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The aim of this study was to evaluate the chemical resistance and mechanical properties of cement class G with n-SiO2 addition after being exposed to CO2-saturated water at HPHT, simulating geological carbon storage condition. Four different amounts of n-SiO2 (0.5, 1, 1.5 and 3 wt%) and a standard cement (STD Cement) were tested with CO2-saturated water at 150 bar and 90 °C for 7 and 56 days. The workability of the slurries was evaluated by mini slump test and helium gas pycnometry was used to measure the specific density of unreacted hardened cement systems. Zones affected by CO2 reactions (bicarbonated, carbonated and portlandite depleted zones) and unreacted core were analyzed using optical and scanning electron microscopes, energy dispersive spectroscopy by line scan, X-ray microtomography and atomic force microscopy. Vickers microhardness and uniaxial compressive strength were used to obtain information about alteration in mechanical properties. The results showed that the addition of n-SiO2 reduced the workability of the slurries and had insignificant influence on specific density of the hardened cement. After 7 days of exposure to CO2 medium, the 1.5% n-SiO2 was the most effective cement system to reduce CO2 degradation, decreasing the chemical altered thickness to 2.63 mm when compared to STD Cement (3.06 mm). Results from 56 days of exposure to CO2 show that only 0.5% n-SiO2 cement system is similar in terms of carbonation to STD Cement. For other n-SiO2 amounts (1%, 1.5% and 3%) the thicknesses of chemically altered layer are bigger than STD Cement. However, changes in chemical composition, microstructure and density from periphery to the core of the cement system were less accentuated in the cement systems with n-SiO2 addition after 56 days of cement systems exposure to CO2. Furthermore, the n-SiO2 cement systems presented a lower loss in compressive strength values when compared to STD Cement after reaction with CO2.

From our work, significant progress has been made in understanding the degradation of cement-casing systems. The CO2 degradation process was evaluated in specimens with a large interfacial defect, such as large annular spaces, voids and/or channels, which may be the result of a poor cementing job. From the experiments showing no interfacial defect, no signs of degradation were observed, while from experiments showing interfacial defect, both the cement and steel undergo significant degradation. In the well casing, the CO2-rich brine affects the steel phase, leaching Fe2+ ions into solution and promoting FeCO3 precipitation on the material surface, while on the cement sheath, two processes are occurring: (i) the portlandite dissolution and (ii) the cement carbonation process. Then, iron (Fe2+) starts to migrate into the cement structure, compromising the material’s self-healing and pore-blocking features, while calcium (Ca2+) starts to compose the corrosion film from the formation of mixed carbonates (FexCayCO3) so reducing the corrosion layer’s protection. Finally, both ions (Ca2+ and Fe2+) become so abundant in the material vicinity that they may form calcium carbonate (CaCO3) on the corrosion layer and iron carbonate (FeCO3) in the cement matrix. Thus, from our results, the degradation mechanisms of the cement-casing system in CO2-rich brine was revised.

The performance of cement class G used in cementation of oilfield wellbores with addition of pozzolans was evaluated under geological carbon storage conditions. Two commercial synthetic zeolites types (4A-1 and 4A-2) and fly ash from a coal-fired plant were used as pozzolanic materials in amounts of 5 and 10% in weight replacing the cement. After curing, the cement samples were submitted to degradation tests in CO2-saturated water at 15 MPa and 90 °C for 7 and 14 days. The cement chemical degradation by CO2 was investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD) and compressive strength tests. The chemically altered layer thickness was averaged 3.46 mm for standard cement after 14 days of exposure to CO2. On the other hand, cement systems with 10% wt. of pozzolanic material varied from 1.70 to 5.50 mm depending on the type of pozzolan and level of cementitious matrix porosity related to pozzolanic particle clustering. In general, 4A-1 zeolite presented better performance in terms of resistance to CO2 attack and higher compressive strength after 14 days when compared to 4A-2 zeolite. The results showed that the addition of fly ash improved the compressive strength of the samples but increased the chemically altered layer due to CO2 diffusion. SEM and XRD analyses showed that the portlandite was consumed and carbonation occurred in the chemical modified layer due to cement reaction with aqueous CO2. Most cement systems with and without pozzolanic material exhibited no expressive loss on compressive strength after being exposed to CO2-rich environment up to 14 days. On the contrary, some cement systems with 4-A1 zeolite and fly ash exhibited a mechanical resistance increase due to the carbonation process.

