Low-Temperature Oxidation Characteristics and Its Effect on the Critical Coking Temperature of Heavy Oils

Abstract

Air assisted steam stimulation (huff and puff) is an innovative process for enhanced heavy oil recovery. In this paper, the low temperature oxidation (LTO) and coking experiments were conducted to reveal the LTO characteristics of heavy oils and its effect on the oil critical coking temperature. The O2 consuming ability and oxidation activity of different oils, in a temperature range of 80-170 o C, were analyzed, including typical light and heavy oil samples. The experimental results show that heavy oils have a higher oxidation activity at low temperature than that of light oils. Heavy oils can effectively consume O2 in the injected air and produce a certain amount of CO2 under reservoir conditions. In terms of the difference in LTO characteristics of different oils, a more general LTO reaction kinetics model of crude oils in reservoir was established. A parameter, Rdec (ratio of hydrocarbon oxides participating in decarbonylation to total hydrocarbon oxides produced from oxidation), is introduced to indicate the O2-to-CO2 conversion features of different oils during LTO process. The Rdec, Ea and ko of heavy oils are usually lower than those of light oils, which can well reflect the LTO characteristics of heavy oils and demonstrate that heavy oils are more easily oxidized at low temperature. However, due to more heavy oil components generated during LTO process, the critical coking temperature of heavy oils exposed to air environment will decrease (by 120 o C from 400 o C to 280 o C for super heavy oil). It should be noted that the effect of LTO process on critical coking temperature is different for different heavy oils, and the induced coking risks are also different in different heavy oil reservoirs, which need further studies in lab and field.

Low-temperature isothermal oxidation of crude oil

“…This relationship is developed basing on the assumption that LTO rate depends largely on the O 2 partial pressure, reaction temperature, and oil concentration, and the variables in Equation (3) has been described previously (Zhang et al, 2014). The Arrhenius equation suggests that activation energy is Downloaded by [University of Sussex Library] at 07:16 21 June 2016 inversely proportional to pressure drop rate.…”

“…Second, the amount of active hydrogens in crude oil is another key indicator controlling the rate of free radical reaction between oxygen and liquid hydrocarbon (Moore and Ursenbach, 2002). As the active hydrogen mainly locates in complex macromolecular chains and heavy oil contains a larger quantity than light oil, the possibility for O 2 to effectively attack carbon chains in heavy oil becomes enormously (Zhang et al, 2014). On the contrary, there are some oxidation inhibitors naturally presenting in light fraction (e.g., saturates).…”

“…The oxidation characteristic of three crude oils was examined by Li et al (2006) using pressure differential scanning calorimeter (PDSC) test and the results indicated that the oxidation extent was highly dependent on oil chemical compositions. Zhang et al (2014)and Niu et al (2011) took advantage of static oxidation reactor to characterize the LTO behaviors of crude oils with different API gravity. Heavier oil with more heavy components possessed stronger O 2 consuming ability and higher pressure drop.…”

In situ catalytic upgrading of heavy crude oil through low-temperature oxidation

“…Hydrocarbon products keep reacting with oxygen, and thus the chains with oxygencontaining functional groups are further broken down to carbon monoxide, carbon dioxide, water, and shorter chain hydrocarbon compounds. Zhang et al (2015) investigated the characteristics of low-temperature oxidation (LTO) of heavy oils through laboratory experiments and found that the heavy components served as the main reactant in the LTO process. During the oxidation, the carbon chains of heavy components were broken, producing some light components and leading to a relative decrease in the total content of heavy components.…”

Influence of Conversion Conditions on Heavy-Oil Coking During in Situ Combustion Process

“…20 Studies have shown that O 2 can be consumed by heavy oil even at reservoir temperature. 21,22 During LTO, oxygen-containing functional groups appear in the gas, liquid, and solid products. 21−26 Appearance of these functional groups influences formation temperature, molecular structure, and yield of coke precursors.…”