Role of Asphaltenes in Compositional Grading of a Reservoir's Fluid Column
Authors: Hirschberg Avraham 
Published: December 2007

Hirschberg, Avraham, Koninklijke/Shell E and P Laboratorium

Summary.
The relationship between compositional variations induced by gravity and reservoir-fluid phase behavior has been investigated. In light oils (stock-tank oil gravity 0.85 g/cm3 35API), strong compositional grading will occur if the reservoir fluid is near critical. In heavier oils, compositional grading is caused by the segregation of asphaltenes, resulting in variation in oil viscosity and the possibility of tar-mat formation.

Introduction
Compositional variations in reservoir fluid with depth have been observed in many reservoirs (within a single oil pool). In most cases, the oil density increases with depth. Such a compositional grading can have a significant influence on various aspects of reservoir development. For example, in the volatile oil field (Brent field in the North Sea) considered by Schulte, the oil FVF varies greatly with depth. This must be taken into account during estimation of the stock tank initially in place of such fields. Also, when considering gas injection, one must be aware that compositional effects (such as the development of miscibility) change with depth. In reservoirs containing heavier oils, compositional variations may also influence field development. An example is a north African field in which strong grading in stock-tank oil gravity and a related variation in reservoir oil viscosity have been observed (see Fig. 1). In this case, the presence of highly viscous oil near the oil/water contact has forced production from updip and would be a serious handicap for downdip water injection. An extreme manifestation of compositional variation is the presence of a tar mat at the bottom of the reservoir, which can presence of a tar mat at the bottom of the reservoir, which can significantly influence waterdrive performance. In view of the impact of compositional grading on field development. it is desirable to assess the importance of these effects as early as possible. In shallow reservoirs and for heavy oils (stock-tank oil gravity possible. In shallow reservoirs and for heavy oils (stock-tank oil gravity &gt;0.93 g/cm3 [less than 20 API), compositional variations with depth often result from a loss of light ends or from biodegradation. Such effects can be inferred from geologic data. For light oils (stock-tank oil gravity less than 0.85 g/cm3 [&gt;35 API]), there appears to be a relationship between oil phase behavior and gravity-induced compositional variations. In the next section, we derive the thermodynamic conditions for strong segregation. This allows a first estimation of compositional grading as soon as PVT data of reservoir fluid are available. In moderately heavy oils (stock-tank oil gravity of 0.93 to 0.88 g/cm3 [20 to 30 API]), segregation of asphaltene appears to be the key factor. The main aspects of asphaltene segregation-i.e., variation in oil viscosity and the formation of tar mats-are considered later.

Equilibrium and Conditions for Strong Segregation
In a reservoir at thermodynamic equilibrium, a segregation profile will be established under the influence of gravitational forces. The time necessary to achieve compositional equilibrium (10 million to 1 billion years) is comparable to the geologic lifetime of a typical reservoir (Appendix A). Complete thermodynamic equilibrium will never be achieved because a uniform temperature, which does not occur in reality, would be required. Theory indicates that the natural temperature gradient (increase of temperature with depth) will enhance compositional grading, but an equilibrium theory yields the correct order of magnitude of the observed compositional variations in light oils.' We therefore restrict ourselves further to the equilibrium theory of gravity segregation.
The gradient drk/dh of the molar fraction Xk of Component k is related to the chemical potential, uk, by
..........................................(1)
where the summation is carried out over all the components present in the reservoir fluid. present in the reservoir fluid. In addition to the condition for compositional equilibrium, we have the conditions for mechanical and thermal equilibrium, respectively:
..........................................(2)
..........................................(3)
Without further assumptions, we can see from Eq.1 that strong segregation of Component i can be expected in two cases: when (Pvi-mi) is large-e.g., for large molecules with a density significantly different from the average reservoir fluid density p-and when the coefficient matrix ( )p, T is singular-e.g., p-and when the coefficient matrix ( )p, T is singular-e.g., when the reservoir pressure and temperature are close to a critical point of the reservoir fluid. point of the reservoir fluid. The first statement can easily be quantified by considering the influence of the molecular weight in an extreme case-i.e., at the limit of the ideal solution behavior, I for which we obtain the well known equation
..........................................(4)
relating the molar fraction xi (h) at elevation h with the molar fraction xi (0) at the reference level. (We use the gas/oil contact as reference level, h=0.) The gradient height, h., is given by
..........................................(5)
Table 1 shows the dependence of hg, on the molecular weight and chemical type of Component i for a typical oil (p=800 kg/m3 [50 lbm/ft 3 ]). From Table 1, we see that in the limit of the ideal solution behavior alkines with a molecular weight on the order of 100 to 1,000 (wax) will not show significant gravity segregation over a reservoir extending over a vertical interval of 100 m [328 ft]. The heaviest polar components present in an oil, asphaltenes, which have a density of about 1200 kg/m3 [75 lbm/ft3 and molecular weights between 1,000 and 10,000, however, will show significant sedimentation over a vertical distance of a few hundred meters. This indicates that heavy polar components, particularly asphaltenes, play a key role in polar components, particularly asphaltenes, play a key role in gravity-induced compositional grading.
JPT
p. 89