A Dynamical Spontaneous Imbibition Model for ShaleConsidering Hydration Damage

Shale spontaneous imbibition is an important and fundamental phenomenon affecting drilling, fracturing, and other engineering measures. Unlike sandstone or carbonate formation, clay mineral is rich in shale. When the water phase enters into shale, it triggers hydration structural damage after contacting with clay, improving the spontaneous imbibition. Thus, if hydration damage during spontaneous imbibition is ignored, it is not consistent with the actual spontaneous imbibition of shale, affecting the design of drilling and completion engineering. Therefore, based on laboratory experiments, a quantitative characterization method of hydration structural damage was established. According to the evolution rule of hydration damage, formulas for the dynamical capillary force of spontaneous imbibition and the dynamical tortuosity were established, and a model of shale dynamic spontaneous imbibition was established. The spontaneous imbibition curve predicted by the shale dynamic spontaneous imbibition model was in good agreement with the actual spontaneous imbibition curve, which verified the accuracy of the model. The factors influencing the shale dynamic spontaneous imbibition were systematically analyzed by using the shale dynamic spontaneous imbibition model. The results show that hydration can significantly promote shale spontaneous imbibition. Under strong hydration, shale porosity increases, and tortuosity decreases, showing stronger driving force and smaller flow resistance and resulting in significantly increased spontaneous imbibition volume and spontaneous imbibition rate, indicating that hydration damage is the main factor affecting shale spontaneous imbibition. The research results deepen the understanding of shale spontaneous imbibition mechanism and provide a theoretical basis for the development of reasonable technical measures in shale drilling and completion engineering.