Effects of a polyamine inhibitor on the microstructure and macromechanical properties of hydrated shale

China is rich in shale gas resources, however, wellbores in shale gas reservoirs are frequently unstable. This has a serious impact on the shale gas drilling cycle. Polyamine, a common additive in water-based shale drilling fluids, can effectively inhibit shale hydration. However, there is a lack of quantitative research on the effect of polyamine inhibitors on the microstructure and macromechanical properties of shale. Therefore, this study investigated those issues via a systematic hydration experiment carried out on shale from the Longmaxi Formation. The results show that microfractures are created and expand during shale hydration, that they also connect to form a complex microfracture network, and that 3% polyamine inhibitors (polyamine solution with volume fraction of 3%) can effectively inhibit their evolution. The ultrasonic velocities and UCS of the Longmaxi shale are significantly anisotropic; the former first increases and then decreases with the laminae angle, reaching its maximum when the laminae angle is 30°. The UCS of the shale is highest and lowest, respectively, when the laminae angles are 0° or 90° and 30°. In general, these UCS appear as a “U" pattern, high on two sides with a dip in the center. Polyamines can effectively inhibit both the expansion of shale and the reduction of P-wave and S-wave velocities, the UCS, and elastic modulus. The UCS of a shale sample was reduced by 28%–40% after immersion for 96 h in water, compared to 2%–20% after immersion in a 3% polyamine inhibitor for the same amount of time. The inhibiting effect of the polyamine was remarkable.