| In recent years,high-temperature deep well cementing project has played an increasingly important role in the exploration,development and utilization of underground resources.When the temperature exceeds 200℃,the conventional cement slurry system is difficult to adapt to this high-temperature working environment,such as the poor pumpability of cement slurry and the poor sealing ability of cement paste due to the failure of additives,which brings difficulties and challenges to ensure cementing quality.Therefore,it is necessary to study the hydration and hardening characteristics of oil well cement at these temperatures,analyze the functions of main additives and their effects on performance,and evaluate the adaptability of additives to cement slurry,which is of great significance to realize the design of high-temperature cement slurry system and effectively solve the cementing construction and quality problems of high-temperature deep wells.In this paper,the composition,product composition,microstructure,thickening time and strength of oil well cement clinker were tested and analyzed by XRD,SEM,high temperature and high pressure thickener and pressure tester at 220℃.The hydration hardening of oil well cement and the role of related additives were studied,and the changing characteristics of hydration product composition,microstructure and cement stone strength of oil well cement were analyzed.The function of retarder and intensity decay inhibitor and their influence on related properties are analyzed,and the adaptability and dosage of the above additives are analyzed.The results show that the thickening time of cement slurry is shortened at high temperature.When choosing polymer retarder,it should be noted that this kind of retarder is sensitive to the composition of cement clinker,and abnormal thickening problems such as "core-covering " will occur in the slurry under certain conditions.The abnormal thickening problem can be solved by selecting the appropriate oil well cement category and retarder dosage,and the reasonable retarder dosage is about 8%～12%;At 220℃,the products of hydrated calcium silicate in cement paste are mainly hydrated dicalcium silicate and polysilicate hydrate,and the microstructure of cement paste is mainly characterized by the connection of large crystalline aggregates of hydrated dicalcium silicate and polysilicate hydrate.The connection between aggregates is weak,the voids are increased,and the strength is greatly reduced.Compared with normal temperature,the strength is reduced by about61.55% after 24 hours of curing.After adding silica sand and other additives,the hydration products are mainly xonotlite,aluminosilicate,xonotlite,etc.The microstructure of cement paste is composed of coarse needle-like and short parallel needle-like xonotlite,fibrous xonotlite and granular xonotlite which can aggregate into lumps,which can improve the strength of cement paste to some extent.The improvement degree is related to the amount and particle size of added siliceous materials,and the reasonable amount and particle size of siliceous materials are about 45% and 0.05mm;The designed cement slurry foundation formula can meet the engineering requirements. |
