Wettability effect on nanoconfined water flow

Understanding and controlling flow of the water confined in nanopores has tremendous implications in theoretical studies and industrial applications. Here we propose a universal model for the confined water flow based on a conception of effective slip, which is linear sum of true slip, only depending on wettability, and apparent slip, caused by the spatial variation of the confined water viscosity as a function of wettability as well as nanopores dimension. Results by the model show that the flow capacity of the confined water is 10−1~107 times of those calculated by no slip Hagen-Poiseuille equation for nanopores with various wettability, in agreement with 47 different cases from the literature. This work may shed light on the controversy over the increase or decrease in flow capacity from the MD simulations and experiments, and guide to tailor the nanopores structure for modulating the confined water flow in many engineering fields, including nanomedicine, water purification, energy storage as well as the flowback of fracture fluid in petroleum industry.

The wettability of a core will strongly affect its waterflood behavior and relative permeability. Wettability affects relative permeability because it is a major factor in the control of the location, flow, and distribution of fluids in a porous medium. In uniformly or fractionally wetted porous media, the water relative permeability increases and the oil relative permeability decreases as the system becomes more oil-wet. In a mixed-wettability system, the continuous oil-wet paths in the larger pores alter the relative permeability curves and allow the system to be waterflooded to a very low residual oil saturation (ROS) after the injection of many PV's of water. The most accurate relative permeability measurements are made on native-state core, where the reservoir wettability is perserved. Serious errors can result when measurements are made on cores with altered wettability, such as cleaned core or core contaminated with drilling-mud surfactants.

Pore-scale modelling of NMR relaxation for the characterization of wettability

Wettability affects water configuration and thereby transport processes and microbial activity in soil. Approaches to visualize the effect of porous media wettability on water films surrounding particles are rarely available in the literature. The aim of this study is therefore (1) to visualize the effect of wettability on area and connectivity of the water phase and (2) to develop a segmentation strategy to enable water films and bulk water to be differentiated. Wettability of silica sand was rendered by silanization using dichlorodimethylsilane. The resulting contact angle was measured using the sessile drop method. Furthermore, wettability was characterized by the water penetration time test in air-dry samples and at a volumetric water content of 8 vol.%. Sulforhodamine B was used to stain distilled water at a concentration of 40 mg/l. By means of the Wilhelmy plate method, the influence of the dye on the liquid surface tension was tested. Confocal laser scanning microscopy (CLSM) was used to visualize the area and connectivity of the fluorescent-dyed water phase and the thickness of water films in untreated and silanized samples at water contents of 8, 16, and 32 vol.%. The silanization significantly increased the contact angle of silica sand, whereas the surface tension of the dye solution did not differ significantly from that of undyed water. CLSM visualized the distribution of water, focused across the surface grain layer. Thresholding of fluorescence signal in two-dimensional intensity projections enables the discrimination between film and bulk water. It was shown that even subcritical water repellency (contact angle <90) leads to a decrease in area and connectivity of the water phase by affecting film instead of bulk water. CLSM detects decreasing effects of wettability with increasing water content, which were no longer significant at a water content of 32 vol.%. CLSM was found to be more sensitive for detecting the effects of wettability than the water drop penetration time test. CLSM provides new insight into wettability-dependent water configuration. One advantage over conventional microscopy arises from the capability to visualize water focused over the pore space. Compared to other three-dimensional imaging techniques, the advantage of CLSM is its simplicity. If only the optically accessible upper layer of the porous media is of interest, sample preparation, measurements, and image analysis can easily be carried out with a cost- and time-saving setup.

Interaction between low molecular weight carboxylic acids and muscovite: Molecular dynamic simulation and experiment study

Diagnostic methods for tracking EOR processes have evolved considerably during recent years. Because each technique can provide different information about specific EOR processes, selection of the best technique for a problem must be done on a case-by-case basis. This paper describes techniques available for evaluating and diagnosing EOR processes and problems and presents selection criteria for each. Two examples are given to illustrate how a suite of techniques can be chosen.

Effects of nanopore geometry on confined water flow: A view of lattice Boltzmann simulation

The characterization of nanoconfined fluid critical properties is critical for nanoconfined fluid phase behavior and flow capacity evaluation. To date, fluid–surface interaction strength is considered as the original cause for the shift of fluid critical properties, which is closely related to pore size and surface wettability. However, the majority of research is devoted to investigating the impact of pore size, while investigations about surface wettability on nanoconfined fluid critical properties are lacking. In order to fill the knowledge gap, in-depth investigations have been carried out to shed light on the wettability effect on nanoconfined fluid critical properties. First, focusing on the relative strength between fluid–fluid interactions and fluid–surface interactions, a robust correlation is proposed to relate surface contact angle, a macroscopic form of wettability effect, to critical property shift. Then, a linear correlation between the wettability effect and the adsorption thickness is established, and as a result, effective pore size can be described as a function of wettability as well. Finally, a prediction model for nanoconfined fluid critical properties is developed, and its reliability is well-clarified against 86 cases, collected from previous molecular simulation results or experiments. The results show the following: (a) The magnitude of the shift of fluid critical property can reach as high as 60%, dominating the variation characteristics of nanoconfined fluid phase behavior. (b) Both the pore size shrinkage and strong fluid-affinity surface are underlying mechanisms for suppressed critical properties at nanoscale. (c) The confined effect arisen from the cylindrical geometry is 2.24 times greater than that for slit nanopores, essentially stemming from the fluid force condition exerted by the pore surface.

