Migration of fines has been observed during water flooding in chalk reservoirs and has been suggested to play a key role in enhanced oil recovery (EOR). But, the exact role of fines is not well studied for carbonate reservoirs. This study shows that addition of water and crude oil on calcite fines leads to formation of soluble oil micelles in the water phase. Formation of these micelles and its implication in EOR has been experimentally analyzed.

To study this phenomenon different water insoluble salts were used as fines including, Li2CO3, MgCO3, CaCO3, CaSO4, SrSO4 and BaSO4 to which oil and water was added. To study conditions of oil micelles formation, design oil was used consisting of hexane and hexadecane. Heptanoic acid and stearic acid were doped in various compositions to mimic the acid number of the oil. Experiments were conducted for pure crude oil and doped oil to understand its implications in EOR. Composition of initial and final floating oil was obtained through gas chromatographic (GC) analysis. The two were thereafter compared to obtain the composition of micelles formed.

The experiments showed that oil micelles are only formed when polar hydrocarbons are present in the oil. Different mixtures of alkanes did not produce any micelle. In oil containing stearic acid it was observed that around 95% of the initial stearic acid was accumulated in the micelles and only 5% was found in the floating oil. In oil samples containing heptanoic acid only 50-60% of its initial amount was accumulated in the micelles. This indicates heavier acids can form more stable micelles. Oil micelles were produced in all the different fines used, but the composition of these micelles where dependent on the salt anions. In all carbonates, lighter acids preferred micelles formation with lighter alkane, (indicated by intensification of heavier alkane in the floating oil compared to the initial oil) and vice versa for heavier acids. While no such selectivity in micelles formation was observed in any of the sulfates. Results obtained with crude oil and doped oil and designed oil were consistent. These results show that fines (of insoluble carbonate) released during core fracturing, or (sparingly soluble sulfates) formed during smart water flooding can form mixed wet water soluble oil micelles and help in mobilization of trapped oil along with increasing sweep efficiency.

The results clearly show oil micelles formation in the water phase from the interactions of oil with fines, and also provide a detailed understanding of its composition under different conditions. The study highlights the significance of fines during smart water flooding in carbonate reservoirs and shows how its role in EOR can be mistakenly underestimated.

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