Synthesis of a novel cationic hydrophobic shale inhibitor with preferable wellbore stability

To solve the problems of wellbore instability and collapse caused by clay hydration during the horizontal drilling process of shale gas, a novel cationic hydrophobic shale inhibitor ADD was synthesized by emulsion polymerization using acrylamide (AM), dioctyl maleate (DOM), dimethyl diallyl ammonium chloride (DMDAAC), and KH570-SiO₂. Fourier transform infrared spectroscopy (FTIR) and Thermo Gravimetric Analysis (TGA) were used to characterize the functional groups and thermostability of synthesized material. The inhibition performance of shale inhibitor ADD was evaluated by the rolling recovery rate, linear expansion rate, and core immersion test. After being treated with 1% ADD for 16 h, the linear expansion rate of the core sample was 18%, the debris rolling recovery rate was 96.3%. The inhibition performance was better than that of polyamine inhibitor YZ and KCl. The Zeta potentials and contact angles measurement of core samples before and after treatment were measured, it was found that the Zeta potentials of core samples decreased significantly with the increasing concentrations of ADD. The possible explanation could be that ADD molecules with the positive charge can neutralize with the negative charge on the clay surface, reducing the repulsion force of the double electric layer of the clay. And the ADD molecules can form a hydrophobic adsorption layer with high roughness and low surface energy on the rock surface, which can reverse the wettability of shale from liquid-wetting to strong hydrophobicity, then preventing the intrusion of drilling fluid and formation water, achieving the inhibition of the hydration and expansion of shale.