Influences of different admixtures on the properties of oil well cement systems at HPHT conditions

Four series of cement slurries were cured for a period of 14 days under high temperature and high pressure condition (200 °C, 20 MPa) to investigate the influences of different admixtures on the physical and mechanical properties of oil well cement. The admixtures studied include coarse and fine silica flour, silica fume, and various colloidal nanomaterials. The property of the cement formulation evaluated includes rheology, compressive strength, Young's modulus, water, and gas permeability, and X-ray diffraction profile. Test results reveal that the total silica dosage is the primary determining factor for the mechanical property of the set cement. Slurry TS60 with 60 % silica exhibits about 400 % increase in compressive strength and more than 1 order of magnitude reduction in permeability compared to slurry TS40 with 40 % silica. Lower strength and higher permeability of TS40 are associated with the formation of excessive xonotlite phase. The adoption of smaller sized silica (fine silica flour and silica fume) and various nanomaterials can serve to reduce the permeability of the set cement (by approximately 50 %), possibly through the well-known filler effect.