Temperatures in excess of 110 ºC result in phase transformations of cement, significantly decreasing its compressive strength. This effect is referred to as strength retrogression. It is frequently observed in cement sheaths of heavy oil wells submitted to steam injection. The present study evaluated the mechanical behavior of Special Class Portland Oilwell Cement (SCPOC) slurries containing silica flour to prevent retrogression. A factorial statistical planning was used to assess the effect of the main variables on the mechanical behavior of cement slurries, i.e., mechanical testing temperature (30, 100, 120, 180 and 230 ºC); contents of silica flour replacing cement (0-18 and 36%) and curing time for rupture (12 h and 7 days). The results revealed that slurries containing 18% of silica flour tested at 230 ºC depicted an increase in compressive strength up to 30% after curing for 12 h and 10% after curing for 7 days, indicating retrogression. On the other hand, testing slurries containing silica flour at temperatures up to 180 ºC revealed strength increase of just 10%, suggesting the mechanical stability of the SCPOC, which prevents retrogression. Such behavior was probably related to the relatively low content of C3A and low specific area of the material. Therefore, strength retrogression at typical bottom hole temperatures of up to 180 ºC can be controlled by small additions of silica flour, economically contributing to the use of SCPOC cementing.
ResumoNo presente trabalho estuda-se a síntese, caracterização e propriedades fotoluminescentes de sistemas contendo óxidos de lantânio, cério e níquel. Estes materiais foram preparados pelo método dos precursores poliméricos, calcinados na faixa de temperatura de 300 a 800 o C e então caracterizados por análise CHN, análise termogravimétrica, infravermelho com transformada de Fourier e difração de raios X. Os resultados revelaram a formação de fase cristalina em 800 o C. Os espectros fotoluminescentes dos sistemas desordenados foram registrados a temperatura ambiente. O sistema de lantânio, cério e níquel apresentou forte intensidade fotoluminescente, centrado em 599 nm, quando excitado a 488 nm. Palavras-chave: óxidos de lantânio, cério e níquel, fotoluminescência, precursor polimérico. AbstractThe synthesis, characterization and photoluminescence property of the systems containing lanthanum, cerium and nickel oxides were studied. These materials were prepared by the polymeric precursors, calcined at temperatures ranging from 300 to 800 o C. Afterwards they were characterized by CHN analysis, thermogravimetric analysis, Fourier-transform infrared spectroscopy and Xray diffraction. The results revealed the formation of the crystalline phases at 800 °C. The photoluminescence spectra of disordered systems have been observed at room temperature. The lanthanum, cerium and nickel system presented strong photoluminescence intensity centered at 599 nm when excited to 488 nm.
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