▎ 摘　　要

Herein, we report mechanically reinforced dual-coated sand proppants for hydraulic fracturing operations. The dual-coated sand proppants were prepared by sequential surface modifications. The sand particles were firstly coated with polystyrene-polymethyl methacrylate copolymer (PS-PMMA) composite with commercial graphene (CG) (first composite layer) and subsequently coated with epoxy-CG composite (second composite layer). The copolymer-CG coating onto sand particles was carried out by mixing 1:1 styrene (S) and methyl methacrylate (MMA) monomers with the initiator AIBN along with CG followed by adding the mixture to sand particles with vigorous stirring and subsequent polymerization at 70 degrees C. The PS-PMMA-CG coated sand particles were then mixed with epoxy resin and a cure in a ratio of 4:1 along with CG and cured at 150 degrees C for 5 min. Chemical, mechanical, thermal, and morphological characterizations for the composite layers and dual-coated sand proppants were investigated in detail by Fourier transform-infrared spectroscopy (FT-IR), nano-indentation, thermogravimetric (TGA), optical, and scanning electron microscopic (SEM) techniques. The FT-IR results revealed that the successful formation of PS-PMMA-CG copolymer composite and cross-linked epoxy-CG composites. The XRD results indicated that the successful surface coverage of the dual composites coating onto the sand. The optical and SEM results revealed that the roundness and sphericity of the reinforced proppant particles were greater than 0.6. Interestingly, elasticity (E), and hardness (H) of the coating shells have reached a maximum of 3.97 GPa and 0.144 GPa, respectively. The thermal stability of the (PS-PMMA-CG)-(epoxy-CG) dual composite shell has reached as high as 363 degrees C. Furthermore, the stress resistance of the optimized dual-coated sand proppant (Sand-(PS-PMMA-CG)-(epoxy-CG)) reached as high as 10000 psi with a fine production of <10 wt%. The mechanically reinforced dual-coated sand including the most desirable characteristics of roundness and sphericity to be greater than 0.6, high-stress resistance with excellent thermal and mechanical stabilities confirmed its great potential to be applied as successful proppants in oil and gas upstream applications.