Physical Upgrading of Heavy Crude Oils by the Application of Heat

Reaction Kinetics of Fuel Formation for In-Situ Combustion

Summary.

Chemical reactions believed to cause fuel formation for in-situ combustion have been studied and modeled. A thin, packed bed of sand/oil mixture is heated under nitrogen flow at linearly increasing temperatures, simulating the approach of a combustion front. Analysis of gases produced from the reaction cell revealed that pyrolysis of crude oil in porous media goes through three overlapping stages: distillation, mild cracking (visbreaking), and severe cracking (coking). Expressions that govern the rates of the two cracking reactions are derived, and a technique is outlined to obtain initial estimates for their parameters from the experimental data. The parameters of a proposed distillation function, as well as refined estimates for the cracking reaction parameters, are obtained by non-linear regression methods based on an parameters, are obtained by non-linear regression methods based on an overall kinetic model. Successful matching of the experimental data, including the total amount of fuel deposited, was achieved with this model. It was found that fuel formation was a result of two successive cracking reactions that the oil undergoes at temperatures above 280 degrees C [536 degrees F]. Also, distillation of crude oil at temperatures below 280 degrees C [536 degrees F] played an important role in shaping the nature and extent of the cracking reactions. The operating pressure and the rate of heating of the sand/oil sample were found to affect the fuel-formation process only through the influence exerted on distillation. Clay minerals showed a catalytic effect on the cracking reactions, especially coking. Finally, the asphaltene fraction of a crude oil was found to correlate with the fuel content of that oil.

Introduction

Fuel formation occurs in a reservoir undergoing in-situ combustion as a result of various physical and chemical changes inflicted upon the reservoir oil, mainly distillation and thermal cracking. An important parameter to be considered in the design of a combustion project is the concentration of fuel deposited ahead of the combustion project is the concentration of fuel deposited ahead of the combustion front, primarily in the evaporation and cracking zones. The nature of fuel varies widely from one reservoir to another. It can be close to the heavy fraction of the parent oil or a solid coke-like residue. Its apparent elemental hydrogen/carbon (H/C) ratio could be computed from effluent gas composition with the carbon oxides and consumed oxygen for a stoichiometric balance. Survey of the literature revealed the following general observations regarding the influence of process variables on fuel properties and concentration.The fuel H/C ratio is generally lower than that of the parent oil to an extent that depends on the operating conditions. The fuel H/C ratio decreases with increasing combustion temperatures, coking temperatures, or pressure.Decrease in oil API gravity or H/C ratio leads to increased fuel deposition. Higher viscosity and higher Conradson carbon residue gave a similar result.Higher combustion-front temperature or slower front velocity reduces fuel deposition.Pressure has no general effect on fuel availability, with every oil showing a different behavior.Higher initial oil saturation was found to cause more fuel deposition; on the other hand, it was argued that the residual oil saturation in the steam plateau was a key factor.The specific surface area of the reservoir rock is of particular importance to fuel deposition, especially when clays are present.

A larger specific surface area facilitates the various heterogeneous reactions that cause fuel formation. While these observations serve as general guidelines for project design, functional relationships between process variables and fuel concentration are imperative for accurate prediction of in-situ combustion performance. Combustion-tube tests yield fuel concentration data, but such data have limited applicability in the light of areal variation of reservoir characteristics. Moreover, the saturation and properties of the residual oil in the steam plateau are expected to properties of the residual oil in the steam plateau are expected to change as the combustion zone advances in the reservoir. Finally, simulation models developed for the in-situ combustion process require accurate mathematical formulation of the kinetics of reactions that a reservoir oil undergoes as the thermal heat wave propagates in the reservoir. To model fuel formation, the sequence of events that lead to it has to be characterized and related. An idealized environment that prevails in a reservoir volume element being approached by the prevails in a reservoir volume element being approached by the combustion front is that of light-hydrocarbon displacement followed by steamdrive, both reducing oil saturation to the residual with sub-stantial light-ends distillation. Further approach by the front cause temperature to rise steadily with time, resulting in more distillation and triggering mild oil pyrolysis. Finally, immediately before the arrival of the front, severe pyrolysis of the trapped hydrocarbons causes fuel deposition. Throughout these events, the gas phase is composed of only nitrogen, carbon oxides, and water vapor.

Pyrolysis Reactions Pyrolysis Reactions Pyrolysis reactions can be represented by the following formula: Pyrolysis reactions can be represented by the following formula:

..........................................(1)

where

Nh(1) = heavy oil, Q = heat, H(1) = hydrocarbon derivatives, and G = gas.

