Ke Xu

Peking University | PKU · Department of Energy & Resources Engineering

We developed a novel method for fabrication of glass micromodels with varying depth (2.5-D) with no additional complexity over the 2-D micromodels' fabrication. Compared to a 2-D micromodel, the 2.5-D micromodel can better represent the 3-D features of multi-phase flow in real porous media, as demonstrated in this paper with three different example...

We show that smaller gas bubbles grow at the expense of larger bubbles and all bubbles approach the same surface curvature after long times in porous media. This anticoarsening effect is contrary to typical Ostwald ripening and leads to uniformly sized bubbles in a homogeneous medium. Evolution dynamics of bubble populations were measured, and math...

Capillary or residual trapping is considered one of the safest geologic CO2 storage mechanisms due to its hydrodynamic stability. We present a first study of the impact of gravity on Ostwald ripening in porous media and show it may render capillary trapping thermodynamically unstable. Gravity induces a vertical chemical potential gradient that lead...

Significance We show how pore geometry modulates the capillary equilibrium states of a trapped bubble inside a porous medium using a simple conceptual model. The model explains recently published data on bubbles trapped in porous rock samples. We explain the counterintuitive observation that bubbles with large surface areas in the subsurface can re...

Bubbles in subsurface porous media spontaneously coarsen to reduce free energy. Bubble coarsening dramatically changes surface area and pore occupancy, which affect the hydraulic conductivity, mass and heat transfer coefficients, and chemical reactions. Coarsening kinetics in porous media is thus critical in modeling geologic CO2 sequestration, hyd...

Droplets of one fluid in a second, immiscible fluid are typically spherical in shape due to the interfacial tension between the two fluids. Shear forces can lead to droplet deformation when they are subjected to flow, and these effects can be further modified when the droplet is stabilized by a surfactant due to a flow-induced gradients in the surf...

Countercurrent imbibition is the process in which the wetting fluid spontaneously displaces the nonwetting fluid, while the nonwetting fluid is recovered at the wetting fluid inlet. It is a major mechanism for the recovery of shale oil, shale gas, and tight oil. In shale and tight formations, microfractures are highly developed. Specifically, some...

Immiscible fluid-fluid displacement in confined geometries is a fundamental process occurring in many natural phenomena and technological applications, from geological CO2 sequestration to microfluidics. Due to the interactions between the fluids and the solid walls, fluid invasion undergoes a wetting transition from complete displacement at low di...

Convective dissolution is an important mechanism for long-term CO $_2$ sequestration in deep saline aquifers. The presence of an unstable geothermal gradient can affect the process of dissolution. In this paper, we present direct numerical simulations in a three-dimensional porous medium at three different concentration Rayleigh numbers $Ra_S$ with...

Miscible displacement of nondilute fluid pairs in subsurface porous media plays a key role in CO2 subsurface sequestration and many other applications. We conduct microfluidic experiments in a near-fracture porous geometry so that nondilute effects are isolated from other disruptive behaviors. We show that although Darcy-Fickian coupling works well...

Countercurrent imbibition is an important mechanism for tight oil recovery, that is, water imbibes spontaneously from the fracture into the porous matrix while oil flows reversely into the fracture. Its significance over cocurrent imbibition and forced imbibition is highlighted when permeability reduces. We used the computed tomography (CT) scannin...

Hypothesis: In a porous medium saturated with oil (containing oleic surfactant) and saline water, salinity reduction alters the thermodynamic equilibrium and induces spatial redistribution of surfactants, changing the local fluid configuration. During fluid-fluid displacement, this local change reshapes global fluid flows, and thus result in improv...

Capillary trapping is considered as one of the safest geologic CO2 storage mechanisms due to its hydrodynamic stability. However, the thermodynamic stability of capillary trapping was questioned by our recent work (Xu in Geophys Res Lett 46(23):13804–13813, 2019). Gravity induces the top bubbles to grow at expense of bottom bubbles through the diff...

Surfactant flooding is widely used in subsurface applications where high displacement efficiency is needed, such as nonaqueous phase liquid (NAPL) remediation and enhanced oil recovery (EOR). For surfactant/water/oil systems, the partitioning of surfactants in each phase is controlled by external factors, such as salinity. The third phase, microemu...

This report summarizes our recent experimental findings [Xie et al., Phys. Rev. Lett., 2022] and pore-scale simulation results [Xie et al., Phys. Rev. Fluids., 2020] on viscoelastic oscillation, which is a new observation of viscoelastic instability in the multiphase flow state. The viscoelastic oscillation causes trapping of droplets in contractio...

