丁向群¹ ，吴彦伯¹ ，魏继军² ，崔殿松¹ ，周书宇¹ ，房延凤¹

1. 沈阳建筑大学 材料科学与工程学院，辽宁 沈阳 110168；2. 中国石油集团 长城钻探工程有限公司固井公司，辽宁 盘锦 124010

摘要：【目的】 探索氧化镁（MgO）活性与油井水泥膨胀性能之间的关系，研究MgO作为膨胀剂在固井工程中的应用。【方法】 选取7种不同活性的MgO掺入油井水泥，分析MgO活性对油井水泥工作性能、力学性能和膨胀性能的影响，利用水化热分析油井水泥的水化进程、 X射线衍射（X⁃ray diffraction，XRD）分析产物变化；利用扫描电子显微镜（scanning electron microscopy，SEM）对水泥的微观结构进行分析。【结果】 较未添加MgO的油井水泥空白试样，添加MgO后油井水泥的流动度和游离液含量降低，油井水泥试样的抗压强度提升，3、7 d龄期的强度最高分别提升28. 5%和33. 1%，油井水泥试样的膨胀率随着 MgO活性的降低而升高，最高可达到 0. 553%。【结论】 掺入不同活性的 MgO均可加快油井水泥的水化进程，细化试样的孔隙结构，提高油井水泥的膨胀性能和抗压强度。

关键词：油井水泥；氧化镁活性；膨胀性能；抗压强度

Abstract

Objective In practical engineering，expansion agents are commonly employed to mitigate cracking in oil well cement. However，the high-temperature and high-pressure conditions characteristic of oil well environments often make most expansion agents unsuitable. Magnesium oxide （MgO）， when used as an expansion agent， forms magnesium hydroxide （Mg（OH）₂） upon hydration， exhibiting exceptional stability under high temperatures. This stability ensures effective and sustained expansion， making it ideal for mitigating cement cracking in oil wells. This study investigates the effects of MgO with varying activities on the expansion performance of oil well cement， offering guidance for the application of MgO with different activity levels in industrial settings.

Methods In the experiments， the water-cement ratio was fixed at 0. 44， and the curing temperature was maintained at 60 ℃.The fluidity and free liquid content of oil well cement containing MgO of different activities were measured. Mortar specimenswith dimensions of 50. 8 mm×50. 8 mm×50. 8 mm and 40 mm×40 mm×160 mm were prepared by mixing MgO of varying activities into the cement and curing them in a water bath at 60 ℃ for 1，3， and 7 days. The compressive strength and expansion rate of the specimens were then evaluated. Furthermore， the effects of MgO with varying activities on the hydration exothermic rate and heat release of the oil well cement were investigated. X-ray diffraction （XRD） and scanning electron microscopy （SEM）were employed to elucidate the mechanism by which MgO influenced the cement expansion.Results and Discussion For MgO at a reaction time of 31 s， the fluidity  and free liquid content of the specimens reached their minimum values， showing reductions  of 15% and 32%， respectively， compared to the control group （without MgO）. However，as the MgO reactivity decreased， the fluidity and free liquid content of the mortar gradually increased. After incorporating MgO，the 1-day compressive strength of the cement specimens increased with decreasing MgO activity， reaching a maximum of 9. 31MPa at a reaction time of 345 s before declining. For the 3-day and 7-day compressive strengths， both exhibited an initialincrease followed by a decrease as MgO activity decreased. The 3-day compressive strength peaked at 26. 16 MPa at a reactiontime of 426 s， representing a 25. 6% increase over the control group. The 7-day compressive strength reached 29. 46 MPa at areaction time of 345 s， which was 18. 7% higher than that of the control group. The expansion rate of the oil well cementincreased gradually as the MgO activity decreased， under constant MgO dosage and curing temperature. The expansion rate peaked at 0. 552% at 426 s， after which it stabilized or even slightly declined. This phenomenon occurred because MgO with a reaction time of 31 s reacted too rapidly at a curing temperature below 60 ℃， causing expansion to occur before internal stresses developed in the cement. As a result， the expansion was absorbed by the mortar， consuming nearly all the MgO， leaving almost no expansion effect during mid-stage hydration of the specimens. When the MgO activity decreased， the formation of Mg（OH）₂ slowed， delaying the expansion reaction. Thus， with decreasing MgO activity， the expansion effect improved during the transition stage. However， when the reaction time exceeded 426 s， the reaction rate became excessively slow， leaving a portion of MgO unreacted at 7 d， which led to a slight reduction in the expansion rate.

Conclusion The fluidity of oil well cement increased as the activity of MgO decreased. The free liquid content rose with thereduction in MgO activity. The compressive strength increased with decreasing MgO activity， peaking at 426 s and 345 s，respectively， and then decreased. The expansion rate of oil well cement exhibited a trend of initial growth followed by a decreaseas MgO activity decreased， reaching its maximum when the reaction time extended to 426 s， after which it stabilized and even slightly declined. The incorporation of MgO effectively enhances the compactness of oil well cement， reducing pores and cracks

and improving both cement strength and expansion rate.

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