Wen Deng | The University of Texas at Austin (original) (raw)

Papers by Wen Deng

Environmental Science & Technology, 2013

Simulations of coupled flow around and inside biofilms in pores were conducted to study the effec... more Simulations of coupled flow around and inside biofilms in pores were conducted to study the effect of porous biofilm on micro- and macro-scale flow and transport. The simulations solved the Navier-Stokes equations coupled with the Brinkman equation representing flow in the pore space and biofilm, respectively, and the advection-diffusion equation. Biofilm structure and distribution were obtained from confocal microscope images. The bulk permeability (k) of bioclogged porous media depends on biofilm permeability (kbr) following a sigmoidal curve on a log-log scale. The upper and lower limits of the curve are the k of biofilm-free media and of bioclogged media with impermeable biofilms, respectively. Based on this, a model is developed that predicts k based solely on kbr and biofilm volume ratio. The simulations show that kbr has a significant impact on the shear stress distribution, and thus potentially affects biofilm erosion and detachment. The sensitivity of flow fields to kbr directly translated to effects on the transport fields by affecting the relative distribution of where advection and diffusion dominated. Both kbr and biofilm volume ratio affect the shape of breakthrough curves.

Water Resources Research, 2013

Seismic waves affect fluid flow and transport processes in porous media. Therefore, quantitative... more Seismic waves affect fluid flow and transport processes in porous media. Therefore, quantitative understanding of the role of seismic waves in subsurface hydrodynamics is important for the development of practical applications and prediction of natural phenomena. We present a theoretical fluid dynamics model to describe how low-frequency elastic waves mobilize isolated droplets trapped in pores by capillary resistance. The ability of the theoretical model to predict the critical mobilization amplitudes (Ac) and the displacement dynamics of the nonwetting droplet are validated against computational fluid dynamics (CFD) simulations. Our theory has the advantage of rapid calculation of Ac for various scenarios. Both theory and CFD simulations show that the Ac increases with increasing wave frequency. The theoretical and computational models agree well in the low-frequency range both in terms of predicting the displacement history of the droplet and its eventual dislodgment, but their results begin to diverge with increasing wave frequency since the Hagen-Poiseuille flow approximation in the model becomes invalid. Relative to a previous “viscous seismic model,” our model compares more favorably to experimental observations. The model is thus appropriate for predicting trapped nonwetting droplet dynamics in and dislodgement from pore constrictions by low-frequency elastic waves.

Water Resources Research, 2013

In this article, the effects of different diverging-converging pore geometries were investigated,... more In this article, the effects of different diverging-converging pore geometries were investigated, and the microscale fluid flow and effective hydraulic properties from these pores were compared with that of a pipe from viscous to inertial laminar flow regimes. The flow fields are obtained using computational fluid dynamics, and the comparative analysis is based on a new dimensionless hydraulic shape factor β, which is the “specific surface” scaled by the length of pores. Results from all diverging-converging pores show an inverse pattern in velocity and vorticity distributions relative to the pipe flow. The hydraulic conductivity K of all pores is dependent on and can be predicted from β with a power function with an exponent of 3/2. The differences in K are due to the differences in distribution of local friction drag on the pore walls. At Reynolds number (Re) ∼ 0 flows, viscous eddies are found to exist almost in all pores in different sizes, but not in the pipe. Eddies grow when Re → 1 and leads to the failure of Darcy's law. During non-Darcy or Forchheimer flows, the apparent hydraulic conductivity Ka decreases due to the growth of eddies, which constricts the bulk flow region. At Re > 1, the rate of decrease in Ka increases, and at Re >> 1, it decreases to where the change in Ka ≈ 0, and flows once again exhibits a Darcy-type relationship. The degree of nonlinearity during non-Darcy flow decreases for pores with increasing β. The nonlinear flow behavior becomes weaker as β increases to its maximum value in the pipe, which shows no nonlinearity in the flow; in essence, Darcy's law stays valid in the pipe at all laminar flow conditions. The diverging-converging geometry in pores plays a critical role in modifying the intrapore fluid flow, implying that this property should be incorporated in effective larger-scale models, e.g., pore-network models.