To facilitate modeling, the reactant is taken as a pseudocomponent of the oil, usually a heavy fraction, although multi-pseudocomponent approaches were attempted. The reaction rate expression for a pseudocomponent adopted by numerous investigators is pseudocomponent adopted by numerous investigators is

...................................................(2)

The reaction rate constant is normally expressed by the Arrhenius equation:

...................................................(3)

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New Kinetic Models for Thermal Cracking of Crude Oils in In-situ Combustion Processes

Summary.

This paper presents two kinetic models for representing the thermal cracking of crude oils, which incorporate the cracking rate parameters and stoichiometric coefficients to correlate experimental data. parameters and stoichiometric coefficients to correlate experimental data. The models presented show that the first-order kinetics generally accepted for pure components are unsatisfactory for multicomponent systems characterized by pseudocomponents. We conclude that three corrections to the existing first-order model are needed for modeling thermal cracking of mixtures. First, the apparent reaction order is always greater than one. Second, the reaction order is a decreasing function of temperature. Third, coke may also be formed from intermediate products. These corrections are incorporated into the models. In the first model, crude oil is split into two pseudocomponents, while in the second model, crude oil is represented by three pseudocomponents. The models can be easily extended to any number of pseudocomponents. The models can be easily extended to any number of pseudocomponents. pseudocomponents. The models successfully correlated experimental data of four systems available in the literature. Furthermore, it was confirmed that coke is not always the same source of the fuel burned in an in-situ combustion process. process.

Introduction

It is generally believed that in in-situ combustion processes, the combustion zone is preceded by a cracking or processes, the combustion zone is preceded by a cracking or superheated steam zone where coke is formed from the thermal cracking (pyrolysis) of crude oil. The kinetics of the cracking reaction may be a crucial process mechanism affecting the performance of combustion processes because it not only produces solid-like coke for combustion but also upgrades the remaining oil, which affects the vaporization behavior. As a result, the cracking reaction will strongly influence the total amount of fuel available in the combustion zone. The effects of cracking reactions on the fuel deposition mechanism and the fuel composition have been discussed elsewhere. The reaction mechanisms of hydrocarbon cracking are very complex. Even for a pure component, it is almost impossible to describe the mechanism precisely. Nevertheless, it is possible to use simple global rate expressions to represent the reaction rate of the reactant. For pure hydrocarbons, it is well established that the cracking reaction can be properly modeled by a first-order rate expression, although self-inhibition (decreasing first-order rate constant with increasing conversion) was generally observed. This is caused by the formation of olefins, which are known to be good inhibitors of free-radical reactions. As a general rule, the reaction rate constant for normal paraffins increases with increasing carbon number. while the activation energy decreases with increasing carbon number. Global rate expressions were also applied to the pyrolysis of gas oil and crude oils by use of first-order pyrolysis of gas oil and crude oils by use of first-order kinetics. Most of these studies lumped the multicomponent mixture into one oil component, while McNab et al. assumed that only the heavy-oil fraction contributes to the cracking reaction and arbitrarily chose 80% of the residue of the distillation of the original crude as the heavy-oil component. This two-pseudocomponent approach was adopted by Henderson and Weber. In all the above studies, only the reaction rates of the crude oil components were considered, and no attempt was made to correlate the stoichiometry of the reaction. In an attempt to match the product distribution of Athabasca bitumen pyrolysis, Hayashitani et al. constructed a number of complex kinetic models. The oil was divided into three to five components, and six to eight first-order reactions were included in each model. They found that these complex models cannot satisfactorily correlate all aspects of experimental behavior of cracking reactions. They concluded that during the course of cracking reactions, each pseudocomponent might have changed its characteristics, A comprehensive reaction scheme for catalytic cracking of gas oils was recently developed by Jacob et al., who simulated the gas-oil cracking using four pseudocomponents (heavy oil, light oil, gasoline, and gas plus coke). pseudocomponents (heavy oil, light oil, gasoline, and gas plus coke). For each of the light- and the heavy-oil components, the pseudocomponent was further split into paraffinic, pseudocomponent was further split into paraffinic, naphthenic, aromatic, and aromatic substitute groups. As a result, the reaction scheme involves 10 species (components) and 20 first-order reactions. The Arrhenius constants and the activation energies for these reactions were assumed to be universal constants (i.e., independent of gas-oil composition), This model was claimed to work well for a wide spectrum of gas oils. However, the applicability of this model to heavy oils with a wider range of composition remains to be proved.

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