Integrated Oil PVT Characterization - Lessons From Four Case Histories
Authors: Wu R. , Rosenegger L. 
Published: March 2010

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

Introduction

Integrated reservoir studies are usually initiated after the reservoir in question has been on production for many years. This provides sufficient data on field performance to ensure that some matching of calculations to actual history can be done. To conduct such studies, engineers generally have to use available data including existing PVT data. In most cases the existing PVT data, which may have been collected right after completion of the initial development wells before field production or after many years of production, can show considerable inconsistency. In addition, some key data may not have been 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 a project to check the consistency of available PVT data with the production test data and with the geological description before using it in a reservoir simulation study.

EOS models are used by engineers to identify such inconsistencies in the PVT data and to calculate data which may not have been measured or is obviously incorrect. Before an EOS can be used, one must first correctly tune its parameters(1), that is, characterize the oil. Hence oil characterization plays an important role in the integrated reservoir study. Correct fluid characterization enables accurate EOS fluid property predictions which can provide useful information about reservoir description and compartmentalization, as well as properly defining fluid behaviour for engineering calculations. In this paper we use four actual field examples to illustrate the procedures used.

EOS Oil Characterization

Reservoir fluids are comprised of a very complex mixture of naturally occurring compounds. These compounds are principally hydrocarbons ranging from methane to asphalt. There are no fixed rules on the distribution of hydrocarbons in reservoirs. Fluids in reservoirs can exist as either single phase (gas or liquid), two phase (gas and liquid) or multi-phase (gas, liquid, liquid) fluids depending on the reservoir conditions and the components present. The ability to predict production rates, optimize production strategies and design production facilities depends on a knowledge of the properties of the reservoir fluid, not just at the original reservoir conditions, but also at a great many conditions on the subsequent depletion and production path.

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