The Influence of Glass Fibers on the Mechanical Performance and Microphase Composition of Sand-Added Oil Well Cement under Ultra-High Temperature

In deep and ultra-deep well operations, complex work conditions such as ultra-high temperature and high pressure impose higher requirements on the mechanical performance of oil well set cement. The most common method of inhibiting the set cement strength degradation is to add quartz sand into the cement slurry; however, the effectiveness of this method weakens at temperatures above 200 ℃. To deal with this situation, the influence of glass fiber on the properties and microphase composition of ultra-high temperature sand-added cement was investigated. Jiahua grade G cement slurries treated with 50% 200-mesh quartz sand and different quantities of glass fiber Z-GF were cured at 240 ℃ and 20.7 MPa. The test results show that glass fiber can mitigate the decline of the strength of the set cement at ultra-high temperatures. The optimal concentration of glass fiber was determined to be 5%, the 28-day compressive strength was 38.3 MPa, which was 32.5% and 18.9% higher than those of the 2-day and the 7-day. After curing for 28 days, the main hydrational product of the cement without Z-GF treatment, which was tobermorite, turned into a hard xonotlite, and a new phase diopside was produced, while the set cement containing 5%Z-GF was dominated by tobermorite. Compared with the cement without Z-GF, the set cement containing 5%Z-GF has the quantity of smaller-sized nanopores (<20 nm) significantly increased from 18.88% to 39.56%. Glass fiber and quartz sand react with CH to consume excess SiO₂, enhancing the long-term strength of the set cement. The increase in the quantity of small-sized nanopores and the generation of tobermorite are the key factors in inhibiting the strength decline of the set cement at elevated temperatures.