A Multiscale Study on Shale Wettability: Spontaneous Imbibition Versus Contact Angle

Environmental and technical issues associated with hydraulic fracturing fluid leak-off and its low recovery in clay-rich shale reservoirs has challenged the energy sector. One of the main hypothesized mechanisms for the uptake of fracturing fluid is fluid imbibition due to capillary forces that are a strong function of the rock's wettability. However, shale wettability estimated by contact angle and spontaneous imbibition experiments are highly inconsistent throughout the literature. It is therefore critical to shed light on the possible reasons for such discrepancies and identify the superior technique. In order to characterize the physical processes from microscale to macroscale, the contact angles and spontaneous imbibition characteristics of shale samples were measured using aqueous ionic solutions (NaCl, KCl, CaCl2, and MgCl2) at different concentrations (0, 0.1, and 0.5 M) and oil (Soltrol-130). The internal structure of the samples exposed to deionized water was imaged by three-dimensional X-ray micro-computed tomography before and after spontaneous imbibition at both unconfined and confined stress states. Additionally, the structure of all samples exposed to each ionic solution was analyzed via environmental scanning electron microscopy. Experimental observations were further substantiated by combining digital image correlation with Poisson-Boltzmann analysis to assess the hydration-induced strains. It was shown that while spontaneous imbibition is a more realistic measurement for liquids, results obtained under unconfined condition can be highly biased due to additional forces acting (e.g., osmotic or hydration forces) and also potential microstructural alterations. Thus, spontaneous imbibition measured under reservoir stress condition is strongly recommended for shale wettability estimation.