Self-healing and toughening well cement system: Application of core-shell microspheres with sodium silicate core, geopolymer shell, and resin-dispersant coating

During the development of tight oil and gas reservoirs, the high pressure generated by fracturing operations can easily induce micro-cracks and micro-annuli in the cement sheath, severely threatening zonal isolation integrity. To address the challenges of conventional toughening materials that struggle to balance strength and toughness while lacking self-healing capability for damage, this study designs and prepares a multifunctional core-shell structured microsphere integrating reinforcement, toughening, and self-healing functions. The microsphere features a sodium silicate core, a geopolymer load-bearing shell, and an epoxy resin-dispersant coating for surface modification, with calcium sulfate whiskers adhered to the outer layer to achieve toughening. Through disc granulation, high-temperature ceramification, and surface coating processes, the Self-healing and Toughening Microspheres were successfully fabricated, exhibiting a concentrated particle size distribution (D50 ≈ 65μm), a dense shell, and excellent thermal stability (decomposition temperature > 360°C). Experimental results show that the microspheres maintain structural integrity under hydraulic pressure of 50 MPa at 70°C, with a particle size retention rate of 94.6%. Protected by the geopolymer-epoxy resin composite shell, the microspheres achieve controlled release of sodium silicate without adversely affecting the engineering properties of the cement slurry, such as rheology, fluid loss, and thickening time. At 75°C, despite the significant increase in yield stress τ₀ from 5.92 Pa to 25.71 Pa, the reduction in fluidity did not exceed 3.2%. Under conditions of 160°C and 40 MPa with a retarder dosage of 3.0%, the thickening time of the cement slurry extended from 180 min for the control group to 210 min. The cement slurry achieved zero free water across the 50–90°C range and reduced API fluid loss by 13.6% at 50°C. Compared with the control cement stone, the cement stone incorporating 7% microspheres exhibited an impact resistance increase of more than 29 times, while the compressive strength retention rate exceeded 95%, demonstrating excellent toughening performance. Self-healing experiments indicate that under curing conditions of 90°C and 150°C, after 7 days of healing, the permeability of the cement stone with 7% microspheres decreased to 0.1883 mD and 0.0952 mD, respectively, with permeability recovery rates reaching 91.27% and 96.39%, respectively. SEM and XRD analyses further reveal that the microspheres rupture upon crack initiation, releasing sodium silicate, which reacts with calcium ions leached from the cement matrix to form dense calcium silicate gel and precipitated phases, thereby achieving effective crack filling and sealing. Through its structure-function integrated design, the proposed core-shell microsphere successfully achieves a synergistic effect of "high strength retention – toughening – damage self-healing" for the cement sheath, providing a novel material solution for ensuring cement sheath sealing integrity after fracturing in tight oil and gas reservoirs.