Abstract

Oil characterization plays an important role in integrated geophysical, geological and reservoir engineering studies. From project inception, correctfluid characterization can provide useful information -about reservoirdescription and compartmentalization as well as properly defining fluidbehavior for engineering calculations. In this paper four field examples arepresented to illustrate (1) how to use equation of state (EOS) oilcharacterization to check the accuracy of fluid sampling procedures andlaboratory fluid property measurements, and (2) how to use EOS oilcharacterization to check for consistency between the geological/geophysicalreservoir description and actual field performance (e.g. produced gas-oilratio). This results in a better reservoir model and makes further engineeringand simulation tasks much easier.

Introduction

Integrated reservoir studies are usually initiated after the reservoir inquestion has been on production for many years. This provides sufficient dataon field performance to ensure that some matching of calculations to actualhistory can be done. To conduct such studies engineers generally have to useavailable data including existing PVT data. In most cases the existing PVTdata, which may have been collected right after completion of the initialdevelopment wells before field production or after many years of production, can show considerable inconsistency. In addition, some key data may not havebeen measured or some of the data may not reflect actual reservoir performance, such as gas-oil ratio (GOR). It is therefore important, from the outset of aproject, to check the consistency of available PVT data with the productiontest data and with the geological description before using it in a reservoirsimulation study.

EOS models are used by engineers to identify such inconsistencies in the PVTdata and to calculate data which may not have been measured or is obviouslyincorrect. Before an EOS can be used, one must first correctly tune itsparametersl, that is, characterize the oil. Hence oilcharacterization plays an important role in the integrated reservoir study. Correct fluid characterization enables accurate EOS fluid property predictionswhich can provide useful information about reservoir description and compartmentalization, as well as properly defining fluid behavior forengineering calculations. In this paper we use four actual field examples toillustrate the procedures used.

EOS OIL CHARACTERIZATION

Reservoir fluids are comprised of a very complex mixture of naturally occurringcompounds. These compounds are principally hydrocarbons ranging from methane toasphalt. There are no fixed rules on the distribution of hydrocarbons inreservoirs. Fluids in reservoirs can exist as either single phase (gas orliquid), two phase (gas and liquid) or multiphase (gas, liquid, liquid) fluidsdepending on the reservoir conditions and the components present. The abilityto predict production rates, optimize production strategies' and designproduction facilities depends on a knowledge of the properties of the reservoirfluid, not just at the original reservoir conditions, but also at a great manyconditions on the subsequent depletion and production path.

It is usually impractical, due to cost, time or lack of sufficient sample toconduct laboratory analyses for such a wide range of conditions.

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