The CaCO3 influence on the corrosion properties of low carbon steel in aqueous solutions saturated with CO2 and NaCl at 80 °C and 15 MPa was investigated over time with respect to morphology, thickness, structure, chemical composition and corrosion rate. The corrosion product formed in CaCO3-based solution was a calcium-enriched siderite and the scales were thinner and more porous than the ones formed in solutions without CaCO3. The CaCO3 reduced the corrosion rate, but the scales produced in the presence of this compound presented depassivation followed by formation of pits during electrochemical measurements effectuated on corroded samples.

In this paper the influence of copper and magnesium content in the microstructural evolution during the solidification of Zn–4 wt.% Al hypoeutectic alloys was investigated using the CA-CCA method (Computer-Aided Cooling Curve Analysis) and SEM (Scanning Electron Microscopy). The identification of chemical composition of the phases and microconstituents was done by SEM using the EDS (Energy Dispersive X-Ray Spectroscopy) operation mode. For that purpose, ternary and quaternary alloys were prepared with different amounts of copper and magnesium. The influence of both copper and magnesium amounts on the transformation temperatures of the Zn–Al based hypoeutectic alloys was evident in the distinct microstructures formed during solidification as well as in the cooling curves obtained by thermal analyses, promoting modifications in solid–liquid temperature range, in the kinetic and also in the chemical compositions of the phase transformation. The addition of extra copper promoted the formation of significant quantities of the copper-rich phase (CuZn 4 precipitate) in the interdendritic region, while the addition of extra magnesium promoted the formation of the magnesium-rich phase and changed not only the morphology of the primary dendrites but also its relative content. Besides, an increase in the relative primary eutectic structure and a decrease in the quantity of the lamellae eutectoid structure were observed. Additionally, the secondary lamellar eutectic became more refined in the presence of higher magnesium content. All the cooling curves are in agreement with the observed microstructure. Both elements, copper and magnesium, also promoted an increase in the hardness of the Zn–4Al hypoeutectic alloys due to the formation of CuZn 4 phase and to the secondary lamellae eutectic refinement, respectively.

We show the results of some test beams performed on a new monolithic strip silicon detector telescope developed in collaboration with the INFN and ST-microelectronics. Using an appropriate design, the induction on the ΔE stages, generated by the charge released in the E stage, was used to obtain the position of the detected particle. The position measurement, together with the low threshold for particle charge identification, allows the new detector to be used for a large variety of applications due to its sensitivity of only a few microns measured in both directions.

This paper reports results of an investigation of the behaviour of several etchants in revealing structural defects in Al-, Te-, Cd- and Te/Cd-doped GaSb. Etchants previously suggested for GaSb, as well as two etchants developed for GaAs and InP, were tested and modified in an attempt to establish the best conditions under which a given etchant may be used and what defects it is likely to reveal. Both the solutions consisting of H 2 O 2 -H 2 SO 4 and 3%Br-methanol provided reproducible etch figures on dislocations, the former on the (111)Ga, (111)Sb, (100) and (110) planes, and the latter only on the (111)Ga plane. In contrast, H 2 O 2 -HC1 and HC1-HNO 3 -H 2 O did not perform satisfactorily in revealing dislocations in the analyzed samples. Using permanganate etchant, it was possible to observe growth striations only in the n-type GaSb. The CrO 3 -HF solution revealed growth striations on both the p-type and the n-type GaSb as well as defects formed due to constitutional supercooling. A new etchant, based on ceric sulfate, was developed to delineate growth striations along the Te-doped GaSb crystals.