Abstract

An important step in the numerical simulation of in-situ combustion for enhanced oil recovery is the selection of the pseudocomponents used to represent the oil. Frequently the oil is separated according 10 distillation cuts. In addition, a hydrocarbon pseudocomponent called "coke" is used to present the heavy residue that results from the reactions immediately ahead of a combustion zone. While this approach provides for some of the general changes that occur during in-situ combustion. it provides a poor basis for representing the overall set of chemical reactions.

Chemical principles and the results of several publications from diverse sources suggest that a modified SARA analytical approach presents a sound basis for representing in-situ combustion chemistry. The potential for such all approach is discussed and a set of pseudocomponents is proposed that could plausibly represent all the major physical and chemical changes that oil experiences in the in-situ combustion process. Finally. the steps that are foreseen to successfully implement such all approach are outlined.

Introduction

In-situ combustion may be a good option for enhanced oil recovery of many reservoirs. Unfortunately, its use is diminished by the lack of reliable methods for predicting performance. Usefully accurate prediction of performance almost certainly requires numerical simulation. The complicated interactions that control firefloods vary with oil and reservoir types and cannot normally be represented in simpler forms. Many of these interactions depend on the chemical reactions that dominate in-situ combustion. Consequently, the selection of a suitable set of pseudocomponents and reactions is vital to the establishment of an accurate numerical model for firefloods.

In the search for a suitable basis to express the chemical reactions of fireflooding, a thought-provoking publication by Ciajolo and Barbella1was found. Their thermal gravimetric analysis data suggested that SARA (saturates, aromatics, resins and asphaltenes) analysis offered this sought-after basis for pseudocomponent selection. Various data in another publication by Bad2 provided a separate indication that the oxidation and cracking behaviour of a wide variety of oils corresponded to their SARA analysis.

In this paper, the SARA-based approach is used to build a plausible and comprehensive reaction scheme that describes the chemistry of in-situ combustion;

PSEUDOCOMPONENT SELECTION
Distillation Cuts

The conventional approach to pseudocomponent selection has been to separate oil into boiling-point ranges.3,4,5,6 This allows reasonable variations in the physical properties of the oil, and can effectively portray distillation effects if at least three distillation cuts are used.4 In addition, gas and a nonvolatile pyrolysis (thermal cracking) product called "coke" are added to the pseudocomponent list. Coke and gas are usually formed by pyrolysis of one or more of the higherboiling cuts.

This approach implicitly assumes that all the material in a selected boiling point range will participate equally in chemical reactions. It also assumes that the high-boiling or non-volatile products of both pyrolysis and low-temperature oxidation reactions are the same. As pointed out below, this is not the case.

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