A microchannel with serial converging-diverging geometries, inspired by porous structures of rock, was designed to enhance the cross-interfacial mass transfer of two-phase flow at a high phase injection ratio up to 200:1. It was experimentally demonstrated that the design enhanced mass transfer coefficient by approximately four times compared to cl...

Ganglia, or bubbles, trapped by capillary forces inside porous materials occur in a wide range of subsurface and manufacturing applications. In geologic CO2 storage, ganglia are desired as they render the injected CO2 hydrodynamically immobile. But they may evolve by dissolution or mass exchange across the brine, called ripening. In fuel cells and...

Permanent trapping of an oscillating, nonwetting droplet is observed in a converging-diverging microchannel when aqueous, viscoelastic fluids are injected. Classical theories based on the balance between capillary and viscous forces suggest that the droplet size should decrease with increasing flow rates of a displacing Newtonian fluid, and the dro...

Ostwald ripening of trapped bubbles in porous media is relevant to several subsurface (e.g., CO2 storage) and manufacturing (e.g., fuel cells) applications. A fundamental question common to both is: how does an initially heterogeneous distribution of bubble sizes, PDFo, evolve towards a stable equilibrium distribution, PDFe? And what is PDFe? We de...

Abundant organic-rich nano-/micropores in unconventional oil reservoirs result in relative hydrophobic pore surface and extreme difficulty to displace the oil stored in the matrix. Hence, it is imperative to reveal the nanomechanical features between crude oil and hydrophobic rock surfaces. In this work, the effects of hydrophobicity of pore surfac...

Profile control is utilized to redirect the injection water to low permeability region where a large amount of crude oil lies. Performed gel particles are the commonly used agent for redistributing water by blocking the pores in high permeability region. But the capability of deep penetration of performed gel particles is poor. Here, we formulate n...

Polymer aqueous solutions are widely used in nonaqueous phase liquid recovery and aquifer remediation processes due to their high viscosity. Additional reduction of residual saturation by polymer's viscoelasticity has recently been discovered which elevates the ultimate displacement efficiency. However, there is no consensus on how, and under what...

Surfactant flooding is an effective enhanced oil recovery method in which the oil/water interfacial tension (IFT) is reduced to ultralow values (<0.01 mN/m). The microscopic fluid-fluid displacement has been extensively studied at high IFT (>10 mN/m). However, the microscopic displacement dynamics can be significantly different when the IFT is ultr...

Microfractures originate in hydrocarbon reservoirs and groundwater aquifers either in the form of pore-scale authigenic apertures or secondary openings during well stimulation. Microfractures are often difficult to detect and can act as an invisible impediment when predicting hydrocarbon recovery. We use microfluidics to identify the flow condition...

Many important applications in the food industry, biological systems, and recovery of fluids from the subsurface involve multiphase flow dynamics where one of the phases is a non-Newtonian, viscoelastic fluid. In this work, we investigate the role of viscoelas-tic displacing fluids in recovering trapped nonwetting fluids. We perform the lattice Bol...

The rapidly increasing global oil/gas demand and gradual depletion of shallow reservoirs require the development of deep oil/gas reservoirs and geothermal reservoirs. However, deep drilling suffers from drilling-fluid failures under ultrahigh temperature, which cause serious accidents such as wellbore collapse, stuck pipe, and even blowouts. In thi...

We investigated the flow of viscoelastic surfactant (VES) solutions, an important type of fracturing fluids for unconventional hydrocarbon recovery, through a diverging–converging microfluidic channel that mimics realistic unit in porous media. Newtonian fluid and viscoelastic hydrolyzed polyacrylamide (HPAM) solution were used as control groups. W...

Fracturing fluid filtration loss is a significant research topic related to fracture propagation, formation damage and the cost of hydraulic fracturing. To reduce the loss, nanoparticles have been used to enhance the fracturing fluid properties such as viscosity and well-building ability. In this work, silica nanoparticles (NPs) with different conc...

Hydraulic fracturing can create a large fracture network that makes hydrocarbon production from low-permeability reservoirs economical. However, water can invade the rock matrix adjacent to the created fractures and generate water blockage that impairs production. Using surfactants as fracturing-fluid additives is a promising method to enhance the...

We present a study of the low salinity effect during oil recovery by microfluidics experiments, trying to narrow the gap between pore-scale observations and porous media flow mechanisms, and to explain one type of low-salinity effect with delayed oil recovery and without the presence of clay. A microfluidic toolbox is utilized, including single-por...

