Mixed wettability

The overall gain of the multiphase mixture model approach above is that the two-phase flow is still considered, but the simulations have only to solve pseudo-one-phase equations. Problems can arise if the equations are not averaged correctly. Also, the pseudo-one-phase treatment may not allow for pore-size distribution and mixed wettability effects to be considered. Furthermore, the multiphase mixture model predicts much lower saturations than those of Natarajan and Nguyen - and Weber and Newman even though the limiting current densities are comparable. However, without good experimental data on relative permeabilities and the like, one cannot say which approach is more valid. 

Figure 15. 3-D liquid water distributions in a hydrophobic and a mixed wettability GDL from the two-phase LB simulations.

These considerations come into play in oil recovery schemes applied to reservoirs of mixed wettability or where the rock is predominantly oilwetting. Another example is the case of the so-called Pickering emulsions. 

Waterflood recoveries were similar in the clean cores with any of the oils, and recoveries were higher in the asphaltene-treated cores (Figure 19). This kind of behavior has been associated with mixed-wettability systems (124—126), defined by Salathiel (125) as systems in which the small pores are water-wet, and the larger pores form continuous oil-wet channels that allow efficient displacement of oil by water. All three criteria used for wettability assessment (contact angle, end-point relative permeabilities, and waterflood recoveries) indicate more oil-wet conditions after asphaltene treatment. 

In the mixed-wettability cores, adsorption is affected only slightly by the presence of oil. The compensating effects of a decrease in the accessible solid—water interface and an increase in oil—water interface could be the reason for the relatively small effect. The most notable effect of the asphaltene treatment is the substantially higher adsorption density on the more oil-wet rock. Surfactant adsorption is higher on hydrophobic surfaces than it is on hydrophilic surfaces, a finding that is consistent with literature data. 

Worden et al. 1998). Wettability would also have to be taken into account the present calculations assume a water-wet system, where the diffusion of solutes in the water phase is retarded by sorption onto mineral surfaces (Ad assumed to be 5 in equation (13)), while components of oil and gas are not significantly slowed (Ad assumed to be 0). Clearly, other wettability scenarios (oil-wet or mixed wettability) could significantly affect the model results. 

In [13], a mixing method is proposed which applies to porous media with mixed wettability, i.e. locally consisting of pores with different wetting characteristics. It is reasonable to assume the same for fuel cell PTLs as well. One outcome of that work is that the discontinuity in the derivative for 5 —> 1 does not occur. 

On one hand, it is possible to fit quite well the behavior of the Brooks-Corey and van Genuchten curves (Figure 8.1), using this relationship for a regularized pressure pressure formulation. On the other hand, fits to measurements for pore spaces of mixed wettability can be performed quite well using this ansatz [1]. 

In an attempt to understand how nature is able to achieve such low residual oil saturations without help of engineers or laboratory specialists, Salathiel in 1973 (34) described a method for changing the wettability of rock surfaces in an unusual way. By treating cores which were saturated with typical values of oil and water, he was able to generate a mixed-wettability condition in the laboratory wherein surfaces in the larger pores were primarily oil-wet and rock surfaces in the smaller pores remained water-wet. 

With this special mixed wettability condition, it appears that the oil relative permeability can remain finite to very low saturations. 

Oil-bearing sedimentary rocks are frequently classified as either water-wet or oil-wet porous media, although categories of intermediate and mixed wettabilities are sometimes also set up. Wettability refers variously to spontaneous imbibition of water or oil, to the shapes of the curves of the relative permeabilities versus saturation (fraction of pore volume occupied by a given fluid) or to an apparent contact angle, i.e. the visually observed angle of intersection of a water-oil meniscus with a smooth surface of the rock or of an ostensibly equivalent solid material. 

Fassi-Fihri et al. [30] used cryomicroscopy on Brent reservoir sandstone to study the placement of oil. Illite was found associated with brine and the reservoir oil had an affinity for kaolinite. Quartz and feldspar were noted to be generally in contact with the brine. Wettability was heterogeneous on the pore scale. The field was considered to have mixed wettability since in laboratory tests both oil and brine would spontaneously imbibe into the rock. 

For multicomponent porous materials with mixed wettability, which are widely used in applied electrochemistry (e.g., fuel cell electrodes containing platinum particles on carbonaceous supports along with different additives), it is possible to investigate separately the structure (pore volume distribution versus pore size) for hydrophilic and hydrophobic (liophilic and liophobic) pores and to evaluate some important parameters, such as the dependence of the fraction p of the pore surface occupied by the hydrophobic (liophobic) components on the pore size. 

Figure 14.2 shows differential distribution functions of the pore volume in terms of pore radii (r) and p values, 9 V/9p, 91ogr for a layer containing 16% PTFE, calculated from the porograms. It can be seen that this function has three maxima two for hydrophilic and one for hydrophobic pores. From these results it can be concluded that the sample investigated contains only two types of pores completely hydrophilic, located between platinum particles, and completely hydrophobic, located between PTFE particles. No pores with mixed wettability were recorded. No functions of this type could be found in the literature. 

Key structural characteristics determining these processes are atomistic surface structure and electronic structure of the catalyst, morphology of the pore network, surface structure and wettability of the support, catalyst nanoparticle shape and size, ionomer structure, mixed wettability of the composite layer, and, last but not least, the electrode thickness, Icl-... 

Demin, W., Muchang, G., and Gang, W. 2002. The Influence of Multi-Pore Volume Water Flooding on Pore Structure and Recovery of Lacustrine Deposit Mixed Wettability Cores. Paper SPE 77873 presented at the SPE Asia Pacific Oil and Gas Conference and Exhibition, Melbourne, Australia, 8-10 October. DPI 10.2118/77873-MS. 

Huifen, X., Demin, W., Junzheng, W., and Fanshun, K. Elasticity of HPAM Solutions Increases Displacement Efficiency Under Mixed Wettability Conditions. Paper SPE 88456 presented at the SPE Asia Pacific Pil and Gas Conference and Exhibition, Perth, Australia, 18-20 Gctober. DPI 10.2118/88456-MS. 

Figure 6 9 Schematic of different liquid water imbibitions and transport behavior in hydrophilic and hydrophobic pores. The catalyst layer and diffusion media are typically mixed wettability media thus hydrophilic and hydrophobic pathways exist for transport of hquid and gas phases.

Gas-Phase Transport in Catalyst, Microporous, and Diffusion Layers The role of the different porous media in the fuel cell (catalyst layer, microporous layer, and diffusion media) in liquid transport can be grasped by understanding capillary flow behavior in mixed wettability porous media. The role of these porous media in controlling gas -phase flow can be simply understood through gas-phase transport relationships. Recall from Chapter 5 that the diffusivity in porous media must be modified to account for the tortuosity and porosity of the porous media ... 

Gerteisen et al. [28] have summarized the PEMFC modeling development and introduced a ID, two-phase, transient model including GDL, catalyst layer, and membrane, under the assumption that GDL, catalyst layer, and membrane are spatially resolved in ID with an agglomerate approach for the structure of catalyst layer. The faults of water flooding and membrane dehydration are anbedded by the assumption that the saturation due to the continuous capillary pressure and immobile saturation due to the mixed wettability of the GDL structure are discontinued. In order to allow dehydration of the ionomer on the anode side, the water content is not a constant but follows the Cauchy boundary condition. The model is validated by voltammetry experiments, and the simnlated cnrrent responses are compared with the measured ones from chronoamperometric experiments. 

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