SLOPE FACTORS AND SLOPE ANALYSIS REVEAL PETROLEUM HISTORY
Slope Analysis provides unique and valuable insight into the nature, history,
and phase behavior of a reservoir fluid, information which is as critical as
the exploration effort itself. It is a powerful data analysis tool, yielding additional
information from gas chromatography, applicable to both reservoir fluids (PVT data)
and C7+ stock tank oils.
Parameters employed here which utilize data in the C1 to C5
range cannot be applied to stock tank oil or separator/line gas analyses as such compounds in
these fluids are fractionated upon collection.
In their primary application, Slope Factors (SF) assess level of maturity by precise
measurement of the pseudo-component profile from PVT data, revealing degree of cracking.
Precision is sufficient to allow the filling history of a field to be mapped on the basis of
the thermal evolution of the charge. In the case of stock tank analyses, a similar
result can be attained for C6+ components by employing either normal-alkane or psuedo-component
data. These slopes differ, but are best related by the equation given below, revised from that
proposed earlier:SF(P10+) = 0.348 +0.676(SF(nC10+)).
Slope, defined as the rate of decrease in component concentration with increasing
carbon number, is expressed as a Slope Factor, SF, modified by indication
of the carbon number range within which it was measured, e.g., SF(C3-nC5),
or SF(P10+). Any ratio of the type [Pn]/[Pn+1] (expressing the concentration ratio of
two adjacent components) is, ideally, equal to the SF value of the series, though generally is
approximate due to "noise". In the studies reviewed here, concentrations are measured in mole percent.
Molar concentrations, whence mole percentages, equal weight concentrations divided by molecular weight.
As detailed in
Section 1 ,
Section 3 of "INVESTIGATIONS", rates of
decrease are measured as exponential series within the ranges C3-nC5, C2-P5 and P6+, although the
range P6+ may become irregular due to secondary alteration, commonly stabilizing beyond P10 or P15.
SF(C3-nC5) is steeper than SF(C2-P5). Both are markedly steeper than SF(P6+).
Slope Factors, "SF" values, are determined by the computerized fitting of an exponential
equation defining the regression of y on x. Values of y comprise the set of component concentrations,
the values of x the associated set of carbon numbers. Details are given in the following table:
SF = e(exp|a|)|
(read "e to the power of a")
where |a| (the absolute value, positive sign) is abstracted from the exponential equation
y = A.e(exp(-an))
fitted to the compositional data.
n is a carbon number,
A is the y axis intercept at n = 0;
"-a" is a logarithmic expression of slope, first devised
by Kissin (1987).
The antilogarithm of "a" (positive) is a linear variable, defined
above as a Slope Factor.
Slope Analysis assesses level of maturity, and four economically significant processes of secondary
alteration: gas injection (mixing), evaporative fractionation, biodegradation and migration
depletion (see ASSESSMENT OF ALTERATION below). All are recognizable at incipient levels employing
but otherwise may remain unnoticed. Detecting gas-liquid interactions is
key to the appraisal of regional GOR, i.e., proneness to oil or to gas.
- INPUT DATA
Molar concentration data, as reported in conventional engineering (PVT) analyses of
reservoir fluids, are the starting point. PVT analyses provide either an abbreviated
table of concentrations, C1 through C6's plus C7+, or extended data. Extended analyses
include determinations of methane, ethane, propane, normal- and isobutane, normal-
and isopentane, plus 24 pseudo-component concentrations: C6's through C29's, plus C30+.
Each pseudo-component sums the concentrations of all compounds of a given carbon number,
for example, P6, P7 etc. P30+ sums all of the heavier remainder. The principal light cyclic
compounds are conventionally excluded from the "hexanes", "heptanes" "octanes" and "nonanes" and are
reported individually. These compounds are cyclopentane, methylcyclopentane, cyclohexane,
benzene, methylcyclohexane, toluene, ethylbenzene + m- & p-xylene, o-xylene and
1,2,4 trimethylbenzene. Their assignment is described below.
Pseudo-components are defined to reduce to manageability the number of "compounds" evaluated
simultaneously. In the work presented here, definitions differ from petroleum engineering
practice. In the latter, the six-carbon pseudo-component, P6, for example, includes all
compounds eluting immediately after n-pentane, up to, and including, n-hexane. Other
pseudo-components are defined analogously. Here, however, treatment differs because of
the assignments of the light cyclic compounds. The polarity of these causes most to elute
outside the expectable carbon number window. For example, methylcyclohexane, C7H14,
elutes after nC7. In engineering practice it would be assigned to P8, but here the light
aromatics and naphthenes are assigned by carbon number to their appropriate
pseudo-component, in this instance P7, regardless of elution anomalies. Engineering protocol is
satisfactory at and beyond P10. Gas-chromatographic integration to quantify pseudo-components
involves peak area determinations measured from the empty column baseline, established separately, not
determinations skimmed from a potentially elevated baseline.
