ADVECTION

The term advection describes the addition of one petroleum fluid to another, the first flowing from a deeper reservoir, frequently over-pressured. Analysis of molecular profiles of reservoir fluids facilitates the recognition of additions of whole oils, gas- condensates, "intermediate" gases, dry gases and methane. The advection of methane, with or without gas-liquids in solution, is the primary, causative, action in evaporative fractionation, the principal mode of generation of gas-condensates (Thompson, 1987).

EVAPORATIVE FRACTIONATION

The term evaporative fractionation (EF) was introduced to describe a major process in the alteration of petroleum (Thompson, 1987). Three steps are visualized: (a) the injection of gas into an oil reservoir; (b) development of saturation at high pressure; (c) pressure release through faulting resulting in rapid degassing and gas migration. Released gases are encountered in other reservoirs as gas-condensates. Oils having undergone EF are deficient in gasoline range components (transferred to gas-condensates) and modified by increase in aromatics and naphthenes. Examination of molecular profiles of reservoir fluid oils has also characterized EF in broad compositional terms, as described in Section 5.

E3

The parameter E3 is defined in Thompson (2010) to provide a measure of gas-liquid addition or depletion. A value of E3 of unity is interpreted as indicating that the degree of cracking evidenced by SF(C3-nC5) is compatible with that evidenced by SF(P15-25). Values greater than unity indicate gas-liquid enrichment, values less than unity, gas-liquid depletion.

E3 compares two slope factors (Eqn. 1), the observed and hypothetical values of SF(C3-nC5). The hypothetical value is determined on the basis of a regression (Eqn. 9, below) presenting the relationship between SF(C3-nC5) and SF(P10+) [= SF(P15-P25] in five pyrolyses of increasing severity of treatment of a petroleum-derived, Type II, clastic- sourced asphaltene, employing the data of Table 1 of Thompson (2002).

E7

E7 provides an index of the degree of addition or depletion of gasoline range compounds in the advection of gas-condensate or loss due to evaporative fractionation.

E7 is calculated in the P6+ fraction and is proportional to the extent of departure of the concentration of the reference pseudo-component, [P7], from a hypothetical concentration predicted on the basis of the slope of the P15-P25 suite, SF(P15-P25), and on the concentration of P15.

In the evaluation of E7 two objectively-determined slopes are compared, that defined by the vector joining P7 and P15 in the molar profile and that joining P15 and P25, the latter yielding the slope factor SF(P15-P25). Ideally, in unaltered oils, these segments form a single, continuous, vector, that is, the graphical projection of the log-linear vector passing through the P15-P25 pseudo-components also passes through those in the P7 -P15 region. In this case the inter-pseudo-component concentration ratios in the two intervals would be statistically equal. P15-P25 data exhibit greater consistency than ranges such as P6-P29, even P10-P29, because of distortion introduced by light end alteration.

E7, defined in Eqn. 2, utilizes the eight carbon-number step P7 to P15, necessitating the use of the eighth root of the concentration ratio [P7]/[P15], to determine the mean slope.

It is assumed that the original distribution from P6 to beyond P30 was exponential. Pseudo-components in the P7-P15 range often fail to form an exponential series and exhibit a slope break because of advective addition of light gas-condensate.

SLOPE FACTORS

Lohrenz and Bray (SPE No. 792, 1964) discovered the exponential distribution by carbon number of P7+ pseudo-components in unaltered petroleums. Kissin (Geochimica et Cosmochimica Acta, 1987, p2445) extended the concept to describe an identical distribution exhibited by normal-alkanes, the principal components.

Exponential series exhibit a constant inter-member ratio. In any unaltered petroleum (reservoir fluid), commencing at P6, the concentration ratio P6/P7 equals P7/P8 which equals P8/P9, and so on, to beyond P30/P31. Values of such ratios increase with the maturity of the oil. Representative values range from 1.08 to 1.17. The ratio value is termed a Slope Factor .

Slope Factors, (SF), can be conveniently determined by curve fitting, employing a statistical-plotting package such as "Kaleidagraph" (Synergy.com). Tabulating molar percent, y, and the associated carbon number, x, for the reservoir fluid analysis of interest, selecting "Curve Fitting, Exponential", an equation of the following form is returned:

where A defines an intercept, e is the base of natural logarithms raised to the power of (-a.x). Solutions yield mole percent of the pseudo-component y, of carbon number x.

The relevant Slope Factor is extracted from Eqn. 1 by evaluating e to the power of a, dropping the negative sign. In this way Slope Factors are rendered as values greater than unity, rather than as decimal fractions, characteristic of an increasing series. The conceptual reason for doing this involves two factors. Firstly, low carbon number components are generated in increasing concentrations by the thermal cracking of those of higher carbon number, suggestive of an increasing series. Secondly, visualizing a reservoir fluid composition from P30 to lower carbon numbers, the following equations obtain:

Thus, given any reference concentration, here P30, any other can be calculated by a multiplication involving the appropriate power of the Slope Factor. Relationships of his type can be employed in calculating amounts of components removed by alteration.

SLOPE FACTOR RANGES

A second series in the same range comprises propane, n-butane and n-pentane represented by

Values here range from 1.40 to 4.06 (n=87).

The third, and most extensive, series comprises P6 through P29, limited by available analytical data. This range may represented by the following slope calculations which, of necessity in unaltered oils only, all yield identical SF values:

SF(C3-nC5) is invariably greater than SF(C2-P5), and the latter greater than SF(P15-P25).

SLOPE FACTOR RELATIONSHIPS IN UNALTERED RESERVOIR FLUIDS

SF(C3-nC5) and SF(C2-P5) are related as follows:

and

In data representing the pyrolysis of asphaltenes, which yields realistic synthetic petroleums, SF(C3-nC5) and SF(P10+) increase simultaneously and are highly correlated (r = 0.87). However, as discussed below, no correlation whatsoever is observed in the great majority of reservoir fluid oils from western Canada because of secondary alteration.

The relationship of SF(C3-nC5) and SF(P15-25) in the pyrolysates is almost identical with that found in a group of oils which are evidently little altered, those of the Rainbow-Virgo fields, NW Alberta (illustrated in Fig. 4, Thompson, 2004). The pyrolysis data yields the following relationship:

SLOPE FACTOR RELATIONSHIPS IN ALTERED RESERVOIR FLUIDS

In the PVT data examined here SF(C3-nC5) and SF(P15-25) are not correlated (r = 0.08). This is due to a variety of commonplace alteration processes, particularly gas advection and evaporative fractionation, defined above, as well as biodegradation (see Section 6).

These additions or depletions in the gas-liquid and gasoline ranges frequently leave the heavier liquid components unaltered, still characterized by SF(P10+) or SF(P15-P25).

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