ON THE MASS BALANCE OF ASPHALTENE PRECIPITATION

In the evaluation of experimental data as well as in calculation of phase equilibria the necessity of the application of mass balances is obvious. In the case of asphaltenes the colloidal nature of these compounds may highly affect the mass balance. In the present paper several experiments are performed in order to check the consistency of mass balances within asphaltene precipitation. Asphaltenes are precipitated in two step processes either by changing temperature or by changes in precipitant with increasing precipitation power. This has been performed for three different oils. The data indicates that in temperature experiments as well as in solvent series experiments the precipitation of heavy asphaltenes affects the following precipitation of lighter asphaltenes. In both cases the mass balance using standard separation techniques cannot be closed, as less material is precipitated in a two step process than in the direct process either at low temperature or by direct precipitation with one precipitant. The different fractions were subject to HPLC size exclusion chromtagraphy showing that the material remaining in solution in the stepwise process was of low molecular weight, and that the material in the second precipitation step was often of higher apparent molecular weight and had an increased overall absorbance coefficient.     

Variation in Composition of Subfractions of Petroleum Asphaltenes

Boscan asphaltenes were precipitated from the crude oil using mixtures of toluene and heptane at temperatures of 24, 50, and 80°C. Another process of extracting solid n-heptane asphaltenes (24°C) using the same solvent systems and temperatures was also investigated. Asphaltene yield is different by the two processes at similar conditions although both increases in temperature or toluene content lead to lower solid yield. This way the asphaltene continuum was investigated from incipient flocculation to total n-heptane precipitated asphaltenes. The asphaltenes were analyzed using elemental analysis, fluorescence spectroscopy, vapor pressure osmometry, and HPLC size exclusion chromatography. The distribution of porphyrins was also measured using absor-bance spectroscopy. The asphaltenes were found to follow a regular trend in elemental ratios, H/C and N/C, indicating increased aromaticity and nitrogen content as the solid yield decreases. Also the molecular weight was seen to increase. Size exclusion chro-matograms and fluorescence spectra were found to be different comparing solids from the two separation processes. This indicates that the asphaltene fractions obtained by extraction of solid asphaltenes are altered relative to asphaltenes obtained by ordinary precipitation. The porphyrin concentration was found to diminish rapidly with solid yield decrease in both precipitation and extraction experiments, the latter fractions, however, containing significantly less asphaltenes indicating an adsorption step in the coprecipitation of porphyrins. Soluble fractions were found to exhibit relatively low molecular weights and an apparent lack of indications of association up to a point of solubles exceeding 50 % of the total asphaltenes. The-latter have implications for the further understanding and experimental investigation. of the associating nature of asphaltenes such as concentration effects during analytical characterization.     

Variation in Composition of Subfractions of Petroleum Asphaltenes

ABSTRACT

Boscan asphaltenes were precipitated from the crude oil using mixtures of toluene and heptane at temperatures of 24, 50, and 80°C. Another process of extracting solid n-heptane asphaltenes (24°C) using the same solvent systems and temperatures was also investigated. Asphaltene yield is different by the two processes at similar conditions although both increases in temperature or toluene content lead to lower solid yield. This way the asphaltene continuum was investigated from incipient flocculation to total n-heptane precipitated asphaltenes. The asphaltenes were analyzed using elemental analysis, fluorescence spectroscopy, vapor pressure osmometry, and HPLC size exclusion chromatography. The distribution of porphyrins was also measured using absor-bance spectroscopy. The asphaltenes were found to follow a regular trend in elemental ratios, H/C and N/C, indicating increased aromaticity and nitrogen content as the solid yield decreases. Also the molecular weight was seen to increase. Size exclusion chro-matograms and fluorescence spectra were found to be different comparing solids from the two separation processes. This indicates that the asphaltene fractions obtained by extraction of solid asphaltenes are altered relative to asphaltenes obtained by ordinary precipitation. The porphyrin concentration was found to diminish rapidly with solid yield decrease in both precipitation and extraction experiments, the latter fractions, however, containing significantly less asphaltenes indicating an adsorption step in the coprecipitation of porphyrins. Soluble fractions were found to exhibit relatively low molecular weights and an apparent lack of indications of association up to a point of solubles exceeding 50 % of the total asphaltenes. The-latter have implications for the further understanding and experimental investigation. of the associating nature of asphaltenes such as concentration effects during analytical characterization.     

