Various admixtures to mitigate the long-term strength retrogression of Portland cement cured under high pressure and high temperature conditions

In order to investigate the problem of long-term strength retrogression in oil well cement systems exposed to high pressure and high temperature (HPHT) curing conditions, various influencing factors, including cement sources, particle sizes of silica flour, and additions of silica fume, alumina, colloidal iron oxide and nano-graphene, were investigated. To simulate the environment of cementing geothermal wells and deep wells, cement slurries were directly cured at 50 MPa and 200 °C. Mineral compositions (as determined by X-ray diffraction Rietveld refinement), water permeability, compressive strength and Young's modulus were used to evaluate the qualities of the set cement. Short-term curing (2–30 d) test results indicated that the adoption of 6 μm ultrafine crystalline silica played the most important role in stabilizing the mechanical properties of oil well cement systems, while the addition of silica fume had a detrimental effect on strength stability. Long-term curing (2–180 d) test results indicated that nano-graphene could stabilize the Young’s modulus of oil well cement systems. However, none of the admixtures studied here can completely prevent the strength retrogression phenomenon due to their inability to stop the conversion of amorphous to crystalline phases.