Effects of magnesium oxide activity on expansion performance of oil well cement

DING Xiangqun; WU Yanbo; WEI Jijun; CUI Diansong; ZHOU Shuyu; FANG Yanfeng

doi:10.13732/j.issn.1008-5548.2026.01.014

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 hydra-tion,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 expan-sion 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 specimens with 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 activi-ties 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 31s,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 9.31 MPa at a reaction time of 345 s before declining.For the 3-day and 7-day compressive strengths,both exhibited an initial increase followed by a decrease as MgO activity decreased.The 3-day compressive strength peaked at 26.16 MPa at a reaction time of 426 s,representing a 25.6％increase over the control group.The 7-day compressive strength reached 29.46 MPa at a reaction time of 345 s,which was 18.7％higher than that of the control group.The expansion rate of the oil well cement increased 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 transi-tion 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 the reduction 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 decrease as 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.