Combustion for Enhanced Recovery of Light Oil at Medium Pressures

Abstract

Using conventional production methods, recovery percentages from oil reservoirs range from 5% for difficult oil to 50% for light oil in highly permeable homogeneous reservoirs. To increase the oil recovery factor, enhanced oil recovery (EOR) methods are used. We distinguish EOR that uses chemical methods, (partially) miscible methods and thermal methods. Air injection is categorized as a thermal recovery method as it leads to combustion and therefore high temperature in the reservoir. However, many oil recovery mechanisms are involved in air injection process, including sweeping by flue gases, field re-pressurization by the injected gas, oil swelling, oil viscosity reduction, stripping off light components in the oil by flue gas and thermal effects generated by the oxidation reactions. Our interest is in recovering light oil from low permeability heterogeneous reservoirs using air injection leading to oil combustion, as the heated oil vaporizes away from the lower permeability parts to be collected in the higher permeability streaks. Due to simultaneous vaporization, the combustion at medium pressures, i.e., at medium depth, occurs at medium temperatures. Our focus is on air injection at medium pressures (? 10?90 bars) to reduce the high compression costs and to avoid fracturing at shallower depth. We study this process at low air injection rates to mimic the processes in the main reaction zone (away from the injection well) in an oil reservoir, which provides a long residence time for the oxygen to be in contact with the oil. The main recovery mechanism that we consider for medium pressures is the interaction between vaporization and combustion of light oil. In the thesis, we consider exclusively modeling and simulation of air injection in light oil leading to medium temperature oxidation (MTO). In MTO, all physical processes, reaction, vaporization, condensation and filtration, are active. The main purpose of the thesis is to elucidate the prevailing mechanisms in MTO. Therefore we developed a 1-D model considering light oil recovery through displacement by air at medium pressures and low injection rates and performed both numerical and laboratory experiments to validate the MTO concept. The presence of liquid fuel, which is mobile and can vaporize or condense, is a challenge for modeling of the combustion process. We only consider the one dimensional flow problem, expecting that its solution contributes to understanding the MTO process and determine the displacement efficiency. The detailed mechanism depends on diffusive processes (capillary, molecular diffusion and heat conductivity), oil composition, air injection rate, pressure, and the presence of reaction water and initial water saturation. Each chapter is summarized as follows: In Chapter 2, the modeling and simulation of the MTO process are exclusively studied including mass-, thermal and capillary-diffusion for air injection in light oil reservoirs. In this case, we consider only single pseudo-component oil, e.g., heptane as liquid fuel in dry porous rock, to improve the understanding of the oxidation/vaporization/condensation mechanisms. It turns out that the oxidation, vaporization and condensation often occur close to each other and move with the same speed in the porous medium (resonant structure). The temperature variation is bounded by the oil boiling temperature and thus not very large. We analyze the effect of capillary pressure, heat conductivity and diffusion and compare the results with the analytical solution in the absence of diffusion processes. The numerical simulation results and the analytical results with zero diffusion processes show qualitatively similar behavior. The solution consists of three types of waves, i.e., a thermal wave, an MTO wave and saturation waves separated by constant state regions. The effect of the diffusive terms is as follows. Molecular diffusion lowers the temperature in the MTO region, but creates a small peak in the vaporization region. Capillary diffusion increases the temperature upstream of the MTO region. Higher capillary diffusion increases the recovery by gas displacement and leaves less oil for combustion. The analytical solution, without diffusive terms, and the numerical solution become qualitatively different at very high capillary diffusion coefficients. The effect of thermal diffusion smoothes the thermal wave and widens the hydrocarbon vapour region. In Chapter 3, we extended 1-D model involving a two-component oil mixture, e.g., light and medium oils as pseudo-components in dry porous rock. The light component (heptane) both vaporizes and combusts, whereas medium fraction in the oil mixture only reacts with oxygen, but its vaporization is disregarded. It was anticipated that at increasing medium oil content the nature of the combustion process would change from MTO to high temperature oxidation (HTO). The main discerning factor in the MTO combustion process is the ratio between vaporization and combustion in the low injection rate regime. It turns out that also with the two-component mixture, oxidation, vaporization and condensation often occur close