ASPHALTENE PRECIPITATION AND SOLVENT PROPERTIES OF CRUDE OILS

ABSTRACT

Improved prediction of the onset of asphaltene precipitation may be achieved using refractive index (RI) to characterize crude oils and their mixtures with precipitants and solvents. Experimental measurements of RI for mixtures of several crude oils with the precipitant n-heptane, are reported at ambient conditions. Theoretical developments are described that will permit extension of these observations to reservoir conditions

Measurements of RJ at the onset of precipitation have shown that the onset occurs at a characteristic RI for each oil/ precipitant combination, supporting the premise that precipitation is dominated by London dispersion interactions and thus, that RI can be used to predict the onset of precipitation. Reports in the literature showing that the onset of precipitation occurs at constant solvent-to-precipitant ratios provide additional confirmation

The theory is developed on the assumption that London dispersion forces dominate aggregation and precipitation of asphaltenes. The interaction energy of asphaltene molecules or aggregates in a medium of oil can be expressed as a function of the difference between the RI of asphaltene and oil. The RI of live crude oil during pressure depletion can be calculated from the RI of the stock tank oil, the molar refraction of the separator gas, the formation volume factor B_o and the solution gas/ oil ratio R_s     

MICROSCOPIC INVESTIGATION OF THE ONSET OF ASPHALTENE PRECIPITATION

A microscopic study of the onset of asphaltene precipitation is reported. The onset conditions can be quantified by measurement of mixture refractive index, together with microscopic observations of particulate formation in mixtures of oil and precipitant, with or without added solvents. For isooctane mixtures with a variety of hydrocarbon solvents and a crude oil from Alaska, the onset of precipitation occurs over a narrow range of solution refractive index. Addition of polar solvents or different precipitating agents can shift the refractive index at which precipitation begins. Refractive index decreases when a crude oil is diluted by precipitant, as in this study, or when changes in temperature and pressure alter the relative molar volumes of species in the oil. If it falls below some critical value, resin/asphaltene aggregates that had been in stable dispersion become unstable and precipitate. These observations provide a method of screening solvents to differentiate between those that prevent precipitation mainly by maintaining a higher mixture refractive index and others that may participate in or disrupt asphaltene/resin interactions.     

MICROSCOPIC INVESTIGATION OF THE ONSET OF ASPHALTENE PRECIPITATION

ABSTRACT

A microscopic study of the onset of asphaltene precipitation is reported. The onset conditions can be quantified by measurement of mixture refractive index, together with microscopic observations of particulate formation in mixtures of oil and precipitant, with or without added solvents. For isooctane mixtures with a variety of hydrocarbon solvents and a crude oil from Alaska, the onset of precipitation occurs over a narrow range of solution refractive index. Addition of polar solvents or different precipitating agents can shift the refractive index at which precipitation begins. Refractive index decreases when a crude oil is diluted by precipitant, as in this study, or when changes in temperature and pressure alter the relative molar volumes of species in the oil. If it falls below some critical value, resin/asphaltene aggregates that had been in stable dispersion become unstable and precipitate. These observations provide a method of screening solvents to differentiate between those that prevent precipitation mainly by maintaining a higher mixture refractive index and others that may participate in or disrupt asphaltene/resin interactions.     

A comprehensive experimental evaluation of asphaltene dispersants for injection under reservoir conditions

As the efficiency of dispersants with different origins is questionable for each typical oil sample, the present study provides a reproducible and reliable method for screening asphaltene dispersants for a typical asphaltenic crude oil. Four different asphaltene dispersants (polyisobutylene succinimide, polyisobutylene succinic ester, nonylphenol-formaldehyde resin modified by polyamines, and rapeseed oil amide) were prepared and their performance on two oils from an Iranian field under laboratory and reservoir conditions was studied. A thorough analysis including ash content and SARA tests was performed on the solid asphaltene particles to characterize the nature of deposits. Then a highly efficient carrier fluid, which is crucial when injecting dispersant into the wells, was selected from a variety of chemicals by comparing their solubility. In the next step, using an optical microscope, a viscometer, and a Turbiscan, the screening of dispersants under laboratory conditions was done on a mixture of dead oil and dispersant to evaluate the onset of asphaltene precipitation and its stability when titrating by a precipitant. Finally, two different mixtures of the efficient dispersants, live oil, and carrier fluid were used with the solid detection system (SDS) and the filtration method to examine their effects on the onset pressure of asphaltene precipitation and the asphaltene content of the crude oil under reservoir conditions. The results show that the combination of experimental methods used in this work could be consistently applied to screening asphaltene dispersants. Among the four different dispersants applied here, the dispersant based on nonylphenol-formaldehyde resin modified by polyamines showed the best performance on the available live oils. This chemical modified the onset pressure of asphaltene precipitation of light oil from 4300 psi to about 3600 psi and decreased the precipitated asphaltene of heavy oil by about 30 %.     

