Reaction Kinetics of Fuel Formation for In-Situ Combustion

Abstract

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) SPERE p. 1308