Salinity-Driven Phase Transformation and Strength Mechanisms of Class G Oilwell Cement at 260°C and 21 MPa

Summary Although the substitution of cement with silica flour is effective in mitigating strength retrogression under high-temperature and high-pressure (HTHP) conditions, the durability of such cementitious systems, primarily due to strength degradation, remains compromised in high-salinity environments, posing significant challenges for deep salt-layer cementing operations. This paper investigated the effect of different concentrations of sodium chloride (NaCl) on the compressive strength, permeability, and microstructure of cement pastes cured at 260℃ and 21 MPa using X-ray diffraction (XRD), scanning electron microscope (SEM), and mercury intrusion porosimetry. Results showed that an increase in NaCl concentration correlated with enhanced compressive strength of hardened cement pastes (HCPs), with no evidence of strength decline observed after 28 days of curing. The increased concentration of NaCl inhibited the formation of high-Ca/Si C-S-H. It was mainly attributed to the Ca leaching from C-S-H. Consequently, NaCl promoted the formation of lower Ca/Si C-S-H with high strength and precipitation of portlandite. These phase changes contributed to refining the pore structure of cement pastes, thereby decreasing the permeability by one to two orders of magnitude.