Wettability is an important property of shales, yet its determination is quite challenging. There is no consensus as to the most appropriate method of assessing shale wettability. The air-liquid-rock and liquid-liquid-rock contact angles (Sessile/Captive) used for finding the wettability of conventional rock samples often give ambiguous/contradictory results for shales, as evident from the published literature showing a broad spectrum of results from water-wet to mixed-wet to oil-wet.
In this study, the Drop Shape Analysis (DSA) approach was employed to evaluate the wettability of Eagle Ford and Wolfcamp outcrop shale samples. Both air-liquid and Sessile/Captive drop contact angles were measured. The effects of heterogeneity (location variability), roughness, solvent rinsing, exposure time, and salinity were analyzed and compared with the water-oil contact angles computed by van Dijke and Sorbie's (2002) approach. Due to the transient nature of contact angles on the shale surface, the instantaneous (initial) contact angles change rapidly. Therefore, it is proposed to use the evolution of 3-phase contact line to identify the time at which the contact angle would be unique and stabilized.
The wettability of Eagle Ford and Wolfcamp shale samples could not be reliably concluded by the DSA method employed alone without the help of some other auxiliary techniques.