Geophysics, 2010

The last decade has seen clarifications of the underlying capillary physics behind stimulation of... more The last decade has seen clarifications of the underlying capillary physics behind stimulation of oil production by seismic waves and vibrations. Computational studies have prevailed, however, and no viscous hydrodynamic theory of the phenomenon has been proposed. For a body of oil entrapped in a pore channel, viscosity effects are naturally incorporated through a model of two-phase core-annular flow. These effects are significant at the postmobilization stage, when the resistance of capillary forces is overcome and viscosity becomes the only force resisting an oil ganglion’s motion. A viscous equation of motion follows, and computational fluid dynamics (CFD) establishes the limits of its applicability. The theory allows inexpensive calculation of important geophysical parameters of reservoir stimulation for given pore geometries, such as the frequency and amplitude of vibrations needed to mobilize the residual oil. The theoretical mobilizing acceleration in seismic waves for a given frequency is accurate to within approximately 30% or better when checked against CFD. The advantages of the viscous theory over the inviscid one are twofold. The former can calculate complete time histories of forced displacement of an oil blob in a pore channel, including retardation by capillary forces, mobilization by vibrations, and an ensuing Haines jump. It also provides an approximately factor-of-two improvement in the calculation of the mobilizing acceleration needed to unplug a static ganglion.

Abstract Connected fractures serve as a dominant path for flow and transport in aquifers, reservo... more Abstract Connected fractures serve as a dominant path for flow and transport in aquifers, reservoirs, and repositories, but detailed characterization of processes even within a single continuous fracture remains a challenge. In this study, flow and solute transport properties, such as the hydraulic conductivity (K) and dispersion coefficient (D), which are critical for the prediction of the fate of injected fluids and solutes, were quantified using two approaches.

Abstract An evolution equation describing the dynamics of an interface between immiscible core an... more Abstract An evolution equation describing the dynamics of an interface between immiscible core and annulus fluids in an axisymmetric tube, in the presence of an imposed flow, can be developed as follows. A solution of the Navier-Stokes equations for a two-phase Poiseuillean flow with non-equal pressure gradients in the core and the film must be obtained.

Abstract The residual trapping of CO2 as an immobile fluid inside pores of geologic reservoirs is... more Abstract The residual trapping of CO2 as an immobile fluid inside pores of geologic reservoirs is one of several mechanisms for securely sequestering CO2 underground which is a potential solution for reducing its atmospheric emission. A dominant mechanism for residual trapping during the imbibition process is snap-off or the disconnection of a continuous stream of the non-wetting CO2 when it passes through pore constrictions and when a criterion based on pressure imbalance is met.

Geophysical Research …, Jan 1, 2011

Geophysical Research …, Jan 1, 2012

Environmental Science & Technology, 2013

Simulations of coupled flow around and inside biofilms in pores were conducted to study the effec... more Simulations of coupled flow around and inside biofilms in pores were conducted to study the effect of porous biofilm on micro- and macro-scale flow and transport. The simulations solved the Navier-Stokes equations coupled with the Brinkman equation representing flow in the pore space and biofilm, respectively, and the advection-diffusion equation. Biofilm structure and distribution were obtained from confocal microscope images. The bulk permeability (k) of bioclogged porous media depends on biofilm permeability (kbr) following a sigmoidal curve on a log-log scale. The upper and lower limits of the curve are the k of biofilm-free media and of bioclogged media with impermeable biofilms, respectively. Based on this, a model is developed that predicts k based solely on kbr and biofilm volume ratio. The simulations show that kbr has a significant impact on the shear stress distribution, and thus potentially affects biofilm erosion and detachment. The sensitivity of flow fields to kbr directly translated to effects on the transport fields by affecting the relative distribution of where advection and diffusion dominated. Both kbr and biofilm volume ratio affect the shape of breakthrough curves.

Water Resources Research, 2013

Seismic waves affect fluid flow and transport processes in porous media. Therefore, quantitative... more Seismic waves affect fluid flow and transport processes in porous media. Therefore, quantitative understanding of the role of seismic waves in subsurface hydrodynamics is important for the development of practical applications and prediction of natural phenomena. We present a theoretical fluid dynamics model to describe how low-frequency elastic waves mobilize isolated droplets trapped in pores by capillary resistance. The ability of the theoretical model to predict the critical mobilization amplitudes (Ac) and the displacement dynamics of the nonwetting droplet are validated against computational fluid dynamics (CFD) simulations. Our theory has the advantage of rapid calculation of Ac for various scenarios. Both theory and CFD simulations show that the Ac increases with increasing wave frequency. The theoretical and computational models agree well in the low-frequency range both in terms of predicting the displacement history of the droplet and its eventual dislodgment, but their results begin to diverge with increasing wave frequency since the Hagen-Poiseuille flow approximation in the model becomes invalid. Relative to a previous “viscous seismic model,” our model compares more favorably to experimental observations. The model is thus appropriate for predicting trapped nonwetting droplet dynamics in and dislodgement from pore constrictions by low-frequency elastic waves.

