纳米SiO_(2)溶胶对高温加砂油井水泥石强度作用规律及机制

Effect of nano-SiO_(2) sol on strength of cement with addition of silica sands under high temperature

测试高温(150℃,20.7 MPa)下纳米SiO_(2)溶胶对加砂油井水泥石抗压强度的影响,分析纳米SiO_(2)溶胶水化产物类型、形貌、微观结构以及作用机制。结果表明:高温下纳米SiO_(2)对加砂油井水泥石有多种作用机制,质量分数在1%~2%时,纳米SiO_(2)主要吸附在水泥颗粒表面,影响水泥正常水化,降低加砂水泥石强度;质量分数为3%~8%时,纳米SiO_(2)通过晶核作用诱导水化产物多点生长,改善水化产物微观结构,同时游离纳米SiO_(2)颗粒以不同团聚体形式充填在不同尺度水化产物孔隙中,提高水泥石致密度,保证加砂水泥石高温强度发展与维持;质量分数超过10%时,大量纳米SiO_(2)吸附在水泥颗粒表面并与Ca(OH)_(2)反应生成CSH凝胶覆盖层,纳米SiO_(2)团聚作用更显著且生成大颗粒聚集体不再具有填充效应,使水泥石微观结构遭到破坏,加砂水泥石强度大幅度降低;在高温固井水泥浆设计中,须确定合理的纳米SiO_(2)加量,纳米SiO_(2)才能通过晶核作用、填充效应等改善水泥石微观结构,有效阻止加砂油井水泥石高温强度衰退。

The effects of nano-SiO_(2) sols on the compressive strength of cement stone containing silica sands were investigated at 150℃and 20.7 MPa,and the influencing mechanisms were analyzed,including the effects of the types of the hydration products formed by the nano-SiO_(2) sols,their morphology and microstructure on the strength of the cement.The results show that the effect of nano-SiO_(2) sol on the properties of cement stone is controlled by its dosage at high temperature.At a low dosage of 1%-2%,the nano-SiO_(2) particles are mainly absorbed onto the surface of cement particles,which can affect the cement hydration process and slightly reduce the strength of the cement stone.At a moderate dosage of 3%-8%,the number of growth sites of hydration products can be increased through the nucleation effect of the nano-SiO_(2) sol,thus,the microstructure of the hydration products are improved.Meanwhile,the nanoparticles can fill in the pores of the hydration products(with different size)in the forms of aggregates.Consequently,the density(due to pore filling)and strength of the cement stones are enhanced and maintained at high temperature.However,at a high dosage of the nano-SiO_(2) sol(>10%),large amount of the nano-SiO_(2) particles are absorbed onto the surface of cement particles,and a dense coating layer made of CSH gel can be formed due to the reaction of nano-SiO_(2) and Ca(OH)_(2).Meanwhile,larger aggregates are formed,which is unfavorable for their filling in the pores of cement slurry.Therefore,the microstructure and strength of the cement stones are undermined significantly.For field application,the dosage of the nano-SiO_(2) sol should be controlled strictly to maximize the nucleation and filling-in effects of the nano-SiO_(2) sols in order to improve the microstructure of the cement stone and increase its compressive strength.