Aging brine-dependent deposition of crude oil components onto mica substrates, and its consequences for wettability

Abstract

While wettability alteration is widely accepted as the dominant mechanism underlying enhanced oil recovery (EOR) in sandstone reservoirs under low salinity water flooding (LSWF) conditions, the effectiveness of LSWF still varies substantially between laboratory experiments and the field indicating our incomplete understanding and control of the process. We use simple mica substrates to investigate the impact of aging conditions on the macroscopic contact angle and the microscopic properties (morphology, composition, stiffness, surface charge) of the surface. Mica samples pre-aged in brines of variable salt content prior to aging in crude oil at elevated temperature displayed macroscopic wettabilities ranging from complete water wetting to intermediate contact angles around 90° in ambient brines of variable salinity. Compared to the macroscopic wettability, variations of the microscopic properties as probed by Atomic Force Microscopy and Confocal Raman microscopy were found to be rather subtle. Clear signatures of asphaltenic deposits were observed ubiquitously, usually consisting of a combination of a thin film and larger aggregates. The deposits typically displayed a salinity-dependent elasticity and (predominantly negative) surface charge. Selective removal of ions from the pre-aging brine lead to strong variations of both macroscopic wettability and microscopic properties, including the response to low ambient salinity: For the same crude oil, reducing the salinity of the ambient brine can either increase or decrease the macroscopic oil contact angle, depending on the composition of the formation brine used to pre-age the mineral surface. These results clearly highlight the complex balance of interactions during brine-oil-rock aging and the subtle role of the connate water composition in extracting and binding different types of crude oil components and thereby setting the initial and boundary conditions for the oil recovery process.