@article{Cheng2024, 
author = {Guo-Dong Cheng and Xue-Yu Pang and Jin-Sheng Sun and Zheng-Song Qiu and Chuang-Chuang Wang and Jian-Kun Qin and Ning Li},
title = {Combined use of fly ash and silica to prevent the long-term strength retrogression of oil well cement set and cured at HPHT conditions},
year = {2024},
journal = {Petroleum Science},
volume = {21},
number = {2},
pages = {1122-1134},
keywords = {Fly ash, Long-term strength retrogression, High temperature, Quantitative X-ray diffraction (QXRD), Partial or no known crystal structure (PONKCS)},
url = {https://www.sciopen.com/article/10.1016/j.petsci.2023.09.010},
doi = {10.1016/j.petsci.2023.09.010},
abstract = {The long-term strength retrogression of silica-enriched oil well cement poses a significant threat to wellbore integrity in deep and ultra-deep wells, which is a major obstacle for deep petroleum and geothermal energy development. Previous attempts to address this problem has been unsatisfactory because they can only reduce the strength decline rate. This study presents a new solution to this problem by incorporating fly ash to the traditional silica-cement systems. The influences of fly ash and silica on the strength retrogression behavior of oil well cement systems directly set and cured under the condition of 200 ℃ and 50 MPa are investigated. Test results indicate that the slurries containing only silica or fly ash experience severe strength retrogression from 2 to 30 d curing, while the slurries containing both fly ash and silica experience strength enhancement from 2 to 90 d. The strength test results are corroborated by further evidences from permeability tests as well as microstructure analysis of set cement. Composition of set cement evaluated by quantitative X-ray diffraction analyses with partial or no known crystal structure (PONKCS) method and thermogravimetry analyses revealed that the conversion of amorphous C-(A)-S-H to crystalline phases is the primary cause of long-term strength retrogression. The addition of fly ash can reduce the initial amount of C-(A)-S-H in the set cement, and its combined use with silica can prevent the crystallization of C-(A)-S-H, which is believed to be the working mechanism of this new admixture in improving long-term strength stability of oil well cement systems.}
}