Natural Convection in Porous Media and Its Effect On Segregated Forward Combustion

Abstract

Abstract This study investigates whether oxygen consumption during segregated forward combustion may be affected by natural convection. Linear theory indicates that thermal instability occurs in a horizontal porous medium when the modified Rayleigh number N'Ra exceeds a critical value of about 40. Most experimental results, including those described here. indicate this value to be about an order of magnitude smaller. N'Ra is derived from "differential similarity" and the proper time scale factor is also obtained. The critical value of N'Ra for a horizontal, water-saturated porous medium at atmospheric pressure subjected to a vertical temperature gradient was found to be about 3.0. The ambiguity in the value of N'Ra arising when fluid properties cannot be assumed constant is indicated. Natural convection was observed for oblique systems below N'Ra while simultaneous horizontal flow did not affect N'Racrit. The expected range of values for the various parameters pertaining to segregated forward combustion is indicated and the corresponding values of N'Ra calculated. Only under very favorable conditions, such as high permeability, high pressure and low top temperature, can the critical value of N'Ra be exceeded. Thickness of the convection layer should not become too large. since the time required to obtain significant mixing is proportional to the square of the thickness. For an oblique combustion front the conditions for significant mixing may be less strenuous. INTRODUCTION Laboratory1,2 and field3,4 experiments have shown that a markedly upgraded oil can be produced by in situ forward combustion. Under certain conditions injected air may move through a high permeability, gas-saturated channel over the oil-saturated layer, while combustion takes place, exclusively, at the horizontal interface (Fig. 1). Such a channel may be a natural gas cap, or, it may result from a forward combustion process in which injected air and combustion gases fingered through the top of the formation, causing a premature breakthrough at the production well. In this particular case of segregated forward combustion, the air stream moves parallel to the combustion front. Molecular diffusion and transverse mechanical dispersion will be insufficient to cause all the oxygen in the air stream to reach the combustion front. When the oxygen in the air stream is not completely consumed in the combustion process, it will reach the production well and create a serious fire hazard. In addition, the reduced efficiency may make the process economically unprofitable while also serious corrosion problems may be experienced in the production well.3 This undesirable situation should, therefore, be prevented.