A positive temperature coefficient is the term which has been used to indicate that an increase in solubility occurs as the temperature is raised, whereas a negative coefficient indicates a decrease in solubility with rise in temperature.

Effect of Temperature

The classical Poisson-Boltzmann (PB) theory of electrolytes assumes a dilute solution of point charges with mean-field electrostatic forces. Even for very dilute solutions, however, it predicts absurdly large ion concentrations (exceeding close packing) for surface potentials of only a few tenths of a volt, which are often exceeded, e.g. in microfluidic pumps and electrochemical sensors. Since the 1950s, several modifications of the PB equation have been proposed to account for the finite size of ions in equilibrium, but in this two-part series, we consider steric effects on diffuse charge dynamics (in the absence of electro-osmotic flow). In this first part, we review the literature and analyze two simple models for the charging of a thin double layer, which must form a condensed layer of close-packed ions near the surface at high voltage. A surprising prediction is that the differential capacitance typically varies non-monotonically with the applied voltage, and thus so does the response time of an electrolytic system. In PB theory, the capacitance blows up exponentially with voltage, but steric effects actually cause it to decrease above a threshold voltage where ions become crowded near the surface. Other nonlinear effects in PB theory are also strongly suppressed by steric effects: The net salt adsorption by the double layers in response to the applied voltage is greatly reduced, and so is the tangential "surface conduction" in the diffuse layer, to the point that it can often be neglected compared to bulk conduction (small Dukhin number).

Steric effects in the dynamics of electrolytes at large applied voltages: I. Double-layer charging

A general description of the mathematical and numerical formulations used in modern numerical reactive transport codes relevant for subsurface environmental simulations is presented. The formulations are followed by short descriptions of commonly used and available subsurface simulators that consider continuum representations of flow, transport, and reactions in porous media. These formulations are applicable to most of the subsurface environmental benchmark problems included in this special issue. The list of codes described briefly here includes PHREEQC, HPx, PHT3D, OpenGeoSys (OGS), HYTEC, ORCHESTRA, TOUGHREACT, eSTOMP, HYDROGEOCHEM, CrunchFlow, MIN3P, and PFLOTRAN. The descriptions include a high-level list of capabilities for each of the codes, along with a selective list of applications that highlight their capabilities and historical development.

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Reactive transport codes for subsurface environmental simulation

Spontaneous imbibition of wetting liquids in porous media is a ubiquitous natural phenomenon which has received much attention in a wide variety of fields over several decades. Many traditional and recently presented capillary-driven flow models are derived based on Hagen–Poiseuille (H–P) flow in cylindrical capillaries. However, some limitations of these models have motivated modifications by taking into account different geometrical factors. In this work, a more generalized spontaneous imbibition model is developed by considering the different sizes and shapes of pores, the tortuosity of imbibition streamlines in random porous media, and the initial wetting-phase saturation. The interrelationships of accumulated imbibition weight, imbibition rate and gas recovery and the properties of the porous media, wetting liquids, and their interactions are derived analytically. A theoretical analysis and comparison denote that the presented equations can generalize several traditional and newly developed models fr...

https://eps.utk.edu/faculty/perfect/Papers/Cai%20et%20al.%20(2014).pdf

Generalized Modeling of Spontaneous Imbibition Based on Hagen− Poiseuille Flow in Tortuous Capillaries with Variably Shaped Apertures

Despite having a more advanced water management system than most Middle Eastern countries, similar to the other countries in the region, Iran is experiencing a serious water crisis. The government blames the current crisis on the changing climate, frequent droughts, and international sanctions, believing that water shortages are periodic. However, the dramatic water security issues of Iran are rooted in decades of disintegrated planning and managerial myopia. Iran has suffered from a symptom-based management paradigm, which mainly focuses on curing the problem symptoms rather than addressing the main causes. This paper reviews the current status of water resources in Iran and recognizes three major causes for the current water crisis: (1) rapid population growth and inappropriate spatial population distribution; (2) inefficient agriculture sector; and (3) mismanagement and thirst for development. The country is faced with serious challenges in the water sector, including but not limited to rising water demand and shortage, declining groundwater levels, deteriorating water quality, and increasing ecosystem losses. If immediate actions are not taken to address these issues, the situation could become more tragic in the near future. The paper suggests some crisis exit strategies that need to be immediately adopted to secure sustainable water resources, if Iran does not want to lose its international reputation for significant success in water resources management over thousands of years in an arid area of the world.

