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.

The effect of dispersion on the stability of miscible displacement in rectilinear porous media is examined. Following a convection–dispersion (CDE) formalism, the base state of Tan and Homsy [Phys. Fluids 29, 3549 (1986)] at conditions of unfavorable mobility contrast is analyzed. Emphasis is placed on the dependence of the dispersion coefficient on flow rate (e.g., mechanical dispersion). It is found that such a dependence induces a destabilizing contribution at short wavelengths. This effect, which is in contrast to the stabilization commonly associated with dispersion, is highly pronounced near the onset of the displacement. It is also near this onset that, for a certain condition, the cutoff wavenumber becomes infinitely large. An analytical expression is derived for this condition and the origin and implications of the instability are discussed. It is also suggested that the present CDE formulation may be inadequate in providing stability criteria for a range of unstable flows.

Reservoir fluid geodynamics (RFG) has recently been launched as a formal technical arena that accounts for fluid redistributions and tar formation in reservoirs largely after trap filling. Elements of RFG, such as analysis of biodegradation, have long been in place; nevertheless, RFG is now strongly enabled by recent developments: 1) downhole fluid analysis (DFA) allows routine elucidation of reservoir fluid gradients, 2) the development of the first equation of state for asphaltene gradients allows identification of equilibrium vs. geodynamic processes of reservoir fluids and 3) RFG analyses of 35 oilfields systematize a multitude of RFG processes and show their direct impact on wide-ranging production concerns. Thermodynamic analyses identifying reservoir fluid geodynamic processes rely heavily on measurement of fluid gradients to avoid ambiguous interpretations. The unique role of asphaltene gradients and their integration with other data streams are the focus herein. RFG oilfield studies have repeatedly shown that analyses of asphaltene gradients are critical to proper evaluation of RFG processes. Naturally, any reservoir concern that directly involves asphaltenes such as heavy oil, viscosity gradients, asphaltene onset pressure, bitumen deposition, tar mat formation, and indirectly, GOR gradients are strongly dependent on asphaltene gradients. Moreover, as shown in numerous case studies herein, asphaltene gradients can be measured with accuracy and the corresponding thermodynamic analyses allow explicit identification of RFG processes not traditionally associated with asphaltenes, such as analysis of connectivity, fault block migration, baffling, spill-fill mechanisms and many others discussed below. In turn, these processes imply other corroborative reservoir and fluid properties that can then be confirmed. Crude oil chemical compositional data, such as ultrahigh resolution two-dimensional gas chromatography, combined with geochemical interpretation, is highly desirable for understanding RFG processes. Nevertheless, biomarkers and other fluid properties often exhibit small gradients relative to standard deviations (except with biodegradation) but often can still corroborate specific RFG processes. In general, integration of fluid gradient analysis with other data streams including petrophysics, core analysis, stratigraphy, geology and geophysics is critical; nevertheless, which integration is most needed depends on particular reservoir attributes and RFG processes that are in question. Examples of data integration are shown for ten reservoirs undergoing various fluid geodynamic processes. Asphaltene gradient analysis is relatively new, yet it is essential for characterization of RFG processes.

A Jurrasic oilfield in Saudi Arabia is characterized by black oil in the crest and with mobile heavy oil underneath and all underlain by a tar mat at the oil-water contact. The viscosities in the black oil section of the column are fairly similar and are quite manageable from a production standpoint. In contrast, the mobile heavy oil section of the column contains a large continuous increase in asphaltene content with increasing depth extending to the tar mat. The tar shows very high asphaltene content but not monotonically increasing with depth. Because viscosity depends exponentially on asphaltene content in these oils, the observed viscosity varies from several to ~ 1000 centipoise in the mobile heavy oil and increases to far greater viscosities in the tar mat. Both the excessive viscosity of the heavy oil and the existence of the tar mat represent major, distinct challenges in oil production. Conventional PVT modeling of this oil column grossly fails to account for these observations. Indeed, the very large height in this oil column represents a stringent challenge for any corresponding fluid model. A simple new formalism to characterize the asphaltene nanoscience in crude oils, the Yen-Mullins model, has enabled the industry's first predictive equation of state (EoS) for asphaltene gradients, the Flory-Huggins-Zuo (FHZ) EoS. For low GOR oils such as those in this field, the FHZ EoS reduces to the simple gravity term. Robust application of the FHZ EoS employing the Yen-Mullins model accounts for the major property variations in the oil column and by extension the tar mat as well. Moreover, as these crude oils are largely equilibrated throughout the field, reservoir connectivity is indicated in this field. This novel asphaltene science is dramatically improving understanding of important constraints on oil production in oil reservoirs.

We consider the parallel flow of two immiscible fluids in a Hele-Shaw cell. The evolution of disturbances on the fluid interfaces is studied both theoretically and experimentally in the large-capillary-number limit. It is shown that such interfaces support wave motion, the amplitude of which for long waves is governed by a set of KdV and Airy equations. The waves are dispersive provided that the fluids have unequal viscosities and that the space occupied by the inner fluid does not pertain to the Saffman-Taylor conditions (symmetric interfaces with half-width spacing). Experiments conducted in a long and narrow Hele-Shaw cell appear to validate the theory in both the symmetric and the non-symmetric cases.