Effect of the hydration rate and microstructure of Portland cement slurry on hydrostatic pressure transfer

Abstract In cementing operations in oil and natural gas wells, the reduction in hydrostatic pressure of the oil-well cement (OWC) slurry is a serious threat to the quality and safety of the operations. To elucidate the mechanism of hydrostatic pressure transfer in OWC slurries, this study investigates the rules for the hydrostatic pressure transfer of OWC slurries using a hydrostatic pressure testing device. In addition, static gel strength (SGS) analysis, isothermal calorimetry, low-field nuclear magnetic resonance (LF-NMR), environmental scanning electron microscopy (ESEM), and X-ray computed tomography (μ-CT) were used to study the effects of the SGS, hydration rate, pore water, and microstructure on the hydrostatic pressure of OWC slurries. The experimental results show that the trends in the hydrostatic pressure curves of the cement slurries are similar. The hydrostatic pressure and SGS of the cement slurries vary exponentially, while the hydrostatic pressure and free water content vary linearly. Combined with the SGS results, the hydrostatic pressure curves can be divided into two stages. The first stage encompasses the range from the initial hydrostatic pressure to a pressure approximately equal to the hydrostatic pressure of water. From the hydration rate and microstructure results, it can be ascertained that this hydration stage is a dynamic balance stage. The microstructure of the cement slurry will be changed from a dispersed particles state to the gelation state, which increase the SGS of the cement slurry and the bonding ability between the cement particles and the interfaces of the casing and borehole. Part of the gravitational force of the cement particles acts on these interfaces, which reduces the hydrostatic pressure. The hydrostatic pressure in the second stage is less than that of water. This hydration stage is an acceleration stage, and the rapid hydration reactions form a large number of hydration products in the pores, which change the microstructure of the cement slurry from the gelation state to a solid state. The LF-NMR results indicate that the hydration products also reduce the pore size of the cement slurry, which causes the free water in the macropores to become capillary water and gel water. However, in the cement slurry, the hydrostatic pressure is not readily transferred by capillary water and gel water.