Anti-strength retrogression cementing materials for deep and ultra-deep wells
Guodong Cheng, Xueyu Pang, Haige Wang, Jinsheng Sun, Zhengsong Qiu
Abstract
Recent studies have brought to light a critical issue pertaining to the conventional silica-enriched Portland cements employed in deep and ultra-deep wells. The traditional silica-enriched oil well cements were found to exhibit substantial strength retrogression during prolonged curing under high temperatures, posing a significant threat to the overall cementing quality and subsequent production safety. This study proposes a novel approach to enhance the long-term stability of traditional silica-cement systems under high temperature conditions by introducing both granulated blast furnace slag (GBFS) and fly ash to the system. Test results indicated that the addition of GBFS increased the strength stability of the traditional silica-cement system but failed to completely prevent its strength retrogression. In contrast, the combined addition of GBFS and fly ash in silica-cement system significantly enhanced its long-term strength measured after 90 d curing. The strength test results were corroborated by further evidences from permeability tests as well mercury intrusion porosimetry analyses. Detailed analysis of the set cement composition suggests that the inhibition of xonotlite phase and the promotion of tobermorite and grossular phases in hydration products are responsible for improved long-term strength stability.