Summary. During in-situ combustion, oxygen can bypass the flame front and react with oil temperatures that are relatively low compared with those in combustion. These reactions, called low-temperature oxidation (LTO), can adversely affect the crude's physical and chemical properties and result in lower oil recovery. This paper presents laboratory study results on the LTO of four oils, ranging from 31.1 to 10.1deg. API and from 14 to 54,300 cp dead-oil viscosity at 85deg.F. They include two biodegraded oils, a mature crude, and an immature heavy oil. The LTO of these oils is reported as a function of reaction time, oxygen partial pressure, and temperature. LTO increased oil viscosity and density. For the 31.1 deg. API oil, these increases are minor and should have insignificant effects on process performance. For the heavier oils, however, the viscosity of the oxidized oils increases exponentially with increasing extent of oxidation, Xo2. Relatively minor oxidation (40 mg02/g oil) results in a six-fold increase in viscosity. The rate of viscosity increase depends on an oil's API gravity, origin, and composition. Compositional changes accompanying LTO are also discussed. LTO is shown to increase the asphaltene content of the oxidized oils significantly, and a correlation is presented between asphaltene content and Xo2. Simulated distillation results demonstrate that LTO does not significantly affect the volatility or carbon number distributions of either the light or heavy oils. The observed physical and compositional property changes are consistent with the LTO mechanism proposed in the literature: oils-resins-asphaltenes/coke. Introduction During in-situ combustion, significant partial oxidation of the reservoir hydrocarbons can occur at temperatures less than that required for complete combustion (less than 400 degrees F). These LTO reactions significantly affect process performance through fuel laydown and oil mobility. LTO, which results when oxygen bypasses the combustion front, can occur in the vaporization, steam, water, and virgin zones ahead of the flame front. Oxygen bypassing arises from both reservoir heterogeneities and insufficient combustion rates to consume all the oxygen. LTO tends to be more pronounced when oxygen, rather than air, is injected into the reservoir. It is also significant in reverse combustion and in steamflooding when oxygen containing gas is injected with the steam. Unlike combustion. which produces CO2, CO, and H2O as its primary reaction products, LTO yields water and oxygenated hydrocarbons, such as carboxylic acids, aldehydes, ketones, alcohols, and hydroperoxides. The resulting oxygenated oils can have significantly higher viscosities, lower volatilities, and lower gravities than the virgin oils. Compositionally, LTO has been found to increase the asphaltene content of the oil and to decrease its aromatic and resin contents. The mechanisms of LTO are extremely complex, but condensation to higher-molecular-weight material has been proposed by several researchers. Adegbesan et al. and Babu and Cormack observed that, of the saturates and aromatics in the oil fraction, the aromatics are significantly more reactive. The saturates appear to be unaffected by LTO. The objective of this work was to characterize the reactivity/ reactions of four crudes and to investigate the effects that LTO has of their physical and chemical properties. Results of a laboratory study on the LTO of these crudes are presented. The experimental matrix includes the effects of reaction time (16 to 336 hours), oxygen partial pressure (150 to 700 psia), and temperature (72 to 450 degrees F). The compositional changes accompanying LTO, noted from elemental analysis, gas composition, asphaltene content, Fourier-transform infrared (IR) spectroscopy, and simulated distillation by gas chromatography (GC) are discussed and related to oil properties (volatility, and), reaction mechanisms, and the extent of oxidation. Experimental The experimental apparatus consisted of a 150-mL, 316 stainless-steel reactor connected to a 100-mL gas reservoir. A high-pressure oxygen cylinder was used to pressurize/purge the reactor. Two air-actuated solenoid valves isolated the reactor from the rest of the apparatus. As a safety precaution, a burst disk was installed in the bottom of the reactor. Furthermore, the reactor assembly was contained in a pit, and the loading and discharging of gases from it were controlled remotely. In Runs 2 through 97, the reactor was held at a constant temperature by heating tapes. These tapes took about 1 hour to heat the reactor to the desired temperature. A similar period was required to cool the reactor to room temperature. In Runs 200 through 214, the reactor was heated by a fluidized sand bath that required less than 5 minutes to bring the reactor to temperature. Reactor pressure was measured by a 5,000-psig pressure transducer. The gas and liquid temperatures in the reactor were measured through two thermowells. Reactor skin temperature was also measured by a thermocouple inserted either in the sand bath or between the heating tapes and the reactor wall. After the initial heatup period, only minor temperature oscillations were observed during a run (less than 10deg.F). The average of the vapor, liquid, and skin (bath) temperatures was taken to be the reaction temperature for each run. The maximum range observed among the three temperatures was 20 deg. F for Runs 2 through 97 and 3deg.F for Runs 200 through 214. In a typical experiment, a known mass (∼75 g) of clean oil (black sediments and water less than 0.5%) was added to the reactor at room temperature. The reactor was then pressurized with oxygen, isolate from the system, and heated to the desired temperature. There was no stirring or shaking of the reactor during oxidation. Two regions generally were observed in the pressure profile: an early rise caused by thermal expansion of fluids in the reactor followed by a gradual decrease in pressure, indicating the consumption of oxygen by LTO. The experiment was ended by fuming off the heating tapes (or by removing the reactor from the sand bath) and allowing the reactor to cool to room temperature. The reactor gas was then vented. The mole percent of CO2 and CO in the vent gas was measured by an Infrared Industries Model 702(TM) infrared analyzer, and the hydrocarbon, H2S, oxygen, and nitrogen content in the gas were measured on a Hewlett Packard Model 5880(T)M gas chromatograph. The oil was decanted from the reactor and centrifuged to remove any solids formed. At higher reaction temperatures, small amounts of coke would be deposited in the reactor. The total solids yield, however, never exceeded 5% of the mass originally charged to the reactor. The liquid-phase viscosity was measured in a capillary-tube viscometer, and its density was determined by pycnometer. The viscosities of the heavier oils were measured on a Haake PK100/RV100(TM) cone and plate viscometer. Distillation was simulated by GC on the liquid phase according to the procedures developed by MacAllister and DeRuiter. SPERE P. 609^