Abstract
ABSTRACT: Maintaining cement sheath integrity under extreme thermal loading is a critical challenge for superhot geothermal wells. This study presents a small-scale laboraty investigation of the pore pressure response and hydraulic diffusivity evolution in a Portland GS cement sheath (Class G with 40% silica flour) subjected to rapid heating up to 320 °C using a modified mini-wellbore simulator. The cement annulus, pre-cured at 140 °C, was instrumented with a mineral-oil-filled capillary tube to monitor pore pressure at the mid-plane. Results show that thermal pressurization of pore fluid occurs during heating but at rates approximately one order of magnitude below undrained theoretical predictions. This discrepancy is attributed to the compliance of the measurement system, including oil compressibility and dissolved gas exsolution, compounded by progressive damage in the cement. A sharp pore pressure turndown at approximately 240 °C signals the onset of effective-stress-driven mechanical failure. Post-heating diffusion tests confirm a 2–3 fold increase in hydraulic diffusivity and indicate discrete crack or micro-annulus formation. These findings provide experimental insight into the thermo-hydro-mechanical mechanisms governing cement sheath failure in high-temperature wells.