Microstructural evolution and sealing performance of silica-flour cement under HPHT conditions: A combined experimental and simulation study

Maintaining long-term zonal isolation in high-temperature wells requires cement systems that withstand extreme thermal stresses while preserving sealing integrity, yet current evaluations emphasize mechanical strength rather than hydraulic sealing. This study directly assessed the leakage performance of a silica-flour-modified Portland cement, BSF28.6 (28.6 wt% silica flour), cured at 220 °C and 1500 psi for 28 days. A custom HPHT leakage apparatus, high-resolution micro-computed tomography (micro-CT), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR) were employed to evaluate sealing capacity, pore architecture, and mineralogical evolution. Despite silica flour stabilization intended to prevent strength retrogression, BSF28.6 failed differential-pressure testing, with immediate pressure equalization indicating continuous leakage pathways. Micro-CT revealed a 192% increase in porosity (from 0.77% to 2.25%) with 1.16% interconnected porosity. and extensive micro-annular cracks along the casing interface. XRD and FTIR analyses confirmed the transformation of gel-like C-S-H into brittle crystalline phases, specifically xonotlite and tobermorite, evidenced by sharp Si-O stretching bands and distinct diffraction peaks. A simulation from the Virtual Cement and Concrete Testing Laboratory (VCCTL) modeling tool supported these observations, indicating accelerated hydration and extensive phase transformation result in lower elastic moduli. These results demonstrate that silica flour fosters hydrate chemistries and heterogeneous microstructures that compromise hydraulic integrity via interfacial debonding under prolonged HPHT exposure. These findings highlight the need for next-generation thermal cement formulations emphasizing gel retention, pore structure refinement, and interfacial bonding to ensure long-term sealing in geothermal and heavy-oil steam-recovery operations.