Asphaltenes at Oil/Gas Interfaces: Foamability Even with No Significant Surface Activity

Abstract

In the oil industry, oil foams can be found at different steps from the crude oil treatment to the gas stations. Their lifetime can sometimes reach several hours and be much longer than the residence times available for gas/liquid separation. However, the conditions of formation and stability of such foams have been poorly studied in the literature, in contrast to the foamability of aqueous systems. On the fields, it is currently observed that crude oils enriched with asphaltenes form particularly stable foams. In this work, we have studied the influence of asphaltenes on the foamability of oil mixtures. All the experiments were performed on model systems of crude oils, that-is-to-say decane/toluene mixtures containing asphaltenes at concentrations ranging from 0.01 to 5 wt%. We in particular demonstrate that, within the investigated concentration range, asphaltenes from two different wells do not have any significant surface active properties despite their contribution to the foamability of oil mixtures. We show that the formation of an asphaltene layer at the interface with air that has been evidenced in the past results from solvent evaporation. Using a recently developed experiment based on the Marangoni effect with our model oils, we demonstrate that asphaltenes are not surface active in those oils. We further characterize the oil foamability by measuring the lifetime of the foam formed by blowing nitrogen through the liquid in a column. At concentrations larger than 1 wt%, asphaltenes significantly enhance the foamability of the oil mixtures. Moreover, the closer the asphaltenes are to their limit of precipitation the larger the foamability. However, we evidence that the oil mixtures themselves foam and we show the importance to consider that effect on the foamability. In addition, we observe that the foamability of the asphaltenes solutions unexpectedly varies with the initial height of the liquid in the column. We suggest that, although not significantly modifying the surface tension, the asphaltenes could be trapped at the oil/gas interface and thus prevent bubble coalescence.

Lifetime of a single bubble on the surface of a water and ethanol bath

“…Indeed, by modifying the structure of the interface, the presence of surfactant molecules can increase the lifetime of air bubbles by several orders of magnitude [5,9,13,14]. The lifetime of bubbles generated from a liquid consisting of a binary solution can also be very long as observed for saline solutions [15][16][17], water-alcohol mixtures [7,18,19], oil [20,21], and molten glass [10,22]. For these binary systems, the bubble stabilization mechanism at the liquid/air interface is less well known.…”

“…To test the validity of this deterministic/stochastic coupling, it is necessary to work on large assemblies of bubbles and measure their lifetime distribution. This has been done recently for oils [20,21], hot or cold clean water-for which thermal rather than solutal Marangoni flows may appear-and salt solutions [7]. However, the case of alcoholic binary solutions has been less studied [7,18,27].…”