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Cobalt-containing mesoporous materials that have been prepared using different procedures have been comparatively characterized by transmission electron microscopy/energy-dispersive X-ray spectroscopy (TEM/EDS), extended X-ray absorption fine structure spectroscopy (EXAFS), X-ray absorption near edge spectroscopy (XANES), and ultraviolet-visible (UV-vis), near-infrared (NIR), and mid-infrared (mid-IR) spectroscopies, and the results provide new insights into the local environment and properties of cobalt in this type of material. TEM/EDS analyses have shown that tetraethyl orthosilicate (TEOS) may be less appropriate as a silicon source during the syntheses of cobalt-containing mesoporous materials, because the distribution of cobalt throughout the framework may become uneven. EXAFS has been determined to be the most suitable method for direct verification of framework incorporation, by identifying silicon as the backscatterer in the second shell. Such a direct verification may not be obtained using UV-vis spectroscopy. From EXAFS analyses, it is also possible to distinguish between surface-bound and framework-incorporated cobalt. There is a good agreement between the results obtained from XANES and UV-vis regarding the coordination symmetry of cobalt in the samples. The presence of cobalt in the silica framework has been determined to create Lewis acid sites, and these acid sites are suggested to be located at tetrahedral cobalt sites at the surface.

The objective of primary cementing is to protect the casing and to ensure zonal isolation. It can be difficult to obtain a good cement job along the full length of a well, and casing centralization is one of the main factors that influence this. Even if the dependence of cement placement on casing centralization is well-known, little information is available on how the degree of casing centralization affects the well during its production phase. Well temperatures cycle up and down as a part of normal production operations – and well barrier materials, in particular steel, cement and rock, will consequently repeatedly expand and contract their volumes. Over time, this is likely to induce debonding and radial cracking of the cement sheath which threatens well integrity. This paper reports the results of an experimental study mapping how, where and when the annular cement loses its sealing ability upon temperature variations, and how this is dependent on casing centralization. The studied samples consisted of rock, cement and casing, and the temperature was cycled in a controlled and programmable manner. In-situ monitoring by Acoustic Emission (AE) sensors detected the development of cracking and debonding in the samples during thermal cycling. Initial and post-experiment computed tomography (CT) scans provided complementary three-dimensional (3D) information on the geometry and location of the induced cracks and debonding. Our study compared the thermal cycling resistance of two samples, one with centralized casing and one with a 50% casing stand-off. The AE monitoring results indicated that most of the cracking/debonding occurred during the actual heating and cooling, and not in between cycles when the temperature was held constant. The CT analyses showed that the thermal cycling caused considerable enlargement of cracks and voids initially present in the cement sheath, and this enlargement was significantly more severe when the casing was not centralized. The paper presents, for the first time, a 3D visualization of cracks and debonded volumes in the cement sheath, and it underlines the importance of obtaining a good initial cement job. Also, it is shown that it is important to obtain a good casing centralization during well construction – not only for optimal cement placement, but also for maintaining well integrity during production.