Inhibitive property and mechanism of polyvinyl alcohol/tannic acid materials on clay hydration and swelling

Shale composed of a large amount of clay minerals is easy to swell and disperse in water, leading to wellbore instability. In recent years, adhesive shale stabilization materials have become a research hotspot because of their good bonding effect. However, many existing materials have problems such as high cost and incompatibility. In this study, polyvinyl alcohol (PVA) and tannic acid (TA) have been used as raw materials to prepare a bonding-type shale stabilizer (abbreviated as PT) for water-based drilling fluids. The shale compressive strength tests, immersion tests, linear swelling tests, and shale hot-rolling recovery tests have been used to evaluate its inhibitive performance. The results showed that the shale recovery of 3 % PT solution with a PVA/TA mass ratio of 2:1 can reach more than 90.0 %, even after hot-rolled at 150°C. In addition, the bentonite soaked in PT aqueous solution had good integrity and small volume expansion at 90°C. After immersing for 5 h, the swelling height of the bentonite sample is only 2.0 mm. Meanwhile, the compressive strength of the artificial core after soaking in 3 % PT aqueous solution at 90°C for 24 h was the highest, reaching 30.19 MPa. The rheological and filtration performance evaluation showed that PT could improve the viscosity and reduce the fluid loss of bentonite dispersion, displaying good compatibility. According to microstructure analysis, it was preferred that PVA polymer with a large number of hydroxyl groups and TA small molecules containing catechol structures in PT could form a network structure through multiple hydrogen bonding interactions, showing a strong adhesive effect. Then, PT easily adsorbed on the surface of shale or clay particles, formed a dense adsorption film on the surface to block the invasion of water molecules, and even enhanced the cementation between these particles. Finally, hydration, swelling, and dispersion of clay minerals were well inhibited, and the borewell stability was enhanced.