Preparation and performance evaluation of microcapsules for self-healing of micro-cracks in oil well cement sheaths

ZHANG Yifei; CHENG Xiaowei; ZHANG Chunmei; MEI Kaiyuan; WEI Tingcong

doi:10.13732/j.issn.1008-5548.2026.02.002

Abstract：Objective This study aims to address the poor compatibility between oil-absorbing resin and oil well cement,which leads to a reduction in the strength of cement stone,and to achieve oil-triggered self-healing of micro-cracks in the cement sheath,thereby ensuring the sealing integrity of the cement sheath in oil and gas wells.In this study,microcapsules with oil-swelling and micro-crack self-healing functions are prepared.The microcapsules are fabricated via a chemical precipitation method using oil-absorbing resin as the core material and reactive silica as the shell material. Methods Scanning electron microscopy(SEM)imaging,thermogravimetric analysis(TG),Fourier-transform infrared(FTIR)spectroscopy,and energy-dispersive spectroscopy(EDS)mapping were employed to test and characterize the oil-absorbing resin and the microcapsule.The oil-swelling and micro-crack self-healing performance of cement samples incorporating the oil-absorbing resin or microcapsule was evaluated by investigating their compressive strength,recovery rate of compressive strength,and permeability.Furthermore,SEM imaging and EDS mapping were conducted on thin sections extracted from crack surfaces of the cement samples to investigate the micro-crack self-healing mechanism in cement systems containing oil-absorbing resin or microcapsule. Results and Discussion The median particle sizes(D50)of the oil-absorbing resin and the microcapsule were 5.601 pm and 6.590 pm,respectively.Within the temperature range of 0-213 ℃,both the oil-absorbing resin and the microcapsule exhib-ited almost no mass loss,demonstrating good thermal stability that met the requirements for cementing operations.Within the temperature range of 214-304 ℃,the mass loss rates for the oil-absorbing resin and the microcapsule were 95.45％and 35.84％,respectively,indicating superior thermal stability of the microcapsule.A vibrational absorption peak for Si-OH was observed at a wavenumber of 941 cm-1,indicating that the oil-absorbing resin was encapsulated within the silica shell.The microcapsule possessed a large specific surface area,which helped improve their dispersion stability and interfacial bonding per-formance within the cement matrix.After 6 hours of oil absorption,the volume expansion rates of the oil-absorbing resin and the microcapsule were 100％and 10％,respectively,demonstrating the integrity and effectiveness of the silica shell encapsulation.The compressive strength of cement sample A1,incorporating microcapsule,reached its maximum value at a curing age of 7 days,indicating an optimal microcapsule dosage of 1％.After a self-healing curing age of 28 days,sample A1 exhibited a com-pressive strength of(29.06±1.61)MPa and a compressive strength recovery rate of 79.05％.Although both the oil-absorbing resin and microcapsule improved the compressive strength recovery capability of the cement stone,the incorporation of oil-absorbing resin led to a decrease in the overall compressive strength.Therefore,microcapsule was more suitable for the self-healing of micro-cracks in cement stone.After a self-healing curing age of 28 days,sample A,showed a permeability of 0.563× 10-3 μm2,representing a reduction of 39.36％,indicating a significant decrease in the permeability of the cement stone. Conclusion The silica shell of the microcapsule possesses high reactivity,enabling it to participate in the cement hydration pro-cess and provide nucleation sites,thereby enhancing the compressive strength of the cement stone after damage and addressing the issue of strength reduction caused by the incorporation of oil-absorbing resin.When a crack forms in cement containing microcapsules,the silica shell is fractured by the stress at the crack tip,exposing the internal oil-absorbing resin.Upon contact with oil,the oil-absorbing resin swells and becomes adsorbed and aggregated among the hydration products and pores,making the microstructure of the cement stone more uniform and dense,thereby effectively promoting the self-healing process of the cement stone.