to each other in the MTO wave. The character of the MTO wave changes by altering the composition of the oil. Vaporization occurs upstream of the combustion process when oil mixture is composed of a higher fraction of light component. This fact confirms previously obtained analytical and numerical solutions for one component volatile oil. The combustion front velocity is high as less oil remains behind in the combustion zone. Whereas, for a predominantly medium oil mixture (0.8 of medium component fraction in volume fraction), the vaporization moves downstream of the combustion zone in the MTO wave. As more oil stays behind in the combustion zone, the velocity of the combustion zone is slower, albeit that the temperatures are much higher. Due to high temperatures, we conjecture a transition to the HTO process in this case. To summarize, numerical calculations establish a range of parameters for the bifurcation point between MTO and HTO in a two-component oil mixture. Indeed, the bifurcation point is mainly determined by the fraction of the non-volatile component. At the bifurcation the character of the combustion process changes from a vaporization-dominated (MTO) to a combustion-dominated process (HTO). In Chapter 4, we investigate the effects of water on the oxidation/vaporization/ condensation mechanisms in the MTO wave by considering a simple three phase model involving a one-component oil (e.g., heptane, pentane or dodecane) and water in porous rock. The single pseudo-component oil vaporizes/condenses as well as combusts, whereas water only vaporizes and condenses. It was anticipated that only if the boiling point of oil is around or modestly higher (below 200oC) than the boiling point of water, the presence of water is conducive to higher and faster oil recovery. The main emphasis of this Chapter is to investigate the relative importance of steam condensation, vaporization/condensation of oil and combustion in the low injection rate regime. The numerical solution consists of a thermal wave, a steam condensation front coinciding with or downstream of the medium temperature oxidation (MTO) wave (oil vaporization and combustion), and a three-phase saturation wave region involving oil, gas and water. Numerical calculations show that the presence of water makes the light oil recovery more efficient and faster and diminishes the adverse effect of high oil boiling points. When the boiling point of the volatile oil is about or slightly higher than the boiling point of water, the speed of the MTO wave (oil vaporization/combustion front) is equal to the speed of the steam condensation front. The volatile oil condenses at the same location as the steam, which leads to complete oil recovery. However, when the boiling point of the oil is much higher than the boiling point of water, the steam condensation front moves ahead of the MTO wave. Numerical calculations make it possible to estimate the bifurcation point (oil boiling point) at which a solution for which steam condensation and combustion occur simultaneously changes to a solution where the steam condenses downstream of the combustion zone. We show that replacing the medium boiling volatile oil by a high boiling point oil (e.g., dodecane) decreases the MTO wave speed with respect to the steam condensation front and leads to delayed recovery. In Chapter 5, a set of experiments have been designed that enables investigation of the medium pressure air injection process at low injection rate in consolidated porous media saturated with one-component oil in a ramped temperature reactor. The initial aim of the laboratory experiments was to validate various aspects considered in Chapters 2-4. The experiments were carried out to evaluate the mechanisms of the combustion reaction at different pressures and injection rates. At slower rates we expect to see details that are not visible for the experiments operating at high rates and high pressures. The most important aspect in this Chapter was to observe that an oxygen sorption step takes place at low temperatures prior to the full combustion reaction. The mechanism of initial uptake of oxygen for later release was established in this work. The sorbed oxygen bonds with hydrocarbon physically or chemically leading to complete uptake of oxygen from the injected air stream at low temperatures. At a later stage, the compound, which contains the chemically or physically adsorbed oxygen, desorbs the oxygen and further undergoes oxidation reactions to produce CO and CO2. The produced liquid is hexadecane; it is not altered by an oxidation reaction because it has the same viscosity and density, which argues against chemisorption. The laboratory experiments indicate displacement efficiencies between 75?90% of the Oil Initially In Place. The amount of oil burned in the air injection process relative to the amount of oil recovered in our laboratory experiments for hexadecane increased from 2% at 10 bar to 18% at 30 bar, and again decreased to 5% at 45 bar, after which it more or less remained constant. This trend was previously obtained by the analytical results of medium temperature oxidation process. It was also shown that the oil recovery is faster at higher pressures.