Evolution of pore structure of oil well cement slurry in suspension–solid transition stage

Abstract

This study investigates the pore structure of oil well cement slurry (OCS) in the suspension–solid transition stage and the effect of the hydrated products on the pore structure via isothermal calorimetry, scanning electron microscopy (SEM), nitrogen adsorption, thermogravimetry (TG), and low-field nuclear magnetic resonance (LF-NMR). The experimental results show that the suspension–solid transition stage of OCS includes a dynamic balance stage and an acceleration stage. The porosity of the fresh cement slurry is approximately 58.09%, and its pore sizes range from 100 nm to 850 nm. In the dynamic balance stage, a small quantity of formed hydrated products between the cement particles cause the OCS macropore structure (>250 nm) to transform into capillary pores (50–250 nm). In the acceleration stage, amorphous calcium silicate hydrate (C–S–H) gel quickly forms in the pores and on the surface of the cement particles, which causes the capillary pores to transform into gel pores (<50 nm). When the OCS is in the solid state (hydration of 720 min), the pore sizes range from 0.6 nm to 99 nm, and the total porosity and porosity of the gel pores are 53.64% and 46.218%, respectively. Further, during the suspension–solid transition stage, the porosity of the cement slurry only decreases from approximately 58.09% to 53.64%. However, the pore size distribution (PSD) decreases from 100–850 nm to 0.6–99 nm, which proves that the pore structure of the cement slurry undergoes a disassembly during this stage. This study provides experimental data for understanding the mechanism of hydration and gas migration in OCS.