Integrated Flow Modeling of Miscible Gas Injection Processes
Authors: Fanchi John R. 
Published: December 2007

Abstract
Sequential application of three-dimensional (3-D) seismic surveys has led to the concept of time-lapse (4-D) seismic monitoring. Changes in reservoir geophysical attributes during the life of a reservoir can be predicted by calculating the attributes as a function of time using a petrophysical algorithm in a flow simulator. The integrated flow model is described, and then applied to the following scenarios: solvent injection into a volatile oil reservoir; and carbon dioxide flooding of an undersaturated oil reservoir. Additional data needed by an integrated flow simulator for miscible gas injection processes are discussed.

Introduction
Time-lapse (4-D) seismic monitoring is the comparison of two seismic surveys conducted at two different points in time in the same area of interest. Differences in seismic response between the two surveys provide information about changes in reservoir properties that effect the transmission of seismic disturbances. These differences are especially useful, when they are significant, because it provides direct information about properties between wells. Time-lapse seismic monitoring, also called 4-D seismic, is becoming a cost-effective tool for improving reservoir characterization, locating bypassed reserves, and identifying the movement of fluid interfaces1. Current research is seeking methodologies for maximizing the amount of information that can be obtained from 4-D seismic. This paper describes a methodology that is based on the calculation of reservoir geophysical attributes using an integrated flow model.
The integrated flow model is the combination of a widely used petrophysical model with a 3-D, three-phase, multicomponent flow simulator to calculate the following reservoir geophysical attributes: acoustic impedance, reflection coefficient, compressional velocity; and shear velocity. These attributes are calculated at different times throughout the life of the reservoir.
The calculation of reservoir geophysical attributes by an integrated flow model has several useful purposes. In addition to determining the feasibility of applying 4-D seismic monitoring to a particular reservoir environment, the integrated flow model is an efficient tool for generating information that will help the reservoir management team optimize a 4-D seismic monitoring program and prepare more accurate production forecasts. The integrated flow model provides information which can be used to predict which reservoir geophysical attribute, or combination of attributes, will yield the optimum response at different points in the life of the reservoir.
The usefulness of information from an integrated flow model to reservoir management is discussed for the following scenarios: solvent injection into a volatile oil reservoir; and carbon dioxide flooding of an undersaturated oil reservoir. One of the difficulties associated with the incorporation of a petrophysical model into a flow simulator is the need for information that is not normally included. The additional data needed by an integrated flow simulator for miscible gas injection processes are discussed.

Integrated Flow Model
The integrated flow model consists of a petrophysical model and a traditional flow model. The flow simulator used in this study is a multicomponent, pseudomiscible simulator called IFLO. It includes a petrophysical model and is a Fortran 90 program that contains the best features of the extended black oil simulator MASTER2 and the black oil simulator with a petrophysical model BOAST4D3.

Fluid Properties Model.
IFLO uses the extended fluid properties model coded in the simulator MASTER. It is an adaptation of Chase and Todd's4 mixing parameter method. Data consistency requires that the extended fluid property model simplifies to the conventional black oil fluid property model when no solvent is present.

Fluid Properties Model.
IFLO uses the extended fluid properties model coded in the simulator MASTER. It is an adaptation of Chase and Todd's4 mixing parameter method. Data consistency requires that the extended fluid property model simplifies to the conventional black oil fluid property model when no solvent is present.