Abstract

Objective In the cementing operation of ultra-high-temperature oil and gas wells， the cement stone of silica-enriched oil wells generally faces serious strength degradation problems， which directly affects the integrity of wellbores and the service life of oil and gas wells. Under high-temperature curing conditions， the compressive strength of cement stone decreases with the extension of curing time， and the degree of decline is closely related to the additive formulation， water-to-cement ratio， and curing temperature. In addition， oil well cement additives are a key technical means to ensure the stability of cement slurry performance and the long-term integrity of cement stone. As the well depth increases， the downhole temperature and pressure rise significantly. Pure cement systems are difficult to meet the comprehensive engineering requirements for thickening time， rheology， fluid loss agent， and compressive strength. Therefore， performance control must be achieved through additives. This study investigates the influence mechanism of additives on the long-term mechanical properties and microstructure evolution of silica-enriched oil well cement under ultra-high-temperature conditions of 240 ℃.

Methods Cement slurries with densities of 1.9 g/cm³ and 1.75 g/cm³ were prepared according to API standards. The compressive strength of cement stone was tested using a universal testing machine， and each group of cement stone samples had three replicates. The thickening performance of cement slurry was tested using a high-temperature and high-pressure thickener. The cement stone sample was placed in the core holder of an automatic permeability tester for permeability measurement. A high-pressure mercury intrusion porosimeter was used to test the pore structure of the cement stone， and the maximum applied pressure was set to 220 MPa. Phase analysis was performed on cement stone using an X-ray diffractometer.

Results and Discussion Under the high-temperature curing condition of 240 ℃， the H70 system containing additives showed excellent early performance. The compressive strength reached 44 MPa after 2 d of curing， the permeability was the lowest， and the median pore size was only 10 nm. However， as the curing time was extended to 90 d， its compressive strength decreased to 33 MPa， the permeability increased significantly （similar phenomenon was observed in the M70 system）， the median pore size coarsened to 20 nm， and the total porosity increased. The analysis of hydration products showed that the content of amorphous C-S-H in the H70 system gradually decreased， and the tobermorite almost completely disappeared after 90 d of curing. In contrast， the early performance of the H70-NA system without additives was poor. The compressive strength was only 33 MPa after 2 d of curing， the permeability was high， and the median pore size reached 100 nm. However， as the curing time was extended to 90 d， its compressive strength increased to 40 MPa， the permeability decreased （similar phenomenon was observed in the M70 system）， the median pore size was refined to 30 nm， and the total porosity remained stable. The content of C-S-H in the hydration products increased， the amount of xonotlite significantly decreased， and the proportion of small pores increased. The performance of the H70-NR system without retarder was between the two （H70 system and H70-NA system）. The thickening time test showed that the system containing additives could initially set at 240 ℃ normally， while the system without additives set prematurely during the heating stage.

Conclusion The absence of additives has a significant impact on the high-temperature mechanical properties of silica-enriched oil well cement. Cement containing an appropriate amount of additives shows significant strength degradation under 240 ℃ curing conditions， while systems lacking retarder， dispersant， and fluid loss agent do not show significant strength degradation. With increasing curing time， the pore structure of cement samples containing appropriate additives coarsens sharply. On the contrary， the pore structure of cement samples without retarder or any additives is relatively stable， and some samples even exhibit pore refinement. In the H70 system containing an appropriate amount of additives， the main reason for the strength degradation is the reduction of amorphous hydration products that have a positive effect on mechanical properties and the decrease in the content of tobermorite. On the contrary， the amorphous hydration products in the H70-NR system lacking retarder and the H70-NA system without any additives increase， while the xonotlite， which has an adverse effect on mechanical properties， significantly decreases during the curing process. This is the main reason for the significant difference in cement hydration products and mechanical properties due to the different setting temperatures of the cement slurry. The setting temperature has a significant impact on the high-temperature mechanical properties of cement， and targeted research should be conducted on the influence of setting temperature on the high-temperature mechanical properties of cement in future efforts.

Keywords：high-temperature mechanical properties; strength retrogression; sand-containing oil well cement; additives

Get Citation：Gao Zixuan， Li Yang， Pang Xueyu， et al. Effect of additives on long-term properties of silica-enriched oil well cement under ultra-high-temperature conditions［J］. China Powder Science and Technology， 2026， 32（4）： 1-10.

Received：2025-12-15， Revised： 2026-05-28，Online： 2026-06-10.

Funding：The research was supported by the Basic Science Center Project of the National Natural Science Foundation of China (Grant No. 52288101) and the Independent Research Project of State Key Laboratory of Deep Oil and Gas (Grant No. SKLDOG2024-ZYTS-11).

DOI：10.13732/j.issn.1008-5548.2026.04.008

CLC No.：TB44；Q39

Type Code：A

Serial No.：1008-5548（2026）04-0001-10