Recent studies had revealed that traditional high-temperature resistant silica-enriched well cements are not suitable for deep well cementing applications due to strength retrogression problems in the long term. This study investigated the use of new admixtures, including fly ash and ground blast furnace slag (GBFS), to mitigate such strength retrogression issue of cement. The combined use of GBFS and silica at optimized dosages effectively prevented well cement strength retrogression up to 30d, but strength reduction and permeability increase were still observed from 30d to 90d. On the other hand, the combined use of fly ash and silica effectively prevented both strength reduction and permeability increase of the well cement. However, the addition of fly ash increased the consistency of cement slurry significantly, making it difficult to pump. Finally, the slurries prepared with mixtures of well cement, silica flour, fly ash, and GBFS exhibited both excellent flowability and long-term strength stability.
The development of deep oil and gas reserves as well as geothermal energy brings significant challenges to oil well cement systems due to the high pressure and high temperature (HPHT) downhole conditions. Recent studies have shown that the traditional silica-enriched oil well cement undergoes significant strength retrogression during long-term curing (≥30d) at 200 °C due to microstructure coarsening (Pang et al.,2021). The strength retrogression issue is primarily caused by the fact that deep well cementing requires the cement slurry to set at high temperature conditions, which is in great contrast to many previous studies that focus on low setting temperature and high exposure temperature. This is because many previous studies of high-temperature resistant well cement systems mainly focused on simulating steam injection wells.
During the pumping operation process in deep wells, the cement slurry should remain in a fluid state at HPHT conditions for several hours; after being pumped in place, it finally set and then was directly exposed to HPHT conditions for a long time (Qin et al.,2021). The optimization of silica admixture (crystallinity, dosage and fineness), increasing curing pressure or adding physical reinforcement materials (such as nanographene and latex fiber) can slow down but not prevent the strength retrogression after a long-term (≥30d) curing period when simulating the deep well cementing condition (Qin et al.,2021; Liu et al.,2021; Qin et al.,2023). Therefore, it is urgent to develop new cementing materials that are applicable to the conditions of deep well cementing.