We study the effects of rock wettability on the flow of oil, water, and gas in hydrocarbon reservoirs. We describe the three-phase fluid configurations and displacement processes in a pore of polygonal cross section. Initially water-filled, water-wet pores are invaded by oil, representing primary oil migration. Where oil directly contacts the solid surface, the surface will change its wettability. We then consider water injection followed by gas injection for any possible combination of oil/water, gas/water, and gas/oil contact angles. We find the capillary pressures for the different displacement processes and determine the circumstances under which the various fluid configurations are stable. Using empirical expressions for the phase conductances, we find three-phase relative permeabilites for a bundle of pores of different sizes with constant triangular cross sections. For gas injection, we show that the oil remains connected in wetting layers down to low oil saturation with a characteristic layer drai...

Molecular dynamics (MD) simulations is currently the most popular and credible tool to model water flow in nanoscale where the conventional continuum equations break down due to the dominance of fluid-surface interactions. However, current MD simulations are computationally challenging for the water flow in complex tube geometries or a network of nanopores, e.g., membrane, shale matrix, and aquaporins. We present a novel mesoscopic lattice Boltzmann method (LBM) for capturing fluctuated density distribution and a nonparabolic velocity profile of water flow through nanochannels. We incorporated molecular interactions between water and the solid inner wall into LBM formulations. Details of the molecular interactions were translated into true and apparent slippage, which were both correlated to the surface wettability, e.g., contact angle. Our proposed LBM was tested against 47 published cases of water flow through infinite-length nanochannels made of different materials and dimensions-flow rates as high as seven orders of magnitude when compared with predictions of the classical no-slip Hagen-Poiseuille (HP) flow. Using the developed LBM model, we also studied water flow through finite-length nanochannels with tube entrance and exit effects. Results were found to be in good agreement with 44 published finite-length cases in the literature. The proposed LBM model is nearly as accurate as MD simulations for a nanochannel, while being computationally efficient enough to allow implications for much larger and more complex geometrical nanostructures.

Droplet evaporation is widespread in natural and industrial application, and the rapid and efficient evaporation can significantly improve energy efficiency. However, the fundamental mechanism of contact line dynamics and the microscopic characteristics of evaporating nanodroplets are not well understood. Moreover, how to design a nanostructure surface to enhance nanodroplet evaporation remains unclear. Here, through molecular dynamics simulation, we investigated the evaporation dynamics of nanodroplets on various nanoring surfaces with different geometric parameters and wettability. By measuring the changes of contact radius and contact angle, the results showed that nanodroplets successively exhibit constant contact angle (CCA), constant contact radius (CCR), and mix mode during evaporation, and the evaporation-induced CCA-CCR transition, in essence, is a Cassie-Wenzel wetting transition, whose onset time is remarkably dependent on the surface roughness and wettability. We found that this evaporation-induced wetting transition is postponed on the surface with small nanostructure spacing and weak hydrophilicity, and the evaporation rate of nanodroplets improves accordingly. The dense and hydrophobic nanostructures can not only restrain the Cassie-Wenzel transition, but also enhance the evaporation rate of nanodroplets. Last, through the potential energy field analysis of nanoring substrates, we revealed that the Cassie-Wenzel wetting transition of nanodroplets is a process of molecule migration to low potential energy regions. Our work provides guidance for designing nanostructure surfaces to effectively control the droplet wetting state and enhance its mass transfer performance of phase change.

