|Fig. 1: Schematic of temperature and saturation profiles in the reservoir during the combustion process.|
In-situ combustion (ISC) is an enhanced oil recovery method in which the air is injected into the reservoir burning the heaviest crude oil components generating heat and combustion gases that enhance recovery by reducing oil viscosity and pressurizing the system, respectively. In this process, highly exothermic reactions occur in the porous medium resulting in significant increases in the temperature. For heavy oils, a 300-400 °C increase in temperature is not uncommon.  Large temperature differences signify heat transfer and also will result in the phase change. ISC involves many phenomena, making modeling complex. So the engineering of the process is more difficult than any other method of crude oil recovery, but the advantages of in-situ combustion motivate researchers to investigate on it.
In this method the air is injected to the crude oil reservoir. After ignition the generated heat by combustion keeps the combustion front moving toward the producer well. Combustion front burns all the fuel in its way. Usually 5 to 10 percent of the crude oil is used as a fuel and the rest is going to be produced in the production well. The heat of reaction vaporizes initial water and also the light components of the oil in front of the combustion front. The steam is condensed while distancing from the hot region. Fig. 1 depicts the temperature and crude oil saturation profile in the reservoir.
|Fig. 2: Kinetic cell experiment set up.|
|Fig. 3: Typical Comparison of O2 consumed and CO2 and CO Produced during oxidation crude oil.|
Although in-situ combustion method has been widely used for heavy crude oil reservoirs, it has been successfully applied to the light crude oil reservoirs. Increase in temperature decreases the viscosity of the crude oil. The fuel of the combustion is mainly composed of asphaltene and heavy fractions of the crude oil. These heavy components hinder the production of the crude oil. So the removing of these components from the crude oil helps the recovery. In-situ combustion not only removes these components but also combusts them to make heat and flue gases. The flue gases are miscible in the crude oil. So in-situ combustion includes miscible gas injection as well. In-situ combustion also benefits from the stable steam front as the condensation of water in front of steam controls the mobility in the steam injection.
There have been many efforts to understand the parameters affecting in-situ combustion processes. [2,3] The two main experiments measure oxidation kinetics and front propagation in a combustion tube. [4,5] Because the combustion of the crude oil is itself complex, kinetic cell measurements aim to capture the reaction kinetics removing the complexity caused by flow in porous media. The combustion tube experiment is a way to understand the in-situ combustion process in one dimension. Both experiments give valuable on the likelihood of successful ISC under field conditions.
A mixture of oil, water, sand and clay is placed in the cell that is subjected to controlled heating and the oxygen is injected at a constant mass rate from the bottom of the cell, as depicted in Fig. 2. The heater is programmed to maintain a ramped temperature increase and the temperature of the cell is measured continuously. The produced gas composition is obtained using a gas analyzer. Fig. 3 demonstrates the temperature of the cell, oxygen consumption and produced gas composition.
|Fig. 4: Schematic of the combustion tube experiment.|
A significant number of compounds can be produced by the oxidation of hydrocarbons. Carbon oxides are produced along with oxygenated compounds including carboxylic acid, aldehyde, ketone, alcohol and hydro peroxide.  Capturing all of the reactions and describing all of the components involved in the combustion of the crude oil seems impossible due to the complexity of the process. Fig. 2 demonstrates two peaks in oxygen consumption that correspond to two increases in the cell temperature profiles that deviate from the temperature schedule. The first hump indicates a sequence of low temperature oxidation (LTO) reactions that apparently produce a fuel that burns at greater temperatures called high temperature oxidation (HTO). In the HTO region, the amount of oxygen consumption is comparable to the amount of carbon oxides (CO2+0.5 CO). This implies a combustion reaction rather than reactions producing oxygenated compound (Fassihi, 1984). In the LTO region, the amount of oxygen consumed is greater than carbon oxides produced implying that the oxygen is added to the oil producing oxygenated compounds. The ratio of CO2/CO in LTO region changes with the temperature implying that many reactions are involved in LTO.
A combustion tube assembly is considered to study the fire flooding of the crude oil. Oil, water, sand, and clay are mixed and then put in the combustion.  The tube is held vertically and the gas is injected from the top. The first 10cm of the tube is heated using electrical band heater and nitrogen is injected while heating the top. Then heaters are turned off and air is injected. Temperature is measured along the tube using thermocouples placed at the center. Fig. 4 shows a schematic of the combustion tube study. The main advantage of the combustion tube is the estimation of the amount of the fuel and the speed of the combustion front.
In-situ combustion is a method of the crude oil recovery in which the air is injected to the crude oil reservoir to burn the heavy fractions of the crude oil and make heat to ease the production. Combustion of the crude oil is very complex to be understood. The complexity of transport makes the process even more difficult to be modeled and engineered. There are two experiments to investigate the combustion process. Kinetic cell experiment is mainly focused on the kinetics of the combustion reaction. Combustion tube experiment is designed to consider the reaction and transport of the fluid simultaneously.
© Mohammad Bazargan. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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