Preparation of Core-Shell Microspheres and Their Influence on the Elasticity and Self-Healing Performance of Set Cement

To deal with Micro cracks and micro annular gap encountered in shale gas wells after cementing and hydraulic fracturing operations, experimental research was conducted on self-expanding polymer microspheres with both elasticity and methane-triggered response, as well as an elastic self-healing cement slurry. The self-expanding polymer microspheres are developed through emulsion polymerization, featuring a core-shell functional structure with “rigidity-toughness complementarity”. On the shell there exist a small amount of carboxyl and sulfonic acid groups which can overcome the limitations of conventional self-expanding polymers when required to uniformly disperse in a cement slurry. The microstructure of the self-expanding microspheres was comprehensively characterized using infrared spectroscopy, particle size measurement, Zeta potential measurement and scanning electron microscope, and the heat resistance and methane-triggered volume expansion of the polymer microspheres were evaluated. The experimental results show that the core-shell self-expanding microspheres are approximately spheric particles with particle sizes of around 200 nm, and are negatively charged on their surfaces. They have a thermal decomposition temperature exceeding 300 ℃; however, the microspheres exhibit “thermal viscosity”, and solidify into lumps after high-temperature treatment followed by cooling. Their “thermal viscosity” can be inhibited by compounding them with fumed silica, an inorganic nanoparticle, and the compounded self-expanding microspheres remain in a powder state after treatment at 160 ℃ for 24 hours, with a methane-triggered rate of volume expansion of over 30%. Using the compound self-expanding microspheres, a normal-density self-healing cement slurry with elastic modulus less than 5.5 GPa can be formulated for operations at temperatures between 50 ℃ and 120 ℃. This cement slurry exhibits a normal thickening curve and a controllable filter loss which meet the requirements of well cementing. Under both pressure-holding and gas-channeling conditions, the set cement containing 10% compound self-expanding microspheres exhibits 100% methane-triggered self-healing efficiency. The cement slurry formulated with the compound self-expanding microspheres has both elasticity and methane-triggered self-healing property, providing a novel solution for inhibiting annular fluid channeling in shale gas wells.