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In this work, the pressure- and temperature-dependent reaction rate constants for the hydrogen abstraction and addition of hydroxyl radicals to the unsaturated cyclopentene were studied. Geometries and vibrational frequencies of reactants, products, and transition states were calculated using density functional theory, with single-point energy corrections determined at the domain-based local pair natural orbital-coupled-cluster single double triple/cc-pVTZ-F12 level. The high-pressure limit rate constants were calculated using the canonical variational transition state theory with the small-curvature tunneling approximation. The vibrational partition functions were corrected by the effects of torsional and ring-puckering anharmonicities of the transition states and cyclopentene, respectively. Variational effects are shown to be relevant for all the hydrogen abstraction reactions. The increasing of the rate constants by tunneling is significant at temperatures below 500 K. The pressure dependence on the rate constants of the addition of [Formula: see text] to cyclopentene was calculated using the system-specific quantum Rice–Ramsperger–Kassel model. The high-pressure limit rate constants decrease with increasing temperature in the range 250–1000 K. The falloff behavior was studied at several temperatures with pressures varying between 10−3 and 103 bar. At temperatures below 500 K, the effect of the pressure on the addition rate constant is very modest. However, at temperatures around and above 1000 K, taking pressure into account is mandatory for an accurate rate constant calculation. Branching ratio analyses reveal that the addition reaction dominates at temperatures below 500 K, decreasing rapidly at higher temperatures. Arrhenius parameters are provided for all reactions and pressure dependent Arrhenius parameters are given for the addition of [Formula: see text] to cyclopentene.

The phase behavior of petroleum fluids under reservoir conditions provides fundamental information for an adequate crude oil production scheme. Recently, complex phase transitions have been studied...

Detailed local and averaged measurements of wax deposition in an annular pipe section were conducted and combined with a comprehensive simulation model. The objective of the study was to contribute...

Multicomponent mixtures may be considered complex because of the overwhelming number of sample constituents and their broad range of physical–chemical properties. Such features translate into chall...

International audience

International audience; In this work, phase behavior was analyzed for recombined mixtures of a Brazilian pre-salt crude oil with carbon dioxide and methane. This oil was sampled at stock tank conditions during a well-test, and mixtures with gas were prepared with a gas molar composition ranging from 20.0 to 86.0 mol %. Their phase behavior was investigated from 293.15 to 378.15 K and up to 100.0 MPa. Direct phase transition detections were achieved by using a full visibility PVT cell, coupled with a short-wave infrared camera. At this wavelength, crude oil has a lower absorbance, and it has been noted that macroscopic phase transitions can be easily detected by visual inspection. For the pseudo-binary system crude oil + carbon dioxide systems, liquid–liquid phase transitions were observed when the gas content is higher than 70.0 mol %. In addition, at the higher carbon dioxide composition, an asphaltic phase was formed at high pressures together with liquid–liquid phases. Similarly, crude oil and methane systems presented a liquid–liquid immiscibility region at methane composition above 60.0 mol % in all temperature domain studied. The Peng–Robinson equation of state was used for modeling purposes, and liquid–liquid and liquid–liquid–vapor transitions were qualitatively described. Additionally, it was observed that liquid–liquid equilibria behavior was highly dependent on the crude oil heavy fraction immiscibility because of the increasing system asymmetry by increasing mixture gas content.

Alkanes are a fundamental part in empirical force fields (FF) not only due to their technological relevance, but also due to the prevalence of alkane moieties in organic molecules, e.g., compounds containing a saturated carbon chain. Therefore, a good description of alkane interactions is crucial for determining the quality of a FF. In this study, the performance of 12 empirical force fields (FF) was evaluated in the context of reproducing liquid properties of alkanes. More specifically, n-octane was chosen as a reference compound since it is a liquid in a broad temperature range and it has numerous experimental data for thermodynamic, transport, and structural properties, as well as for their temperature dependencies. A normalized root-mean-square deviation (NRMSD) analysis was used to rank the force fields in their ability to reproduce the experimental data. Five out of the six best force fields considered were united-atom models. The GROMOS force field showed the smallest deviation in terms of NRMSD, followed by TRAPPE-EH, NERD, CHARMM-UA, TRAPPE-UA, and OPLS-UA. This overall better performance of the united-atom force fields indicates that complexity does not always bring quality.

An efficient strategy for developing a petroleum reservoir as well as designing a robust production system depends on the knowledge of the phase behavior of the fluids to be produced. Enhanced oil ...

