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1 Introduction Tungsten trioxide (WO 3 ) thin films have interesting physicochemical properties for many applications such as catalysts in oxidation and reduction reactions, electro-chromic devices and gas sensors [1][2][3][4][5]. Tungsten trioxide-based gas sensors are efficient for detecting numerous gases; more significant are NO 2 , H 2 S and NH 3 [3-7]. In this field various studies were devoted to two important effects. First: these devices are also capable of detecting many other compounds present in vapor states within the atmosphere. Thus, in order to be able to use these devices for characterizing various pollutants in the atmosphere, it is necessary to first understand their behavior when placed in contact with WO 3 -based gas sensors. Second: the physicochemical properties of WO 3 thin films similar to other transition-metal oxides depend mainly on the structure and morphology, which are strongly dependent on the conditions of preparation [7][8][9]. In the case of the WO 3 thin films, the degree of structural order depends on the methods of preparation and annealing treatments. As-deposited WO 3 thin films are usually either polycrystalline with short-range order or amorphous. However, for applications such as gas sensors, thin films have to be an-

Application of matrix stimulation treatments in horizontal wells with openhole completions is a challenging task. Acid diversion is one of the key factors that determine the success of these treatments. This is especially true when the target zone has high permeability streaks. Proper acid distribution in carbonate formations can be achieved by increasing the viscosity of the acid in-situ. This can be done by using polymer/cross-linker or viscoelastic surfactant acid systems. The latter system was selected in the present study because of the tight nature of the reservoir and the presence of hydrogen sulfide in the disposal water. Both factors do not favor the use of polymer-based acids. The surfactant used in the diversion stage is amphoteric. It carries a positive charge in the presence of live HCl acid. Once the acid reacts with the carbonate minerals, the pH of the solution rises and the concentrations of divalent cations (calcium and magnesium) increase. Both factors enhance the viscosity of the solution in-situ. This high viscosity will help in diverting the live acid into other un-stimulated zones. Once the acid stimulation is complete, the high viscosity of spent acid can be broken either by dilution with the injection water or by adding mutual solvent in the pre and post-flushes. Viscoelastic surfactant-based acids have been successfully used to stimulate oil and gas wells over the last three years. This is the first time these acids were used to matrix acidize water disposal wells worldwide. No operational problems were encountered. Injectivity tests conducted after the acid treatment indicated that the treatment removed the damage; iron sulfide, calcium carbonate particles and trapped oil droplets. The skin factor decreased from + 22 before the treatment to nearly zero after the treatment. The pumping rate increased by a factor of three and the injectivity index increased by a factor of 14. The well maintained its injectivity up to the present time (more than seven months after the treatment) and still showing no signs of decline. An additional benefit of the viscoelastic surfactant acid system is its ability to decrease friction losses during acid pumping. Field data are given to support this new benefit. Introduction One of the main goals of acid stimulation treatments is to remove formation damage induced by drilling mud filter cake. However, the stimulation fluid will flow through the path of least resistance where the permeability is high or the damage (skin) is low. Hydrochloric acid reacts with carbonate materials very fast creating high conductivity flow paths in the formation (also known as wormholes). As a result, most of the injected acid would flow through these paths leaving the majority of the formation untreated. Therefore, there is a need for a proper fluid diversion to enhance the outcome of the treatment.1–4 Mechanical and chemical means have been utilized for diversion during matrix acidizing treatments with various degrees of success. Unlike cased wells, mechanical means e.g., isolation packers, are less efficient in horizontal wells with open hole completion. Polymer-based acids and foam are the most preferred chemical means for diversion during matrix stimulation of carbonate formations. However, there were some concerns raised when polymer-based acids were used in tight, sour formations.5,6 Viscoelastic surfactant-based fluids have been used for diversion in carbonate reservoirs. They were used to enhance the properties of foams used for diversion in water injectors.7 They were also used to increase the viscosity of HCl during matrix acidizing of horizontal oil producers.8 This paper presents the first application of a viscoelastic surfactant-based acid system to enhance acid diversion in a horizontal water disposal well with an openhole completion. The objective of the stimulation treatment was to increase the injectivity index by at least four folds. This was needed to avoid drilling additional wells for water disposal in this area.

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The effect of annealed (0001) α-Al 2 O 3 surfaces on heteroepitaxial growth of silver nanoparticles were analysed by reflection high-energy electron diffraction, transmission electron microscope and selected area electron diffraction. Ag nanoparticles were deposited on 1 × 1 stoichiometric and reconstructed (111)Al//(0001) α-Al 2 O 3 with the Knudsen cell. The maximum cluster density method and the Lifethenz theory of Van der Waals energy were used to investigate the Ag//(0001)α-Al 2 O 3 interface parameters. The growth modes, lattice parameters, nanoparticle forms and sizes are strongly dependent on the substrate surface structures. Initially, three-dimensional islands of Ag nanoparticles grow on both kinds of surfaces with partial hexagonal shapes. Ag nanoparticles on stoichiometric surface create the (111)Ag//(0001)α-Al 2 O 3 interface without any preferred epitaxial direction. On this surface, Gaussian distribution is characteristic of an atom-by-atom growth mode with density of Ag nanoparticles lower than saturation density while a coalescence growth mode appears due to binary collisions between Ag nanoparticles accompanied by a liquid-like behaviour after saturation density. In case of reconstruction substrates, the epitaxial relationships between Ag nanoparticles and the surface are formed (111)Ag// (0001) • around the epitaxial orientations 1100 or 1230 . Only the atom-by-atom growth mode were found at all Ag nanoparticles growth processes.

Thick lms of zinc oxide (ZnO) nanopowders have been prepared by high energy ball-milling for various spans of mill time (318 h). The morphology and crystal structure of the prepared ZnO powder were characterized by scanning electron microscope and X-ray diraction. The ZnO thick lms were then used to construct a gas sensor for O,O-dimethyl dithiophosphate of diethyl mercaptosuccinate (malathion) at dierent operating temperatures. The sensor response at 100 ppm of malathion was found to reach a maximum as large as 80 at 6 h of high energy ball-milling, four times larger than that found for ethanol. Scanning electron microscope observation of the granular state and pore size distribution analyses indicated that increasing high energy ball-milling time gave rise especially to an increase in the volume of pores in the pore size range of 635 nm. It is suggested that such a change in nanostructure is responsible for the marked promotion of the response to malathion.