Wettability Literature Survey- Part 4: Effects of Wettability Part 4: Effects of Wettability on Capillary Pressure Summary. The capillary-pressure/saturation relationship depends on the interaction of wettability, pore structure, initial saturation, and saturation history. No simple relationship exists that relates the capillary pressures determined at two different wettabilities. Therefore, the most accurate measurements are made with cores that have native reservoir wettability. In a uniformly wetted porous medium, pore geometry effects and the extremely rough surfaces of the porous medium make the capillary pressure curve insensitive to wettability for small contact angles (less than about 50 deg.[0.87 rad] for drainage capillary pressure curves and less than about 20 deg. [0.35 rad] for spontaneous-imbibition capillary pressure curves). When the porous medium has fractional or mixed wettability, both the amount and distribution of the oil-wet and water-wet surfaces are important in determining the capillary pressure curve, residual saturations, and imbibition behavior. Imbibition also depends on the interaction of wettability, pore structure, initial saturation, and saturation history. Because of these interactions, there is a large range of contact angles where neither oil nor water will imbibe freely into a uniformly wetted reservoir core. In contrast, it is sometimes possible for both fluids to imbibe freely into a core with fractional or mixed wettability. Contact Angles, Capillary Pressure, and Wettability This paper is the fourth in a series of literature surveys covering the effects of wettability on core analysis. Changes in the wettability of cores have been shown to affect electrical properties, capillary pressure, waterflood behavior, relative permeability, dispersion, simulated tertiary recovery, irreducible water saturation (IWS), and residual oil saturation (ROS). When oil and water are placed together on a surface, a curved interface between the oil and water is formed, with a contact angle at the surface that can range from 0 to 180 deg. [0 to 3.15 rad]. By convention, the contact angle, 0, is measured through the water. Generally, when 0 is between 0 and 60 to 75 deg. [0 and 1.05 to 1.31 rad], the system is defined as water-wet. When 0 is between 180 and 105 to 120 deg. [3.15 and 1.83 to 2.09 rad], the system is defined as oil-wet. In the middle range of contact angles, a system is neutrally or intermediately wet. It can be shown that whenever an oil/water interface is curved, the pressure will abruptly increase across the interface to balance the interfacial tension (IFT) forces. This pressure jump, which is the capillary pressure, is given by Laplace's equation : (1) where sigma = IFT, P = capillary pressure, p = pressure in the oil, p = pressure in the water, and r1, r2 = radii of curvature of the interface, measured perpendicular to each other. By convention, the capillary pressure is defined as po-pw. Because of this definition, a radius of curvature po-pw. Because of this definition, a radius of curvature directed into the oil is positive, while one directed into the water is negative. Depending on the curvature of the surface, the capillary pressure can be positive or negative. When the interface is flat, the capillary pressure is zero. When fluids other than oil and water are used, the capillary pressure is usually defined as (2) where pNW is the pressure in the nonwetting fluid and pWET is the pressure in the wetting fluid. pWET is the pressure in the wetting fluid. The radii of curvature of the interface, and hence the capillary pressure, are determined by local pore geometry, wettability, saturation, and saturation history. For most porous media, the equations for the interfacial curvature are much too complicated to be solved analytically, and capillary pressure must be determined experimentally. In these cases, a simple relationship between contact angle and capillary pressure cannot be derived. One geometry where capillary pressure can be calculated as a function of geometry, wettability, and IFT is a capillary tube. Laplace's equation can be used to solve for the capillary pressure as a function of IFT, contact angle, and rt, the radius of the tube. P. 1283

Effect of surface wettability on microfluidic EDGE emulsification

Nanostructured materials have attracted intensive scientific interests during the past two decades due to their outstanding physical and mechanical properties. However, the brittleness of nanostructured materials posed a great challenge for their engineering applications. Recently, several strategies were successfully adopted to produce nanostructured materials with both high strength and ductility such as surface-nanocrystallized (SNC) materials, nanocrystalline materials with stress-induced nanograin growth and nanotwinned metals. A lot of molecular dynamics (MD) simulations, modelling and experiments have been conducted to investigate the deformation mechanisms and the correlated exceptional mechanical properties and considerable progress has been made. However, some problems remain unsolved. For example, the complicated structure of SNC materials due to its grain size gradient (GSG) surface layer makes it difficult to establish a quantitative model for prediction of their strength and ductility; the main mode of nanograin growth in nanostructured materials, i.e., shear-coupled migration of grain boundaries (GBs), was experimentally observed as contributing to their enhanced ductility, but the mechanism of the enhancement remains unclear. In addition, there exist contradictory results for the grain size dependence of transitional twin thickness that corresponds to the maximum strength of nanotwinned metals. All these issues should be addressed to gain a better understanding of the mechanism-ductility correlation in order to provide some guidelines for designing lighter, stronger and ductile nanostructured materials. Therefore, an attempt was made to study the plastic deformation of nanostructured materials with high strength and ductility by theoretical modelling and numerical simulations.&#13;\nFirstly, the enhanced balance of strength and ductility of SNC materials was studied using a combination of theoretical analysis and finite element simulation. A criterion was established for determining the ductility of SNC materials. The results obtained showed that the ductility of a SNC sample could be comparable to that of its coarse-grained counterpart, while it simultaneously possessed a much higher strength than that of the latter if optimal GSG thickness and topmost phase grain size were adopted. Then a dislocation-density-based model was proposed to quantitatively predict the plastic deformation of SNC materials; the stress-driven nanograin growth was also incorporated in the said model. The capability of the model in predicting the strength and work hardening of SNC materials was validated by the existing experimental results. Thirdly, physical models for shear-coupled migration of GBs in nanostructured materials were developed to explain the general coupling between the shear and the normal migration of GBs observed in MD simulations and experiments. The coupled migration process was found to be a general and effective toughening mechanism in nanostructured materials. Moreover, our study showed that the shear-coupled migration is able to enhance the intrinsic ductility considerably when it cooperates with GB sliding. Finally, an elastic-viscoplastic constitutive model based on the competition of intra-twin and twin-boundary-mediated deformation mechanisms was proposed to predict the grain size dependent transitional twin thickness of nanotwinned metals. A linear relation between the transitional twin thickness and the grain size was predicted, which was in excellent agreement with the results obtained from MD simulations and experiments available in the literatures.

Enhanced water flow and apparent viscosity model considering wettability and shape effects