GAS - DRIVEN
Terms employed are defined in the Glossary.
Conceptual experiments performed with an equation of state reservoir fluid
simulation package, WinProp (CMG Group, Calagry), illustrate progressive stages in the
modification of an oil by evaporative fractionation. The oil is subjected to a series of
flash separations at elevated temperature and pressure after having been serially charged with
a "stripping gas". This results in a series of phase separations with the generation of a gas phase
and a residual liquid. The gas is a gas/condensate, rich in liquid components vaporized
from the oil. Consequently, as the oil is depleted, the secondary maximum recedes from
an original position at P6 to higher carbon numbers. Thus, progressively depleted oils are designated here as P7, P8, P9 etc., fluids.
illustrates four of six calculated stages in
the development of residual oils, R1, R2, R4 and R6.
illustrates the corresponding gas/condensates, G1 through G6.
The stripping gas in
Figures 19a and b
comprises decreasing quantities of C1 to P5 components,
so that these compounds tend to be depleted slowly as they are replenished by each gas charge.
P6 is depleted most rapidly, heavier components less, because of their progressively
diminishing vapor pressures (fugacities). In nature, the added stripping gas proves
to principally comprise methane. All heavier compounds are depleted.
On the basis of the addition of methane and the progressive, decreasing, loss of other heavier
compounds, two parameters were developed to index advancing levels of evaporative fractionation in
western Canadian oils. These parameters are the methane/ethane ratio, C1/C2, which increases, and P4/P8,
which decreases. Further, the reservoir fluids were classified as P6, P7 and P8 types.
Evaporative fractionation seldom advances beyond a P8 stage in western Canada.Figures 20a, 20b and 20c
, illustrate the values
of the two index variables in P6, P7 and P8 oils, respectively. To ensure that only the effects of
evaporative fractionation are represented, biodegraded oils were excluded from this subset of
146 analyses. The criteria for exclusion are discussed in
Section 6 .
EVAPORATIVE FRACTIONATION IN MOLAR PROFILES
Molar profiles in Figures 21 through 23 illustrate significant stages of evaporative fractionation
and the wide variety of combined conditions of enrichment and depletion. Early stages are shown
in Figures 21a and 21b, and progressively advancing stages in following figures, as the secondary
maximum migrates from P6 to P8. Illustrated cases are also identified in Figures 20a - 20c. In the
profiles, two rows of data points are shown in P4 to P9 regions. The lower row represents n-butane,
n-pentane, other normal-alkanes and their branched isomers, also the unresolved complex below peak
bases. The upper row represents pseudo-components, that is, the sum of the lower row and the
concentrations of isobutane, isopentane and the nine cyclic compounds identified in Section 3.1.
is postulated to illustrate the earliest evidence of gas-driven alteration detectable
in PVT data. E3 is elevated and the initial stage of movement of the secondary maximum is shown by a
slight deficiency at P6. The added gas in
induces "stripping", true evaporative fractionation.
The addend is inferred to comprise methane plus minor ethane, as C3, P4 and P5 are strongly depleted.
The secondary maximum remains at P6 by a narrow margin but there is also loss at P7.
illustrates shift of the secondary maximum to P7 in a case where the addend is inferred to
be a gas-condensate, disrupting the liquid-range exponential and conspicuously raising both E3 and E7.
Evaporative fractionation is evidenced by loss of P6, but it cannot be determined where fractionation
took place, whether in the reservoir represented by the PVT data, or in a distant reservoir where
fractionation developed a residual oil which eventually gave rise to a gasoline-range-depleted
gas-condensate (with diminished concentrations of P6, P7 or P8), the latter finally being emplaced
in the sampled reservoir.
is analogous to Figure 21b but represents a slight advance, the secondary maximum having moved to P7.
The appearance is similar to theoretical cases generated by equation of state models.
Figures 23a and 23b illustrate cases in which the secondary maximum has migrated to P8.
Taber North, Case 600, is of interest as, in addition to evaporative effects in a deep reservoir, it has
undergone fractionation due either to migration depletion or uplift and erosion. This is
evidenced by a low saturation pressure and GOR, depletion in methane (to 10 mole percent) and slope
reversal at C2-C3. It is noteworthy that neither degassing in a separator, nor migration depletion,
nor similar laboratory experiment, shifts the position of the secondary maximum unless the excess of
the latter is very small. If, for example, P6 exceeds P7 in the reservoir fluid, this condition will be
preserved in the separator liquid. Thus, in the case of Taber North, migration of the secondary maximum
to P8 took place by evaporative fractionation. Additional loss, particularly of methane, C2 and C3,
occurred during migration depletion with little effect on P4+, evidenced by the inflexion at P4.
represents a Triassic-sourced and reservoired oil from British Columbia in which both
biodegradation and fractionation are evident, as discussed in
Section 6 .
Characteristics and selected engineering data relating to the oils represented in Figures 21
(Section 6) are summarized in
Table 3 of Section 6.
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