Water Resources Research, 2013

In this article, the effects of different diverging-converging pore geometries were investigated,... more In this article, the effects of different diverging-converging pore geometries were investigated, and the microscale fluid flow and effective hydraulic properties from these pores were compared with that of a pipe from viscous to inertial laminar flow regimes. The flow fields are obtained using computational fluid dynamics, and the comparative analysis is based on a new dimensionless hydraulic shape factor β, which is the “specific surface” scaled by the length of pores. Results from all diverging-converging pores show an inverse pattern in velocity and vorticity distributions relative to the pipe flow. The hydraulic conductivity K of all pores is dependent on and can be predicted from β with a power function with an exponent of 3/2. The differences in K are due to the differences in distribution of local friction drag on the pore walls. At Reynolds number (Re) ∼ 0 flows, viscous eddies are found to exist almost in all pores in different sizes, but not in the pipe. Eddies grow when Re → 1 and leads to the failure of Darcy's law. During non-Darcy or Forchheimer flows, the apparent hydraulic conductivity Ka decreases due to the growth of eddies, which constricts the bulk flow region. At Re > 1, the rate of decrease in Ka increases, and at Re >> 1, it decreases to where the change in Ka ≈ 0, and flows once again exhibits a Darcy-type relationship. The degree of nonlinearity during non-Darcy flow decreases for pores with increasing β. The nonlinear flow behavior becomes weaker as β increases to its maximum value in the pipe, which shows no nonlinearity in the flow; in essence, Darcy's law stays valid in the pipe at all laminar flow conditions. The diverging-converging geometry in pores plays a critical role in modifying the intrapore fluid flow, implying that this property should be incorporated in effective larger-scale models, e.g., pore-network models.

Geophysics, 2010

The last decade has seen clarifications of the underlying capillary physics behind stimulation of... more The last decade has seen clarifications of the underlying capillary physics behind stimulation of oil production by seismic waves and vibrations. Computational studies have prevailed, however, and no viscous hydrodynamic theory of the phenomenon has been proposed. For a body of oil entrapped in a pore channel, viscosity effects are naturally incorporated through a model of two-phase core-annular flow. These effects are significant at the postmobilization stage, when the resistance of capillary forces is overcome and viscosity becomes the only force resisting an oil ganglion’s motion. A viscous equation of motion follows, and computational fluid dynamics (CFD) establishes the limits of its applicability. The theory allows inexpensive calculation of important geophysical parameters of reservoir stimulation for given pore geometries, such as the frequency and amplitude of vibrations needed to mobilize the residual oil. The theoretical mobilizing acceleration in seismic waves for a given frequency is accurate to within approximately 30% or better when checked against CFD. The advantages of the viscous theory over the inviscid one are twofold. The former can calculate complete time histories of forced displacement of an oil blob in a pore channel, including retardation by capillary forces, mobilization by vibrations, and an ensuing Haines jump. It also provides an approximately factor-of-two improvement in the calculation of the mobilizing acceleration needed to unplug a static ganglion.

Abstract Connected fractures serve as a dominant path for flow and transport in aquifers, reservo... more Abstract Connected fractures serve as a dominant path for flow and transport in aquifers, reservoirs, and repositories, but detailed characterization of processes even within a single continuous fracture remains a challenge. In this study, flow and solute transport properties, such as the hydraulic conductivity (K) and dispersion coefficient (D), which are critical for the prediction of the fate of injected fluids and solutes, were quantified using two approaches.

Abstract An evolution equation describing the dynamics of an interface between immiscible core an... more Abstract An evolution equation describing the dynamics of an interface between immiscible core and annulus fluids in an axisymmetric tube, in the presence of an imposed flow, can be developed as follows. A solution of the Navier-Stokes equations for a two-phase Poiseuillean flow with non-equal pressure gradients in the core and the film must be obtained.

Abstract The residual trapping of CO2 as an immobile fluid inside pores of geologic reservoirs is... more Abstract The residual trapping of CO2 as an immobile fluid inside pores of geologic reservoirs is one of several mechanisms for securely sequestering CO2 underground which is a potential solution for reducing its atmospheric emission. A dominant mechanism for residual trapping during the imbibition process is snap-off or the disconnection of a continuous stream of the non-wetting CO2 when it passes through pore constrictions and when a criterion based on pressure imbalance is met.

Geophysical Research …, Jan 1, 2011

Geophysical Research …, Jan 1, 2012