With the increasing depth of oil and gas exploration, high-temperature and high-pressure conditions become common in deep and ultra-deep wells. Under such harsh environments, the long-term integrity of wellbore cement is frequently compromised due to the strength retrogression, posing serious risks to zonal isolation and production safety. Although silica sand is widely used to mitigate strength degradation via modifying the Ca/Si ratio of hydration products, recent studies show that significant strength loss still occurs at ultra-high temperatures.

Previous studies focused on curing regimes involving low-temperature pre-curing followed by high-temperature exposure, which mainly represent upper wellbore conditions. However, in deeper sections of high-temperature wells, cement typically sets and cures directly at elevated temperatures, resulting in more severe degradation. In addition, low-density cement is widely applied in cementing operations, its long-term performance under such conditions remains insufficiently understood, particularly beyond short-term curing periods. This study was to investigate the long-term performance evolution and underlying mechanisms of low-density sand-added oil well cement under different high-temperature conditions, providing a guidance for the design of durable cement systems for deep well applications.

Six low-density cement slurry formulations with a target density of 1.6 g/cm³ were designed, incorporating silica sand, silica fume, and hollow glass microspheres. Two curing regimes were adopted to simulate different downhole environments, i.e., (1) direct curing at 180 ℃ to represent high-temperature deep well conditions, and (2) curing at 80 ℃ for 2 d and then long-term curing at 180 ℃ to simulate upper wellbore conditions. The long-term performance of cement samples was evaluated after curing at 180 ℃ for 2, 30 d, and 90 d, respectively. The compressive strength, Young's modulus, and permeability were measured to assess macroscopic properties. The microstructural and mineralogical evolution was characterized by mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), thermogravimetric analysis (TG), and scanning electron microscopy (SEM), enabling a comprehensive analysis of pore structure development and hydration product transformation.

The results indicate that curing conditions have a decisive influence on the long-term performance of low-density cement systems. Under direct high-temperature curing at 180 ℃, all formulations exhibit severe strength retrogression. The compressive strength of silica sand-based systems remains approximately 28 MPa within 30 d, but decreased to below 10 MPa after 90 d. The incorporation of hollow glass microspheres significantly enhances the early strength (i.e., up to 45 MPa at 2 d). However, these systems experience a rapid deterioration, with strength nearly disappearing after 30 d. In contrast, the samples subjected to low-temperature pre-curing and followed by high-temperature exposure show significantly improved stability. For formulations without hollow glass microspheres, the compressive strength remains stable or slightly increases for 90 d. However, the systems containing hollow glass microspheres still exhibit a noticeable strength reduction, indicating their limited suitability for high-temperature applications. The evolution of Young's modulus and permeability is consistent with the strength results. Under direct high-temperature curing, Young's modulus decreases markedly, while permeability increases in orders of magnitude, indicating severe degradation of structural integrity. Conversely, under low-temperature pre-curing conditions, the parameters both remain relatively stable, reflecting an improved durability. The microstructural analysis reveals that pore structure coarsening plays a key role in the performance degradation. Under direct high-temperature curing, the median pore size increases significantly from tens to hundreds of nanometers, accompanied by a transformation from harmless pores to harmful large pores. In contrast, the samples subjected to low-temperature pre-curing exhibit slower pore coarsening and more stable pore size distributions. The mineralogical evolution further explains the observed macroscopic behavior. Under direct high-temperature curing, beneficial amorphous hydration products and tobermorite gradually transform into more crystalline phases such as xonotlite, leading to the strength loss. The thermogravimetric analysis confirms significant changes in decomposition peaks, indicating alterations in hydration product composition. In contrast, under low-temperature pre-curing conditions, the content of amorphous phases and tobermorite remains relatively stable, contributing to sustained mechanical performance. The SEM images show that hollow glass microspheres progressively react and disappear under prolonged high-temperature exposure, leaving large voids that significantly deteriorate the microstructure. Although these microspheres enhance the early strength, their long-term effect is detrimental due to the pore enlargement and structural weakening.

This study demonstrated that curing conditions critically affected the long-term performance of low-density sand-added oil well cement. Direct high-temperature curing led to severe strength retrogression, pore structure coarsening, and unfavorable phase transformations, whereas low-temperature pre-curing significantly improved long-term stability. Hollow glass microspheres were effective in enhancing early strength but are not suitable for high-temperature applications due to their negative impact on long-term durability. The stability of hydration products, particularly amorphous phases and tobermorite, could be identified as a key factor governing strength retention. These findings could provide important insights into the design of durable cement systems for high-temperature oil and gas wells and highlight the necessity of considering both curing conditions and material composition in practical applications.