Abstract
The various microscopic processes that take place during enhanced oil-recovery upon injecting low salinity brines are quite complex, particularly for carbonate reservoirs. In this study, we characterize the in-situ microscopic responses of the organic layers deposited on flat Iceland spar calcite surface to brines of different salinity using Atomic force Microscopy (AFM). Organic layers were deposited from crude oil at the end of a two-step aging procedure. AFM topography images reveal that the organic layers remain stable in high-salinity brines and desorb upon exposure to low-salinity brines. In addition, the organic layers swell in low-salinity brines, and the stiffness of the organic layers is found to directly proportional to the brine salinity. These observations are explained in terms of ‘salting-out’ effects, where the affinity of organic layers to solvent molecules increases upon reducing the brine salinity. The swelling and desorption of organic materials provide access for the brine to mineral surface causing dissolution and change in wetting properties of the surface. Our results show the significance of de-stabilizing the organic layer on rock surfaces in order to design any successful improved oil recovery (IOR) strategy.
Highlights
Understanding the response of crude oil deposits on mineral surfaces when exposed to various brines is one of the key aspects of improved oil recovery (IOR) strategy that can lead to unravelling the physics underlying the low-salinity effect (LSE)
The microscopic contact-angles determined from high resolution scanning electron microscopy (SEM) im ages in this study showed lower water contact angles for low salinity brine when compared to the high salinity brine
The Raman intensity for the poly-aromatic hydrocarbons (PAH) is about 3 times higher for eqCROaged samples relative to equilibrated formation water (eqFW)-aged samples (Fig. S5). This indicates that the surface deposition of organic materials from Equilibrated crude oil (eqCRO) is substantially higher than from eqFW treatment
Summary
Understanding the response of crude oil deposits on mineral surfaces when exposed to various brines is one of the key aspects of improved oil recovery (IOR) strategy that can lead to unravelling the physics underlying the low-salinity effect (LSE). Enhancing oil recovery by LSE involves a hierarchy of processes ranging from molecular affinity between oil and the rock [13,14] over contact angle variations [15] on the pore scale [16,17,18,19] up to reservoirscale two-phase flow phenomena [20] All together, they determine the overall recovery factor. Any effect of potential determining ions, sulfates, calcium, and magnesium on wettability alteration in carbonate reser voirs becomes strongly dependent on temperature [30] This taken together with the geothermal gradients that exist in the reservoirs makes it more complicated for carbonates than sandstone reservoirs
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