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Nanoparticles of silver chloride have been synthesized by the method of mixing of two microemulsions, one containing silver ions and other containing chloride ions. The effects of changing the intermicellar exchange rate by varying the continuous phase, by adding benzyl alcohol, and by varying the water to surfactant molar ratio as well as the effect of cations (metal chlorides from the first and second group of the periodic table) on the particle size and the size distribution and the number density have been studied. The particle diameters are measured from the photomicrographs obtained by transmission electron microscopy. The average particle size, the polydispersity, and the number of particles formed are shown to be dependent on the intermicellar exchange rate and/or the rigidity of the surfactant shell. This dependency can be qualitatively explained by means of nucleation and growth phenomena, as mediated by the intermicellar exchange of contents.

We report the effects of intermicellar exchange rate on absorption spectra and particle size of silver nanoparticles synthesized in reverse micelles of AOT. The silver nanoparticles are prepared by the method of mixing of two microemulsions, one containing the silver nitrate and the other containing sodium borohydride. The intermicellar exchange rate is varied by changing the organic solvent, surfactant (SDS, NP-5, and DTAB), and organic (benzyl alcohol and toluene) additives. The higher intermicellar exchange rate is found to give smaller particle size and blue shift in the absorption spectra. An interesting and potentially useful effect has been observed in that addition of a small amount of a nonionic surfactant significantly reduces the particle size.

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A critical salt concentration (CSC) was found to exist in the water sensitivity of Berea sandstone. If the salinity of the permeating fluid falls below the CSC, the sandstone permeability is significantly reduced as a result of clay particles being released from the pore walls and blocking the pore throats. However, when changes in salinity occur above this threshold value, clay particles are not released from the pore walls in the sandstone and therefore no reduction in core permeability occurs. The CSC was determined by core flood experiments in conjunction with particle analysis of core effluent samples. The CSC exists only in the case of monovalent cations and is virtually nonexistent for cations having a valence greater than one, Even among monovalent cations the CSC varies considerably and it decreases with increasing ion exchange affinity of the clay for the counterion. For polyvalent counterions, the release of clay particles is effectively prevented due to the strong ion-exchange affinity of clay for the counterion. The critical salt concentration was also found to depend on the temperature, but not on the flowrate, of the electrolyte solution. The temperature dependence has been explained by using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of stability of colloidal dispersions.

Experimental and theoretical studies have been carried out to elucidate the mechanism of water sensitivity of Berea sandstone and to quantify a number of important parameters. Based on the results of a number of novel experiments, a physical model has been developed. In this model, clay particles are released only when the salt concentration falls below a critical salt concentration. These colloidal clay particles remain dispersed in fresh water and are carried with the flowing fluid until they are captured at a local pore constriction, thereby decreasing the permeability. A mathematical model based on this mechanism has been developed. This model contains two parameters stemming from the rate equations of the release and capture of clay particles. Correlations of these parameters with flow rate and temperature are presented. Introduction The water sensitivity of sandstone is a colloidal phenomenon whereby the permeability of the sandstone is decreased rapidly and significantly after the sandstone is contacted with fresh water. This phenomenon is demonstrated by a standard water shock experiment in which the flow through a sandstone core is changed abruptly from salt water to fresh water. The results of a standard water shock experiment are shown in Fig. 1. The normalized permeability (k/kl) drops from 1.0 to about 0.01 after only 2 or 3 PV of fresh water have been forced through the core. Permeability reduction resulting from water sensitivity is of serious concern to the oil- and gas-producing industries. Water sensitivity, first recognized during waterflooding of petroleum reservoirs, is now a concern in many other field operations that require aqueous solutions, such as drilling, solution mining, and stimulation. Even though water sensitivity in sandstone has been recognized for 35 years, the literature on this subject is limited. The works of Gray,1 Mungan,2,3 Jones,4 and Hewitt5 are among the most widely cited. These papers document the phenomenon and concur that the water sensitivity results from clay swelling, clay particle migration, or a combination of these effects, depending on the composition of the sandstone. Clay particle migration is the most important mechanism of permeability reduction since sandstones containing very little or no swelling clays and a considerable amount of migratory or dispersible clays such as kaolinite and illite are water-sensitive. Gray,1 Mungan,2,3 and Jones4 have reported results relating permeability reduction to clay particle migration. Previous studies investigated the effects of salt solutions, pH, and rate of decrease in salinity on the water sensitivity of sandstone. However, an in-depth analysis of the processes of dispersion and plugging of clay particles and how these processes are affected by flow rate, temperature, and salt concentration has not been reported in the literature. Some effects have been explained inadequately or incorrectly. These are discussed in detail elsewhere.6 In addition, a mathematical model describing quantitatively the permeability reduction with time and other parameters has not been developed. Such a model would aid in understanding the dynamics of this phenomenon as well as in designing preventive measures. The study of water sensitivity is also of general scientific interest since the phenomenon involves a number of colloidal and interfacial phenomena, such as flocculation, peptization, filtration, and adsorption. Practical considerations and scientific interests warrant a comprehensive study of this phenomenon. In this paper, a mathematical model is developed, and comparisons with experimental observations are made. These observations include permeability restoration with saltwater reversal, sequential permeability reduction, effect of flow rate, and core length.