Formula of cementing slurry system for ultra-high temperature conditions of deep wells and its strength retrogression mechanism

The quality of set cement is an important factor influencing the production and service life of oil and gas wells, while the high-temperature and high-pressure (HTHP) environment of deep and ultra-deep wells has a significant impact on the long-term stability and safety of cement sheath, which is one of the important reasons for the failed cement sheath integrity of oil and gas wells. Thus, a set of cementing slurry system, whose 440 min thickening time is adjustable and rheology, settlement stability and fluid loss property can meet the requirements of ultra-deep well under the circulating temperature of 180℃ and pressure of 120 MPa, was designed. Additionally, the long-term changes of the physical mechanical properties of set cement under the curing temperature of 200℃ and 260℃ and their control mechanisms were analyzed using several testing techniques, e.g., uniaxial compression, X-ray diffraction, mercury intrusion, and scanning electron microscopy. Under the curing condition of high temperature of 200℃, more amorphous C-S-H gel hydration products were formed in the silica sand enriched well cement in the early stage, which contributes excellent physical mechanical properties to the set cement. However, the crystallization transformation of such amorphous products under HTHP curing conditions and the coarsening of its own internal structure can lead to a long-term continuous decline of the macroscopic strength of the set cement. Under the curing condition of 260℃, the hydration products of silica sand enriched well cement were dominated by stable crystalline type, so the strength change of set cement in the period of 2-90 d is within the error range and the long-term performance is relatively stable, even though the initial strength of set cement is low and the permeability is high under such temperature.