Compositional data are transformed into a curve, the molecular or molar profile,
which is evaluated for Slope Factors by fitting an exponential equations within the chosen regions of n.
Data are best presented on log-linear axes, plotting molar concentrations on a logarithmic ordinate
scale (y) versus carbon number (x). Ideal exponential concentration distributions
are thus seen as straight lines, providing visual appraisal of perfection or alteration
in a reservoir fluid composition.
- ORIGINAL COMPOSITIONS
Slope Analysis involves a theoretical model of petroleum composition and comparison with
observed compositions. A summary of ideal, or model, compositions is given in
Concepts are based on pyrolysis data representing model precursor compounds
such as n-hexadecane, also asphaltenes which generate realistic synthetic petroleums.
(Thompson, 2002, ( Bibliography )).
In unaltered oils, slope is constant from C6 to beyond C30. The four major alteration
processes principally modify only the configuration or slope of the front end. After
modification the front end slope is no longer compatible with that of the heavy ends.
Slope Analysis facilitates comparison of the actual light end profile with
that theoretically predictable from the heavy end distribution, facilitating a means of
recognition and quantification of gas-related alteration. These relationships are discussed
Section 4 .
Techniques were developed using databases representing over one thousand analyses of reservoir
fluids (PVT data) and stock tank oils.
- ASSESSMENT OF LEVEL OF MATURITY
Slope Analysis facilitates the assessment of level of maturity, as well as the diagnosis
and quantification of secondary alteration. These processes affect GOR, API gravity, saturation
pressure, viscosity, formation volume factor, and other economically significant petroleum properties.
Increasing maturity in oils is expressed through the thermal cracking of precursor
entities (long chains of normal-alkane type) and secondary reactions, leading to progressive
decrease in mean molecular weight. Heavy ends are progressively cracked to light ends. This results
in increasing API gravity and gas/oil ratio (GOR), decreasing viscosity, accompanied by the progressive
modification of all petroleum properties.
illustrates the slope aspects of a fluid of low maturity and
those of one of relatively high maturity.
- ASSESSMENT OF ALTERATION
1. GAS INJECTION
The most significant modes of alteration of oil involve the injection of dry gas or gas-
condensate into an oil reservoir. The gas is postulated to migrate from
deeper higher pressure sources. The effects are illustrated in
Figure 3 ,
Figure 4 and
Figure 5 . They
result in enhanced GOR and API gravity, and frequently in the generation of elevated
pressure in the oil reservoir.
2. EVAPORATIVE FRACTIONATION
Evaporative fractionation involves several essential processes. Firstly, advection of gas
into an oil reservoir, secondly, mixing, then depressurization of the gas-saturated oil due to
fault activation. Degasification follows, with resulting loss of gas/condensate, generally into faults
connecting to shallower reservoirs
(Thompson, 1987). The products are light end-depleted oils of increasing aromatic content
and non-thermal gas-condensates which contain the more volatile components from the oil.
illustrates the features of a strongly fractionated oil,
residual after generation of gas-condensate. The fractionating gas is methane, the only
light component which is not depleted in the process. C2 - P5 are removed in proportion to their
fugacities. Liquid components are less affected.
Bacterial activity in shallow reservoirs at temperatures below approximately 80C removes
hydrocarbons, particularly normal-alkanes. Removal commences in the nC10 - nC12 region,
where a “valley” develops in the profile. As bacterial activity proceeds the valley
widens towards both higher and lower carbon numbers, as shown in
4. MIGRATION DEPLETION
Migration depletion involves loss of dissolved light hydrocarbons from saturated oils, principally
methane, ethane and propane, extending to higher carbon numbers in greatly reduced amounts, proportional
to their fugacities under the conditions of decreasing temperature and pressure prevailing
along migration paths.
Migration depletion progressively lowers GOR and saturation
pressure (bubble point), maintaining the latter at the local pressure along the path. Effects
on a typical oil are illustrated in
Figure 8 .
In contrast to the behavior of liquids, migration depletion in gas-condensates has
comparatively little effect on the light end composition but is extremely effective in
controlling the composition of the dissolved liquid components. Pressure decrease causes
precipitation of heavier dissolved components in amounts inversely proportional to
their fugacities, to the extent that path or reservoir pressure is the dominant factor
controlling SF(P6+) and therefore the API gravity of the condensate liquid (Thompson,
2004, p21). Again, saturation pressure (dew point) is maintained at the local pressure
along the path. As opposed to the case with oils, SF(P6+) and the API gravity of gas-
condensate liquids is unrelated to maturity, except in the instance of thermal gases.
Ultimately, these also probably have an origin in evaporative fractionation, with pressure
control of liquid slope. At elevated pressures, greater than, say, 10,000 psi, SF(P6+) in
gas-condensate liquids is as low as that in many oils, with comparable API gravities. The
relationship between SF(P6+) in gas-condensates and their saturation pressure (dew point)
is shown in
Figure 9 .
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