Two-step hydrothermal synthesis of active carbon-dots with high temperature and salt resistance for injection pressure reduction and enhanced oil recovery

Abstract

Ultrasmall carbon nanomaterials show enormous potential in the development of unconventional oil resources for reducing injection pressure and improving oil recovery. However, the high temperature and salinity reservoir conditions and the poor activity of carbon nanoparticles hinder their practical applications. In this study, carbon nanoparticles (CDs) are readily synthesized by citric acid (CA) and ethylenediamine (EDA) through a hydrothermal reaction. The results show that a change in the type and density of oxygen hydrophilic groups by adjusting the molar ratio of reactants can improve CDs dispersibility and stability at a temperature of 90 °C and salinity of 16 × 104 mg/L. The active carbon nanoparticle (sm-CDs) is obtained by surface modification of CDs with erucate amidopropyl betaine (EAB), which enhances surface activity while retaining the original temperature and salt resistance. Core flooding tests show 0.03 wt% sm-CDs nanofluid provides a pressure-reduced rate of 63.5% and an enhancement in oil recovery of 23.0%. The results of atomic force microscopy (AFM) with hydrophobic probe reveal sm-CDs can effectively reduce the adhesion between alkanes and lipophilic surfaces, which alter microscopic surface wetting from oil-wet to uniformly water-wet. This study provides a feasible strategy for the efficient development of carbon-based active nanofluids, especially in unconventional reservoirs.