This paper presents the finding of a study investigating the effects of silica additive on certain properties of Hoang Thach Portland cement and evaluates its suitability for oil well construction at a depth of 1000 m. In this study, silica was added to the cement in varying proportions from 0 to 25% by mass. The results indicate that as the silica content increased, the water requirement of the mixtures gradually rose, while the slurry density slightly declined. At a curing temperature of 52 °C, both the initial and final setting times of the cement paste decreased, and the 1-day compressive strength of the hardened samples increased sharply. Notably, sample M3, containing 15% silica, achieved the highest compressive strength, 1.28 MPa—approximately three times higher than that of the reference sample. However, under standard conditions, the strength of silica-modified samples dropped significantly. These results suggest that Hoang Thach Portland cement, when combined with an appropriate amount of silica, can be effectively used in oil well construction.