The hydration and swelling of shale caused by water in water-based drilling fluids is one of the most important problems that causes wellbore instability. The development of high-performance shale inhibitors is an important prerequisite for ensuring the drilling of shale formations. In this study, four deep eutectic solvents (DESs) based on choline chloride (urea-DES, Gly-DES, Oxa-DES, and Cit-DES) were synthesized and introduced as promising shale inhibitors. Fourier transform infrared spectroscopy (FT-IR) and hydrogen nuclear magnetic resonance spectroscopy (1H NMR) were used to characterize the structural information on DESs. The inhibition ability of DESs was evaluated by a bentonite inhibition test, a linear swelling test, and a settlement test. Compared with the traditional inhibitors, such as potassium chloride (KCl) and polyether amine, the four DESs can effectively inhibit the hydration and swelling of shale. Five wt % DESs can maintain low rheological parameters when the content of sodium bentonite (Na-bent) was as high as 40 wt %. Among them, Gly-DES not only exhibited the most efficient inhibition capability but also had the advantage of not affecting the basic performance of the drilling fluid. Inhibitory mechanism analysis showed that the electrostatic interaction and hydrogen bonding between DESs and sodium bentonite (Na-bent) are the most important factors to inhibit clay hydration. Besides, the reduction of the surface tension of the liquid by DESs effectively weakened the driving force for water to penetrate the Na-bent layer. Finally, it is important to note that, although DESs exhibited strong inhibitory capabilities, some DESs, such as Oxa-DES and Cit-DES, are prone to adversely affect the basic performance of drilling fluids. Therefore, a careful selection is required.

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The wellbore instability caused by the penetration of drilling fluids into the formation is a vital problem in the drilling process. In this study, we synthesized a polymer/graphene oxide composite (PAAN-G) as a fluid loss additive in water-based drilling fluids. The three monomers (acrylamide (AM), 2-acrylamide-2-methyl-1-propane sulfonic acid (AMPS), N -vinylpyrrolidone (NVP)) and graphene oxide (GO) were copolymerized using aqueous free radical polymerization. The composition, micromorphology, and thermal stability properties of PAAN-G were characterized by Fourier transform infrared (FT-IR) spectroscopy and thermogravimetric analysis (TGA). According to the American Petroleum Institute (API) standards, the influence of PAAN-G on the rheological and filtration properties of bentonite-based mud was evaluated. Compared with PAAN, PAAN-0.2G has more stable rheological properties at high temperatures. The experimental results showed that even at a high temperature of 240 °C, PAAN-G can still maintain a stable fluid loss reduction ability. In addition, PAAN-G is also suitable for high-salt formations; it can still obtain satisfactory filtration volume when the concentration of sodium chloride (NaCl) and calcium chloride (CaCl 2 ) reached 25 wt %. Besides, we discussed the fluid loss control mechanism of PAAN-G through particle size distribution and scanning electron microscopy (SEM).