Oil Wettability as an Indicator of Early-Time Productivity in Duvernay Organic-Rich Shales

Multistage-fractured horizontal wells completed in the Duvernay Formation exhibit substantial variation in production performance, with wells located approximately 135 km apart differing in productivity by up to 9-fold. This study investigates the relationship between the oil wettability index (WIo) and early-time hydrocarbon production and explains the mechanisms governing this relationship. Core samples spanning a wide range of thermal maturities, together with production, completion-design, and reservoir pressure data, are analyzed. Wettability is quantified by spontaneous imbibition experiments, and well production is normalized by the gross perforated interval of the horizontal well. The relationship between WIo and well productivity is interpreted in terms of thermal maturity, petrophysical properties, organic matter morphology at the pore scale, and mineralogical composition. The results reveal that WIo exhibits a strong positive correlation with normalized early-time production, following a power-law relationship that accounts for more than 85% of the production variation. This behavior indicates that WIo serves as a reliable proxy for determining the extent of mobile hydrocarbon in the Duvernay Formation, consistent with elevated gas-oil ratios in higher-producing oil wells and elevated gas-condensate ratios in gas-condensate wells. The latter trend can be associated with higher thermal maturity and secondary cracking of oil into gas. WIo is primarily controlled by thermal maturity. It correlates positively with Tmax and negatively with the hydrogen index. Scanning electron microscopy observations indicate that progressive thermal maturation drives the development of organic matter-hosted pores. This is consistent with the higher hydrocarbon saturation and greater hydrocarbon storage capacity observed in the more thermally mature samples. WIo shows no correlation with porosity or permeability. WIo represents the oil-wet fraction of the connected pore network. This network comprises the organic matter hosted pores and the oil wet mineral hosted pores. In contrast, bulk porosity and permeability include both the oil wet and water wet pore networks. Bulk mineralogy also shows no correlation with WIo.