Novel N, N-dimethylacrylamide copolymer containing multiple rigid comonomers as a filtrate reducer in water-based drilling fluids and mechanism study

Ultra-high temperature dramatically deteriorates rheological properties and filtration performance of water-based drilling fluids (WBDFs), especially for a salt-gypsum formation that greatly restricts the application of WBDFs in ultra-deep well drilling operations. Hence, the research in this article used an anionic copolymer (DANS) prepared by N, N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, N-vinylpyrrolidone, and sodium 4-styrenesulfonate as an anti-ultra-high temperature and anti-salt contamination filtrate reducer. Due to its multiple bulky cyclic structures in the side chain, DANS exhibits excellent thermal stability in the thermal gravimetric, rheological, and filtration tests. By adding 1.0 wt% DANS, the American Petroleum Institute (API) filtration volume of sodium bentonite-based fluids (SBT-BFs) decreased from 56.0 to 9.2 ml after aging at 240 degrees C. Even under both 20 wt% NaCl contamination and 240 degrees C aging, SBT-BFs with 2.0 wt% DANS could maintain a filtration volume of 9.4 ml, whereas SBT-BFs without DANS reached a filtration volume of 203.0 ml. The possible filtration control mechanism of DANS was further investigated via the Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), zeta potential, particle size distribution, filter cake micro-morphology, and transmission electron microscopy (TEM). Confirmed by FTIR and AFM results, the effective adsorption was formed between DANS and bentonite even under a high-salinity environment. Thus, a firm "dot net" structure formed by bentonite/DANS was observed by TEM, significantly improving the colloidal stability, dispersibility, and filter cake compactness of SBT-BFs. Finally, by utilizing DANS as the core treatment agent, a high-density WBDFs system with a temperature resistance of 240 degrees C and a salt-tolerance of 20 wt% has been successfully prepared.