Modifying In-Situ Combustion Performance by the Use of Water-Soluble Additives

Abstract

Summary Experiments were performed to study the effects of various additives on theoxidation kinetics of Californian and Venezuelan oils. Aqueous solutions of 10metallic salts were indexed with sand and Huntington Beach, CA, oil. Themixtures were subjected to a constant flow of air and a linear heating schedulewhile the effluent gases were analyzed for composition. The variation in theoxygen consumption was analyzed with a model of three competing oxidation reactions. Values for the important kinetic parameters for the three reactionswere obtained for each additive. Iron and tin salts were found to enhance fuelformation, while copper, nickel, and cadmium salts bad no significant effects. Other experiments with a heavy Venezuelan oil showed that, contrary to earliersuggestions, the use of a ketal did not decrease fuel formation. Introduction The major constraint limiting the applicability of the in-situ combustionoil recovery process is the propensity of the reservoir oil to form fuel in thereservoir matrix ahead of the combustion zone. If insufficient fuel isdeposited, as is often the case for light oils, the combustion front will notbe self-sustaining and will die out quickly. Conversely, if excessive fuel isdeposited, the advance rate of the combustion front will be slow and thequantity of the oxidizing gas (usually air) required to sustain combustion willbe uneconomically high. If methods can be developed to modify the tendency foroil to deposit fuel, then the in-situ combustion process could be made feasiblefor a wider range of crude oils. The amount of fuel formed and the velocity ofthe combustion front are governed by the kinetics of the oil oxidation andpyrolysis reactions. Catalytic and organic compounds affect the kinetics ofthese reactions and so influence the amount of fuel formed. Oil oxidationduring in-situ combustion involves numerous competing reactions occurring overdifferent temperature ranges. A number of experimental studies haveinvestigated the kinetics of these oxidation reactions. A common procedureinvolves subjecting a mixture of oil, water, and sand to a linear heatingschedule. The effluent gas produced by passing air through the mixture duringthe heating process is analyzed for its oxygen and carbon oxides content. Thesestudies established that the overall oxidation mechanism of crude oils inporous media may be represented by grouping the reactions into three classes ofcompeting reactions occurring over different temperature ranges:(1)low-temperature oxidation (LTO) reactions, which are heterogeneous (gas/liquid)and produce no carbon oxides;(2)medium-temperature oxidation produce nocarbon oxides;(2)medium-temperature oxidation (MTO), fuel-formationreactions, which are homogeneous (gas phase) and involve the oxidation of theproducts of distillation phase) and involve the oxidation of the products ofdistillation and pyrolysis; and(3)high temperature oxidation (HTO), fuel-combustion reactions, which are heterogeneous, in which the oxygen reacts with the fuel formed during the medium-temperature reactions. LTO reactions canoccur in high-permeability streaks, which enable oxygen to travel so rapidlythrough the combustion zone that it contacts insufficient fuel for completeoxygen utilization to occur. LTO results in the production of partiallyoxygenated compounds such as aldehydes, ketones, and alcohols. This partialoxidation increases the viscosity and boiling range of the fluid. The regionahead of the combustion front is heated by conduction, by convection of thecombustion gases, and by condensation of volatiles and steam generated frominterstitial water. As the temperature rises, the crude oil undergoes threeoverlapping stages of pyrolysis: distillation, visbreaking, and coking. Athigher temperatures (250 to 300 degrees, mild cracking of the oil occurs(visbreaking) in which the hydrocarbons lose small side groups and hydrogenatoms to form more-stable, less-branched compounds. At temperatures above about300 degrees C, the residual oil cracks into a volatile fraction and anonvolatile heavy residue of coke, tar, and pitch, which constitutes theprimary fuel for combustion. In the combustion zone, exothermic heterogeneousreactions occur between oxygen in the gas phase and the heavy residue of oildeposited on the rock matrix at lower temperatures.