A wide variation in composition and thermodynamic properties is expected for different reservoir fluids, from simplest one containing only methane and light compounds, to those with high compositional complexity. In this work, phase behavior of six different reservoir fluids recombined with methane was compared and contrasted. From our past work [Romero Yanes et. al. (2019), DOI:10.1016/j.fuel.2019.115850], a low asphaltene crude oil (28.0 °API, 0.68 wt % n-C7 asphaltenes), here referred as BRO, has presented a complex phase behavior when mixed with methane, especially when methane content is around 75.0 mol%. Based on these results, it was studied in this work the phase behavior of six different crude oil mixed with 75.0 mol% of methane. Samples main characteristics can be summarized as follows: a light condensate fluid (P1, 44.0 °API, and not detected n-C7 asphaltenes), three medium crude oils [(P2, 30.8 °API, and 0.09 wt % n-C7 asphaltenes); (P3, 26.0 °API, and 0.54 wt % n-C7 asphaltenes); and (P4, 25....

The thermal conductivity of even carbon number n-alkanes from n-dodecane to n-dotriacontane (C22H46 to C32H66) was measured in both the liquid and solid phases, in the temperature range from 297.15 to 353.15 K at 0.1 MPa. To assure the purity and map the solid phase of the different samples during the thermal conductivity measurements of the solid samples, the melting point, solid-solid transition temperature and their respective enthalpies were also evaluated. The thermal conductivity measurements for the liquid and solid samples were obtained within uncertainty levels better than 3% and 5%, respectively. To the best of our knowledge, this is the first report on the thermal conductivity of liquid n-hexacontane, n-octacontane, n-triacontane and n-dotriacontane, and, for solid samples, from n-docosane to n-dotriacontane.

DSC and wax solubility curves were measured for two diesel fuel samples. Compositional profiles were also measured for the solubility curve points. The paraffin distributions of the samples were determined and the non-paraffinic components were lumped into pseudocomponents. The liquid phase was modeled using the Peng-Robinson equation of state with the classical van der Waals mixing rule. For the solid phase, the modified UNIQUAC model was employed. The models were fitted to the DSC data by tuning only four multipliers acting on the molecular weights, boiling and critical temperatures of the pseudocomponents as well as on the paraffins’ pair interaction energies from the UNIQUAC model. Then the model is used to predict wax appearance temperature, amount and composition of solid deposit as a function of temperature during cooling. Excellent results were obtained for the two samples analyzed. This way, the methodology developed is able to predict adequately the formation of solid paraffin from diesel fuels.

This study aimed to use different types of ion source gases [synthetic air, nitrogen (N2), and helium (He)] to compare the ionization efficiency of linear, branched, and cyclic hydrocarbon (HC) standards (i.e., hexatriacontane, squalene, and 5-α-cholestane, respectively), condensed aromatics (CA) standards (i.e., coronene, benz[a]anthracene, and n,n′-bis(3-pentyl)perylene-3,4,9,10-bis(dicarboximide)), one n-paraffin standard, containing carbon numbers ranging from C5 to C120, and two saturated HC fractions. In all cases, a positive-ion mode atmospheric pressure chemical ionization coupled to a Fourier transform ion cyclotron resonance mass spectrometer was used. Isooctane reagent was used to facilitate the ionization of n-paraffins. Three ionization mechanisms were observed: electron transfer ([M]rad+), proton transfer ([M+H]+), and hydride abstraction ([M−H]+). For the ionization of HC standards, synthetic air and He gases presented better ionization efficiency and produced mass spectra with greater mass accuracy and signal-to-noise rate. Moreover, linear HCs were preferentially ionized through hydride abstraction (production of [M−H]+ ions), whereas the ionization of unsaturated and cyclic HCs mostly occurred through the production of [M+H]+ and [Mrad+] species. The unique exception is related to 5-α-cholestane, which is ionized as [M−H]+ in the presence of synthetic air. For the CA standards, N2 and synthetic air promoted the detection of CA standards mainly by electron transfer mechanism, [M]rad+ species. Conversely, He favored the proton transfer ionization ([M+H]+) with minimal fragmentation or oxidation of the analyte. In all cases, synthetic air provided mass spectra with excellent signal-to-noise ratio. This performance was attributed to the high reactive-ionizing power of O2 gas over the HC and CA molecules. For the n-paraffin samples, synthetic air and He provided better ionization performance through hydride abstraction ([M−H]+). N2 favored the production and ionization of heteroatomic compound classes (Ox and NOx).