原油正构烷烃沥青质聚沉机理研究及沉淀量测定

用IP 143标准方法测定了我国孤岛和草桥原油正构烷沥青质沉淀量。结果表明两种原油的沥青质沉淀量均随沉淀剂分子量增大而减小、随剂油比增大而增大。在原油沥青质 胶质胶束模型的基础上提出了一种新的沥青质聚沉机理 ,该机理的基本假设是原油中沥青质分子以胶束形式存在 ,其中胶核为沥青质缔合物 ,溶剂化层为胶质和溶剂分子。通过分析沉淀剂性质、剂油比、体系温度和压力等对沥青质 胶质胶束稳定性的影响得出了沥青质沉淀点、沉淀量、沉淀物平均分子量以及沉淀物平均颗粒大小随沉淀剂性质和剂油比等因素变化的规律。经比较说明 ,这些规律与本文及文献实验结果相符     

SEPARATION OF ASPHALTENES BY POLARITY USING LIQUID-LIQUID EXTRACTION

ABSTRACT

In order to investigate the nature of petroleum asphaltenes in terms of polarity a process was developed using initial liquid-liquid extraction of the oil phase followed by precipitation of the asphaltenes using n-heptane. The liquid-liquid extraction was performed using toluene-methanol mixtures with increasing content of toluene. Although large fractions of the crude oil (Alaska '93) was extracted in the higher polarity solvents (high concentration of methanol), the asphaltene content of the dissolved material was low As the toluene content increased more asphaltenes were transferred to the solvent phase. The asphaltenes were analysed using FTir, Elemental analysis, and HPLC-SEC with a diode array detector. With increasing content of toluene in the methanol the molecular weight distribution of the asphaltenes significantly move to higher molecular weights. The content of nitrogen and sulfur of the mallene phase also increase while H/C decreases. The content of heteroatoms in the asphaltenes are relatively higher and apparently increase with the polarity of the solvent. It is concluded that these asphaltenes are indeed dominated by high molecular weight substances that cannot be extracted in the high polarity solvents     

INVESTIGATION OF ASPHALTENE PRECIPITATION AT ELEVATED TEMPERATURE

In order to obtain quantitative data on the asphaltene precipitation induced by the addition of n-alkane (heptane) at temperatures above the normal boiling point of the precipitant, a high temperature/ high pressure filtration apparatus has been constructed. Oil and alkane are mixed at the appropriate temperature and the pressure in closed vessels keeping the mixture at the liquid state. The filtration is performed with a small differential pressure over the filter so as to avoid flashing the mixture. The technique requires a low dead volume in the system to minimize the content of maltenes in the extracted fraction, hence there is no room for stirring. The equipment as well as solutions to some of the problems are presented along with precipitation data from 40 to 200° C The asphaltenes separated are analyzed using FT-ir. The filtrate containing the maltenes was cooled to room temperature and the asphaltenes separating upon cooling was collected and analyzed. The oil and selected maltene fractions and extraction/ cleaning solvents were analyzed using GC     

AN EXPERIMENTAL STUDY OF THE EFFECT OF PARAFFINIC SOLVENTS ON THE ONSET AND BULK PRECIPITATION OF ASPHALTENES

Asphaltene onset concentration and bulk deposition were measured for a typical live reservoir oil titrated with n-C₆H₁₄, n-C₅H₁₂, n-C₄H₁₀, C₃H₈, C₂H₆, CH₄ and CO₂ at 100° C (212 ° F) and 29.9 MPa (4340 psia). The concentration of titrant at asphaltene onset was observed to decrease approximately in a linear fashion with decreasing molecular weight of the paraffinic solvent; CH₄ did not induce any asphaltene precipitation. Bulk deposition experiments were performed using a solvent: oil volume ratio of 10:1; the results indicated that the weight percent of asphaltenes precipitated increased exponentially with decreasing molecular weight of the paraffinic solvents. More importantly, the asphaltene molecular weight showed a maximum for n-C₄H₁₀ precipitated asphaltenes. Possible explanations for this unusual result are presented.     

Phase behavior modeling of asphaltene precipitation utilizing RBF-ANN approach

Precipitation of heavy hydrocarbons, particularly asphaltenes, is the reason for numerous operational and production problems in the petroleum industry. Hence, knowing the amount of asphaltene precipitation is a critical commission for petroleum engineers to overcome its problems. The aim of this study was to predict the amount of asphaltene precipitation as a function of temperature, dilution ratio, and molecular weight of different n-alkanes utilizing radial basis function artificial neural network (RBF-ANN). Additionally, this model has been compared with previous correlations, and its great accuracy was proved to predict the precipitated asphaltene. The values of R-squared and mean squared error obtained were 0.998 and 0.007, respectively. The efforts confirmed brilliant forecasting skill of RBF-ANN for the approximation of the precipitated asphaltene as a function of temperature, dilution ratio, and molecular weight of different n-alkanes.     