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Water management in Iran: what is causing the looming crisis?

Improved oil recovery by low salinity flooding (LSF) in sandstone reservoirs is hypothesized to be the result of a wettability change of the crude oil, brine, rock (COBR) system to a more water-wet state. The exact mechanism behind the wettability change upon lowering the ionic strength of the brine is, however, not yet fully understood. It is generally accepted that a strong low salinity effect requires the presence of clay minerals in the reservoir rock and preferably a high salinity of the formation water containing divalent cations. Still, COBR systems that obey these minimum requirements may give a highly variable response to low salinity flooding. To create enhanced understanding of the critical parameter(s) controlling the low salinity effect, crude oil, rock and brine from three different reservoir systems were varied in all possible combinations in a series of spontaneous imbibition tests. These tests show that, for the COBR systems analyzed here, the rock is the most critical parameter for a strong low salinity effect. Cross-correlation of the change in water saturation upon exposure to low salinity, ΔSw LS, with various rock parameters indicated the strongest correlation with rock zeta potentials.

The Critical Parameter for Low Salinity Flooding-The Relative Importance of Crude Oil, Brine and Rock

Upscaling electroosmosis in porous media is a challenge due to the complexity and scale-dependent nonlinearities of this coupled phenomenon. “Pore-network modeling” for upscaling electroosmosis from pore scale to Darcy scale can be considered as a promising approach. However, this method requires analytical solutions for flow and transport at pore scale. This study concentrates on the development of analytical solutions of flow and transport in a single rectangular channel under combined effects of electrohydrodynamic forces. These relations will be used in future works for pore-network modeling. The analytical solutions are valid for all regimes of overlapping electrical double layers and have the potential to be extended to nonlinear Boltzmann distribution. The innovative aspects of this study are (a) contribution of overlapping of electrical double layers to the Stokes flow as well as Nernst–Planck transport has been carefully included in the analytical solutions. (b) All important transport mechanisms including advection, diffusion, and electromigration have been included in the analytical solutions. (c) Fully algebraic relations developed in this study can be easily employed to upscale electroosmosis to Darcy scale using pore-network modeling.

Analytical solution of electrohydrodynamic flow and transport in rectangular channels: inclusion of double layer effects

Pore-scale modelling is now a well-established progressive research field, which has significantly contributed to fundamental understanding of flow and transport in porous media in the last five decades. Examples can be (and not limited to) reservoir engineering [1–4], environmental hydrogeology [5], paper and pulp industry [6], fuel cells [7], biology, and medical science [8]. The first pore-scale model was developed by Fatt in 1956 [9], who simulated the capillary pressure curve as a function of saturation using a pore-network model. After this legendary work, pore-network models, mainly dominated by quasi-static pore-network models, were developed and applied to reservoir engineering and hydrogeology problems. The pore-network modelling opened up a new horizon in understanding the constitutive relations for two-phase flow such as capillary pressure and relative permeability curves [10, 11], and at a later stage exploring the transport phenomena in porous media [12]. With further technological developments in micromodel and X-ray imaging facilities and computational infrastructures, quantitative pore-network models were further developed [13]. Almost all pore-network models for different applications share the common principles: they require analytical or semianalytical solutions for a given specific research problem at the pore scale, meaning that the domain geometry requires

/pdf/pore-scale-modelling-techniques-balancing-efficiency-4xg25hirj6.pdf

Vahid Joekar-Niasar

Papers

The Critical Parameter for Low Salinity Flooding-The Relative Importance of Crude Oil, Brine and Rock

Analytical solution of electrohydrodynamic flow and transport in rectangular channels: inclusion of double layer effects

Pore-scale modelling techniques: balancing efficiency, performance, and robustness

Comparison of modified effective-medium approximation to pore-network theory for relative permeabilities

Large scale modeling of nitrogen transformation in the unsaturated zone???A case study of Tehran City, Iran