Based on Bairds rule, which states that the excited state of an aromatic compound is antiaromatic, a new method for hydrogenating aromatic molecules by metal-free photochemical reaction employing 1,4-cyclohexadiene (1,4-CHD) as hydrogen source is proposed. Here only light and 1,4-CHD are required to hydrogenate aromatic compounds without metal catalysts or temperature. The proposed mechanism is related to the reduction potential of the aromatic probes on their first triplet excited state. In such methodology, besides the non-aromatic character of the excited state, the aromatization of the hydrogen donor turns this reaction feasible as a hydrogen storage procedure at low cost, expanding the chemical interest for materials like graphene.

Experimental phase equilibrium data were measured for systems containing a mixture of heavy n-alkanes (C22H46, C23H48 and C24H50) and a light n-alkane (C12H26) in the presence of 20, 40, 60 and 80% mole fraction of CO2. The experiments were carried out in the range of 290–353 K and up to 70 MPa. The measured phase equilibrium data presented regions of vapor-liquid, solid-liquid and solid-liquid-vapor coexistence. To model the data obtained, the solid phase was described using the multiple solid solution approach, which employs the modified UNIQUAC equation to calculate the activity coefficients. The Poynting correction factor was used to take into account the pressure effect in the solid-liquid equilibrium. The liquid and vapor phases were described using the Peng-Robinson equation of state with classical mixing rule. Further, a correlation was developed for the calculation of the molar volumes difference between the liquid and the solid phases. The phase equilibrium predictions were in good agreement with the measured phase transitions data, indicating that the set of models employed is adequate to describe the phase equilibrium behavior of mixtures containing paraffins and CO2 at low and high pressures.

The detailed understanding of the combustion process differences between saturated and unsaturated hydrocarbons is of paramount importance for the rational design of more efficient fuel blends. In this work, extensive ReaxFF molecular dynamics simulations have been employed to characterize and differentiate the initial combustion profiles of cyclopentane and cyclopentene. The main goal is the comprehensive identification of the main intermediates and reaction channels. The results show that, while homolytic carbon-carbon bond cleavage constitute the main initiation pathway, the rate of such type of elementary reaction depends highly on the internal energy content of the system. Thus, towards lower temperatures, hydrogen abstraction by dioxygen and other radicals become more prominent. The presence of a single unsaturation leads to a greater number of distinct points of attack on cyclopentene, but such feature does not implicate in a faster decay of the olefin during combustion. A pool of radicals, especially HO · and HO 2 · , was found to be present over the entirety of both processes, contributing to chain propagation and branching. Therefore, a cogent picture of the similarities and differences between the combustion process of an unsaturated and a saturated hydrocarbon is presented and discussed.

Heavy oil fractions studies are of great importance for the oil industry, receiving particular attention in recent decades due to interest in obtaining compounds with higher added value derived from heavy oil fractions. Moreover, these studies also aim to offer solutions to problems caused by these fractions during all oil processing steps. Asphaltenes placed in n-methylpyrrolidone (NMP) produce two fractions with distinct characteristics, soluble and insoluble in NMP (called NS and NI, respectively). In previously studies it was suggested that the NI fraction presents low content of aromatic compounds, opening a new debate on the composition of asphaltene as is widely discussed in the literature. Here, we show the deep characterization of three set of asphaltenes fractions obtained in NMP using Fourier transform ion cyclotron resonance mass spectrometry coupled to positive and negative-ions mode electrospray ionization (ESI(±)-FT-ICR MS), Fourier transform infrared spectroscopy (FTIR), molecular fluorescence and p roton nuclear magnetic resonance ( 1 H NMR) spectroscopy. FTIR, 1 H NMR and molecular fluorescence demonstrated differences among asphaltene fractions studied, in which, NS fractions exhibited a more polar and aromatic character. On the other hand, NI fractions showed nonpolar profile, being proved by the presence of high amounts of alkyl and naphthenic hydrocarbons showing high abundance of alkyl and naphthenic hydrocarbon related compounds. ESI(±)FT-ICR MS data corroborate the FTIR, 1 H NMR and molecular fluorescence data, which show NS fractions with a high proportion of heteroatomic species and higher aromaticity (higher DBE values) compared with NI fractions.

