Transport in Porous Media最新文献

Abstract

Colloid particle size plays an important role in contaminant adsorption and clogging in the hyporheic zone, but it remains unclear how the particle size changes during the transport of colloids. This study investigated the variation of the particle size of colloids in the overlying water and the effects of settlement and hyporheic exchange via laboratory experiments and numerical simulations with two main factors settlement and hyporheic exchange being considered. The results show that the particle size distribution varies when colloids transport in hyporheic zone, and both settlement and hyporheic exchange are involved in the exchange of colloids between stream and streambed. Large-sized particles are mainly controlled by settlement and advection and thus their concentration in the overlying water decreases more quickly; but small-sized particles are mainly controlled by hyporheic exchange and thus their concentration decreases more slowly, and some particles can be resuspended. The increase of retention coefficient and settling velocity will accelerate the transfer of colloids into the streambed. This study may provide important insights into the variation of the particle size of colloids in the overlying water and the effects of settlement and hyporheic exchange.

The pore-scale numerical modeling of CO₂ injection into natural rock saturated with oil–water mixture was performed using the density functional hydrodynamics approach. The detailed 3D digital model of the sandstone core sample contained over 7 billion cells, which allowed us to perform analysis of oil displacement efficiency at different scales. Utilization of large-size detailed numerical models make it possible to characterize, both qualitatively and quantitatively, the processes at pore scale to the level of detail not achievable on smaller models. The obtained results indicate large-scale effects even on relatively heterogeneous core indicating possible need for multiscale hierarchical models even in heterogeneous cases. This fact imposes the demand for scalability performance on both the software and hardware used in such simulations, as well as the need for adequate modeling upscaling methods.

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.

Abstract

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.

Abstract

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.

Abstract

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.