Tuning
The calculation of petroleum phase behaviour using an EOS approach is best performed using the compositional resolution commonly used by reservoir engineers. The most basic type of fluid description used in PVT simulators consists of a detailed gas composition (molar contents of C1 through C5 alkanes and iso-alkanes) and a bulk characterisation of a minimum of two compounds belonging to the liquid range (C6 and C7+). Accordingly a compositional description derived from source rock analysis should adhere to the same type of compositional format.
The data used for PVT evaluations consists of a detailed gas composition and a bulk characterisation of a minimum of two compounds belonging to the liquid range (C6 and C7+). Non hydrocarbon gases such as N2, H2S and CO2 are commonly part of the compositional description of reservoir fluids, as these are, however, not determined routinely via pyrolysis we chose to disregard them in the compositional model developed here. The minimal compositional description for phase prediction in basin modelling, therefore, contains seven compounds in the gas range (C1, C2, C3, i-C4, n-C4, i-C5 and n-C5), a pseudo C6 (corresponding to all compounds eluting between n-C5 and n-C6 excluding n-C5 and including n-C6) and a plus fraction, C7+, which theoretically contains all other compounds present in the fluid eluting after n-C6.
Gas composition tuning
As outlined above gas compositions of pyrolysates are consistently methane-deficient. On the other hand, our results concerning the GOR of pyrolysates indicates that the total proportion of gas generated in at least the closed system pyrolysis approach used here (MSSV) is comparable to that of naturally generated petroleums. Hence, it becomes obvious that only the composition of the pyrolysate gas phase and not its amount requires correction for the prediction of hydrocarbon phase behaviour.
Methane is generally the most abundant compound within the gas phase of petroleums. It also is the single compound with the strongest effect on phase behaviour. Especially the saturation pressure of a fluid is strongly controlled by the methane proportion. Mechanistically, changing preferences in radical cleavage positions as a function of temperature appear to explain the observed differences in gas wetness (Horsfield, di Primio & van Duin, unpublished).
Characterisation of the liquid phase
The liquid compositional description has a major effect on the assessment of petroleum fluid physical properties. As discussed by di Primio et al. (1998) the liquid composition strongly controls the physical properties of the liquid phase, the relative miscibility of the individual compounds or compound groups of the fluid, the extent of a fluids phase envelope in the high temperature range , the location of the critical point.
PVT simulation details
In addition to the C7+ amount the molecular weight and density of this fraction must be defined. In PVTsim (the PVT simulation software package we use, from Calsep a.s., DK) the C7+ fraction definition is used to calculate a distribution of components representing the total liquid phase. As discussed in di Primio and Skeie (2004) this so called C7+ characterisation consists in representing the hydrocarbons with seven and more carbon atoms as a reasonable number (generally 12) of pseudo-components (with specific EOS parameters) whereby a logarithmic relationship between the molar concentration zN, of a given fraction and the corresponding carbon number, CN, for CN >7 is assumed. The characterisation leads to the automatic definition of a set of additional compounds of increasing molecular weight (usually extending up to the molecular weight range corresponding to alkane chain lengths of C80-C100). The variables from which the definition of these additional compounds is performed are the amount, molecular weight and density of the C7+ fraction. The extrapolated mixture may consist of more than 200 components and pseudo-components. Commercial PVT simulators can handle only a limited number of components such that a reduction is required, which is accomplished through a lumping procedure leading to a total of 12 lumped pseudo-components each containing approximately the same weight amount.
This type of characterisation of the plus fraction is, as yet, not available in the PVT modules of current basin modelling programs. The characterisation procedure in the PVT simulation software used leads to a balanced description of the fluid composition, where the gas range is defined based on analytical results and the liquid range based on a mathematical extrapolation of the C7+ properties. Hence, the characterised fluid contains a distribution of compounds from methane up to C80 or C100. If, instead of this characterisation procedure, a small number of individual compound classes is chosen to represent the liquid fraction of the fluid under study (as assumed in the conventional multi-component kinetic datasets), the phase behaviour will not be comparable to that of a characterised fluid.