Effect of lignin source and initiation conditions on graft copolymerization of lignin with acrylamide and performance of graft copolymer as additive in water- based drilling fluid

In our previous study, as effective and eco-friendly polymer additive in the water-based drilling fluids (WBDFs), lignosulfonate-g-polyacrylamide (LS-g-PAAm) graft copolymers were successfully synthesized and characterized [J. Petroleum Science and Engineering, 171(2018) 484–494]. The aim of this study was to investigate effect of the functional groups (sources) of lignin as well as initiation condition on the chemical modification of lignin via graft copolymerization method. Kraft lignin (KL) extracted from black liquor (as a waste of paper mills) and sulfonated lignin (SL) were used as lignins with the different source and chemical structure. KL and SL were then modified by graft radical copolymerization of acrylamide initiated with thermal or redox initiator. The aliphatic hydroxyl groups were identified as the active sites in the graft copolymerization, where the number of these functional groups in the lignin chain had a significant effect on the progress of the graft copolymerization reaction. Structure of the copolymers was investigated qualitatively and quantitatively using Fourier transform infrared (FTIR) and proton nuclear magnetic resonance (¹HNMR) spectroscopies. The performance of lignin and lignin-based graft copolymers (with different grafting percentage) as an additive in the water-based drilling fluids (WBDFs) was studied. Before and after hot rolling, rheological properties including apparent viscosity, plastic viscosity, yield point as well as fluid loss were measured in the absence and presence of the salt contamination. In all fluids, except fluids containing unmodified lignin, an increase in apparent and plastic viscosities was observed. Also, thermal stability and resistance to the salt contamination were observed in fluids formulated with the graft copolymers. Best performance was observed for a fluid containing of kraft lignin graft copolymer 1 (KLGC1) where grafting percentage was as high as 452.9 wt%. Also, results showed that higher amounts of the aliphatic hydroxyl functional groups in the KL in comparison with the SL provides higher rate of the reaction progress, leading to a superior performance for use of corresponding graft copolymer as an additive in the WBDFs. The KL-based graft copolymers were able to maintain rheological and fluid loss properties during drilling operations under the hot rolling and salt contamination.