Mechanism of synergistic reinforcement of cement paste using inorganic fibres and nano powder under high-temperature conditions and fracturing loads: Multi-scale structural evolution

Oil well cement paste serves in high-temperature and high-pressure wellbore environments and bears multiple fracturing loads during deep shale-gas resource development. Formation sealing failure caused by strength attenuation and micro-crack propagation seriously threatens wellbore integrity and shale-gas production safety. To prevent high-temperature strength degradation and brittle cracking failure under fracturing load, this study used inorganic wollastonite fibre (WF) and nano-silica powder (NS) as typical representatives to systematically explore synergistic improvement of strength and toughness of cement paste by fibres and nano-powder as well as the corresponding mechanisms. The influence and mechanism of WF and NS on mechanical properties, sealing ability, and microstructure were revealed via multi-scale characterization, including macroscopic mechanical tests, meso-CT scanning, and microstructure analyses. Results show that cement paste with 7.5 wt% WF and 1 wt% NS after curing at 180 °C and 20.7 MPa for 7 d exhibited compressive and tensile strengths 129.68 % and 29.61 % higher than those of the reference paste. Moreover, the composite cement paste could withstand 17 cyclic loads under simulated conditions of 180 °C, 30 MPa in-situ stress, and 70 MPa internal pressure. Mesoscopic CT imaging confirmed the composite system significantly reduced crack volume and effectively inhibited crack propagation. Microscopic analyses showed WF and NS synergistically promoted growth of hydration products tobermorite and xonotlite, improving the interface structure between WF and cement paste and improved the high-temperature strength of cement paste. In addition, the toughness of cement paste was enhanced by dissipating energy and dispersing stress through fibre pull-out mechanism.