Effects of hydration swelling and wettability on propagation mechanism of shale formation crack

The shale formation cracks have an important influence on the borehole stability of shale well, and the interaction between drilling fluidand shale (capillary effect and hydration swelling) would impact on the propagation mechanism of shale formation crack. In this thesis, experiments were done for the outcrop and well core of the Longmaxi Formation shale of the southern Sichuan Basin to determine shale wettability and hydration swelling. Based on fracture mechanics, the propagation model of shale formation crack was established with the hydration swelling stress and capillary force, and the effects of hydration swelling and wettability on the propagation mechanism of shale formation crack were studied. The result shows that the wettability of the Longmaxi Formation shale is both water-wet and oil-wet, and is more likely to be oil-wet. The hydration swelling stress of the shale tend to ascent first and then become stable with the increase in time. For the shale rocks first soak in oil or KCl, the rise velocity of hydration swelling stress would decrease.The fractures on the sample surface are parallel plane of bedding in the process of immersing in water. With the increasing of immersion time, the sample would keep its integrality or spall to pieces. While the number of the fractures soaking in KCl is less and the cracked degree of the rocks is lighter. The hydration swelling has a great influence on the increment of the stress intensity factor, and the drilling fluid systems need to decrease the drilling fluid filter loss and increase the clay minerals hydrate inhibitor. The wetting behavior has a great influence on the propagation mechanism, and the drilling fluid systems need to reduce the drilling fluid interfacial tension and enlarge the wetting angle between drilling fluid and rock medium. The contact angles of oil-based drilling fluids and shale surface and the interfacial tension are little, which result in small capillary force. Therefore, the oil-based drilling fluid is used to inhibit crack propagation, hence the wellbore will be easy to maintain stable.