Double Layer Expansion Research Articles

Effect of seawater ionic composition modified by nanofiltration on enhanced oil recovery in Berea sandstone

Water ionic composition significantly influences reservoir wettability and hence microscopic oil sweep. However, there is no clear explanation of how the ion composition of injection water influences the oil displacement. In this work, aqueous solutions were formulated from seawater using nanofiltration (NF) membranes, which has been currently used to remove sulfate from seawater in offshore oil prospecting. The effect of the interfacial tension (IFT) of water/crude oil, zeta potential and contact angle (CA) at water/crude oil/rock interface was evaluated to identify changes in wettability and also to relate with spontaneous imbibition experiments. The main objective of this study is to investigate the effect of the injection aqueous solutions composition on wettability and oil recovery in sandstone reservoirs.The results demonstrate that it is possible to improve the oil recovery efficiency by controlling the injection water salinity using NF process. Increased water salinity resulted in lower electrokinetic potential near the sandstone surface, approaching null values, suggesting adsorption of cations. The oil droplet interface also was affected in a similar way. Furthermore, in the presence of low salinity water, both rock and oil zeta potential was negative, which leads the electrical double layer (EDL) expansion, changing wettability and intensifying spontaneous imbibition.

Salinity-Dependent Contact Angle Alteration in Oil/Brine/Silicate Systems: the Critical Role of Divalent Cations.

The effectiveness of water flooding oil recovery depends to an important extent on the competitive wetting of oil and water on the solid rock matrix. Here, we use macroscopic contact angle goniometry in highly idealized model systems to evaluate how brine salinity affects the balance of wetting forces and to infer the microscopic origin of the resultant contact angle alteration. We focus, in particular, on two competing mechanisms debated in the literature, namely, double-layer expansion and divalent cation bridging. Our experiments involve aqueous droplets with a variable content of chloride salts of Na+, K+, Ca2+, and Mg2+, wetting surfaces of muscovite and amorphous silica, and an environment of ambient decane containing small amounts of fatty acids to represent polar oil components. By diluting the salt content in various manners, we demonstrate that the water contact angle on muscovite, not on silica, decreases by up to 25° as the divalent cation concentration is reduced from typical concentrations in seawater to zero. Decreasing the ionic strength at a constant divalent ion concentration, however, has a negligible effect on the contact angle. We discuss the consequences for the interpretation of core flooding experiments and the identification of a microscopic mechanism of low salinity water flooding, an increasingly popular, inexpensive, and environment-friendly technique for enhanced oil recovery.

A Review on the Progress of Ion-Engineered Water Flooding

In the oil and gas industry, Enhanced Oil Recovery (EOR) plays a major role to meet the global requirement for energy. Many types of EOR are being applied depending on the formations, fluid types, and the condition of the field. One of the latest and promising EOR techniques is application of ion-engineered water, also known as low salinity or smart water flooding. This EOR technique has been studied by researchers for different types of rocks. The mechanisms behind ion-engineered water flooding have not been confirmed yet, but there are many proposed mechanisms. Most of the authors believe that the main mechanism behind smart water flooding is the wettability alteration. However, other proposed mechanisms are interfacial tension (IFT) reduction between oil and injected brine, rock dissolution, and electrical double layer expansion. Theoretically, all the mechanisms have an effect on the oil recovery. There are some evidences of success of smart water injection on the field scale. Chemical reactions that happen with injection of smart water are different in sandstone and carbonate reservoirs. It is important to understand how these mechanisms work. In this review paper, the possible mechanisms behind smart water injection into the carbonate reservoir with brief history are discussed.

Geometrical properties of materials for energy production by salinity exchange

Environmental contextOceans and seas have the potential to play a significant role in providing renewable and clean energy. In particular, salinity difference energy aims to extract the enormous amount of energy that is released when fresh water rivers flow into the oceans. Capmix methods are focused on this challenge by using capacitive carbon electrodes whose optimisation will certainly help in developing salinity difference energy. AbstractOne of the most powerful marine renewable resources is salinity difference energy, also termed blue energy. Numerous techniques have been investigated to harvest this energy but, recently, the capmix proposal has increased in importance due to its easy implementation and use of low cost materials, very often activated carbon. Two methods based on this principle are tested in this work, namely CDLE (energy production by double layer expansion in bare electrodes) and SE (the electrodes are made ‘soft’ by polyelectrolyte coating). The characteristics of the carbon materials play a central role in capmix energy production. In this work, we focus on understanding the required pore structure that might be demanded from carbon samples. The balance between micro- and mesopores, the wettability of the material and its electrical resistance are explored by using hierarchical carbons, and their combination with graphene oxide and carbon nanotubes. It is found that the CDLE technique requires a large fraction of mesopores for easy solution exchange, while SE performance improves with a large amount of micropores. The addition of carbon nanotubes to the activated carbon reduces the capmix cycle duration, increasing the extracted power. In the case of electrodes containing graphene the internal resistance decreases, but the hydrophobicity of graphene oxide works against the improvement in energy extraction.