SEPARATION OF ASPHALTENES BY POLARITY USING LIQUID-LIQUID EXTRACTION

In order to investigate the nature of petroleum asphaltenes in terms of polarity a process was developed using initial liquid-liquid extraction of the oil phase followed by precipitation of the asphaltenes using n-heptane. The liquid-liquid extraction was performed using toluene-methanol mixtures with increasing content of toluene. Although large fractions of the crude oil (Alaska '93) was extracted in the higher polarity solvents (high concentration of methanol), the asphaltene content of the dissolved material was low As the toluene content increased more asphaltenes were transferred to the solvent phase. The asphaltenes were analysed using FTir, Elemental analysis, and HPLC-SEC with a diode array detector. With increasing content of toluene in the methanol the molecular weight distribution of the asphaltenes significantly move to higher molecular weights. The content of nitrogen and sulfur of the mallene phase also increase while H/C decreases. The content of heteroatoms in the asphaltenes are relatively higher and apparently increase with the polarity of the solvent. It is concluded that these asphaltenes are indeed dominated by high molecular weight substances that cannot be extracted in the high polarity solvents     

ORGANIC PRECIPITATES IN OIL PRODUCTION OF A VENEZUELAN OIL FIELD

A very light crude oil is being produced in an oil field in Venezuela. Such oils are susceptible to asphaltene precipitation as the reservoir pressure decreases during production. An asphaltene deposit has been found in the tubing of one of the wells and in the bottom sediment of the tanks on site. Dead oil samples from the well head of seven different producers in the field were analyzed by three different companies. The laboratory results covering the following aspects of different solvents show significant differences: asphaltene content, asphaltene/resin ratio, molecular mass of the precipitated asphaltenes, flocculation point, Vanadium/nickel ratio, precipitated asphaltenes, and the Hildebrand parameter. Using different consistency checks and establishing correlations among the results, the analysis of one lab is strongly favored against the others. The analytical results are discussed in detail, including the correlation between asphaltenes and the well situation regarding perforation depth and the phase behavior of the produced oil.     

EFFECT OF PRECIPITATION TEMPERATURE ON THE COMPOSITION OF N-HEPTANE ASPHALTENES

Petroleum asphaltenes were separated from Boscan and Kuwait crude oils by n-heptane at constant temperatures ranging from -2 to 80 ° C. A decreased yield with increase in temperature was observed. The precipitated material was characterized using infrared spectroscopic functional group analysis, HPLC-size exclusion chromatography, vapor pressure osmometry, and elemental analysis. The change in vanadium porphyrin content was estimated for Boscan using UV-Vis. With elevated temperature the asphaltenes are more aromatic (lower H/C) and have higher apparent molecular weight, whereas the pattern of changes in functional groups and heteroatoms (NSO) are more complex and apparently dependent on the crude oil. Porphyrins and low molecular weight types were seen to stay in solution at increased temperature, and a specific extraction of these takes place above 40 ° C. Asphaltene solubility is seen to approximate the Flory-Huggins theory as small molecules go into solution before large molecules. Several other mechanisms may, however, be involved in the phase separation. The molecular size distribution curves obtained by HPLC-SEC were found to explain the trends found in the molecular weights by VPO.     

注气对固溶物沉淀影响的研究与应用

注气采油是提高原油采收率的主要方式之一,在此过程中准确描述含有沥青质等高分子有机固相物质的油气体系相平衡十分必要。为此,将沉淀的沥青质视为固相,假设标准状态下必须有沥青质沉淀,将标准状态压力和温度引入沥青质固相逸度计算,并同时考虑了标准状态压力和温度对沥青质固相逸度的影响,建立了能模拟沥青质沉淀的气、液、固三相相平衡热力学模型。据该模型计算的结果表明:①能通过比较液相沥青质逸度和固相沥青质逸度大小来判断固相沥青质沉淀的出现。②当注入某油的气体为烃类混合气体时,烃类混合气体的添加使得含沥青质原油的组分发生变化;温度相同时,注气浓度越高,沉淀的压力越大;浓度相同时,温度越低,沉淀的压力越大;当沉淀量一定时,随着注气浓度增加,油品的饱和压力随之增大;相同注气浓度下,当压力高于饱和压力时,随着压力增大,沉淀量减少。③在温度不变的情况下,注入某油的气体为CO2时,其沥青质沉淀量是注CO2浓度的函数且随着CO2浓度的增加,固相(沥青质)的沉淀量不断增大。④在注气驱油过程中,气体的注入极易引发含沥青质原油中沥青质等重质有机物的沉积。     

