| In-situ combustion (ISC) is a kind of thermal recovery method with obvious technical advantages and potential. As a result of the oil drying up for the past years, ISC draws more and more attention from domestic and foreign scholars. However, ISC is very difficult to monitor and control, because heavy oil combustion process in the underground, reservoir is very complex and the combustion mechanism is still unclear, so the study of ISC is necessary.In this paper, numerical simulation and experiment are explored to study the ignition and stable combustion mechanism of heavy oil in the underground reservoir. One-dimensional and two-dimensional combustion heat transfer model of heavy oil burning inside the porous medium are established,where the porous structure of the sandstone is similar to underground reservoir. In order to understand the combustion mechanism of oil in the underground reservoir, a simulation experiment system of ISC is built in laboratory, based on the basic characteristic experimental study of the crude oil.According to the characteristics of the porous structure within oil layer and the mechanism of heat transfer in the oil burning, we take the in-situ combustion as a filtration combustion process of crude oil in porous medium. The heat transfer model with a mobile inner heat source is established. The differential equation is solved by Matlab software. We get the temperature distribution of the combustion of heavy oil in the porous sandstone and analysis the influence of air injection rate, saturation, porosity and preheating temperature to the distribution of temperature. The result shows that after igniting, the combustion zone gets wider and the high temperature region moves forward with burning. An apparent characteristic temperature partition appears and the maximum combustion temperature keeps relatively stable. The combustion wave propagation velocity is around 0.1 m/h order of magnitude, and it close proportional relationship with air injection rate. Simulation results agree well with the experimental results, which proves the validity of the model. Based on simulation results, suitable air mass flow, porosity, saturation are estimated to ensure the ignition and steady-state burning of the indoor experiments.Physical and chemical parameters of heavy oil, which closely related to the combustion, are measured. Experiments include crude oil viscosity measurement experiment, thermogravimetric experiment and crude distillation experiment. The viscosity of crude oil with temperature changes is analyzed. The thermogravimetric (TG) curve and derivative thermogravimetric (DTG) curve are analyzed through Coasts-Redfern method, universal method and Segal method, etc. The kinetic parameters of crude oil in the different combustion stages are obtained. The results shows that the suitable kinetic parameters can not be obtained through Coasts-Redfern method at the low temperature stage, and the activation energy in the high temperature which obtained through Coasts-Redfern method is lower than through other methods. The activation energy obtained through Segal method is higher than other methods. The kinetic parameters which obtained through thermogravimetric experiment can be used in simulations as important data.An indoor simulation experiment system of ISC is built and three times combustion tests are carried out, by which the ignition and stable combustion of crude oil in the porous medium are realized. The results shows that the highest burning temperature is up to 1210 K, and the temperature in the center of combustion chamber stabilizes at about 750 K, which lasts about four hours. It can be found that insufficient air supply after ignition and high heating location might cause a flameout, and the combustion wave is difficult to move forward at a low amount of gas. In order to maintain a continuous combustion wave propagation, the location of heat source and air inlet should be arranged at the bottom of the reservoir. |
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