Wettability alteration and oil recovery by spontaneous imbibition of low salinity brine into carbonates: Impact of Mg2+, SO42− and cationic surfactant

A large amount of the discovered oil reserves are reserved in carbonate formations, which are mostly naturally fractured oil-wet. Wettability alteration towards more water-wet state reduces the capillary barrier, hence improving the oil recovery efficiency in such reservoirs. In this study, wettability alteration towards favorable wetting state was investigated by combining modified low salinity brine with surfactant during water flooding. The diluted brine was modified by increasing the concentration of Mg2+ and SO42−, individually as well as both ions in combination. Different brine formulations were tested experimentally through the observations of contact angle measurements and spontaneous imbibition tests for oil-wet limestone core samples both in the presence and absence of surfactant. Contact angle measurements revealed that both magnesium and sulfate ions can act as wettability modifying agents for the oil-wet calcite and all of the modified low salinity brine solutions changed the wettability of oil-wet surface towards more water-wet state. In the presence of cationic surfactant, utilizing both wettability influencing ions and surfactant molecules resulted in more water-wet surfaces. The spontaneous imbibition results showed that the combination of salting-in effect, mineral dissolution and electric double layer expansion, cause wettability alteration towards more water-wet state and result in critical increase in oil recovery efficiency during spontaneous imbibition of diluted brine into limestone core sample. Modified brine which contains a higher concentration of sulfate ions was more efficient in wettability alteration in comparison to the brine modified with magnesium ions. High oil recovery efficiency can be achieved by optimizing the brine composition without using costly surfactant solutions. However, the combination of the effect of wettability influencing ions (Mg2+ and SO42−) along with the cationic surfactant can result in a remarkable oil production during the spontaneous imbibition process.

Effect of multi-component ions exchange on low salinity EOR: Coupled geochemical simulation study

Abstract The mechanism(s) of Low salinity water flooding (LSWF) has been extensively investigated for 15–20 years, as a cost-effective and environmentally friendly technique for improved oil recovery. However, there is still no consensus on the dominant mechanism(s) behind low salinity effect due to the complexity of interactions in the Crude oil/Brine/Rock (COBR) system. While wettability is most agreed mechanism of low salinity EOR effect. Nevertheless, the mechanism(s) behind the wettability change is debated between multi-component ion exchange (MIE) and double layer expansion (DLE) in sandstone reservoirs. This paper aims to investigate the effectiveness of MIE with a coupled geochemical-reservoir model using published experimental data reported by Nasralla and Nasr-El-Din [1]. We created core-scale numerical models with parameters identical to those used in the experiments. We simulated the low salinity effect using a commercial reservoir simulator, CMG-GEM, by coupling three chemical reactions: (1) aqueous reaction, (2) multi-component ion exchange, and (3) mineral dissolution and precipitation. We modelled the adsorption of divalent cations on the surface of the clay minerals during low salinity water injection. Simulation results were compared with the experimental results. Simulation results show that the fractional adsorption of divalent cations (Ca 2+ ) increased almost 25% by injecting a 2000 ppm NaCl solution, compared to initial 10,000 ppm NaCl. Injecting a 2000 ppm of CaCl 2 solution, however, significantly increased the adsorbed Ca 2+ from 0.1 to 1, which implies the complete saturation of mineral surface with divalent cations. Moreover, injecting 50,000 ppm of CaCl 2 solution also demonstrated the same effect as the 2000 ppm CaCl 2 solution but with a faster rate. Upon combining the simulation and experimental results, we concluded that the multi-component ion exchange is not the sole mechanism behind low salinity effect for two reasons. First, almost 10% additional oil recovery was observed from the experiments by injecting the 2000 ppm CaCl 2 compared with 50,000 ppm CaCl 2 solutions. Even though in both cases the surface is expected to be fully saturated with Ca 2+ according to the geochemical modelling. Second, 6% incremental oil recovery was achieved from the experiments by injecting 2000 ppm NaCl solution compared with that of 50,000 ppm NaCl. Although 25% incremental adsorption of divalent cations (Ca 2+ ) were presented during the flooding of the 2000 ppm NaCl solution. Therefore, it is worth noting that the electrical double layer expansion due to the ion exchange needs to be taken into account to pinpoint the mechanism(s) of low-salinity water effect.

