Constant Asphaltene Molecular and Nanoaggregate Mass in a Gravitationally Segregated Reservoir

Abstract

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 (L 2 MS), 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.

Diffusion Model Coupled with the Flory–Huggins–Zuo Equation of State and Yen–Mullins Model Accounts for Large Viscosity and Asphaltene Variations in a Reservoir Undergoing Active Biodegradation

“…[25] The assumption of a single asphaltene chemistry in this highly graded column was confirmed by sulfur X-ray absorption near edge spectroscopy [26] and by establishing the same asphaltene molecular weight and nanoaggregate aggregation number independent of the location within the oil column. [27] With the equilibrium model of reservoir fluids determined with the FHZ EoS, it is now possibly to treat dynamic process from a thermodynamic perspective, where a transient is occurring in the reservoir. One transient process that is of great significance in oilfields is biodegradation.…”

A Geological Model for the Origin of Fluid Compositional Gradients in a Large Saudi Arabian Oilfield: An Investigation by Two-Dimensional Gas Chromatography (GC × GC) and Asphaltene Chemistry

“…7,26 Additionally, L 2 MS and SALDI-MS were used to determine that there is no observed variation in molecular weight or nanoaggregate weight for the asphaltenes in this reservoir. 33 Thus, the large variation of asphaltenes with height correlates with different concentrations of asphaltenes as given by the barometric equation for asphaltene clusters, not from a large chemical change in the asphaltenes. 6 …”

“…[26][27] Further evidence for 1.5 nm asphaltene structures has been observed in tandem mass spectrometry coupled with collisional activation decomposition, 28 in interfacial studies at an oil-water interface, [29][30][31] and in sum-frequency generation experiments. 32 2.0 nm nanoaggregates of about seven molecules have also been confirmed by SALDI-MS. 26,33 These weakly bound nanoaggregates are readily disrupted. In toluene, the critical nanoaggregate concentration (CNAC) has been determined by high-Q ultrasonics, 34 DC-conductivity, 35 NMR, 36 centrifugation, 37 and mass spectrometry.…”

Avaliação De Asfaltenos Precipitados Em Diferentes Condições De Composição, Temperatura E Pressão

“…Essa característica é condizente com espectros encontrados em outros trabalhos para amostras de diferentes origens e obtidos por outros métodos da espectrometria de massas. 42,[49][50][51][52][53][54] As massas molares médias dos petróleos obtidas por LDI-MS foram 622 Da e 643 Da para os petróleos P01 e P02, respectivamente. As massas molares médias das frações resultantes das extrações com n-pentano (C5I/ C5S) foram sempre inferiores àquelas com n-heptano (C7I/ C7S): 592/ 579 Da para as frações C5 do P01 e 745/ 621 Da para as do P02 contra 759/ 615 Da para as frações C7 do P01 e 814/ 651 Da para as do P02.…”