Comparing Black-Oil PVT Tables with Laboratory PVT Data

Abstract This paper provides the methods and equations to make a direct comparison between PVT properties derived from a black-oil PVT (BOPVT) table with laboratory PVT data from conventional depletion PVT experiments – Constant Composition Expansion (CCE), Constant Volume Depletion (CVD), and Differential Liberation Expansion (DLE). Example data would be oil and gas densities, relative total and relative oil volumes, and produced gas volumes in the CVD and DLE lab tests. The methods presented are based on a material balance of the two surface products ("components") surface gas and surface oil used in the black-oil PVT formulation. As well known, the black-oil PVT formulation uses two invariant "pseudo-components" – surface oil and surface gas – to describe phase and volumetric behavior of oil and gas phases. It assumes constant surface "component" densities, independent of the total GOR, and whether existing phases are saturated or undersaturated. Saturated phase behavior is described by the pressure-dependent functions defining (a) solution-GOR Rs for saturated oil phase at a bubblepoint pressure, and (b) solution-OGR rs for saturated gas phase at an (upper) dewpoint pressure. Phase volumetric behavior is described by FVFs Bo and Bgd at both saturated and undersaturated conditions. Phase gas and oil densities are computed from invariant surface densities and the pressure-dependent BOPVT properties Rs, Bo, rs, and Bgd. BOPVT phase viscosities are tabulated, and they represent the depletion process used when generating the BOPVT table (CCE, CVD, or DLE test). The proposed material balance is straight-forward and relatively simple to use, with a complete set of equations to convert BOPVT table properties to PVT lab data equivalents for comparison with PVT reported quantities. The conversion depends on which depletion experiment is used by the laboratory (CCE, CVD, or DLE). The same methodology can be used to compare BOPVT properties versus EOS-based simulated experimental laboratory tests, where the same EOS model is used to create the BOPVT table and to simulate lab PVT tests. Significant differences may suggest inconsistencies or inaccuracy of the BOPVT method, where important discrepancies require reconciliation. It has been well established that a single EOS model can describe a wide range of fluids in complex reservoirs (and entire basins)1. Such models use 10s or 100s of samples with lab PVT data to tune EOS model parameters (C7+ characterization and binary interaction parameters). If each sample in such a study is used to generate a BOPVT table, one might see that some samples group nicely into a similar pressure trend for all (and particularly) BOPVT properties (Rs, Bo, μo, rs, Bgd, μg), where those samples can be described by a single BOPVT table. Other samples in the same field / basin may show a different trend for BOPVT properties (Rs, Bo, μo, rs, Bgd, μg), thus requiring a separate BOPVT table. A reservoir simulation same model might use a single PVT region defined by a common EOS model but where spatial variation in composition is used to map the spatial variations in PVT properties. A black-oil PVT formulation for the same reservoir simulation model might require multiple PVT regions, each one with a separate BOPVT table. How does one best establish the need for multiple BOPVT tables? The proposed method can be used to correlate samples into similar "PVT groups" or "PVT regions" where a single BOPVT table gives a good representation of all lab PVT data from wells belonging to a particular "PVT group". Lab PVT data might also suggest multiple PVT regions, and which samples belong to each region; this helps choose a sample to generate an appropriate BOPVT table for that region. Our standard approach is to create a BOPVT table for each and every sample used in developing a single EOS model, then identifying which samples can be grouped into a PVT region that are adequately described by a single BOPVT table. And finally, the proposed method can help identify isolated "outlier" lab PVT data.