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Exploring Compositional Changes along In-Situ Combustion and Their Implications on Emulsion Stabilization by FT-ICR MS Fernando A. Rojas-Ruiz, Hernando Bottia-Ramirez, Lilia Rodriguez-Rodriguez, and Jorge Armando Orrego-Ruiz Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02421 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 19, 2017
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Energy & Fuels
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Exploring
Compositional
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Combustion
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Stabilization by FT-ICR MS
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Fernando A. Rojas-Ruiz,† Hernando Bottia-Ramirez, Lilia Rodríguez-Rodríguez,† Jorge A.
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Orrego-Ruiz*†
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†
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ABSTRACT
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In-situ combustion (ISC) is one of the highest potential Enhance oil recovery (EOR) process for
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heavy oils. However, several operational issues, including the formation of highly stable
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emulsions, have limited its application. Disclosing the physicochemical proprieties of these
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emulsions, especially the chemical nature of the compounds involved in the stabilization process,
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become relevant for ISC projects success. In the present work, the physicochemical changes at a
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laboratory scale low temperature oxidation (LTO) regimen performed over a Colombian heavy
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crude oil were followed by mass spectrometry. The compositional analyses were performed
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using both positive-ion atmospheric pressure photoionization (APPI) and negative-ion
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electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry
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(FT-ICR MS). Further isolation of acidic compounds and surface-active species, allowed
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detecting that the process incorporates a wide variety of compounds to build up the O/W
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(oil/water) interface and thus, increasing the emulsions stabilizing tendency. During the
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combustion, oxygen is chemically incorporated to the crude over hydrocarbon compounds, as
and
Their
Changes
along
Implications
on
In-Situ Emulsion
ECOPETROL, Instituto Colombiano del Petróleo, Piedecuesta, Santander 681018, Colombia.
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well as over sulfur and nitrogen-containing compounds, generating classes such as O, O2, O3,
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O4, OS, NO2 and NO3 that explain the high viscosity and the high stability of the emulsions.
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1.
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Heavy oils are characterized by their high viscosity and extremely complex composition, which
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set up a challenge for oil recovery, transportation and refining/upgrading processes.1 In-situ
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combustion (ISC) has been getting the attention worldwide as a promising thermal EOR
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methodology, since is thermally efficient.2-4 ISC not only increases the recovery factor, but also
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has the potential of upgrading the oil, and thereby increasing their value.2,5 During in-situ
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combustion (ISC), the increasing of temperature, and the injection of external gases into the
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reservoir prompt chemical and physical variations on the different constituents of the system
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(water, reservoir rock minerals and organic matter).5,6 Under these conditions, the susceptibility
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of heavy oils to form stable emulsions may be extremely heightened. Therefore, with the purpose
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of making feasible this EOR methodology, it is mandatory to understand the compositional
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changes along In-Situ combustion and their implications on emulsion stabilization. It is well
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known that water in oil (W/O) emulsions stability can be explained by the presence of inorganic
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solids such as clay, sandstone and corrosion products, or the action of petroleum’s natural
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surfactants.6-11 These compounds interact amid the W/O interface as they bear functional groups
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allowing them to interact both with oil and water phases. Thus, it can be considered that some of
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these surfactants can be potentially generated along the oxidation of crude oils at low
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temperature.12
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Although the asphaltenes has been acknowledged as natural surfactants and considered as the
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main emulsion stabilizers,10,11,13-16 it is more precise to describe the stabilization in emulsions
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water-crude oil in terms of ionizable species. Many compounds of the petroleum can suffer
INTRODUCTION
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ionization in the oil-water interface. Some of them are carboxylic acids (fatty acids); naphthenic
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acids and alkyl benzene acid.17-21 Although some of these species are part of the asphaltenes, it is
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more precise to say that the asphaltenes stabilize the emulsions by forming an intefacial film via
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molecular aggregation at the oil-water interface. Xi Wang et al.22 studied the interaction between
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asphaltenes and stearic acid, identyfing a decrease in oil-water interfacial tension, but that
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asphaltenes and acid did not reduce the interfacial tension separately, as the mixture shown.
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Teklebrhan et al.23 assessed the co-adsorption of naphthenic acids and polyaromatic molecules
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(asphaltenes analogs) in a toluene-water interface. Clingenpeel, et.al. showed that a continuum of
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acidic species can contribute to stable emulsion formation.24 These outcomes suggest that both
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the size and structural characteristics of acids affect the tension and the distribution of the
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molecules in the interface. Pauchard et al.25 and Muller et al.26 described a field case where a
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medium crude oil (27°API) of low TAN (0.24 mg KOH/g) and medium asphaltenes content,
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formed emulsions particularly stable. In that research was determined that the composition of the
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interfacial material corresponded mainly to linear carboxylic acids with carbon number between
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20 and 40. These compounds were also determined in the oil asphaltene fraction, which means
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that the asphaltenes-acids film was the responsible of stabilizing the emulsion under study.
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Recently Zhao et al.27 showed how the acidity of a crude under ISC varies as a function of the
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oxidation temperature and the reaction time. They found that there is not a linear relation
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between the total acid number (TAN) and the progress of the process at high temperatures (up to
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320°C), as a consequence of the generation and decomposition of carboxylic acids at different
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ISC stages. The oxidized oils were characterized by negative ESI FT-ICR-MS finding structural
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changes in classes O2 and O1 that were further associated with the oil properties. In a different
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work, the same group found that the acidity of a light crude at Low-Temperature Oxidation
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increased with temperature.28 In this case, the temperature was up to 200°C and the TAN
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increased up to 7.29 mg KOH/g as a function of oxygen consumption. It is well known that the
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carboxylic acids may act as W/O emulsion stabilizers.9,13,25,26 In a previous work, this
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predominant contribution of naphthenic acids (CcHhO2) to the total acidity of a crude oil was
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confirmed by FT-ICR MS.29 However, it was also noticed that additional species (CcHhNO2,
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CcHhSO2, CcHhO3, and CcHhO4) detected by (−) ESI can markedly affect TAN. This means
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that an increasing TAN implies an increase in the concentration of a wide variety of acidic
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species.
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In the present work, in an attempt to understand the emulsion stability of fluids produced from an
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ISC process and to emulate field scenarios, five samples were assessed rheologically and
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characterized by FT-ICR MS to evaluate the compositional changes along In-Situ Combustion
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and how this can affect the emulsion stabilization.
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2. EXPERIMENTAL DEVELOPMENT
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2.1. Materials
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The crude oil chosen for this study is a heavy crude oil from one of the most important fields in
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Colombia in terms of production and reserves. The main properties for this sample are included
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in Table 1.
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Table 1. Crude oil general characterization Property API at 15°C C H N S
Value 9.43° 84.8%wt 10.4%wt 0.55%wt 3.34%wt
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Saturates Aromatic Resins Asphaltenes TAN
12.9% 40.9% 19.4% 17.3%