Analysis and simulation of a blue energy cycle

The mixing process of fresh water and seawater releases a significant amount of energy and is a potential source of renewable energy. The so called ‘blue energy’ or salinity-gradient energy can be harvested by a device consisting of carbon electrodes immersed in an electrolyte solution, based on the principle of capacitive double layer expansion (CDLE). In this study, we have investigated the feasibility of energy production based on the CDLE principle. Experiments and computer simulations were used to study the process. Mesoporous carbon materials, synthesized at the Oak Ridge National Laboratory, were used as electrode materials in the experiments. Neutron imaging of the blue energy cycle was conducted with cylindrical mesoporous carbon electrodes and 0.5 M lithium chloride as the electrolyte solution. For experiments conducted at 0.6 V and 0.9 V applied potential, a voltage increase of 0.061 V and 0.054 V was observed, respectively. From sequences of neutron images obtained for each step of the blue energy cycle, information on the direction and magnitude of lithium ion transport was obtained. A computer code was developed to simulate the process. Experimental data and computer simulations allowed us to predict energy production.

Molecular Dynamic Simulations of Montmorillonite–Organic Interactions under Varying Salinity: An Insight into Enhanced Oil Recovery

Enhanced oil recovery is becoming commonplace in order to maximize recovery from oil fields. One of these methods, low-salinity enhanced oil recovery (EOR), has shown promise; however, the fundamental underlying chemistry requires elucidating. Here, three mechanisms proposed to account for low-salinity enhanced oil recovery in sandstone reservoirs are investigated using molecular dynamic simulations. The mechanisms probed are electric double layer expansion, multicomponent ionic exchange, and pH effects arising at clay mineral surfaces. Simulations of smectite basal planes interacting with uncharged nonpolar decane, uncharged polar decanoic acid, and charged Na decanoate model compounds are used to this end. Various salt concentrations of NaCl are modeled: 0‰, 1‰, 5‰, and 35‰ to determine the role of salinity upon the three separate mechanisms. Furthermore, the initial oil/water-wetness of the clay surface is modeled. Results show that electric double layer expansion is not able to fully explain the effec...

Numerical Simulation of the LSWI Effect on Hydrocarbon Recovery From Carbonate Rocks

Wettability alteration mechanism is still believed to be the main contributor to low salinity water injection (LSWI) effect on oil recovery from carbonate rocks. The authors investigated the main mechanism and contributor to the LSWI effect on oil recovery from carbonate rocks through history matching recently published corefloods using the UTCHEM simulator. The history match results suggest that wettability alteration has negligible effect on water relative permeability, which could be related to the double electric layer expansion mechanism. This paper provides a basis to develop a LSWI model to predict oil recovery from carbonate rocks.

A molecular theory for optimal blue energy extraction by electrical double layer expansion

Electrical double layer expansion (CDLE) has been proposed as a promising alternative to reverse electrodialysis (RED) and pressure retarded osmosis (PRO) processes for extracting osmotic power generated by the salinity difference between freshwater and seawater. The performance of the CDLE process is sensitive to the configuration of porous electrodes and operation parameters for ion extraction and release cycles. In this work, we use a classical density functional theory (CDFT) to examine how the electrode pore size and charging/discharging potentials influence the thermodynamic efficiency of the CDLE cycle. The existence of an optimal charging potential that maximizes the energy output for a given pore configuration is predicted, which varies substantially with the pore size, especially when it is smaller than 2 nm. The thermodynamic efficiency is maximized when the electrode has a pore size about twice the ion diameter.

Effect of strong acid functional groups on electrode rise potential in capacitive mixing by double layer expansion.

