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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°C. Even under both 20 wt% NaCl contamination and 240°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), ζ 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°C and a salt‐tolerance of 20 wt% has been successfully prepared.

This work investigated the effect of the alkyl chain length of soluble methylimidazolium bromide ionic liquids (ILs) on their inhibition performance. The IL with a shorter alkyl chain length showed superior inhibition performance by suppressing clay swelling, mitigating clay dispersion, at room temperature. Particularly, the IL with an alkyl chain length of two (EmBr) reduced the sodium bentonite (Na-BT) swelling degree to 89% and achieved a cutting recovery of 81.9% after being rolled at room temperature, performing the best among all ILs. To systematically analyze the inhibition mechanism of ILs, X-ray diffraction (XRD), ζ potential, and particle size distribution have been carried out. The results revealed that the methylimidazolium with shorter alkyl chain length had better ability to enter the interlayer void by ion exchange and decrease interlayer distance, suppress the electrical double layer of the Na-BT particles and decrease the ζ potential, and promote the aggregation of Na-BT in water. It is also observed that high hot rolling temperature reduced the shale inhibiting performance of all ILs, and ILs with longer alkyl chain length had better ability to prevent cutting disintegration at high temperature. It is attributed to the variation of the hydrophilic characteristic of Na-BT at high temperature where EmBr no longer adsorbed the most on the surface and entered the interlayer voids of Na-BT. This study can be used as a reference to systematically explore the effect of the structure of shale inhibitors on their inhibiting performance and develop effective shale inhibitors.