Application of a Cationic Polymer Flocculant for the Treatment and Recycling of Spent Oil-Based Drilling Fluids

Prolonged use of oil-based drilling fluids often leads to the accumulation of inferior solids, resulting in gradual performance degradation, requiring eventual disposal. Current recycling technologies exhibit significant limitations in terms of cost-effectiveness, operational feasibility, and removal efficiency. For efficient recycling and reuse, this study synthesized a cationic polymer flocculant (CPF) with a narrow molecular-weight distribution via free-radical solution polymerization, using lauryl methacrylate and methacrylatoethyl trimethylammonium chloride as monomers. The chemical structure and molecular weight of CPF were systematically characterized via Fourier transform infrared spectroscopy, proton nuclear magnetic resonance, and gel permeation chromatography. Thermogravimetric analysis indicated that CPF exhibits good thermal stability, with a decomposition temperature of 226 °C. Based on fractal dimension analysis, the optimal dosage of CPF was determined to be 1%. Mechanistic studies revealed that CPF acts through electrostatic adsorption on the surface of low-density solids, effectively neutralizing particle surface charges, reducing interparticle electrostatic repulsion, promoting floc formation, and improving settling efficiency. Meanwhile, its bridging effect enhances adhesion, ensuring structural stability of flocs under shear conditions. Performance evaluation results demonstrated that, at a CPF dosage of 1%, the initial settling velocity of low-density solids in mineral oil increased to 0.3 mm/s, and the transmittance of the supernatant reached 60%. After CPF assisted flocculation centrifugation treatment, the density of the spent oil-based drilling fluid, solid content, and apparent viscosity decreased by 0.54 g/cm³, 15%, and 25 mPa·s, respectively, indicating significantly improved rheological properties. These results demonstrate that CPF shows promising potential for the treatment and recycling of spent oil-based drilling fluids.