Global Component Lumping for EOS Calculations

Abstract

Abstract To reduce CPU time in compositional reservoir simulations, a minimum number of components should be used in the equation of state (EOS) to describe the fluid phase and volumetric behavior. A "detailed" EOS model often contains from 20 to 40 components, with the first 10 components representing pure compounds H2S, CO2, N2, C1, C2, C3, i-C4, n-C4, i-C5, and n-C5. The remaining components represent a split of the heavier C6+ material in single-carbon-number (SCN) fractions such as C6, C7, C8 and C9, or groups of SCN fractions such as C10-C12, C13-C19, C20-C29, and C30+. Occasionally the light aromatics BTX (benzene, toluene, and xylene isomers) are also kept as separate components for process modeling. Today's typical laboratory compositional analysis provides 50-60 components, including isomers with carbon numbers 6 to 10, SCN fractions out to C35 and a residual C36+. This is in contrast to the 11-12 components (through C7+) reported in most commercial laboratory reports pre-1980. A "pseudoized" EOS model might contain only 6-9 lumped components – e.g. lumping "similar" components such N2+C1, i-C4+n-C4+i-C5+n-C5, and some 3-5 C6+ fractions. The selection of which components to lump together is difficult because of the huge number of possible combinations. This paper describes a systematic, automated method1 to search a vast number of feasible pseudoized EOS models based on an initial, detailed EOS model. The obvious application of pseudoized EOS models is compositional reservoir simulation, where run time is an important issue and fewer components may be important. The method we present is based on (1) quantifying the "quality of match" between a pseudoized EOS model and the detailed EOS model from which it is derived, and (2) systematically testing all plausible lumping combinations. The method allows for a set of constraints to be imposed on the lumping of components, such as (1) not lumping certain non-hydrocarbons (e.g. CO2), (2) forcing the first plus fraction to contain a specific carbon-number component (e.g. C6), and (3) the last component in the original EOS not being lumped with other heavy fractions (e.g. C30+). The proposed pseudoization procedure is comprehensive, and founded in the ability of an EOS with fewer components to describe a wide range of phase and volumetric properties covering all of the relevant pressure-temperature-composition (p-T-z) space expected for a given reservoir development. The litmus test of quality is how well the pseudoized EOS compares with the detailed EOS model from which it is derived, an EOS that accurately describes all key measured laboratory PVT data. The method proposed will find an optimal pseudoized EOS model to describe all PVT data that are relevant to a particular reservoir development – e.g. depletion performance, immiscible and miscible gas injection, compositional variation, and surface processing.