplet is formed as it is pulled by perpendicular fluid flow, as it is pulled by co-axial fluid flow, and or as it splits due to collision with a post. Our results show that oil breakthroughs only occur when the oil is in the form of mobile droplets, suggesting that droplet formation can be an important condition for the mobility of residual oil in porous media. Additionally, this post-array microfluidic device can be used for the production of monodisperse droplets whose size can be controlled by the spacing of the posts.

Convective drying of porous media is central to many engineering applications, ranging from spray drying over water management in fuel cells to food drying. To improve these processes, a deep understanding of drying phenomena in porous media is crucial. Therefore, detailed simulation of multiphase flows with phase change is of great importance to investigate the complex processes involved in drying porous media. While many studies aim to access the phenomena solely by simulations, here we succeed to compare comprehensively simulations with an experimental methodology based on microfluidic multiphase flow studies in engineered porous media. In this contribution, we propose a Navier–Stokes Cahn–Hilliard model coupled with balance equations for heat and moisture to simulate the two-phase flow with phase change. The phase distribution of the two fluids air and water is modeled by the Phase-Field equation. Comparisons with experiments are rare in the literature and usually involve very simple cases. We compare our simulation with convective drying experiments of porous media. Experimentally, the interface propagation of the water–air interface was visualized in detail during drying in a structured microfluidic cell made from PDMS. The drying pattern and the drying time in the experiment are very well reproduced by our simulation. This validation will enable the application for the presented Navier–Stokes Cahn–Hilliard model in more complex cases focused more on applications, e.g., in the field of fibrous materials.