严重生物降解稠油沥青质包裹体中生标的研究

对孤岛原油中的可溶有机质生物标志化合物进行了气相色谱—质谱分析,结果表明,该原油受到了严重的生物降解,部分用于对比的生物标志物受到了破坏,致使有关参数值发生了变化,给油—油对比、油—源对比带来了困难。采用超声萃取技术,分离出沥青质包裹体中抗生物降解的生物标志物,并与原油中的生物标志物进行对比。其结果显示沥青质包裹体组分与原油组分的甾、萜烷非常相似,C27,C28,C29规则甾烷类分布均呈"L"型,而与生物降解程度无关,表明此方法可完全用于强烈生物降解原油的油—油对比和油—源对比研究,在有机地球化学领域的研究中具有重要的指导意义。     

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     

PARTICLE SIZE, SOLVENT, OXYGEN, TEMPERATURE AND PRESSURE EFFECTS ON THE CONTENTS OF INSOLUBLE FRACTIONS FROM SOLID VACUUM RESIDUA

n-Heptane insoluble contents were determined for an extraheavy-oil vacuum residue, one petroleum and one coal pitch as a function of particle size. This variable was found to affect the yields of insoluble material, aspect which was also observed during extractions carried out with aromatic and halogenated solvents. The precision of results was observed to be independent on the operators. A study was carried out to determine the effect of sample drying time on fraction yields. It was found that maltene fractions were strongly dependent on this variable, whereas asphaltene fractions were totally independent

Predissolution in aromatic solvents proved worthless from the repeatability point of view. However, predissolution of the oil residue in CH₂Cl₂ followed by filtration and asphaltene precipitation with simultaneous solvent removal, was deemed to be a suitable technique to remove mineral matter and isolate asphaltenes in a single step

Sample oxidation during asphaltene precipitation and isolation was detected by gravimetric and spectroscopic techniques. This fact is very important since common precipitation techniques do not address this issue

Preliminary accelerated extraction experiments carried out with pitch samples at high temperature and pressure, revealed that this approach can be a very interesting research tool to investigate solubility properties.     

Effect of pressure of the onset of asphaltene precipitation during gas flooding

In order to prevent and eliminate the asphaltene precipitation during gas flooding, the rules of the changes of the pressure under different conditions are systematically studied by the laser solid detection system, the light transmission method, and the P–T phase diagram when the asphaltene precipitation occurs. When the pressure is reducing to a certain value, the penetrating light intensity of the crude oil is markedly decreased, which shows that the pressure at this point is the pressure of the onset of asphaltene precipitation (AOPP). The AOPP value decreases with an increase in the temperature. During gas flooding, the AOPP value increases with an increase in the content of gas in the crude oil, and these two nearly do have a linear relationship. According to the P–T phase behaviors of the gas–crude oil system, with an increase in the content of gas in the crude oil, the asphaltene precipitation envelope curve (APE) is gradually moved up, which shows that the possibility of asphaltene precipitation is much higher. During oil production, for the oil reservoirs with rich asphaltene, the production pressure drop should be kept a small value to ensure that the flowing bottom-hole pressure is higher than AOPP and it is necessary to help complement producing energy timely by using water injection, gas injection, etc. Besides, according to the intersection of T–P curve and APE curve in the P–T phase diagram, the location where the asphaltene precipitation occurs in the wellbore can be roughly predicted. The experimental results can provide the theoretical foundation for taking measures to prevent and eliminate the asphaltene precipitation.     

