Synthesis of carboxymethyl chitosan as an eco-friendly amphoteric shale inhibitor in water-based drilling fluid and an assessment of its inhibition mechanism

To achieve potent inhibition, reduced environmental impacts and excellent compatibility between drilling fluid and shale inhibitors, an eco-friendly amphoteric inhibitor based on carboxymethyl chitosan (CMCS) was synthesized. The molecular structure of CMCS was characterized by potentiometric titration, Fourier transform infrared spectroscopy and nuclear magnetic resonance, and the carboxymethyl substitution degree was revealed to be approximately 0.94. The inhibition performance was evaluated through linear swelling and hot-rolling dispersion tests, and the results were compared with those obtained with low-molecular-weight cationic polyether diamine, high-molecular-weight non-ionic polysaccharide encapsulator, and potassium chloride. The interaction between clay particles and CMCS molecules was investigated by X-ray diffraction, scanning electron microscopy, surface wettability, particle size distribution, zeta potential and surface tension tests. The results showed that CMCS with a specific amphoteric molecular structure exhibited a different inhibition behaviour compared with conventional inhibitors, more effectively prevented clay hydration and swelling and inhibited shale disintegration at both 77 degrees C and 150 degrees C. CMCS could absorb onto the clay surface via electrostatic force and hydrogen bonding, and an encapsulation film was formed through intermolecular interactions to immobilize outer free water molecules and to provide hydrophobic characteristics. CMCS intercalates into clay lattice, expels inner water and reduces the interlayer space. Additionally, CMCS reduces the surface tension of the bulk solution and hinders fluid invasion. Furthermore, CMCS was compatible with bentonite dispersion and improved the rheological and filtration properties at a suitable concentration.