Correction for ‘Bio-inspired intelligent structural color materials’ by Luoran Shang et al. , Mater. Horiz. , 2019, DOI: 10.1039/c9mh00101h.

Bioinspired structural color material is a curiosity-led, technology-driven field with multidisciplinary researches aiming at mimicking and recapitulating natural coloration and functions. Over the past several decades, great achievements have been made, including natural structural coloration mechanisms, fabrication of artificial structural color...

Recovery in carbonate reservoirs is challenging because they are often oil wet and highly fractured. Surfactant flooding has been proposed as a possible enhanced oil recovery method to address these problems. To better understand the mechanisms of oil recovery from oil-wet, fractured rocks using surfactants, we created oil-wet glass micromodels, tr...

Improved understanding of the interplay between fracture and matrix fluid transport in the presence of injected water is important for the development of unconventional reservoirs. We make use of microfluidic experiments to investigate the impacts of fracture connectivity and matrix wettability on injected water sweep efficiency and viscous fingeri...

Nanoparticle (NP) based enhanced oil recovery (Nano-EOR) has been considered as a promising future EOR strategy. However, although many mechanisms of Nano-EOR have been proposed, a lack of direct connections between the pore-scale mechanisms and the macro-scale oil recovery performance makes it hard to determine which mechanisms are dominant. In th...

We discover a novel nanoparticle (NP) - crude oil interaction and propose a mechanism of NP-based enhanced oil recovery. This NP–crude oil interaction and its’ effect on oil recovery are systematically investigated conducting microfluidic experiments in both single-pore scale and “reservoir-on-a-chip” scale. It is confirmed that hydrophilic silica...

Emulsions have been widely applied in subterranean enhanced oil recovery processes, cosmetics and chemistry engineering fields, etc. Current research interests are focusing on improving the stability of emulsion by involving surfactants or colloidal materials. This work aims to enhance the emulsion stabilization by introducing silica nanoparticles...

A new microfluidic approach to preparing anisotropic colloidal photonic crystal microparticles is developed and the self-assembly kinetics of colloidal nanoparticles is discussed. Based on the ?coffee ring? effect in the self-assembly process of colloidal silica particle in strong solvent extraction environment, we successfully prepared anisotropic...

Injecting oil-in-water (O/W) emulsions stabilized with nanoparticles (NPs) or surfactants is a promising option for enhanced oil recovery (EOR) in harsh-condition reservoirs. Stability and rheology of the flowing emulsion in porous media are key factors for the effectiveness of the EOR method. The objective of this study is to use microfluidics to...

njecting oil-in-water (O/W) emulsions stabilized with nanoparticles or surfactants is a promising option for enhanced oil recovery (EOR) in harsh-condition reservoirs. Stability and rheology of flowing emulsion in porous media are key factors for the effectiveness of the EOR method. The objective of this study is to use microfluidics to (1) quantit...

Injecting oil-in-water (O/W) emulsions stabilized with nanoparticles or surfactants is a promising option for enhanced oil recovery (EOR) in harsh-condition reservoirs. Stability and rheology of flowing emulsion in porous media are key factors for the effectiveness of the EOR method. The objective of this study is to use microfluidics to (1) quanti...

The flow of multiple fluid phases in porous media often results in trapped droplets of the nonwetting phase. Recent experimental and theoretical studies have suggested that nanoparticle aqueous dispersions may be effective at mobilizing trapped droplets of nonwetting fluid (oil) in porous media. Hypotheses to explain the observation include the nan...

Here we developed a novel and facile microfluidic approach to produce W–O Janus droplets with a wide operating range (Ca number and volume ratio of the two dispersed phases). We then used this approach to fabricate hydrophilic–hydrophobic anisotropic Janus particles (HHAJPs). The volume ratio of the two dispersed phases and the exposure area of a s...

In this study, we developed a new method for the direct measurement of differential pressures in a co-flow junction microfluidic device using a Capillary Laplace Gauge (CLG). The CLG - used inside the microchannel device - was designed using a tapered glass-capillary set up in co-flow junction architecture with a three-phase liquid-liquid-gas syste...

The optical properties of close-packed polystyrene colloidal crystal particles could be improved and controlled via adjusting their shape, by first forming colloidal suspension–gas Janus droplets with different structures in a double coaxial microfluidic device, and then solvent-extraction-derived self-assembly to form nonspherical microparticles w...

In this work, we developed a novel and simple microfluidic method for the fabrication of self-assembled monodispersed photonic crystal microbeads with core–shell structures using solvent extraction. Monodispersed aqueous droplets encapsulating colloidal photonic crystal particles were produced in a T-junction microfluidic device, and the controlled...