Effects of Curing Pressure on the Long-Term Strength Retrogression of Oil Well Cement Cured under 200 degrees C

The influences of curing pressure on the physical and mechanical property development of oil well cement during long-term curing were studied. Five silica-enriched cement slurries designed without and with reinforcement materials (latex fiber and nano-graphene) were autoclaved at 200 degrees C under two different pressures. The low pressure (50 MPa) curing was conducted for 2, 60, 90 and 180 days; the high pressure (150 MPa) curing was conducted for 2 and 360 days. The physical and mechanical properties of set cement were characterized by compressive strength, Young's modulus, and water/gas permeability; the mineral composition and microstructure were determined by X-ray diffraction, mercury intrusion porosimetry, thermogravimetry and scanning electron microscope. Test results showed that high pressure (150 MPa) curing led to a more compact microstructure, which reduced the rate of strength retrogression in the long term. Samples with reinforcement materials, especially the latex fiber, showed higher compressive strength, Young's modulus and lower permeability during long-term curing at both pressures.