Impact of Silica Blend Towards Early Compressive Strength Development of Well Cement Class G

Abstract

Abstract In this paper we evaluate the effects of silica on early compressive strength development of well cement Class G during the first 24 hours. The paper’s scope is a 15.8 lbm/galUS system with 35% by weight of cement (BWOC) silica varied between fine, medium, and coarse grades, followed by an optimized version. The addition of silica at 35% BWOC is an accepted industry norm for exposure of 110°C or greater. Slurries are evaluated at 120 and 160°C by an ultrasonic cement analyzer (UCA) to determine the compressive strength development. API 10A compliant class G cement is added with different grades of silica with an average size of 4.5 µ (fine), 16 µ (medium), and 330 µ (coarse) at 35% BWOC separately. It is mixed according to API RP 10B-2 and the compressive strength development is evaluated using the ultrasonic cement analyzer (UCA) at 120 and 160°C. The time taken to reach 50 and 500 psi, together with the strength value at 12 and 24 hours, are compared for differing silica grades. The slurry’s rheological behavior is evaluated to eliminate unfavorable slurry properties due to silica size. Testing shows that silica size is a significant contributor to the early compressive strength development. The fine silica slurry achieved higher strength development compared to medium and coarse silica. The drawback is that the use of a fine product alone causes unacceptable high slurry gel strength. An optimized silica blend being a mixture of the three grades provides the highest compressive strength development and favorable rheological property. At 120°C, the coarse silica alone has the lowest strength at 24 hours, while the medium-sized silica provides a 49% increase over the coarse grade. The fine grade provides a 67% increase over the medium grade. The optimized silica blend can provide an additional 7% increase over the pure fine-grade silica. An evaluation done at 160°C shows that course silica provides the least compressive strength while the medium silica provides about six times the value of the medium, and the fine about 5% more than the medium. The optimized silica blend provided an additional 5% over the fine grade. Although the industry is clear about the need to add a minimum 35% BWOC of silica to prevent strength retrogression for high-temperature applications, there is little information on how silica grades affect the early compressive strength development of the cement system. This study has clarified that proper optimization of silica grades can bring out the best early compressive strength for elevated temperature applications.