Elsevier

Thermochimica Acta

Volume 690, August 2020, 178690
Thermochimica Acta

Evaluation of heat release caused by low-temperature oxidation of heavy oil and its SARA fractions under isothermal conditions

https://doi.org/10.1016/j.tca.2020.178690Get rights and content

Highlights

  • Heat release of heavy oil and its SARA fractions at isothermal LTO modes was studied.

  • Heat release of oil, aromatics and resins reduced from 250 to 300 ℃ due to NTC effect.

  • Asphaltenes hold the fastest exothermic rate from 250 to 350 ℃ among SARA fractions.

Abstract

During in-situ combusiton, the oils in most of the area before the combustion front encounter low-temperature oxidation (LTO) reactions under relatively isothermal conditions in a long time, and heat release of LTO significantly influences the quality of formed fuel. This work is aimed at examining the heat release of LTO of one Xinjiang heavy oil and its saturates-aromatics-resins-asphaltenes fractions using isothermal high-pressure differential scanning calorimetry tests. Furthermore, the widely-accepted LTO reaction pathways of hydrocarbons are summarized. The results indicated that the Xinjiang heavy oil, aromatics, and resins experienced the increased heat release from 200 to 250 ℃ and from 300 to 350 ℃ due to their elevated oxidation rate, and the decreased heat release from 250 to 300 ℃. Thus, to build a stable combustion of the oil, it is key to overcome the action energy barriers between 250 and 300 ℃.

Introduction

As a result of the continuously increasing global energy demand and limited availability of conventional oil reservoirs, extensive attention has been put on the exploitation of hard-to-produce unconventional oils (heavy oils, extra-heavy oils, bitumens, and so forth) that account for approximately 70 % of the total oil reserves [1]. Of the well-known enhanced oil recovery strategies, in-situ combustion (ISC) turns out to be an economical and efficient technology for heavy oil production [2,3].

As is widely accepted, the entire ISC process is generally divided into three main intervals, that is, low-temperature oxidation (LTO), fuel deposition (FD), and high-temperature oxidation (HTO) [[4], [5], [6], [7], [8], [9], [10]]. Among these intervals, LTO is fairly complex as a consequence of many different types of physical and chemical reactions taking place in this interval. Meanwhile, LTO is of great significance because its products are the precursors of the formed fuel that directly affects the sustainability and stability of the combustion front [9,11]. Furthermore, during an ISC process, the heat liberated at the LTO stage plays a crucial role in the quality of LTO products and the induction period required to ignite and establish a stable combustion [12,13]. It has been agreed that the oxidation behavior is mainly controlled by the reactivity of oils that is predominantly dependent on their fractions. Usually, oils are separated into four fractions, termed as saturates, aromatics, resins, and asphaltenes (SARA) [14,15]. Therefore, numerous researchers paid much effort to the investigations concerning the thermal release of LTO of heavy oils and their SARA fractions in recent decades.

Kuppe et al. [16] claimed that saturates and aromatics encountered greater heat liberated in the LTO regime relative to resins and asphaltenes, as observed from the ramped differential scanning calorimetry (DSC) profiles. Varfolomeev et al. [17] also performed the non-isothermal DSC experiments to evaluate the thermal effect of LTO of one heavy oil sample and its SARA fractions. The resulting profiles indicated that at the LTO regime, the oil generated close enthalpy change at different heating rates, but the highest enthalpy change was obtained at the lowest heating rate for SARA fractions. Recently, more researchers used high-pressure DSC (HP-DSC) to monitor the thermal release of crude oils and pure hydrocarbons during combustion, arising from the fact that this instrument could offer high-pressure conditions and the pressure generated significant influences on the heat release of oxidation reactions [[18], [19], [20], [21], [22], [23]]. The ramped HP-DSC tests conducted by our research group elucidated that the heavy oil, saturates, and aromatics experienced higher heat release produced by LTO rather than HTO, different from resins and asphaltenes [3]. Until now, most investigations on this topic focus on characterizing the oxidation behavior of crude oils under given heating rates that can simulate the dynamic heating process of the combustion front. However, the crude oils in most of the area before the combustion front were susceptible to different degrees of LTO reactions [9,24], and did not encounter a significant temperature rise in a short time because of the very slow advancing speed of fire front during an actual ISC process [25]. In other words, these oils were subjected to LTO reactions under relatively isothermal conditions over a long period of time. Therefore, it was believed that the investigations on the oxidation behavior of crude oils at the LTO stage under isothermal conditions should be of much importance. Khansari et al. [7,26] studied the mass loss of heavy oils at the LTO stage through a sequence of isothermal thermogravimetry (TG) tests, from which the LTO interval was separated into four subintervals. Nevertheless, there is a lack of studies regarding the heat release of LTO of oils and their SARA fractions under isothermal modes.

In this work, an HP-DSC instrument was utilized to explore the thermal effect produced by LTO of one heavy oil sample and its SARA fractions under isothermal modes between 50 and 350 ℃ where LTO dominated. Additionally, the LTO reaction pathways of hydrocarbons were briefly summarized, which essentially explained the phenomenon observed from the HP-DSC profiles. This study was expected to add deep insights into the heat release of LTO, which were highly valuable with regard to field applications of ISC.

Section snippets

Materials

The heavy crude oil was obtained from Xinjiang Oilfield, located in northwestern China. Table 1 lists its basic properties. The SARA fractions were separated from the oil sample according to the industrial standard of NB/SH/T 0509−2010. The chemicals used herein, including toluene, heptane, ethanol, and so forth, were all supplied by Kelong Chemical Co. (Chengdu, China) and used as received.

Isothermal HP-DSC experiments

NETZSCH DSC 204 HP Phoenix was adopted to characterize the heat release of the oil sample and its SARA

Heat release caused by LTO of heavy crude oil at different temperatures

Fig. 1 displays the isothermal HP-DSC profiles of heavy crude oil between 50 and 350 ℃. It was found that all the profiles appeared similar endothermic phenomenon at the initial period of reaction because evaporation (endothermic effect) played a more important role than oxidation reactions (exothermic effect) at this stage in terms of thermal release. The HP-DSC curves at 50, 100, and 150 ℃ only presented the endothermic activity, so evaporation was the main reaction at temperatures no greater

Conclusion

In this work, the heat release produced by LTO of the oil sample and its SARA fractions under isothermal modes was thoroughly examined. The LTO reaction pathways of hydrocarbons was summarized. The obtained conclusions can be drawn as follows.

(1) For the heavy oil, aromatics, resins, and asphaltenes, the oxidation reactions dominated the LTO stage between 200 and 350 ℃. However, regarding saturates, the oxidation reactions began to prevail at 150 ℃, which partly accounted for why the light oils

CRediT authorship contribution statement

Shuai Zhao: Conceptualization, Writing - review & editing, Methodology. Wanfen Pu: Writing - review & editing. Mikhail A. Varfolomeev: Methodology, Validation. Hao Ren: Resources. Ai Kenjiang: Resources. Dmitrii A. Emelianov: Investigation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the Russian Government Program of Competitive Growth of Kazan Federal University.

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