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SummaryPreformed particle gel (PPG) holds promising potential for conformance control in fractured tight reservoirs as it enables mitigation of fracture channeling with insignificant leak off to matrix. However, conventional PPG is very susceptible to shrinkage, breakage, fatigue, and even degradation when extruding through narrow fractures due to its weak and brittle network. This hampers its development and application in the oilfields. This paper presents a comprehensive laboratory evaluation of a new kind of nanocellulose (NCF)-engineered robust particle gel (N-PPG) for this application. The results demonstrated that the presence of NCF noticeably improved the mechanical properties of N-PPG. The swelling kinetics and swelling ratio (SR) of N-PPG were almost independent of salinity. We packed porous media using millimeter-sized glass beads to replicate proppant-filled fractures after hydraulic fracturing. As anticipated, N-PPG exhibited a greater resistance factor (Fr) and residual resistance factor (Frr), and its plugging efficiency reached more than 99.3%. N-PPG was hardly broken even after extruding from pore-throat geometries with Dg/Dp up to 21.4, whereas the control PPG was notably ruptured at Dg/Dp = 14.7. Herein, this tough N-PPG could provide a solution to conformance control of fractured tight reservoirs.

Preformed-particle-gel (PPG) holds promising potential for conformance control in fractured tight reservoirs because it enables mitigation of fracture channeling with insignificant leakoff. However, conventional PPG with weak and brittle network is severely vulnerable to shrinkage, breakage, fatigue and even degradation during extruding through narrow fractures that were much smaller than themselves, and finally results in the failure of gel treatment. Therefore, in this work, a new kind of nanocellulose-regulated robust particle-gel (N-PPG) was designed and prepared using high-modulus and green nanocellulose (NCF). The mechanical properties of N-PPG including hardness, springiness, resilience, chewiness and cohesiveness were assessed using a texture analyzer at the grain-scale. The results demonstrated that the presence of NCF (0.1 wt%) noticeably improved the mechanical properties of PPG, 49.5% increment of hardness, 29.3% of resilience, 86.3% of chewiness and 25% of cohesiveness. The swelling test showed that the salinity had slight effect on the swelling kinetics and equilibrium swelling ratio (SR) of N-PPG. N-PPG exhibited excellent tolerance to the acidic solution. After aging for 44 days, SR fluctuated slightly and maintained at 9 cm3/g. The gel skeleton was not collapsed, and the microstructure was similar to the control group (aged in the neutral solution). Upon transporting through the fractures, the porous media was packed using millimetric-sized glass beads to replicate proppant-filled fractures after hydraulic fracturing. N-PPG exhibited significantly higher resistance factor (Fr) and residual resistance factor (Frr), indicative of better performance in conformance control. The influence of particle size, velocity, and PPG elasticity on the transport and placement of PPG in fractures were investigated. Due to the preeminent mechanical properties, N-PPG was hardly broken even after being extruded out from pore-throat geometries with up to a particle-throat diameter ratio (Dg/Dp) of 15, whereas the control PPG was notably crushed, implying the low efficiency in deep applications of conformance control. The Fr of PPG in fractures analogously depended on Dg/Dp even if the superficial velocity (u) was varied from 0.72 m/d to 4.32 m/d. To fully consider the interaction between deformable particle and fluid in the complex pore-throat geometries, an Immersed Boundary-Lattice Boltzmann modeling (IB-LBM) was developed to numerically simulate PPG passing through a throat (50 μm). A spring-network model was used to capture the deformation of PPG. The grain-scale modeling yielded the pressure profile of PPG, from which a clog-deform-pass procession mode was defined.

Multiphase displacement in porous media can be adjusted by micro/nanoparticle suspensions, which is widespread in many scientific and industrial contexts. Direct visualization of suspension flow dynamics and corresponding multiphase patterns is crucial to understanding displacement mechanisms and eventually optimizing these processes in geological, biological, chemical, and other engineering systems. However, suspension flow inside the opaque realistic porous media makes direct observation challenging. The advances in microfluidic experiments have provided us with alternative methods to observe suspension influence on the interface and multiphase flow behaviors at high temporal and spatial resolutions. Macroscale processes are controlled by microscale interfacial behaviors, which are affected by multiple physical factors, such as particle adsorption, capillarity, and hydrodynamics. These properties exerted on the suspension flow in porous media may lead to interesting interfacial phenomena and new displacement consequences. As an underpinning science, understanding and controlling the suspension transport process from interface to flow patterns in porous media is critical for a lower operating cost to improve resource production while reducing harmful emissions and other environmental impacts. This review summarizes the basic properties of different micro/nanoparticle suspensions and the state-of-the-art microfluidic techniques for displacement research activities in porous media. Various suspension transport behaviors and displacement mechanisms explored by microfluidic experiments are comprehensively reviewed. This review is expected to boost both experimental and theoretical understanding of suspension transport and interfacial interaction processes in porous media. It also brings forward the challenges and opportunities for future research in controlling complex fluid flow in porous media for diverse applications.