The amount of salinity-gradient energy that can be obtained through capacitive mixing based on double layer expansion depends on the extent the electric double layer (EDL) is altered in a low salt concentration (LC) electrolyte (e.g., river water). We show that the electrode-rise potential, which is a measure of the EDL perturbation process, was significantly (P = 10(–5)) correlated to the concentration of strong acid surface functional groups using five types of activated carbon. Electrodes with the lowest concentration of strong acids (0.05 mmol g(–1)) had a positive rise potential of 59 ± 4 mV in the LC solution, whereas the carbon with the highest concentration (0.36 mmol g(–1)) had a negative rise potential (−31 ± 5 mV). Chemical oxidation of a carbon (YP50) using nitric acid decreased the electrode rise potential from 46 ± 2 mV (unaltered) to −6 ± 0.5 mV (oxidized), producing a whole cell potential (53 ± 1.7 mV) that was 4.4 times larger than that obtained with identical electrode materials (from 12 ± 1 mV). Changes in the EDL were linked to the behavior of specific ions in a LC solution using molecular dynamics and metadynamics simulations. The EDL expanded in the LC solution when a carbon surface (pristine graphene) lacked strong acid functional groups, producing a positive-rise potential at the electrode. In contrast, the EDL was compressed for an oxidized surface (graphene oxide), producing a negative-rise electrode potential. These results established the linkage between rise potentials and specific surface functional groups (strong acids) and demonstrated on a molecular scale changes in the EDL using oxidized or pristine carbons.

Multi-ionic effects on energy production based on double layer expansion by salinity exchange

It has been recently shown that the free energy change upon salinity mixing in river mouths can be harvested taking advantage of the fact that the capacitance of charged solid/liquid interfaces (electrical double layers, EDLs) depends strongly on the ionic composition of the liquid medium. This has led to a new generation of techniques called Capmix technologies, one of them (CDLE or capacitive energy extraction based on DL expansion) based precisely on such dependence. Despite the solution composition playing a crucial role on the whole process, most of the research carried out so far has mainly focused on pure sodium chloride solutions. However, the effect of other species usually present in river and seawaters should be considered both theoretically and experimentally in order to succeed in optimizing a future device. In this paper, we analyse solutions of a more realistic composition from two points of view. Firstly, we find both experimentally and theoretically that the presence of ions other than sodium and chloride, even at low concentrations, may lead to a lower energy extraction in the process. Secondly, we experimentally consider the possible effects of other materials usually dispersed in natural water (mineral particles, microbes, shells, pollutants) by checking their accumulation in the carbon films used, after being exposed for a long period to natural sea water during CDLE cycles.

Impact of cation type and concentration in injected brine on oil recovery in sandstone reservoirs

Low salinity waterflood has a high potential to improve oil recovery. However, the optimum water chemistry for waterflooding is still not well known, and the dominant mechanisms are debated. In the present work, the effect of cation type and concentration in the injected water on the oil recovery to identify the optimum water salinity and composition was studied by performing coreflood experiments. In addition, zeta potentials at oil/brine and rock/brine interfaces were measured to examine the impact of brine composition on electrical double layer expansion and investigate if it is one of the dominate mechanisms. Furthermore, ionic exchange tests were conducted to improve the understanding of the rock/brines interactions. Berea sandstone cores were used for waterflooding and ion exchange experiments; all coreflood experiments were performed at high-pressure/high-temperature (HP/HT) conditions. Different concentrations of NaCl, CaCl2, and MgCl2 solutions were tested with two crude oils of different compositions and properties. Ion concentrations in the core effluent fluids were analyzed for both waterflooding and ionic exchange tests. The zeta potential was measured for solutions of oil/brine, and solid/brine; the solids were Berea sandstone, quartz, feldspar, and four types of clays. This work contributes to the understandings of the impacts of double layer expansion on oil recovery during waterflooding. Zeta potential results showed that Na+ changes the electrical charge at both oil/brine and rock/brine interfaces to highly negative, which results in higher repulsive forces between the two interfaces, and hence wettability alteration. Moreover, waterflood experiments with NaCl solutions improved oil recovery significantly compared to CaCl2 and MgCl2. These results showed that there seems to be a correlation between zeta-potential and oil recovery which implies that double-layer expansion could be a primary mechanism of oil recovery by low salinity waterflood. This study demonstrates that cation type has a significant impact on oil recovery, and it could be more dominant than the effect of total salinity of the injected brines. These findings can help in screening the brines that have higher potentials for oil recovery improvement.

