Clusters of Asphaltene Nanoaggregates Observed in Oilfield Reservoirs

Many studies have indicated that asphaltenes have two hierarchical nanocolloidal species, the nanoaggregate and the cluster of nanoaggregates. These two species, along with the dominant molecular architecture of asphaltenes comprise the Yen-Mullins model of asphaltenes. Delineating different nanocolloidal species is a challenge, and moreover, elucidating their corresponding practical importance is a necessity in this applied science. Moreover, it is necessary to continue testing this asphaltene nanoscience model especially for crude oils, as opposed to simply asphaltene solutions in laboratory solvents. Both the validity and applicability of this model are addressed by observing gravitational gradients of asphaltenes in black oil and mobile heavy oil columns in oilfield reservoirs. In this paper, we examine stacked oil reservoirs in an oilfield in Saudi Arabia using simple predictions from the Flory-Huggins-Zuo equation of state (FHZ EoS), with its foundation in the Yen-Mullins model. The extraordinary finding is that, for two mobile heavy oil columns in this oilfield, the asphaltene clusters alone account for very large asphaltene concentration gradients of a factor of 6 over a circumference of many tens of kilometers of the oil field. In certain local sections of the field, the predictions match exactly. Saturates, aromatics, resins, and asphaltenes (SARA) analyses of the crude oils are consistent with the simple model employed. The large gravitation gradients produced by asphaltene clusters dwarf corresponding predictions for nanoaggregates, confirming the existence and size of clusters per the Yen-Mullins model. Moreover, these results confirm the utility of reservoir analysis through application of the FHZ EoS. In addition, several wells were analyzed that penetrated the tar mat at the base of the oil column. Analysis of this tar mat is again consistent with the gravitation accumulation of asphaltenes and reinforces the FHZ EoS analysis of the oil column.