We report non-monotonic wettability effects on displacement efficiency in heterogeneous porous structures at the post-breakthrough stage, in contrast to the monotonic ones in homogeneous porous structures. Experiments on designed microfluidic chips show that there exists a critical wettability to attain the highest efficiency of displacement in the porous matrix structure combined with a preferential flow pathway, while a stronger wettability of the displacing fluid leads to a higher displacement efficiency on the same matrix structure only. The porous structure with or without a preferential flow pathway results in totally different topological characteristics of phase distribution during displacement. Pore-scale mechanisms are identified to elucidate the formation of this non-monotonic wettability rule: cooperative pore filling under weakly water-wet conditions yields the best displacement; corner flow under strongly water-wet conditions and Haines events under strongly oil-wet conditions decrease the displacement efficiency. The pore-scale findings may provide unique insights into the joint effects of both wettability and flow heterogeneity on fluid displacement in porous media.

Preferential flow in porous media is commonly encountered and decreases the multiphase displacement efficiency. Here, we synthesized microgel-in-oil in suspension and demonstrated that microgel-in-oil as a novel additive could present self-adaptive transport behavior and introduce a novel multiphase displacement mode for improving displacement efficiency in heterogeneous porous media.We investigated the microgel-in-oil formation process and characterized their morphology with fluorescence microscopy and Cryo-SEM. The suspension displacement performance in heterogeneous porous media was evaluated using a microfluidic chip containing a preferential flow pathway (PFP) and a parallel matrix region. The displacement results of microgel-in-oil were compared to plain microgel particles and analyzed from pore-scale particle transport behavior to macroscopic multiphase flow patterns.The results show that suspension with moderate microgel-in-oil yields the optimal displacement efficiency. Fewer microgel-in-oil cannot alter the flow direction, while too many microgel-in-oil would block the PFP region. The topological analysis identified that suspensions with moderate microgel-in-oil content could achieve the strongest sweeping and carrying abilities that contribute to the highest displacement efficiency. The synergistic transport of microgel-in-oil and plain microgel particles would result in local pressure fluctuations to divert displacing fluid from PFP into the matrix region, which explains the above flow behavior.

Micro‐gel particle suspensions (MGPS) have been proposed for enhanced oil recovery (EOR) in reservoirs with harsh conditions in recent years, yet the mechanisms are still not clear because of the complex property of MGPS and the complex geometry of rocks. In this paper, the micro‐gel particle‐based flooding has been studied by our microfluidic experiments on both bi‐permeability micromodels and reservoir‐on‐a‐chip. A method for reservoir‐on‐a‐chip design has been proposed based on QSGS (quartet structure generation set) to ensure that the flow geometry on chip owns the most important statistical features of real rock microstructures. In the micromodel experiments with heterogeneous microstructures, even if the MGPS has the same macroscopic rheology as the hydrolyzed polyacrylamides (HPAM) solution for flooding, MGPS may lead to significant fluctuations of pressure field caused by the nonuniform concentration distribution of particles. In the reservoir‐on‐a‐chip experiments, clustered oil trapped in the swept pores can be recovered by MGPS because of pressure fluctuation, which hardly happens in the HPAM flooding. Compared with the water flooding, the HPAM solution flooding leads to approximately 17% incremental oil recovery, while the MGPS results in approximately 49.8% incremental oil recovery in the laboratory.

Commercial-scale photoelectrochemical (PEC) water splitting devices have been proved to be cost-competitive with fossil-based fuels. Nevertheless, large-scale PEC water splitting cells with high performance have not yet been demonstrated, and controllably scaling up high-performance photoelectrodes still remains challenging. Typically, bismuth vanadate (BiVO4) has been regarded as one of the most efficient photoanodes for use in PEC water splitting because of its well-suited band structure. However, state-of-the-art PEC performances achieved with BiVO4 photoanodes to date have generally been tested on small scales (