Thermal conductivities of wax deposits formed on the cooled wall of a channel were measured under flowing conditions, for different values of the Reynolds number (Re) in laminar flow. To this end, a laboratory-scale experiment was especially designed, employing a rectangular channel flow loop. The thermal conductivities were directly measured for steady-state conditions in deposits formed after 7 h of deposition. The cooled wall on which the deposits were formed was equipped with embedded heat flux and thermocouple sensors. A micrometer-driven temperature probe of small dimensions was installed at the opposite channel wall and could be traversed across the channel, yielding the temperature profile of the deposited wax layer, as well as the liquid/deposit interface temperature. Deposit compositions were obtained by high-temperature gas chromatography. Thickness-averaged deposit thermal conductivities were obtained from the deposit thickness, heat flux, and interface-to-wall temperature measurements. The th...

ACL; International audience; Wax deposition receives increasing attention as oil industries moves towards hostile environments. Production in deep water scenarios and even the Artic pushes the boundaries of flow assurance, especially those related to wax precipitation and deposition. Thus, modeling wax phase behavior becomes central to minimize production costs. Although different thermodynamic models are available in the literature, only the solubility curve, either entirely or only its first point, the wax appearance temperature, are used to differentiate them. It is alternatively proposed to compare these models against DSC thermograms. The results show that a liquid model based on Peng-Robinson EoS using van der Waals mixing and combining rules together with a solid model based on the modified UNIQUAC scheme can reproduce the experimental thermograms for the studied systems. As EoS's are widely used by oil companies to describe live oil fluid behavior, the proposed scheme can be readily applied to real fluids, reducing uncertainties associated to model tuning. Moreover, we show that the multiple solid solutions model, using modified UNIQUAC, presents a better description of solubility and calorimetric data when compared with the multisolid and ideal solid solution models. © 2017 Elsevier B.V.

ACL; International audience; Among the flow assurance subjects, wax deposition plays a major role. As exploration an oil production moves towards deeper water frontiers, longer tiebacks increase the risk of wax deposition, which is assessed through modeling. In modeling schemes, the phase behavior of solid wax is essential and it is available through either a measured solubility curve or through thermodynamic models. These thermodynamic models normally need tuning and, due to the experimental difficulties of measuring solubility curves for real fluids, are tuned to the wax appearance temperature (WAT). To reduce the uncertainties of adjusting a model to just one point, a procedure to directly simulate DSC curves from a liquid-solid thermodynamic model is proposed. This new strategy allows a full thermogram adjustment without going through the empirical integration of the experimental DSC. Experimental DSC thermograms and simulated curves are presented for five standard single wax mixtures. Then, the solubility curves obtained from the model are compared to the experimental literature data and the difference between simulated wax disappearance temperature (WDT), experimental WDT and experimental wax appearance temperature (WAT) are discussed. © 2017 Elsevier B.V.

Pre-salt reservoirs are located in ultra-deep Brazilian waters. The petroleum production from these reservoirs is characterized by a high gas-oil ratio (GOR) and high content of CO2 in the associated gas. The challenges for production in such the pre-salt reservoir conditions include understanding the unusual phase behavior observed in the laboratory from field samples. In this paper, we discuss the phase equilibrium data and thermodynamic modeling of the systems containing CO2/n-C16H34, CH4/CO2/n-C16H34 and CH4/CO2/n-C16H34/C14H10. Transitions observed during this study were liquid-liquid (LL), vapor-liquid (VL), and vapor-liquid-liquid (VLL). The phase behavior investigated includes equilibrium conditions observed at temperatures from 293.1 K to 353.1 K and at pressures up to 58 MPa. The Peng-Robinson equation of state with classical van der Waals mixing rule was used to correlate the obtained experimental data. Results show a complex phase behavior for the binary system CO2/n-C16H34 at 298 K, featuring two liquid-liquid regions and barotropic inversion near the VLL transition. The addition of methane to the previous system (1/1 mol ratio relative to CO2) induces a phase behavior alteration in which the liquid-liquid transition disappears. The addition of a polyaromatic component (phenanthrene), whose influence over the mean molecular weight of the mixture under test is little, increased the saturation pressure. This study contributes to show that the use of a model representing the live-oil is a good strategy to understand how chemical and physical-chemical characteristics can induce complex phase behaviors, such as those observed in reservoirs with high carbon dioxide content.