Study of Asphaltene Precipitation Using Refractive Index Measurement

The measurements of the refractive index of crude oils were utilized in this work to enhance the understanding of the behavior of asphaltenes in crude oil, specifically, their tendency to precipitate from crude oil. The onset of asphaltene precipitation was measured in eight crude oil samples, which were titrated with either heptane or pentane in order to induce precipitation of the asphaltenes. The refractive index of each sample was measured to find its relationship to asphaltene precipitation. The assumption that refractive index of a mixture is a linear combination of the refractive indexes of the individual components was verified. It was also found that mixtures of heptane or pentane and crude oil also followed this same behavior. However, as asphaltenes began to precipitate from the solution, the refractive index no longer followed this linear mixing rule. Careful analysis of the refractive index data for each of the crude oil samples revealed many interesting relationships between the refractive index data and the content of the different polar asphaltene fractions present. The refractive index of asphaltenes was predicted from the refractive index data of crude oils. The results suggest the possibility predicting the properties and characteristics of the asphaltenes contained in a crude oil simply by measuring the refractive index.     

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。     

PRECIPITATION OF ASPHALTENES AT HIGH PRESSURES; EXPERIMENTAL TECHNIQUE AND RESULTS

Precipitation of asphaltenes in crude oil has been measured both as a function of pressure and composition. The onset of precipitation has been determined by the measurement of conductivity. A specially designed high pressure conductivity cell has been constructed. Due to the low conductivity of most crude oils the electrode spacing has to be very narrow and the effective electrode area as large as practical. The cell has a coaxial design with inlet and outlet on opposite sides on the cylinder. The cell is rated to 700 bar and 120 deg.C but can be constructed for both higher pressure and temperature. The onset of precipitation is detected by a maximum in the weight normalized conductivity, i.e. the conductivity divided by the weight fraction of oil

By using normal alkanes from ethane to pentane the precipitation onset is shown to increase linearly as a function of n-alkane carbon number. A smaller but significant increase in the onset is found by increasing the hydrostatic pressure. For monophasic live crude the onset is shown by a break in the conductivity curve.     

Study of Asphaltene Precipitation Using Refractive Index Measurement

Abstract

The measurements of the refractive index of crude oils were utilized in this work to enhance the understanding of the behavior of asphaltenes in crude oil, specifically, their tendency to precipitate from crude oil. The onset of asphaltene precipitation was measured in eight crude oil samples, which were titrated with either heptane or pentane in order to induce precipitation of the asphaltenes. The refractive index of each sample was measured to find its relationship to asphaltene precipitation. The assumption that refractive index of a mixture is a linear combination of the refractive indexes of the individual components was verified. It was also found that mixtures of heptane or pentane and crude oil also followed this same behavior. However, as asphaltenes began to precipitate from the solution, the refractive index no longer followed this linear mixing rule. Careful analysis of the refractive index data for each of the crude oil samples revealed many interesting relationships between the refractive index data and the content of the different polar asphaltene fractions present. The refractive index of asphaltenes was predicted from the refractive index data of crude oils. The results suggest the possibility predicting the properties and characteristics of the asphaltenes contained in a crude oil simply by measuring the refractive index.     

Comparison of the inhibitory capacity of vegetable oils,and their nonionic surfactants on Iran crude oil asphaltene precipitation using Quartz crystal microbalance

Asphaltene deposition is one of the main problems in some oil field production. Using the cheap chemical compounds to control asphaltene precipitation is one of the important tools in crude oil field development. Vegetable oil are natural chemical compounds which can be used as asphaltene stabilizer. In this work, the inhibitory capacity of the vegetable oils, their binary mixtures, and their binary mixtures with nonionic surfactants were studied by Quartz Crystal Microbalances. The results show that the turnip oil, binary mixtures of rosemary oil/sesame oil, sweet almond oil/camomile oil, and olive oil/tritoonX-100 are more efficient than other additives and the percentages of the inhibitory capacity are 87%, 89%, 90, and 90% respectively.     

