Proper Use of Equations of State for Compositional Reservoir Simulation
Authors: Peng Wang , Pope Gary A. 
Published: June 2014

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized to be experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering.

Summary
Equation-of-state (EOS) compositional reservoir simulation is an accurate and powerful means to model complicated phase and flow behavior involved in the displacement of oil and gas in porous media. The use of EOS simulation becomes even more effective when the process involves solvent injection for miscible or near-miscible displacement of crude oil in reservoirs. The current trend of combining the simulation of reservoirs and surface facilities increases the demand for the use of EOS compositional models.
One key to proper use of compositional simulators is development of an EOS for describing the phase behavior of the fluids. This paper is a summary of experiences and developments in the use of EOS's for compositional reservoir simulation studies. The reader is referred to the recent monograph by Whitson and Brulé1 for a more complete treatment of the volumetric and phase behavior of petroleum fluids and the use of EOS's to model these fluids.

Introduction
When the displacement process depends on pressure and fluid composition, an EOS should be used to simulate the equilibrium mass transfer between phases and the pressure/volume/temperature (PVT) behavior of the fluids. PVT laboratory measurements are made on only a small portion of the composition path encountered throughout the displacement, whereas an EOS can be used to predict behavior for the entire composition path and pressure range of the process. Many field development projects have strong composition dependence, such as production from gas/condensate reservoirs, miscible and near-miscible gas injection, or water-alternating-gas injection for enhanced oil recovery. Until very recently, industry practice was to use a black-oil model and/or its modified versions2,3 or a limited-composition reservoir simulator4,5 to approximate the complicated phase behavior. These simplified approaches require less computer time and memory and can provide a quick preliminary evaluation of reservoir performance. However, efforts by industry and universities have led to rapid advancements in parallel-computer hardware and software during the last several years, which have increased simulation efficiency considerably6–9 and now make compositional simulation practical. Also, most of the commercially available EOS compositional simulators and in-house models have been improved significantly in terms of robustness, efficiency, and features. More frequent use of EOS compositional simulators should be expected in the future.
However, uncertainties still exist that are associated with the use of an EOS to model petroleum reservoir fluids because of the complicated nature of such fluids and limitations of the most commonly used EOS's. Tuning the adjustable EOS parameters to match experimental fluid PVT data is a common practice. The proper use of EOS models for reservoir applications has been an active research topic since the 1970's. Although significant progress in this area has been made, engineers still face several challenging questions: How many fluid components should be used? How much experimental data and what kind of data are needed to tune an EOS? How good is the predictive power of an EOS that matches all of the experimental data available? What are limitations of the commonly used EOS's for hydrocarbons when the gas or oil contains significant concentrations of components other than hydrocarbons? Which EOS should be used when more than one option is available in a reservoir simulator?

Representative-Sample Collection
The accuracy of any EOS model depends on the quality of the laboratory PVT data and procedures used to obtain the EOS parameters. The value of the PVT data depends on the quality of the fluid samples. They must be representative of the reservoir fluid. Three common methods are used to sample the reservoir fluid.

Downhole Sampling.
A sample collector with a capacity of several hundred cubic centimeters is used to obtain a fluid sample from within the wellbore. The downhole flowing pressure at the sampling point must be greater than the saturation pressure of the fluid so that the sample is a single phase. At least three fluid samples should be collected to determine the appropriate fluid sample for the PVT measurements.

Wellhead Sampling.
A fluid sample can be collected directly from the wellhead. The fluid needs to be in the single-phase region at wellhead flowing conditions. Normally, the sample needs to be taken at a pressure of at least 100 psi greater than the bubblepoint to be valid. Again, taking multiple samples is recommended.