Reaction Kinetics of In-Situ Combustion: Part 1-Observations

Continuous analysis of produced gases from a small packed bed reactor, at both isothermal and linearly increasing temperatures, has shown that combustion of crude oil in porous media follows several consecutive reactions. Molar carbon dioxide/carbon monoxide (CO2/CO) and apparent hydrogen/carbon (H/C) ratios were used to observe the transition between these reactions at different temperature levels. A new kinetic model for oxidation of crude oil in porous media is presented in Part 2 of this paper (Page 408) Introduction The quantity of fuel consumed and the reaction rate within the burning zone have been studied intensively for two reasons. First, the maximum oil recovery is the difference of the original oil in place (OOIP) at the start of the operation and the oil consumed as fuel. Second, one of the most important factors in the economic evaluation of any in-situ combustion project is the cost of air compression. Excessive fuel deposition causes a slow rate of advance of the burning front and large air compression costs. However, if the fuel concentration is too low, the heat of combustion will not be sufficient to raise the temperature of the rock and the contained fluids to a level of self-sustained combustion. This may lead to combustion failure. Thus, it is necessary to understand the reactions occurring at different temperatures as the combustion front moves in the porous medium. The most crucial and yet least understood zone of insitu combustion oil recovery is the burning front, where temperature reaches a maximum value. The velocity of the burning front is controlled by the chemical reactions involved. However, since crude oil is a mixture of hydrocarbons, it is necessary to consider a global description of the reaction mechanism. Reaction Mechanism The reaction between fuel and oxygen in a forward combustion process is a heterogeneous flow reaction. Injected oxidant gas must pass through the burning zone to make the burning front move. Within the burning zone, four known transport processes occur:oxygen diffuses from the bulk gas stream to the fuel interface; then, perhaps,the oxygen absorbs and reacts with the fuel;then combustion products desorb; andproducts finally transfer into the bulk gas stream. If any of these steps is inherently much slower than the remaining ones, the overall rate will be controlled by that step. Also, the rate of each series of steps must be equal in the steadystate condition. However, there are no useful correlations for computing absorption and desorption of oxygen in a porous medium. Consequently, consideration of these phenomena becomes extremely difficult for even simple reactions. Theoretical expressions for postulated mechanisms often contain 10 or more arbitrary constants. Because of the large number of arbitrary constants, sever-al expressions developed for widely different mechanisms often will match experimental data equally well. In general, the combustion rate, Rc, of crude oil in a porous medium can be described as dCm m nRc = - ------ = kpo2 Cm,............................(1)dt whereCm = instantaneous concentration of fuel, k = rate constant, Po2 = partial pressure of oxygen, andm, n = reaction orders. The reaction constant, k, is often a function of temperature, T, as expressed by k=w exp(– E/RT).......................................(2) where E is the activation energy, w is the Arrhenius constant, and R is the universal gas constant. For heterogeneous reactions, the constant w is a function of the surface area of the rock. Early studies of crude oil oxidation in a porous medium were mostly qualitative. Differential thermal analysis (DTA) was performed on samples of crude oil, and the resulting thermograms represented the thermal history of each sample as it was heated at a uniform rate (usually 18 degrees F/min [10 degrees C/min]) in a constant air flow (usually 277 mL/min [277 cm3/min]). These thermograms indicated the presence of a number of exothermic reactions. Another method of analysis is thermogravimetric analysis (TGA). Here, a sample of crude oil is weighed continuously as it is heated at a constant rate. The resulting curve of weight change vs. time or temperature indicates the occurrence of at least two reactions at different temperatures. SPEJ P. 399^