This work provides a detailed atom probe tomography study of clustering in the Al–Mg–Si system. Focus is on separating and understanding the influence of natural aging, preaging, and alloy composition on the clustering behavior of solute atoms. Two dilute alloys with the same total solute content have been studied, one Mg-rich and one Si-rich. The detrimental effect of natural aging for these alloys is investigated by comparing directly preaged samples to samples stored at room temperature before the preaging treatment. Clusters were identified in the atom probe datasets by the maximum separation method employing heuristically determined input parameters. It was found that seven days of intermediate natural aging gave a five times lower number density of clusters as compared to direct preaging for both alloy types. The clusters were of comparable size but their compositions depended on heat treatment history. Preaging promoted the formation of clusters with an Mg:Si ratio close to 1 in both alloys, while natural aging produced clusters with Mg:Si ratios more similar to those of the alloys.
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.
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