Experimental and model analysis on shale spontaneous imbibition and its influence factors

Spontaneous imbibition is a natural process in the interaction between liquid-rock and also a vital influencing factor of hydraulic fracturing in shale gas exploitation. Currently, researches of shale spontaneous imbibition are based on methods from spontaneous imbibition of tight sandstone, ignoring hydration effect during spontaneous imbibition. Due to high content of clay mineral, when liquid is contacting with shale, hydration effect occurs, triggering osmosis and structural damage, which are able to affect driving force and flow path of spontaneous imbibition. Therefore, this paper introduced hydration effect into shale spontaneous imbibition and built a novel analytical model by taking into account osmotic pressure and hydration fracture. This model was validated through spontaneous imbibition experiments of 12 shale samples. Besides, influencing degrees of all factors of shale spontaneous imbibition have been analyzed on the basis of their weight coefficients. Research results indicate hydration effect increases driving force of spontaneous imbibition by forming osmotic pressure. However, osmotic pressure occupies small part of driving force and capillary force is still the major one. Hydration fracture has relatively larger impact on imbibition amount, compared with osmosis, capillary effect and pore structure. In other words, hydration fracture is the main influencing mechanism of shale spontaneous imbibition. In addition, shale spontaneous imbibition is susceptible to saline ion. Saline ion has the inhibiting effect on shale spontaneous imbibition and its inhibiting mechanism is to decrease osmotic pressure, weaken hydration structural damage and shale water-wet. Findings of this study could provide a better understanding of shale spontaneous imbibition, offering theoretical guidance for shale gas development.