The intrinsically elusive concepts of electronic “delocalization” and “chemical resonance” are briefly reviewed emphasizing their connection with Spin-Coupled (SC) descriptions of electronic structure. Multiconfiguration Spin-Coupled (MC-SC) calculations are performed to describe the three-center two-electron (3c-2e) bonding in some representative carbenium and nonclassical carbonium ions. Within the MC-SC approach, it is found that these cations present significant electronic energy stabilization when described by more than one valence SC spatial orbital configuration. It is shown that it is necessary to have a superposition of two chemical structures to completely span the orbital valence space of these cations. Two characteristic bonding themes are clearly distinguished. One specific to allyl-type carbenium ions and another specific to the nonclassical carbonium ions. In both situations, the 3c-2e bond is described by two chemical structures. The 3c-2e bond present in these carbocations is described clearly within this conceptual framework. The results point out for the robustness of the Spin-Coupled description in yielding a general picture of bonding, even when considering valence-bond type multiconfiguration effects.

The analysis of hydrocarbon using atmospheric or ambient ionization techniques still remains a challenge in mass spectrometry. Traditionally, the ionization occurs via protonation or deprotonation and thus the molecules of interest must have a basic or acidic group to generated [M+H]+ or [M−H]− ions. To overcome such limitation, it is proposed a simple, easy, fast and powerful analytical methodology to ionize saturated (linear and branched), unsaturated, and cyclic hydrocarbons as well as polyaromatic hydrocarbons by atmospheric pressure chemical ionization (APCI) using small hydrocarbons as reagents in a FT-ICR mass spectrometer. These molecules may be present in hydrocarbon fraction samples and paraffin/crude oil blends. Among the APCI hydrocarbon reagents, isooctane provided the best results when compared to pentane, hexane, cyclohexane and heptane. The method renders the ionization of hydrocarbons to yield [M−H]+ ions with no associated fragmentation using nitrogen as sheath gas.

N-Heptane and 2,2,4-trimethylpentane (isooctane) are the key species in the modeling of ignition of hydrocarbon-based fuel formulations. Isooctane is knock-resistant whereas n-heptane is a very knock-prone hydrocarbon. It has been suggested that interconversion of their associated alkylperoxy and hydroperoxyalkyl species via hydrogen-transfer isomerization reaction is the key step to understand their different knocking behavior. In this work, the kinetics of unimolecular hydrogen-transfer reactions of n-heptylperoxy and isooctylperoxy are determined using canonical variational transition-state theory and multidimensional small curvature tunneling. Internal rotation of involved molecules is taken explicitly into account in the molecular partition function. The rate coefficients are calculated in the temperature range 300-900 K, relevant to low-temperature autoignition. The concerted HO2 elimination is an important reaction that competes with some H-transfer and is associated with chain termination. Thus, the branching ratio between these reaction channels is analyzed. We show that variational and multidimensional tunneling effects cannot be neglected for the H-transfer reaction. In particular, the pre-exponential Arrhenius fitting parameter derived from our rate constants shows a strong dependence on the temperature, because tunneling increases quickly at temperatures below 500 K. On the basis of our results, the existing qualitative model for the reasons for different knock behavior observed for n-heptane and isooctane is quantitatively validated at the molecular level.

Asphaltenes analysis still remains a challenge due to their unknown molecular structure and self-associative behavior. FT-ICR MS analyses using five different ionization methods, ESI, APCI, APPI, LDI and MALDI, in both positive and negative mode were performed. A characteristic and typical behavior was observed for the experiments using LDI(±) and MALDI(±)FT-ICR analyses: a broadband mass spectrum ranging from m/z 500 to 3000 was observed with a 24 Da interval. This phenomenon was attributed to fullerene cluster formation. The most abundant allotropic form of fullerene, C 60 , was observed at m/ z 719.99914. These species were not observed in APCI(+)/APPI(+) analyses. This last analysis of asphaltene samples doped with the C 60 solution promptly reveals the typical fullerene ions. LDI(+) analyses of pure C 60 showed condensation products. These results indicate a clusterization of asphaltenes in the LDI/MALDI plume, which showed a highly dependency on the laser power density. The dispersion of LDI results on carbonaceus samples in the literature is associated with the lack of control of these parameters. However, to access the sample composition, APPI and APCI should be more suitable.