Deformable micro-gel particles (DMP) have been used to enhanced oil recovery (EOR) in reservoirs with unfavourable conditions. Direct pore-scale understanding of the DMP transport mechanism is important for further improvements of its EOR performance. To consider the interaction between soft particle and fluid in complex pore-throat geometries, we perform an Immersed Boundary-Lattice Boltzmann (IB-LB) simulation of DMP passing through a throat. A spring-network model is used to capture the deformation of DMP. In order to obtain appropriate simulation parameters that represent the real mechanical properties of DMP, we propose a procedure via fitting the DMP elastic modulus data measured by the nano-indentation experiments using Atomic Force Microscope (AFM). The pore-scale modelling obtains the critical pressure of the DMP for different particle-throat diameter ratios and elastic modulus. It is found that two-clog particle transport mode is observed in a contracted throat, the relationship between the critical pressure and the elastic modulus/particle-throat diameter ratio follows a power law. The particle-throat diameter ratio shows a greater impact on the critical pressure difference than the elastic modulus of particles.

Preferential flow that leads to non-uniform displacement, especially in heterogeneous porous media, is usually unwelcome in most practical processes. We propose a self-adaptive preferential flow control mechanism by using dispersed polymers, which is supported strongly by experimental and numerical evidence. Our experiments are performed on a microchip with heterogeneous porous structures where oil is displaced by dispersed polymer microsphere particles. Even though the size of the particles is much smaller than the pore-throat size, the diversion effect by the dispersed microspheres is still proved. Therefore, the plugging effect is not the major mechanism for preferential flow control by dispersed polymers. The mechanisms are further investigated by pore-scale modelling, which indicates that the dispersed polymers exhibit an adaption ability to pressure and resistance in the porous flow field. In such an intelligent way, the displacing fluid with dispersed polymers smartly controls the preferential flow by inducing pressure fluctuations, and demonstrates better performance in both efficiency and economic aspects than the traditional method by simply increasing the viscosity. These insights can be applied to improve techniques in the field, such as enhanced oil recovery and soil wetting.

This study focuses on the dynamic process of deformation and start-up of an oil droplet on a wall driven by an external shear flow. A mesoscopic numerical method is developed based on the lattice Boltzmann framework for multiphase coupled with the color-gradient model. After validations of the numerical method, we performed force analyses based on the flow field, considering the pressure difference and the shear stress inside the droplet simultaneously. It was intriguing to find that the shear stress is a driving force instead of a resistance due to the interesting phenomenon that a vortex was formed inside the droplet and the direction of reaction shear stress acted by solid wall accorded with the direction of the start-up. A force-balance model was established on the basis of the force analyses to predict the yield criteria in terms of the critical capillary number and pressure difference. Finally, by comparing the flow field before and after the start-up of the droplet, it was found that the proportion of the wall applying shear driving force decreases but the values of the shear stress increase significantly after the start-up compared to before the start-up.

A lattice Boltzmann (LB) framework is developed for simulation of three-phase viscoelastic fluid flows in complex geometries. This model is based on a Rothman-Keller type model for immiscible multiphase flows which ensures mass conservation of each component in porous media even for a high density ratio. To account for the viscoelastic effects, the Maxwell constitutive relation is correctly introduced into the momentum equation, which leads to a modified lattice Boltzmann evolution equation for Maxwell fluids by removing the normal but excess viscous term. Our simulation tests indicate that this excess viscous term may induce significant errors. After three benchmark cases, the displacement processes of oil by dispersed polymer are studied as a typical example of three-phase viscoelastic fluid flow. The results show that increasing either the polymer intrinsic viscosity or the elastic modulus will enhance the oil recovery.

Since graphene is a type of 2D carbon material with excellent mechanical, electrical, thermal, and optical properties, the efficient preparation of graphene macroscopic assemblies is significant in the potentially large-scale application of graphene sheets. Conventional preparation methods of graphene macroscopic assemblies need strict conditions, and, once formed, the assemblies cannot be edited, reshaped, or recycled. Herein, inspired by the biomineralization process, a feasible approach of shapeable, multimanipulatable, and recyclable gel-like composite consisting of graphene oxide/poly(acrylic acid)/amorphous calcium carbonate (GO-PAA-ACC) is designed. This GO-PAA-ACC material can be facilely synthesized at room temperature with a cross-linking network structure formed during the preparation process. Remarkably, it is stretchable, malleable, self-healable, and easy to process in the wet state, but tough and rigid in the dried state. In addition, these two states can be readily switched by adjusting the water content, which shows recyclability and can be used for 3D printing to form varied architectures. Furthermore, GO-PAA-ACC can be functionalized or processed to meet a variety of specific application requirements (e.g., energy-storage, actuators). The preparation method of GO-PAA-ACC composite in this work also provides a novel strategy for the versatile macroscopic assembly of other materials, which is low-cost, efficient, and convenient for broad application.