Ions tuning water flooding experiments and interpretation by thermodynamics of wettability

Abstract In recent years, ions tuning water flooding (ITWF) has become a promising technique to recover oil in sandstone reservoirs. In view of research results for the last decade, it is acknowledged that substantial oil recovery beyond conventional waterflooding from sandstone is due to the wettability alteration. However, the major contributor to wettability alteration is still uncertain. Therefore, this paper investigates this major mechanism and shows how it is involved in the process of IOR. Rock surface chemistry and wettability alteration studies were conducted to explain the mechanisms involved for improving oil recovery in low permeability sandstone reservoirs. Moreover, the thermodynamics of wettability during the ITWF was analyzed to characterize the surface forces between the surfaces of oil/water and water/rock. The major mechanism of ITWF to recover incremental oil was confirmed by coreflood experiments with five different brines. Zeta potential results showed that decreasing divalent cations and salinity makes the electrical charges become strongly negative at both oil/brine and brine/rock interfaces, which result in elevating the repulsive forces between the interface of oil/water and interface of water/rock, and as a result the rock turns more water-wet, which was confirmed by thermodynamics characterization. Different coreflood experiments showed that ITW (0.5569%) with 0.023% divalent cation as secondary (starting at Swi) and tertiary (starting at Sorw) modes recovered 15.6% and 15.1% OOIP, respectively. It demonstrates that the ITW can improve oil recovery in secondary and tertiary modes. Injecting brine (0.2% NaCl) as tertiary mode recovered 10.3% OOIP after 5.8% NaCl solution was flooded at the secondary mode. Furthermore, 10% NaCl as tertiary mode only recovered 3.3% OOIP after the formation brine flooded the secondary mode. It proved that the multi-component ions exchange is not dominant to recover additional recovery without double layer expansion. In conclusion, double layer expansion was caused by the highly negative zeta potential as a result of lower salinity and the divalent cation plays a major role in recovering additional oil. These findings can help in ions tuning waterflood to maintain higher potential to recover oil.

Double-Layer Expansion: Is It a Primary Mechanism of Improved Oil Recovery by Low-Salinity Waterflooding?

Summary Literature review shows that improved oil recovery (IOR) by low-salinity waterflooding could be attributed to several mechanisms, such as sweep-efficiency improvement, interfacial-tension (IFT) reduction, multicomponent ionic exchange, and electrical-double-layer (EDL) expansion. Although these mechanisms might contribute to IOR by low-salinity water, they may not be the primary mechanism. Therefore, the main objective of this study is to investigate if the mechanism of EDL expansion could be the principal reason for IOR during low-salinity waterflooding. Low-salinity water results in a thicker EDL when compared to high-salinity water, so we tried to eliminate the effect of low-salinity brines on double-layer expansion to show to what extent IOR is related to EDL expansion caused by low-salinity water. The double-layer expansion is dependent on the electric surface charge, which is a function of the pH of brine; therefore, the pH levels of low-salinity brines were decreased in this study to provide low-salinity brines that can produce a thinner EDL, similar to high-salinity brines. ζ-potential measurements were performed on both rock/brine and oil/brine interfaces to demonstrate the effect of brine pH and salinity on EDL. Contact angle and coreflood experiments were conducted to test different brine salinities at different pH values, which could assess the effect of water salinity and pH on rock wettability and oil recovery, and hence involvement of EDL expansion in the IOR process. ζ-potential results in this study showed that decreasing the pH of low-salinity brines makes the electrical charges at both oil/brine and brine/rock interfaces slightly negative, which reduces the double-layer expansion caused by low-salinity brine. As a result, the rock becomes more oil-wet, which was confirmed by contact-angle measurements. Moreover, coreflood experiments indicated that injecting low-salinity brine at lower pH values recovered smaller amounts of oil when compared to the original pH because of the elimination of the low-salinity-water effect on the thickness of the double layer. In conclusion, this study demonstrates that expansion of the double layer is a dominant mechanism of oil-recovery improvement by low-salinity waterflooding.

Materials selection for optimum energy production by double layer expansion methods

The capacitive mixing procedure for energy extraction based on Double Layer Expansion (CDLE) belongs to the group of so-called CAPMIX techniques, which aim at obtaining energy from the salinity difference between fresh and sea waters. Specifically, the CDLE technique takes advantage of the voltage rise that occurs when sea water is exchanged for river water in a pair of porous electrodes which jointly behave as an electrical double layer supercapacitor. In this article, we deal with some experimental aspects that appear essential for optimizing the extracted energy, and have not yet been analyzed with sufficient detail. This investigation will help in evaluating those parameters which need to be fixed in a future CDLE device. These include the charging potential, the durations of the different cycle steps, the load resistance used, and the porosity and hydrophilicity of the carbon.