Although the term “naphthenic acids” was originally used to describe acids that contain naphthenic rings, today this term is used in a more general sense and refers to all components in the acid extractable fraction. In crude oil, naphthenic acids exist as a complex mixture of compounds with broad polydispersity with respect to both molecular weight and structure. There has been increasing interest in the naphthenic acids in crude oil because of the corrosion problems that cause during oil refining. Herein, two powerful analytical tools, negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry, ESI(-)FT-ICR MS and atomic force microscopy (AFM), were combined to monitor the thermal degradation of naphthenic acids and their corrosion effects on AISI 1020 steel, respectively. Two different acidic crude oils (TAN = 2.38 and 4.79 mg KOH g −1 , and total sulfur = 0.7993 and 1.0220 wt%) have been submitted to thermal treatment at 280, 300 and 350 °C during 2, 4 and 6 h, and characterized by ESI(-)-FT-ICR MS, total acid number (TAN), and total sulfur. The AISI 1020 steel was analyzed by scanning electron microscopy (SEM) and AFM. Generally, heating the crude oil at 350 °C in a period of 6 h, it was observed that a high efficiency (≅80%) and selectivity of thermal decarboxylation process was monitored by decay of TAN (4.79 → 0.44 mg KOH g −1 ). ESI(-)-FT-ICR MS results showed that naphthenic acid species remained after the heating have DBE ranging 1–12 and carbon number from C 15 to C 45 . AFM topographic profile evidenced that the naphthenic acid corrosion of the crude oil with TAN of 4.73 mg KOH g −1 on AISI 1020 steel was profoundly altered and a marked reduction in peak to peak height values (obtained by subtracting the value of the lowest peak by the highest peak in the topographic area examined). Optical images and microphotographs confirmed the presence of irregularities, characterizing the corrosion mechanism as pitting type. The naphthenic corrosion was also evidenced in samples with low TAN value (0.44 mg KOH g −1 ).

Asphaltenes can be considered like the ‘bad guys‘ when heavy petroleum are transported or upgraded. These compounds are not classified by the their chemical structures but by the solubility class, that is defined as solubles in aromatic solvents, such as toluene, and insolubles in n -alkanes, such as n -heptane. Here, we analyze three crude oil samples (classified as extra-heavy, heavy and asphaltic having API degree of 17.3, 22.0 and 13.5, respectively) and their asphaltenes by atmospheric pressure photoionization (APPI) coupled to Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in positive and negative ionization modes, APPI(±)-FT-ICR MS. We apply the DBE (double bond equivalent) versus carbon number (CN) plots as the aim of to predict the aromaticity of crude oil and their asphaltenes. Additionally, average molecular weight, M w , heteroatomic-containing compounds profile and van Krevelen diagrams were constructed to visualize and interpret the MS data. MS data were compared and discussed with aromaticity parameters obtained from elemental analyses and 1 H NMR measurements. For three crude oils analyzed with different magnitude of the viscosities (III > II > I), the amount of the asphaltene extracted follower the behavior analogous (7.63 ± 0.65 wt%; 2.23 ± 0.25 wt%; and 0.41 ± 0.01 wt%, respectively). The aromatic hydrogen content ( H ar , in molar%) was determined from 1 H NMR, where the heavier crude oil and its asphaltene ( H ar = 4.90 and 27.3 M%, respectively) were slightly more aromatic than other. An inverse correlation was observed between H ar values and the N and O concentrations determined from elemental analyses. For APPI(±)FT-ICR MS data, the values of M w observed for asphaltenes ( m / z 200–650) were significantly distinct, being correlated with the physico-chemical characteristics of their parent crude oil. Using the concept of planar limited, the line generated by connecting maximum DBE values at given CN in the DBE versus carbon number plots, it was possible to extract the aromaticity degree from APPI FT-ICR MS data. The slopes of these lines determined by DBE/carbon number ratio were calculated by linear regression for protonated hydrocarbons (HC[H]) and basic nitrogen (N[H]) compounds classes. We observed an increase in slopes in function of an increasing of the H ar values.

