SECTION 5,
GAS - DRIVEN
ALTERATION

  • EVAPORATIVE FRACTIONATION

    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. Figure 19a illustrates four of six calculated stages in the development of residual oils, R1, R2, R4 and R6. Figure 19b 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.

    Figure 21a 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 Figure 21b 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.

    Figure 22a 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. Figure 22b 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. Figure 23a, 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. Figure 23b 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 through Figure 24 (Section 6) are summarized in Table 3 of Section 6.


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