EFFECT OF PRECIPITATION TEMPERATURE ON THE COMPOSITION OF N-HEPTANE ASPHALTENES Part 2

Petroleum asphaltenes precipitated by n-heptane at temperatures between ambient and 80^°C from two crude oils have been characterized regarding hydrocarbon structures using ¹H and ¹³C nmr, and fluorescence spectroscopy. This indicates a very complex and apparently very small change in structural features except for a molecular weight increase as more material stays in solution at elevated temperature. Aromaticity increase and apparently alkyl chains diminish. The latter trend is however affected by the structural equation used. According to the fluorescence spectroscopy the content of large and complex chromophores increases but a significant effect of the presence of porphyrins on the spectra is also observed. The data is analyzed in combination with previous characterizations of the same asphaltenes presented in the first part of this work (Andersen, 1994). This indicates that as smaller molecules are extracted at elevated temperature the precipitated asphaltenes tend to associate more. This implys that these small molecules block association sites which therefore become available for further association at elevated temperature. For Boscan part of the smaller easy to extract porphyrins may have this behavior.     

AN EXPERIMENTAL STUDY OF THE EFFECT OF PARAFFINIC SOLVENTS ON THE ONSET AND BULK PRECIPITATION OF ASPHALTENES

ABSTRACT

Asphaltene onset concentration and bulk deposition were measured for a typical live reservoir oil titrated with n-C₆H₁₄, n-C₅H₁₂, n-C₄H₁₀, C₃H₈, C₂H₆, CH₄ and CO₂ at 100° C (212 ° F) and 29.9 MPa (4340 psia). The concentration of titrant at asphaltene onset was observed to decrease approximately in a linear fashion with decreasing molecular weight of the paraffinic solvent; CH₄ did not induce any asphaltene precipitation. Bulk deposition experiments were performed using a solvent: oil volume ratio of 10:1; the results indicated that the weight percent of asphaltenes precipitated increased exponentially with decreasing molecular weight of the paraffinic solvents. More importantly, the asphaltene molecular weight showed a maximum for n-C₄H₁₀ precipitated asphaltenes. Possible explanations for this unusual result are presented.     

Asphaltene Self-Association: Modeling and Effect of Fractionation With A Polar Solvent

The self-association of asphaltenes in toluene is believed to occur step-wise, rather than by the formation of micelles. A number of step-wise models have been used to fit the calorimetric titration of asphaltenes in dried toluene solutions, with excellent results. All the models are based on chemical reactions equivalent to the ones found in polymerization. The study shows that the choice of the average properties of asphaltenes, such as the molecular weight, is critical in the final value of the parameter of interest, namely the average heat of self-association ΔH_a. The low values of ΔH_a obtained suggest that a fraction of asphaltenes is not active in the calorimetric experiments. Asphaltenes from Venezuela (LM1) and Mexico (KU) have been fractionated by precipitation with a mixture of acetone and toluene. It is considered that the most polar compounds are collected in the soluble fraction. A calorimetry study was performed on the two fractions, and the results show that the soluble fraction (SOL) has a much higher heat developed than the insoluble fraction (INS). This suggests again that a fraction of asphaltenes is not active in the calorimetric experiments, either because it does not self-associate or because the dilution effect is not strong enough to break the aggregates. Fluorescence and IR spectroscopy experiments confirm there is self-association in INS fraction, leading to the conclusion that asphaltene aggregates are formed by bonds of different strengths. The stronger aggregates would be predominantly in INS fraction and would be inactive in the calorimetric experiments.     

EFFECT OF PRECIPITATION TEMPERATURE ON THE COMPOSITION OF N-HEPTANE ASPHALTENES Part 2

ABSTRACT

Petroleum asphaltenes precipitated by n-heptane at temperatures between ambient and 80^°C from two crude oils have been characterized regarding hydrocarbon structures using ¹H and ¹³C nmr, and fluorescence spectroscopy. This indicates a very complex and apparently very small change in structural features except for a molecular weight increase as more material stays in solution at elevated temperature. Aromaticity increase and apparently alkyl chains diminish. The latter trend is however affected by the structural equation used. According to the fluorescence spectroscopy the content of large and complex chromophores increases but a significant effect of the presence of porphyrins on the spectra is also observed. The data is analyzed in combination with previous characterizations of the same asphaltenes presented in the first part of this work (Andersen, 1994). This indicates that as smaller molecules are extracted at elevated temperature the precipitated asphaltenes tend to associate more. This implys that these small molecules block association sites which therefore become available for further association at elevated temperature. For Boscan part of the smaller easy to extract porphyrins may have this behavior.     

