One of the principal conundrums in drilling operations is addressing wellbore instability caused by shale hydration. Therefore, it is crucial to develop high-performance shale inhibitors. In this work, a hyperbranched polyethyleneimine/graphene composite (HPEI-G) was prepared by blending at 60 °C, and it was then used as a shale inhibitor. The inhibition performance of HPEI-G was verified using mud making test, linear swelling test and sedimentation test. The mechanism of HPEI-G was researched and determined using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), particle size distribution test and scanning electron microscopy (SEM). The compatibility of HPEI-G with the basic water-based drilling mud (WBM) was also verified. It can be observed from the results of the linear swelling test that 0.5 wt% HPEI-G reduced the swelling rate of montmorillonite (MMT) to 30.36%, and 1 wt% of KCl only decreased the swelling rate of MMT to 43.83%. In addition, HPEI-G is compatible with WBDF. The inhibition mechanism of HPEI-G included chemical adsorption and physical blockage. HPEI-G was adsorbed on the surface and interlayer of MMT by hydrogen bonding and electrostatic attraction, reducing the diffuse electric double layer to inhibit the hydration of MMT. The sheets of graphene in HPEI-G allowed it to stick on the surface of the shale and plug the nanopores of the shale, preventing the access of water. The inhibition effect of HPEI-G over a temperature range from room temperature to 150 °C was considered to be excellent.
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