Constant Asphaltene Molecular and Nanoaggregate Mass in a Gravitationally Segregated Reservoir

Spatial gradients in the chemical composition of crude oil are now routinely observed; oils near the top of a reservoir can be enriched in lighter ends, whereas oils near the bottom of a reservoir are typically enriched in asphaltenes. Equations of state capable of modeling these gradients have numerous practical applications, such as predicting variations in fluid properties, the presence of tar mats, and flow connectivity. Because of the great chemical complexity of crude oil, successful modeling of these gradients requires both an understanding of the physical drivers of the gradients and a set of reasonable simplifying assumptions for describing the composition of the crude oil. Gravity is often a dominant force driving fluid gradients, resulting in separation of low-density gas above medium-density oil above high-density water when those isolated phases are present and frequently driving gradients within a phase as well. The impact of gravity in driving gradients depends in part upon the mass and volume of molecules or aggregates in the crude oil. Here, we explore the impact of gravity in segregating asphaltenes of different masses. Asphaltenes were extracted from crude oils from a connected reservoir with a large gradient in asphaltene content and studied with two mass spectrometric techniques: laser desorption laser ionization mass spectrometry (L2MS), which measures the mass of asphaltene molecules, and surface-assisted laser desorption/ionization (SALDI) mass spectrometry, which measures the mass of asphaltene nanoaggregates. No significant gradients in the molecule or nanoaggregate mass were observed, suggesting that gravity causes almost no segregation of different molecular or nanoaggregate masses within the asphaltene class. That is, the large concentration gradient of asphaltenes in the reservoir is not accompanied by a large molecular weight gradient within the asphaltene class. These results indicate that asphaltene gradients can be modeled with the simplifying assumption that gravity drives gradients in the concentration of asphaltenes but not the chemical composition of asphaltenes in crude oil.