注天然气油井沥青质析出规律研究

目的 解决东河区块原油在注气开采过程中沥青质沉积堵塞井筒问题。方法 采用高温高压固相沉积规律测试装置,基于光散射理论,研究了温度、压力、气油比等因素对沥青质析出特征的影响。结果 温度升高会增加沥青质在原油中的溶解度,促进原油稳定;等温降压过程中,沥青质随着压力降低逐渐析出,在泡点压力附近达到最大析出量,发生沥青质沉积堵塞油井的风险最大。DH-1井泡点压力对应井深2 140 m,与油井生产实际遇阻位置1 969 m接近,泡点压力可初步用于预测油井堵塞位置;溶解注气量越大,沥青质初始析出压力越大,沥青质析出压力区间也增大,沥青质沉积位置向油井深度下移。结论 研究揭示了注气过程沥青质的析出规律,对注天然气油井沥青质析出防治具有重要指导作用。     

FLOCCULATION OF ASPHALTENES AT HIGH PRESSURE. I. EXPERIMENTAL DETERMINATION OF THE ONSET OF FLOCCULATION

The influence of pressure on the onset of flocculation of asphaltenes was investigated in the pressure range from 1 to 300 bar and at temperatures of 55, 75, and 100°C. Methane was employed as gas component, and i-octane was used as precipitant. The onset of flocculation was determined with a light intensity meter. Three crude oils of different provenance were chosen for the investigations. The crude oils were stabilized in a rotary evaporator at 50 mbar and 180°C, and a part of stabilized products were mixed with different amounts of n-octane or methylcyclohexane. The dependence of the onset of flocculation on the pressure is analogous for all products. The required volume of i-octane decreases strongly with pressure up to 100 bar. Above 100 bar the influence of the pressure diminishes. This dependence corresponds with the solubility of methane in crude oil products. The amount of light hydrocarbons determines the position of the pressure dependence of the onset of flocculation. Without the addition of i-octane, the onset of flocculation could not be ascertained. The observed kinetics of flocculation depends on the asphaltene content as well as on the pressure. The influence of dispersing agents was also examined.     

高压注烃类气体过程中沥青质初始沉淀压力试验研究

为预防注烃类气体提高采收率过程中产生沥青质沉淀,对沥青质初始沉淀压力进行了试验研究.在分析注烃类气体过程中沥青质沉淀机理的基础上,通过自主研发的固相沉积激光探测装置,采用透光强度法测定了原油样品在不同温度下高压注气过程中沥青质的初始沉淀压力,并确定了沥青质沉淀的深度.试验得出,原油沥青质初始沉淀压力随温度升高而下降,测得44,80和123 ℃温度下原油的沥青质初始沉淀压力分别为44.1,39.7和35.2 MPa;每注入物质的量分数为1%的烃类气体,试验油样的沥青质初始沉淀压力升高0.5~0.6 MPa;井筒温度压力曲线与沥青质沉淀相包络线相结合预测井筒中出现沥青质沉淀的深度在1 800 m左右,与现场情况吻合较好.研究表明,原油中沥青质初始沉淀压力与注气量之间呈线性关系,可为现场注气驱油预防和清除沥青质沉积物提供理论依据.      

Self-organization behavior of asphaltene molecules in heavy oils with varying the oil: Precipitant ratio

Ashal’chinskoe and Mordovo-Karmal’skoe crude oils and their components have been studied by thermal analysis and ESR spectroscopy. Directly from the oils as such, two sets of asphaltene fractions (I and II) have been obtained by the two-step addition of 40-fold excess of a precipitant with varying the oil: precipitant ratio in the first step to be 1 : 5, 1 : 10, and 1 : 20. A comprehensive analysis of the results of studying the oils and asphaltene fractions revealed specific features of aggregation of asphaltenes in different oils.     

Prediction of Asphaltene Precipitation in Live and Tank Crude Oil Using Gaussian Process Regression

The study of asphaltene precipitation properties has been motivated by their propensity to aggregate, flocculate, precipitate, and adsorb onto interfaces. The tendency of asphaltenes to precipitation has posed great challenges for the petroleum industry. Since the nature of asphaltene solubility is yet unknown and several unmodeled dynamics are hidden in the original systems, the existing models may fail in prediction the asphaltene precipitation in crude oil systems. The authors developed some Gaussian process regression models to predict asphaltene precipitation in crude oil systems based on different subsets of properties and components of crude oil. Using feature selection techniques they found some subsets of properties of crude oil that are more predictive of asphaltene precipitation. Then they developed prediction models based on selected feature sets. Results of this research indicate that the proposed predictive models can successfully predict and model asphaltene precipitation in tank and live crude oils with good accuracy.