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Organic-rich shale wettability and its controlling factors: Comparative investigations of typical marine, continental, and transitional shales in the Middle Yangtze Platform (China)

Abstract

Wettability is a physico-chemical rock property to play a crucial role in the efficient development of shale gas and oil. However, a precise characterization of wettability for shale reservoirs is still a challenging task, due to their complex pore structure (with a large proportion of pores in nm scale) and shale composition (variable at μm scale). In this work, we conducted fluid drop wettability test, air-liquid contact angle measurement and spontaneous imbibition (SI) with two fluids (deionized [DI] water and n-decane [an oil-phase]) to evaluate the wettability of typical organic-rich shales (marine Longmaxi, continental Dongyuemiao, and transitional Dalong and Longtan shales), and discussed the effects of total organic carbon (TOC), thermal maturity, mineral composition, effective porosity and pore size distribution on shale wettability. Contact angles results show that marine Longmaxi shales have strong or moderately-strong water-wet properties. In contrast, continental Dongyuemiao and transitional Dalong-Longtan shales show much stronger water-repellant property with contact angles in a range of 21.2°-46.4° and 57.8°-61.7°, respectively. N-decane spreads quickly on the flat sample surfaces to generate zero contact angles, indicating all these shale are strongly oil-wet. This is consistent with the much larger imbibition slopes of n-decane in the plot of log dimensionless weight vs. log dimensionless time than DI water. Shale samples with higher TOC content, higher quartz content, lower clay content have stronger water-repellant property in marine Longmaxi and continental Dongyuemiao shales. In contrast, relationships between DI water contact angles and quartz content, clay content are not obvious in transitional shales. With thermal maturities increase from mature to high-mature and over-mature, the wetting affinity to DI water trends to increase rapidly, which could be related to the wettability alteration at high thermal maturity. Effective porosity has a positive effect on DI water contact angles for argillaceous limestone in continental Dongyuemaio, and for siliceous shale in marine Longmaxi and transitional shales, while a negative effect in the silty shales from continental Dongyuemiao and argillaceous siliceous shale from marine Longmaxi.