Pore connectivity of oil well cement in the early hydration stage by in situ electrical resistivity measurements and low-field nuclear magnetic resonance.

• LF-NMR and ERM was used to evaluated the pore connectivity of cement slurry; • First quantitative analysis of the pore connectivity in early hydration of cement slurry; • Pore connectivity of oil-well cement slurry in the first 20 h was studied; • Formation of chain C-S-H rapidly decreased the pore connectivity of cement slurry. The pore connectivity of cement slurry is an important parameter to be considered when investigating short-term gas migration in the cementing engineering of natural gas wells. This study combined electrical resistivity measurements (ERM), low-field nuclear magnetic resonance, and multi-phase phenomenological models to create an in situ evaluation method for the pore connectivity of cement slurry. This method was effective for investigating the porosity, pore size, and pore connectivity of the cement slurry in the early hydration stage. The results showed that the initial formation factor of the cement slurry was determined by the water-to-cement ratio (w/c). In the initial hydration stage, the porosity of the cement slurry (w/c = 0.44) was approximately 58%. Additionally, the pore size ranged from 11 to 75 nm, and the pore connectivity was 0.65–0.70. The small amount of Q1 calcium silicate hydrate (C-S-H) generated during the hydration induction period did not affect the dispersed-particle microstructure of the cement slurry and had little effect on its pore structure. During the hydration acceleration period, numerous Q1 and Q2 SiO 4 species were detected, which converted the microstructure of the cement slurry to a cross-linked structure, thereby reducing the pore size and pore connectivity of the cement slurry. The porosity of the cement slurry reduced to 52% after hydration for 12 h at 30 °C. However, the pore size distribution decreased to 4.5–35 nm, and the pore connectivity varied from 0.5 to 0.58. [ABSTRACT FROM AUTHOR]