Experimental study on crack propagation in shale formations considering hydration and wettability

Cracks in shale formations exhibit an important influence over wellbore stability of shale gas wells, and the interaction between the drilling fluid and shale affect the crack propagation mechanism in shale formation. In this paper, the outcrop shale samples from the Lower Silurian Longmaxi Formation (LF) in the Sichuan Basin of China are carried out laboratory experiments to investigate the wettability and hydration by spontaneous imbibition. Based on the fracture mechanics, a crack propagation model was established for the shale formation with the hydration stress and capillary force, and the influences of hydration by spontaneous imbibition and capillary effect on the crack propagation mechanism in shale formation were discussed. The results indicate the LF shale wettability is both water-wet and oil-wet and tends to be more oil-wet. The hydration stress of the shale tends to ascend first before stabilizing with increasing immersion time. The surface fractures of the shale samples exhibit parallel bedding plane immersion in water. Increasing the immersion time causes the shale samples to spall into pieces. The capillary effect influences the stress intensity factor with a critical width of 0.6 mm. The hydration and capillary effect positively influence the stress intensity factor, and drilling fluid systems should decrease their drilling fluid filter loss, increase their clay minerals hydrate inhibitor, reduce their drilling fluid interfacial tension and enlarge the contact angle between the drilling fluid and rock medium. The water-base fluids have a significant positive influence on crack propagation in shale formation, whereas the oil-base fluids have a less positive impact on crack propagation in shale formations. Therefore, the oil-based drilling fluids better maintain wellbore stability during when drilling in shale formations.