Phase behavior of reservoir fluids is crucial for an efficient production development project. In this paper, a new dataset of phase transitions of Brazilian pre-salt crude oil mixed with methane systems is presented and phase behavior discussed under different analytical technics. A series of mixtures of a dead crude (API 28.0, 0.68 wt% of asphaltenes) with 65.0, 67.5, 70.0, 72.5, and 75.0 mol% of methane were studied by constant mass expansion in a PVT cell, coupled with a high-pressure microscopy (HPM). Pressure-volume data shows slight phase transition, with a not evident break due to difference on compressibility, specially by increasing methane molar content. For this reason, beside of visual observation, a near-infrared (NIR) transmittance was used for a proper identification of bubble point pressure. NIR transmittance also allow the detection of phase transitions above bubble point, that was confirmed by intermediate of by HPM analysis. A dispersion of fine particles was detected, and it was associated with asphaltenes onset, for the systems with high content of gas (72.5 and 75.0 mol% of methane). NIR transmittance along with micrographs taken during system pressurization confirm a low asphaltene hysteresis redissolution, with no flocs and mayor aggregates formation and rapid redissolution after system pressurization. This phenomenon could be related to the low content of asphaltenes on the crude oil sample.

Protonated methane, CH5+, is a key reactive intermediate in hydrocarbon chemistry and a borderline case for chemical structure theory, being the simplest example of hypercoordinated carbon. Early quantum mechanical calculations predicted that the properties of this species could not be associated with only one structure, because it presents serious limitations of the Born−Oppenheimer approximation. However, ab initio molecular dynamics and diffusion Monte Carlo calculations showed that the most populated structure could be pictured as a CH3 tripod linked to a H2 moiety. Despite this controversy, a model for the chemical bonds involved in this ion still lacks. Here we present a modern valence bond model for the electronic structure of CH5+. The chemical bond scheme derived directly from our calculations pictures this ion as H3C···H2+. The fluxionality can be seen as the result of a proton transfer between C−H bonds. A new insight on the vibrational bands at ∼2400 and ∼2700 cm-1 is suggested. Our results sh...

Abstract Paraffinic fractions cause two mostly known problems under deepwater cold environments: wax deposition and gelation. These two topics have been addressed for a long time and industry design standards are well established today. However field operating experience and R&D results have shown that there is room for improving design standards. This paper will present a few aspects on this subject. The first one concerns determining yield stress for gelled oil, a major concern for projects with high wax content crudes. The usual procedure may be overly conservative thus leading either to high pressure pipeline rating or excess of chemical consumption (PPD). A new approach is presented in order to enable a more accurate estimate of the stress needed to break the gel and flow restart. Regarding wax deposition the general rule is to design the system so to keep the minimum flowing temperature above WAT. It is common to allow the flow slightly below WAT with no deposition, provided the wax solids amount is low enough, as will be presented here. Even so this solution may not be economical for some cases, such as a small deepwater field located northeast Brazil. The project was made to cope with wax deposition by pigging. Operation experience has shown that deposition rate was negligible despite the oil being waxy and flowing well below WAT. This case is analyzed in this paper and an explanation for this behavior is proposed. In general increasing production flow rate raises stream temperature so reducing wax deposition and other flow assurance issues. However a case history is presented where, given fluid characteristics (high GOR) and riser geometry, decreasing flow rates leads to increasing fluid temperatures. Introduction Flow assurance problems caused by wax fractions are known for a long time and the deposition mechanisms were well established since Burger et al (1981). Despite this fact, wax related problems are still challenging industry in 21st century. Considering Brazilian environment, the discovery of Pre-Salt areas brought back problems that were supposedly already solved. It comes from two facts that together bring difficulties for the flow assurance project. The first one is the Pre-Salt oils wax contents which are much higher than that usually observed in oils produced from zones above the salt layer. The second point is the typical reservoir temperature at the Pre-Salt, lower than at other areas. These two facts mean oils with higher WAT coming from reservoirs at lower temperatures, thus lowering the margins for temperature drop. Besides that, there is still the higher amount of gas (or GOR) that causes fluid cooling while decompressing in upward flow in the risers. This situation requires thicker thermal insulation layer in the flow lines and bottom riser, considering that at riser top the thermal insulation is not effective to prevent temperature drop. This shorter margin between reservoir temperature and WAT requires a more careful heat transfer specification and reducing as much as possible any clearance in the WAT determination. Oil gelling is a problem that used to happen in some onshore waxy oil fields, located in state of Bahia, Brazil. However these fields are located at a hot environment area and gelling is restricted to specific cases during the winter. Once again some Pre-Salt oils have a tendency to form strong gel if cooled down to temperatures typical of deepwater regions. This adds a new issue to be addressed in order to allow production of such fields.

Modem valence bond (VB) theories such as Spin-Coupled theory, together with DFT and Molller-Plesset MO methods, and ab initio molecular dynamics, were employed to study structure/dynamics in representative carbonium ions.