Abstract
<jats:title>Abstract</jats:title> <jats:p>Achieving reliable long-term isolation in wellbore environments depends critically on the integrity of the cement-casing interface and its ability to sustain sealing and mechanical resistance over time. This study quantifies curing-induced expansion pressure at the steel-cement interface and evaluates its effect on sealability and interfacial friction for oil-well cement plugs incorporating an MgO-based expanding additive. Cement plugs were cast inside 5-in steel casing sections using Class G cement containing 0%, 3% and 6% dead-burned MgO (by weight of cement). All specimens were cured for 50 days at 70 °C and 50 bar external water pressure with free-water access. Axial and hoop strains on the outer casing wall were continuously monitored and corrected for thermal effects using the non-expansive reference plug. Hoop strain was converted to radial expansion pressure through an elastic thin-walled cylinder formulation. Following curing, plug performance was assessed through single-phase water seepage tests under differential pressures of 30-50 bar and mechanical push-out tests to obtain force-displacement response and the mechanical work done by the piston pushing the cement plug. Companion cement cylinders were tested to determine uniaxial compressive strength, Young's modulus, and Poisson's ratio. The MgO-modified plugs developed end-of-curing expansion pressures of approximately 2.11 MPa and 3.35 MPa for the 3% and 6% MgO formulations, respectively. Seepage testing showed reduced effective permeability relative to the reference, with the lowest permeability observed for the 3% MgO mixture, while the 6% mixture exhibited a slightly higher leakage rate. Push-out resistance increased consistently with MgO content, with peak loads increasingly linearly with MgO content from about 866 kN for the reference to 1,060 kN and 1,365 kN for the 3% and 6% MgO plugs, respectively. Increased MgO content was also found to be accompanied by higher post-peak work by the piston displacing the plug. Conversely, uniaxial compressive strength decreased with increasing MgO dosage, indicating a trade-off between expansion-induced sealing, push-out resistance and bulk mechanical strength. By simultaneously linking hoop-strain-derived radial pressure to sealability and push-out behavior at casing scale, this work provides quantitative guidance for MgO dosage selection and valuable input for the calibration of empirical and numerical models of plug-casing interaction in well-abandonment design.</jats:p>