Relationship Between the Microstructure/Pore Structure of Oil-Well Cement and Hydrostatic Pressure

Abstract

In cementing operations, hydrostatic pressure reduction in cement slurries is a serious threat to operation safety and cementing quality. This study combines X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, low-field nuclear magnetic resonance, and nano-computed tomography to investigate the mechanism of hydrostatic pressure reduction in cement slurries. The experimental results reveal that hydrostatic pressure transmission in a fresh cement slurry follows Pascal’s law. However, in the slurry, some cement particles undergo sedimentation with an increase in the hydration time, which reduces a part of the hydrostatic pressure. Further, another part of the hydrostatic pressure, which is maintained by the free water in the slurry pores, is reduced during the hardening stage. When the hydration reaction of the slurry is accelerated, the pore water in the slurry is supersaturated and hydration products start to rapidly nucleate and grow between the cement particles. These hydration products are porous gel structures and can change the pore structure of the cement slurry; a macro-pore is divided into many micropores, such as capillary pores and gel pores. Because these pores are filled by water in the slurry, during this process, free water in the macro-pores is changed to capillary water and gel water. However, gel water and capillary water cannot transmit hydrostatic pressure in the cement slurry. Meanwhile, in the fresh cement slurry, many pores containing free water are connected and some hydration products rapidly grow in the macro-pores and fill them, which may reduce the column height of the free water in the pores and lead to hydrostatic pressure reduction in the slurry.