Wettability alteration in petroleum systems: the role of polar non-hydrocarbons

Compositional changes amongst surface active components of petroleum are brought about by crude oil/brine/rock interactions. The behaviour of petroleum components following simulated petroleum migration through an initially water-saturated continuous siltstone (89.7 cm x 5 cm) core was investigated. A suite of produced oils and core extract petroleum from the core-flood experiment were analysed by Iatroscan and Gas Chromatography-Mass Spectrometry (GC-MS). Dramatic changes were observed in the composition and distributions of polar non-hydrocarbons, namely, fluoren-9-ones, carbazoles, benzocarbazoles and C-0-C-3 phenols indicating varying degrees of surface activity within the core-flood system. Following flooding, environmental scanning electron microscopy (ESEM) was used to view water condensation behaviour on freshly exposed core chips. Samples prepared from the original unflooded core and core material sectioned towards the outlet of the core flood displayed surfaces exhibiting hydrophilic tendencies. Meanwhile, hydrophobic behaviour was observed in a sample from the core inlet. The ESEM descriptions of the interaction of water on core surfaces appear to be consistent with the compositional changes encountered in the core extract petroleum. Wettability was altered most strongly at the core inlet where surface active compounds are rapidly removed from the migrating petroleum. In particular, small hydrophilic moieties, such as alkylphenols, appear to rapidly precondition mineral surfaces, changing wettability and subsequently allowing larger hydrophobic molecules to sorb. Hence, petroleum surface active compounds are capable of rapidly (days to months) facilitating wettability alteration in crude oil/brine/rock systems that are initially water wet.