纳米SiO_2溶胶缓解油井水泥高温强度衰退的作用机理

油井水泥石强度衰退是高温固井所面临的主要难题之一,而添加纳米SiO_2能否缓解水泥石的高温强度衰退以及其作用机理是什么,还有待于验证和确认。为此,通过室内试验,基于X射线衍射仪、扫描电子显微镜和能谱仪分析水泥石的矿物组成、微观结构和水化产物的元素,测定了高温条件下(150℃/35 MPa)纳米SiO_2溶胶对G级油井水泥石抗压强度的影响及变化规律,据此研究纳米SiO_2溶胶在高温下对水泥水化产物的作用机理。研究结果表明:①纳米SiO_2溶胶可以提高G级油井水泥浆的稠度系数,对水泥浆的流变性会产生不利的影响;②在高温养护初期,纳米SiO_2溶胶会降低水泥石的抗压强度,但加入纳米SiO_2溶胶的水泥石的抗压强度不会随着养护时间的增加而产生明显的变化;③加入少量纳米SiO_2溶胶的G级油井水泥中的纳米SiO_2颗粒吸附在水泥矿物表面阻碍水化反应,能够缓解水泥水化产物的高温脱水变质,纳米SiO_2颗粒还可以提高水泥微观结构的致密性;④加入大量纳米SiO_2溶胶的G级油井水泥中的纳米SiO_2与氢氧化钙发生火山灰反应生成一种新型的、结构松散的薄片蜂窝状CSH产物,难以提供较高的抗压强度。结论认为,纳米SiO_2可以作为水泥添加剂以缓解油井水泥高温强度衰退,该研究成果为高温固井水泥浆体系的设计提供了一条新的思路。

Working mechanism of nano-SiO2 sol to alleviate the strength decline of oil well cement under high temperature

Strength decline of oil well set cement is one of the major challenges during well cementing under high temperature. Therefore, it is necessary to verify and confirm whether nano-SiO2 can mitigate this problem and its working mechanism. In this paper, the mineral composition and microstructure of set cement and the elements of its hydration product were experimentally analyzed by using a X-ray diffractometer, a scanning electron microscope and an energy spectrometer. Then, the effect of nano-SiO2 sol on the compressive strength of Class G oil well set cement and its change laws under high temperature (150 ℃/35 MPa) were investigated. Based on this, the working mechanism of nano-SiO2 sol on the hydration product of cement under high temperature was studied. And the following research results were obtained. First, nano-SiO2 sol can increase the consistency coefficient of Grade G oil well slurry and has an adverse impact on the rheological property of slurry. Second, nano-SiO2 sol can decrease the compressive strength of set cement in the early stage of high temperature curing, but the compressive strength of the set cement with nano-SiO2 sol will not change obviously as the curing goes. Third, if a small amount of nano-SiO2 sol is added into Grade G oil well cement, nano-SiO2 particles are adsorbed to the surface of cement minerals to hinder the hydration reaction, so it can mitigate the evaporation metamorphism of the hydration product of cement under high temperature. Besides, nano-SiO2 particles can improve the micro-structural compactness of the cement. Fourth, if a large amount of nano-SiO2 sol is added into Grade G oil well cement, pozzolanic reaction happens between nano-SiO2 and calcium hydroxide, generating a new type of structurally loose honeycomb calcium silicate hydrate (CSH) product, which cannot provide higher compressive strength. In conclusion, nano-SiO2, as cement additive, can mitigate the strength retrogression of oil well cement under high temperature. These research results provide a new idea for the design of a high-temperature cement slurry system.