Preparation of salt-resistant carbon dots-based nanofluids from coal tar pitch and their properties for enhanced oil recovery

Abstract

Nanomaterials are important enhancers for low-permeability reservoir engineering, while conventional nanomaterials still face limitations in size distribution, stability, and environmental compatibility. To address these challenges, the coal tar pitch (CTP), a low-cost by-product of coal chemical industry, was used as a carbon precursor to prepare coal tar pitch-based carbon dots (CTP-CDs, 2.61 ± 0.3 nm) with high stability, graphitized structure, and rich in carboxyl and hydroxyl groups through oxidation and hydrothermal methods, achieving high-value conversion of industrial waste. Structural characterization analysis through Fourier-transform infrared (FT-IR) peak deconvolution, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) reveals a carbonization pathway dominated by polyaromatic core condensation and periphery oxidation. The CTP-CDs were further mixed with sodium fatty alcohol polyoxyethylene ether carboxylate (AEC-9Na), which achieved the synergistic enhancement of interfacial activity, wetting control, and enhanced oil recovery (EOR) efficiency. The AEC-9Na/CTP-CDs system (ω_CTP-CDs = 0.5, c = 500 mg/L) can reduce the toluene-water interfacial tension to 0.66 mN/m at the salinity of 200,000 mg/L. The mixed system remains stable at the salinity of 300,000 mg/L or pH = 13, and the oil-wet glass contact angle decreases from 106.0° to 64.3°. It enhances oil recovery through the dual mechanism of “oil film removal-emulsification", and the crude oil recovery rate is increased by 17.4 %. This study provides a theoretical basis for the development of carbon-based nanofluids in low-permeability reservoirs and has important engineering value for realizing the resource utilization of coal-based solid waste.