Capacitive Mixing for Harvesting the Free Energy of Solutions at Different Concentrations

An enormous dissipation of the order of 2 kJ/L takes place during the natural mixing process of fresh river water entering the salty sea. “Capacitive mixing” is a promising technique to efficiently harvest this energy in an environmentally clean and sustainable fashion. This method has its roots in the ability to store a very large amount of electric charge inside supercapacitor or battery electrodes dipped in a saline solution. Three different schemes have been studied so far, namely, Capacitive Double Layer Expansion (CDLE), Capacitive Donnan Potential (CDP) and Mixing Entropy Battery (MEB), respectively based on the variation upon salinity change of the electric double layer capacity, on the Donnan membrane potential, and on the electrochemical energy of intercalated ions.

Predictions of the maximum energy extracted from salinity exchange inside porous electrodes

Capacitive energy extraction based on double layer expansion (CDLE) is the name of a new method devised for extracting energy from the exchange of fresh and salty water in porous electrodes. It is based on the change of the capacitance of electrical double layers (EDLs) at the electrode/solution interface when the concentration of the bulk electrolyte solution is modified. The use of porous electrodes provides huge amounts of surface area, but given the typically small pore size, the curvature of the interface and EDL overlap should affect the final result. This is the first aspect dealt with in this contribution: we envisage the electrode as a swarm of spherical particles, and from the knowledge of their EDL structure, we evaluate the stored charge, the differential capacitance and the extracted energy per CDLE cycle. In all cases, different pore radii and particle sizes and possible EDL overlap are taken into account. The second aspect is the consideration of finite ion size instead of the usual point-like ion model: given the size of the pores and the relatively high potentials that can be applied to the electrode, excluded volume effects can have a significant role. We find an extremely strong effect: the double layer capacitance is maximum for a certain value of the surface potential. This is a consequence of the limited ionic concentration at the particle-solution interface imposed by the finite size of ions, and leads to the presence of two potential ranges: for low electric potentials the capacitance increases with the ionic strength, while for large potentials we find the opposite trend. The consequences of these facts on the possibility of net energy extraction from porous electrodes, upon changing the solution in contact with them, are evaluated.

Electro-diffusion of ions in porous electrodes for capacitive extraction of renewable energy from salinity differences

One dimensional theory of the electro-diffusion of ions in activated carbon porous electrodes is applied to describe the dynamic cycle of the capacitive mixing (CAPMIX) based on the double layer expansion (CDLE) technique to harvest renewable energy from salinity gradients. The model combines the electro-diffusion of ions with adsorption and desorption of charge and neutral salt into the double layers at the solid liquid interface, providing a comprehensive and accurate description of the full CAPMIX cycle experimentally measured in prototype cells. A careful analysis of the simulated cycles identifies key parameters for the optimization of the extracted power, like the appropriate thickness and micro-structure of the electrodes, best materials and operation conditions of the electrochemical cell. These directions will be fundamental in the development of this technique as an economically competitive renewable energy source.

Faster Time Response by the Use of Wire Electrodes in Capacitive Salinity Gradient Energy Systems

Capacitive energy extraction based on Donnan potential (CDP) and capacitive energy extraction based on double layer expansion (CDLE) are novel electroctrochemical processes to convert the potential free energy of mixing sea and river water into electric work. This is done by the use of supercapacitor electrodes with and without ion exchange membranes. Currently, these techniques rely on improved mass transport in order to become more efficient and give higher power output. In this paper we evaluate the transport phenomena by diffusion and the electrode geometry when switching between sea and river water at open circuit potential (OCP). By changing the electrode geometry from a flat plate to a cylindrical one, experiments and analytical models in combination show that mass transport by diffusion is increased. This is demonstrated without any changes in the hydrodynamic conditions. Improving mass transport without changing the hydrodynamic conditions breaks with what has been the convention in the scientific community of salinity gradient power. Moreover, in sea water the transport phenomena appear to be controlled by diffusion, and the response time for building open circuit potential in CDP and CDLE under this condition is reduced by a factor of 2 when using wire electrodes instead of flat plate electrodes. In river water, the trend is similar though the response time is generally larger.