Supporting data set for manuscript ‘Impact of thermal cycling and porewater freeze-thaw processes on CO2 injection well cement integrity’

Thermal loads associated with Carbon Capture and Storage (CCS) operations in depleted hydrocarbon reservoirs are known to affect the performance of cement seals along CO2 injection wells, yet wellbore cement behaviour under realistic downhole temperature and pressure conditions remains poorly constrained. This study presents confined thermal cycling and porewater freeze–thaw experiments on API Class G HSR Portland cement to evaluate its volumetric response, mechanical integrity, and permeability evolution under conditions representative of CO2 injection operations. Imposing temperature variations from 80 °C to –20 °C at constant confining pressures of up to 20 MPa produced reversible contraction–expansion behaviour, superimposed by progressive permanent compaction, particularly at higher confinement. Under near drained conditions, apparent volumetric thermal expansion coefficients averaged (5±3)·10-5 m3 m-3 K-1 and reached up to 25·10-5 m3 m-3 K-1, markedly exceeding values commonly adopted in wellbore integrity models. Subsequent triaxial testing shows thermally cycled samples exhibit a 5–30% reduction in compressive strength and a transition from discrete shear failure to more pervasive microcracking, likely linked to freeze–thaw induced damage. Thermal cycling treatment had limited impact on cement matrix permeability, which remained within industry guidelines for effective zonal isolation. Our results show that downhole-representative thermal expansion behaviour must be incorporated into predictive modelling of CO₂ injection well integrity, as commonly assumed values for the cement thermal expansion coefficient derived from atmospheric laboratory testing lead to underestimation of thermally induced stresses and debonding potential.