高温高压环境中MgO膨胀剂的性能与作用机理

Behavior and Expanding Mechanism of MgO Expansive Agent at High Temperature and High Pressure

观察了不同MgO掺量的补偿收缩水泥浆在高温高压环境中的自由膨胀率,并结合显微形貌、浆体孔隙率等微观测试手段,分析了高温高压环境中MgO膨胀剂的作用机理。结果表明:掺加适量适当活性MgO膨胀剂,可以在对浆体强度影响不大的前提下有效补偿油井水泥浆在高温高压环境中的收缩;在150℃、20 MPa下,宜使用低活性的S型MgO膨胀剂,掺量应控制在3%左右;在高温、高压环境中,Mg^(2+)的迁移速度更快,Mg(OH)2晶体的生长范围更广,体积更大;除扩张微裂缝外,Mg(OH)_(2)晶体还与水泥水化产物雪硅钙石晶簇相互穿插,扩大水化产物间的距离,所产生的膨胀率远高于常温常压条件时的。

High-temperature high-pressure(HTHP)working conditions of oil-well cement paste alter the expansive mechanism of MgO expansive agent,thus significantly accelerating the expansion speed and increasing the ultimate expansion.The free expansion rate of shrinkage-compensating pastes with MgO expansive agent at different contents was investigated.The microstructural characteristics,including morphology and porosity,were evaluated to reveal the expansive mechanism of MgO expansive agent in HTHP environment.It is indicated that the shrinkage of oil-well cement paste can be compensated by admixing MgO expansive agent at proper contents and reactivity.At 150℃and 20 MPa,S-type(220 s)MgO with a relatively low reactivity is recommended and its dosage is less than 4%.At HTHP,the migration of Mg^(2+)from MgO particles is accelerated.Besides the formation of expanding mircocracks,Mg(OH)_(2) crystals also are interlocked with tobermorite crystals as a hydration product of cement,thus enlarging their gap and causing an extraordinary expansion,compared to the expansive binder cured at normal temperature and pressure.