Mechanism of the reversibility of asphaltene precipitation in crude oil

Asphaltene precipitation and deposition occur in petroleum reservoirs as a change in pressure, temperature and liquid phase composition and reduce the oil recovery considerably. In addition to these, asphaltene precipitates may deposit in the pore spaces of reservoir rock and form plugging, which is referred to as a type of formation damage, i.e. permeability reduction. In all cases above, it is of great importance to know under which conditions the asphaltenes precipitate and to what extent precipitated asphaltenes can be re-dissolved. In other words, to what extent the process of asphaltene precipitation is reversible with respect to change in thermodynamic conditions. In present work, a series of experiments was designed and carried out to quantitatively distinguish the reversibility of asphaltene precipitation upon the change in pressure, temperature and liquid composition. Experiments were conducted in non-porous media. Generally it was observed that the asphaltene precipitation is a partial reversible process for oil under study upon temperature change with hysteresis. However, the precipitation of asphaltene as a function of mixture composition and pressure is nearly reversible with a little hysteresis.     

Asphaltene Self-Association: Modeling and Effect of Fractionation With A Polar Solvent

Abstract

The self-association of asphaltenes in toluene is believed to occur step-wise, rather than by the formation of micelles. A number of step-wise models have been used to fit the calorimetric titration of asphaltenes in dried toluene solutions, with excellent results. All the models are based on chemical reactions equivalent to the ones found in polymerization. The study shows that the choice of the average properties of asphaltenes, such as the molecular weight, is critical in the final value of the parameter of interest, namely the average heat of self-association ΔH _a . The low values of ΔH _a obtained suggest that a fraction of asphaltenes is not active in the calorimetric experiments. Asphaltenes from Venezuela (LM1) and Mexico (KU) have been fractionated by precipitation with a mixture of acetone and toluene. It is considered that the most polar compounds are collected in the soluble fraction. A calorimetry study was performed on the two fractions, and the results show that the soluble fraction (SOL) has a much higher heat developed than the insoluble fraction (INS). This suggests again that a fraction of asphaltenes is not active in the calorimetric experiments, either because it does not self-associate or because the dilution effect is not strong enough to break the aggregates. Fluorescence and IR spectroscopy experiments confirm there is self-association in INS fraction, leading to the conclusion that asphaltene aggregates are formed by bonds of different strengths. The stronger aggregates would be predominantly in INS fraction and would be inactive in the calorimetric experiments.     

Comparisons Between Asphaltenes from the Dead and Live-Oil Samples of the Same Crude Oils

Asphaltenes precipitated from pressure-preserve bottomhole oil samples have been obtained for three oils at different pressures, using a bulk high-pressure filtration apparatus. The precipitates captured on the filter were recovered, the asphaltenes defined by the n-heptane insolubility were extracted and analyzed. These pressure-driven asphaltenes found on the filter were found to make up in the range between 50 and 100 ppm of the whole crude oil. Opening of the cell did not reveal asphaltenes retained due to wall adhesion. Size exclusion chromatography tests performed on both the live-oil-derived asphaltenes and the standard asphaltenes as precipitated by atmospheric titration on the same crude oil, revealed that the live-oil asphaltenes had apparent smaller hydrodynamic volume and narrower distributions than the standard asphaltenes for two oils. Further FTIR tests also showed large differences between standard asphaltenes and the asphaltenes obtained at high pressure filter. The latter appeared to contain more functional groups and be less saturated. Implication of these structural differences on precipitation modeling is discussed.     

THE CONCEPT OF ASPHALTENES REVISITED

Petroleum asphaltenes are defined as material insoluble in pentane or heptane and the nature of the molecular species that constitute asphaltenes has been a well researched subject over the past three decades. These investigations have led to a general consensus of the majority of researchers that asphaltenes contain condensed aromatic systems carrying alkyl, cycloalkyl, and heteroatom substituents. A variety of molecular models has been developed which have been used to explain the behavior of asphaltenes in feedstocks during recovery and processing operations

However, an alternate concept introduced the idea that asphaltenes can be defined as a complex mixture of organic compounds of varying molecular weight and polarity. This allows asphaltenes to be described more fully in terms of their behavior during processes and properties in products, such as asphalt

Thus, the concept can be used to explain, even predict, the incompatibility of asphaltenes with the other petroleum constituents as well as the deposition of asphaltic material on reservoir rock and the occurrence of sediments during refinery operations.