A SARA-Based Model for Simulating the Pyrolysis Reactions That Occur in High-Temperature EOR Processes
Authors: Freitag N.P. , Exelby D.R. 
Published: March 2010

Although there is a need for forecasting the performance of enhanced oil recovery processes involving air injection, the capability to do so is still modest. One of the limitations to such forecasting is the lack of knowledge of the reaction chemistry, which leaves questions as to which or how many reactions are needed, and how to obtain values for the associated rate parameters.

A model is presented to describe one of the three major categories of reaction that must be considered when simulating air injection: the heat-induced cracking of oil components. The model is well suited for the numerical simulation of air-injection EOR processes with commercial simulators. It is based on the measured rates of pyrolysis/coking reactions of purified SARA fractions separated from two very different sources: a Lloydminster heavy oil, and a Cold Lake bitumen. Most of the results for the two oils were fairly similar, which suggested that the model might apply readily to a broad range of oils. This paper also outlines a modified SARA analytical procedure that proved to be more reliable for this type of study than conventional methods of SARA analysis.

Introduction

One of the essential steps in the development of any enhanced oil recovery (EOR) project is the forecasting of oil production. Such forecasts are normally performed by numerical simulation. For any process that involves heating of part of the oil reservoir to high temperature, as often occurs for example in EOR by air injection, the effects of pyrolytic reactions upon the oil must be considered. However, only a moderate number of publications provide the information that reservoir simulators need for pyrolysis to be included.

The first widely accepted simulation models(1, 2) of air-injection processes already recognized the need to use several separate fractions to represent the oil. The fractions were determined from distillation cuts. Coke, a solid hydrocarbon resulting from pyrolysis, was also included. This approach was refined(3), but soon alternative approaches appeared that divided the oil along the lines of solubility(4, 5) (separation of asphaltenes), or used lumped SARA (saturates, aromatics, resins, asphaltenes) fractions(6). Within a few years, these descriptions were followed with characterizations(7–13)that used each SARA fraction distinctly, in addition to coke and various gaseous components.

A few of these studies(7, 8, 12, 13) were performed on individual SARA fractions that had been isolated from crude oil. Studying the chemical reactions in this fashion greatly improves the accuracy of the experimental measurements. Although the fractions have a modest effect(8, 13) upon the reaction rates and products of the other fractions in the oil, thermal analytical evidence(8, 14) indicates that this effect is small. Therefore, the advantages of studying the reactions of the isolated fractions instead of mixtures appear normally to outweigh the disadvantages. In addition, even fewer(7, 13) of the studies carried out the tests isothermally and in reactors from which the products could be recovered and examined; the others employed merely temperature-ramped thermal analysis.