Improved Fluid Characterization For Miscible Gas Floods
Authors: Egwuenu Azubuike Michael , Johns Russell T. , Li Yinghui 
Published: December 2007

Equations-of-state (EOS) are typically tuned to black-oil PVT data such as constant volume depletion, constant composition expansion, differential liberation, and separator tests.Other PVT data more appropriate for gas injection could include multicontact and swelling tests.The standard method of tuning, however, does not typically incorporate important displacement parameters such as the minimum miscibility pressure or enrichment (MMP or MME) or the likely compositions that result in a reservoir from condensing-vaporizing (CV) displacements.

This paper demonstrates an improved reservoir fluid characterization procedure for miscible gas floods that can more accurately represent the interaction of flow and phase behavior.We demonstrate the approach for two displacements, an eleven-component CO2 flood and a twelve-component enriched gas flood.The method-of-characteristic (MOC) theory is used to determine the MME (or MMP) of both lumped and unlumped models.The results show that by tuning to the MME/MMP, fewer pseudocomponents are required to characterize the fluid than with conventional tuning methods.For the cases studied, fluids lumped to as few as four pseudocomponents can match well the composition profiles and oil recoveries of the unlumped models.

Introduction

Gas injection into oil reservoirs results in complex interactions of flow with phase behavior that are often not modeled accurately by black oil simulation.This is especially true for miscible or nearly miscible floods where significant mass transfer occurs between the hydrocarbon phases.Such floods are best modeled by compositional simulation.

A significant disadvantage of compositional simulation, however, is that it is more computationally intensive than black-oil simulation.The primary reason for the increased computational time is the result of solving repeated flash calculations with cubic equations-of-state (EOS).The use of fewer pseudocomponents could reduce the flash computation time, but fewer components results in poor fluid characterizations and less accuracy.

Reservoir oils are typically subjected to standard black-oil PVT experiments that give volumetric behavior for recovery predictions from conventional methods such as waterflooding.These experiments include constant volume depletion, differential liberation, constant composition expansion, and separator tests.Standard PVT experiments, however, do not provide sufficient phase behavior data in the range of compositions that result from mixing of gas with resident oil.

For gas floods, multicontact experiments, along with swelling tests and slim-tube experiments are sometimes performed.1Most gas floods, such as those with CO2 and enriched-gas injection, however, have features of both condensing and vaporizing (CV) drives.[2–4]Miscibility in these CV drives is developed in the transition zone between the condensing and vaporizing regions at an equilibrium tie line known as the crossover tie line.[3,5,6]Multicontact tests attempt to mimic the composition paths that result from either vaporizing or condensing drives, but not both.Thus, these tests do not provide sufficient PVT data in the compositional range of interest, especially in the transition zone near the critical region where miscibility is developed in CV drives.

Slim-tube tests can be used to tune an EOS by matching the experimental recoveries with 1-D compositional simulations.[7]Slim-tube tests, however, are expensive and time consuming to obtain, and their recoveries can be affected by dispersion and relative permeabilities.[8,9]Slim-tube tests are not always available and even if they are, it would be helpful to have a method that is not dependent on the level of dispersion or relative permeability parameters.Recent research has shown how to calculate the dispersion-free MMP/MME from an EOS by method of characteristics.[10–13]

Equations-of-state are used to predict the compositions and volumetric behavior that result when oil and gas mix in the reservoir.These EOS fluid characterizations must be tuned to match the PVT behavior of the original reservoir fluid.The process of tuning an EOS involves:selection of the pseudocomponents;determination of EOS properties for the pseudocomponents; andadjustment of pseudocomponent EOS properties by regression to the PVT data.