Spontaneous Ignition in Oil Reservoirs

Abstract

Abstract Typical experimental data on low-temperature oil oxidation in porous media are presented. Using these kinetic data, numerical models in radial and longitudinal flow are proposed for computing the spontaneous ignition conditions when air is injected into oil reservoirs. The models account for heat transport by conduction and convection in the formation. The computed ignition times are compared with the values obtained by an analytical approach. The distance from the well to the injection zone is also discussed. Introduction Ignition is the first step of the in-situ combustion process. It may be obtained by heating the formation around the ignition well with a burner or an electrical heater. The heat required to initiate a combustion front also may be provided by an exothermic reaction between air and an appropriate chemical compound. This compound may be an easily oxidizable organic liquid injected into the formation in the vicinity of the ignition well before air injection. But if the formation crude oil itself has sufficient reactivity with oxygen under reservoir conditions, the formation may ignite spontaneously when air is injected for a prolonged period. As a matter of fact, spontaneous prolonged period. As a matter of fact, spontaneous ignition has been obtained in some reservoirs in a reasonably short time. The ignition delay may vary from a few days to an indefinite period if the rate of heat dissipation to the surroundings of the reacting volume prematurely becomes as high as the rate of heat prematurely becomes as high as the rate of heat release. In the approximate analytical approach of Tadema and Weijdema, heat dissipation by conduction and convection was neglected. Hence, ignition would occur for all reservoir conditions eventually after an ignition delay of several years. However, if Tadema and Weijdema's approach gives a good approximation of the ignition delay when the oxidation rate is high under reservoir conditions, this delay is likely to be significantly underestimated when crude oil reactivity is low. A numerical model was built for computing the ignition conditions (ignition delay and position of the ignition zone) as a function of reservoir, oil, and gas flow characteristics. The kinetic parameters of oil oxidation were determined experimentally. DESCRIPTION OF THE NUMERICAL MODEL Gas flow is assumed to be one dimensional and linear in x or radial in r. Longitudinal conduction and convection are accounted for. Transversal conduction is neglected since, during the ignition delay, the vertical temperature gradient is very low, especially if the central part of the oil layer is considered. Fluid and solid temperatures are supposed to be equal. Air is injected into the ignition well. Pressure and gas flow rate are assumed to be approximately constant in the investigated zone around the well. Water and oil flow or vaporization are neglected in the heat balance. Diffusion-dispersion of oxygen is not considered. SPEJ P. 73