Compositional Effects in Light/Medium Oil Recovery by Air Injection: Vaporization vs. Combustion

Abstract

Combustion can be used to enhance recovery of heavy, medium, or light oil in highly heterogeneous reservoirs. Such broad range of applicability is attained because not only do the high temperatures increase the mobility of viscous oils but also the high thermal diffusion spreads the heat evenly and reduces heterogeneity effects. For the latter reason, combustion is also used for the recovery of light oils. The reaction mechanisms are different for light oils, where vaporization is dominant, whereas for medium nonvolatile oils combustion is dominant. We will only consider combustion of medium and light oils. Therefore we ignore coke formation and coke combustion. It is our goal to investigate the relative importance of vaporization and combustion in a two-component mixture of volatile and nonvolatile oils in a low air injection rate regime. By changing the composition we can continuously change the character of the combustion process. We derive a simplified model for the vaporization/combustion process, and implement it in a finite element package, COMSOL. For light oil mixtures, the solution consists of a thermal wave upstream, a combined vaporization/combustion wave in the middle (with vaporization upstream of combustion) and a saturation wave downstream. For medium mixtures the vaporization/condensation sequence is reversed and vaporization moves ahead of the combustion. Due to its low viscosity, the light oil is displaced by the gases to a region outside the reach of oxygen and therefore less oil remains behind to reach the combustion zone. This leads to a high combustion front velocity. For oil with more nonvolatile components, vaporization occurs downstream of the combustion zone. As more oil stays behind to feed the combustion zone, the velocity of the combustion zone is slower, albeit the temperatures are much higher. The relative importance of vaporization/combustion depends also on the injection rate, pressure, initial temperature, and oil viscosity. Numerical calculations allow to estimate the bifurcation points where the character of the combustion changes from a vaporization-dominated to a combustion-dominated process.

Recovery of light oil by air injection at medium temperature: Experiments

“…In a previous theoretical study based on two-component oil model, we found that the ratio between vaporization and reaction determines the effectiveness of the air injection process [31]. If a large amount of heavy components is present in oil, the vaporization is weak and most oil is left behind for combustion.…”

“…One of the purposes of our research is to investigate whether we can find experimental evidence for the medium temperature combustion mechanism described theoretically in [40,31]. We perform and interpret experiments involving air injection in sandstone rock filled with n-hexadecane (modeling light oil) at medium pressures and conditions that are typical away from the injection well.…”

Oxidation Behavior of Light Crude Oil and Its SARA Fractions Characterized by TG and DSC Techniques: Differences and Connections

“…Air injection techniques can offer unique benefits and technical opportunities for enhanced oil recovery (EOR) in many candidate reservoirs for both conventional and unconventional oil sources, such as high pressure air injection (HPAI) for water-flooded light oil reservoirs and low permeability reservoirs where water injection is difficult (conventional oil sources), in situ combustion (ISC) for heavy oil reservoirs, bitumen sands and shale oil (unconventional oil sources). − After air is injected into reservoirs, various oxidation reactions take place between crude oils and oxygen contained in the injected air, which generates heat to reduce oil viscosity (especially for ISC in heavy oil reservoirs) and flue gas to improve swept volume and increase reservoir pressure . It is acknowledged that oxidation reactions between crude oil and injected air dictate the overall success of the air injection processes .…”

In Situ Combustion: A Comprehensive Review of the Current State of Knowledge

“…During in situ combustion processes, it is expected that hydrocarbons/heavy oil/ bitumen and the oxygen-enriched injected air react and interact, resulting in inevitable chemical changes due to the chemical reactions involved. As shown in Figure 3, the reaction zones across the reservoir can be categorized into three different temperature ranges: (1) low-temperature reactions (reservoir temperature up to 300 • C; LTO), (2) medium-temperature reactions (300-350 • C; MTO), and (3) high-temperature reactions (350-525 • C; HTO) [24,[47][48][49][50][51]. The three ISC kinetic regimes are briefly summarized in Table 3, and further details are provided in the following subsections.…”