Formation damage in production wells has been less studied than in injection wells. In injection wells, decline in well injectivity can occur, due to fine particles suspended in the water. These particles are deposited in the reservoir, blocking the pores, causing reduction in the permeability of reservoir rock near the well. Thus, the decline is due to injected water quality and reservoir properties.In production wells, particles can be transported through the reservoir and cause formation damage. The damage is caused by deposition near the wellbore as a result of the converging flow geometry. As is briefly discussed, it is a common problem in the water industry, where water production wells suffer from productivity decline due clogging of the aquifer near the wellbore. The accumulation of particles causes plugging of pores and decreases the permeability creating a damaged zone near the well. This results in extra, expensive cleaning operations and early shut-in of producers. This paper reports on an investigation on particle deposition in converging flow geometry, modeling oil and water production wells. Parameters are varied to study the effect on particle deposition of flow rate, particle concentration and particle/grain aspect ratio. The experiments use a converging-flow unconsolidated sand-pack. A hematite particle suspension is injected into the sand pack to observe clogging effects. With the aid of a CT-scanner (X-ray tomography), deposition profiles in time are obtained. Effluent particle concentrations and pressure profiles are also measured in real time. The results of the experiments are interpreted using deep bed filtration theory in converging flow geometry. The experiments show a clear effect of converging flow geometry on particle deposition.
Rock-salt lead selenide nanocrystals can be used as building blocks for large scale square superlattices via two-dimensional assembly of nanocrystals at a liquid-air interface followed by oriented attachment. Here we report Scanning Tunneling Spectroscopy (STS) measurements of the local density of states of an atomically coherent superlattice with square geometry made from PbSe nanocrystals. Controlled annealing of the sample permits the imaging of a clean structure and to reproducibly probe the band gap and the valence hole and conduction electron states. The measured band gap and peak positions are compared to the results of optical spectroscopy and atomistic tight-binding calculations of the square superlattice band Electronic properties of atomically coherent square PbSe nanocrystal superlattice resolved by Scanning Tunne structure. In spite of the crystalline connections between nanocrystals that induce significant electronic couplings, the electronic structure of the superlattices remains very strongly influenced by the effects of disorder and variability.
Inorganic–Organic lead halide materials have been recognized as potential high-energy X-ray detectors because of their high quantum efficiencies and radiation hardness. Surprisingly little is known about whether the same is true for extreme-ultraviolet (XUV) radiation, despite applications in nuclear fusion research and astrophysics. We used a table-top high-harmonic generation setup in the XUV range between 20 and 45 eV to photoexcite methylammonium lead bromide (MAPbBr 3 ) and measure its scintillation properties. The strong absorbance combined with multiple carriers being excited per photon yield a very high carrier density at the surface, triggering photobleaching reactions that rapidly reduce the emission intensity. Concurrent to and in spite of this photobleaching, a recovery of the emission intensity as a function of dose was observed. X-ray photoelectron spectroscopy and X-ray diffraction measurements of XUV-exposed and unexposed areas show that this recovery is caused by XUV-induced oxidation of MAPbBr 3 , which removes trap states that normally quench emission, thus counteracting the rapid photobleaching caused by the extremely high carrier densities. Furthermore, it was found that preoxidizing the sample with ozone was able to prolong and improve this intensity recovery, highlighting the impact of surface passivation on the scintillation properties of perovskite materials in the XUV range.
We present a new coordination polymer, {[VO(pzdc)(H2O)2] H2O}n, built from vanadyl and pyrazine-2,5-dicarboxylate (pzdc) ions. It consists of a one-dimensional chain of vanadyl ions linked by pzdc ions. The carboxylate groups show monodentate coordination, while the pyrazine ring is present both in non-coordinated and coordinated modes. This novel structure is stabilized by an intricate network of hydrogen bonds. The material is highly robust, and thermally stable up to 400 K. It is also antiferromagnetic, with a maximum magnetic susceptibility at ca. 50 K. The orbital shape and population analysis by means of DFT analysis confirm the π-acceptor role of the aromatic nitrogen function of the ligand, while the oxygen-based moieties (carboxylates from pzdc, the aqua ligands and oxo from V=O group) behave as normal donors. Charting the density flow related with significant transitions computed by time-dependent DFT, we determined the ligand-to-metal charge transfer processes. The topology of the chain complex implies two different types of connecting bridges. Using Broken Symmetry DFT modelling gives evidence for two different exchange coupling mechanisms between the vanadyl ions along each of these two molecular bridges. One is strongly antiferromagnetic, practically reducing the chain to 'vanadyl dimers'. The other is almost uncoupled, due to the large distance between the vanadyl ions.
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