Effective zonal isolation is the main purpose of oil well cement. The cement bond can fail by the stresses incurred either during drilling, completion/stimulation or production phases. It is a great challenge to prevent strength retrogression, especially for cementing deep and thermal wells. In this study, we investigated the effect of different forms of silica on oil well cement under high temperature, and revealed its mechanism through acoustic emission, acoustic velocities, strain and scanning electron microscope (SEM).
Rock deformation is usually accompanied by acoustic emissions which occur due to stress relaxation resulting from crack growth, dislocation motion or twinning. Comparison of the acoustic emission count rate and the changes in P and S-wave velocities during deformation provide considerable insight into the mechanical processes. Hence, compressive strength of different formulations of cement with additives is measured along with acoustic sensors and strain gauges in a triaxial setup. The microstructure of the samples were studied through SEM.
Silica flour (SF) improves the compressive strength of cement under high temperatures by forming a denser cement structure. Although, Nanosilica (NS) and silica flour have the same chemical composition, i.e., SiO2, but the effect of NS is not as significant as SF on cement strength improvement. NS has small particle size and large specific surface area compared to SF. Hence in cement slurry, NS has the ability of agglomeration and adsorption on cement particles and inhibit the cement hydration. Therefore, NS is not very suitable as a single additive for oil well cement to prevent strength retrogression under high temperatures. But NS come in various sizes and forms. Some studies have found optimum amount of specific NS acting as filler for a particular cement type and increasing its strength.
Acoustic measurements along with strain measurements give additional insight into the cement strength studies.