The Asphaltene and Wax Deposition Envelopes

ABSTRACT

Asphaltene and wax phase behavior is quite different than the conventional “PVT” phase behavior. Asphaltenes exhibit a behavior at some thermodynamic states called flocculation. That is, asphaltene particles or micelles aggregate or flocculate into larger aggregates or flocs. The locus of all thermodynamic points in a P-T-x phase diagram at which flocculation occurs is called the Asphaltene Deposition Envelope (ADE). Paraffin waxes, on the other hand, at some thermodynamic states, exhibit the phenomenon of crystallization. The locus of all thermodynamic points in a P-T-x phase diagram at which wax crystallization occurs is called the Wax Deposition Envelope (WDE). Asphaltene flocculation can be both reversible and irreversible (as expected from the asphaltene colloidal nature). Wax crystallization is generally a reversible process. However, paraffin waxes more than often precipitate together with resins and asphaltenes (which are said to be responsible for the observed irreversible thermodynamic phenomena). Hence, some wax precipitation is occasionally reported as irreversible. Measurement of the ADE and WDE boundaries, at in-situ conditions, is a challenging task. Measurement of asphaltene and wax solubility at in-situ conditions inside the ADE and WDE is even more challenging. The ADE and WDE data have practical significance and are very useful for modeling of reservoir fluid behavior. Asphaltene and wax data for a number of oils are presented and analyzed. This paper introduces and compares two powerful thermodynamic diagrams that summarize the phase behavior of asphaltenes and waxes, the ADE and WDE.     

EFFECT OF ASPHALTENE AND RESINS ON THE STABILITY OF WATER-IN-WAXY OIL EMULSIONS

Asphaltene, resins and paraffin waxes, their mutual interactions and their influence on the stability of water-in-oil emulsions have been studied. 20 wt % paraffin wax dissolved in decalin was used to model the waxy crude oil. Asphaltene and resins separated from a crude oil were used to stabilize the water-in-oil emulsions. Synthetic formation water was utilized as the aqueous phase of the emulsion. The emulsion stability increased with increasing the concentration of asphaltene with a subsequent decrease in the average particle size distribution of the emulsion. Resins alone are not capable of stabilizing the emulsion, however, in the presence of asphaltene they form very stable emulsions. Dynamic viscosity and pour point measurements provided evidence for resins-paraffin waxes interactions. Asphaltene in the form of solid aggregates form suitable nuclei for the wax crystallites to build over with a mechanism similar to that of paraffin wax crystal-modifiers. As asphaltene are polar in nature they are derived at the oil/water interface which was proved by the ability of asphaltene to reduce oil/water interfacial tension. Consequently, nucleation of the wax crystallites by asphaltene and resins at the interface will add to the thickness of the oil-water interfacial film and hence increase the stability of the emulsion.     

EFFECT OF ASPHALTENE AND RESINS ON THE STABILITY OF WATER-IN-WAXY OIL EMULSIONS

ABSTRACT

Asphaltene, resins and paraffin waxes, their mutual interactions and their influence on the stability of water-in-oil emulsions have been studied. 20 wt % paraffin wax dissolved in decalin was used to model the waxy crude oil. Asphaltene and resins separated from a crude oil were used to stabilize the water-in-oil emulsions. Synthetic formation water was utilized as the aqueous phase of the emulsion. The emulsion stability increased with increasing the concentration of asphaltene with a subsequent decrease in the average particle size distribution of the emulsion. Resins alone are not capable of stabilizing the emulsion, however, in the presence of asphaltene they form very stable emulsions. Dynamic viscosity and pour point measurements provided evidence for resins-paraffin waxes interactions. Asphaltene in the form of solid aggregates form suitable nuclei for the wax crystallites to build over with a mechanism similar to that of paraffin wax crystal-modifiers. As asphaltene are polar in nature they are derived at the oil/water interface which was proved by the ability of asphaltene to reduce oil/water interfacial tension. Consequently, nucleation of the wax crystallites by asphaltene and resins at the interface will add to the thickness of the oil-water interfacial film and hence increase the stability of the emulsion.     

The Thermodynamic Modeling of Asphaltene Precipitation Equilibrium during the Natural Production of Crude Oil: The Role of Pressure and Temperature

Asphaltene precipitation is a sophisticated issue in the upstream oil industry, worldwide, and has detrimental effect on a verity of production processes; it damages the properties of the reservoir and causes an unfavorable and significant decrease in oil production. In spite of numerous studies to predict asphaltene behavior, the effect of temperature on asphaltene precipitation during pressure depletion at reservoir conditions is still obscure in the literature. In this study the PVT data as well as experimental data of asphaltene precipitation at reservoir conditions of an Iranian light oil samples is used, and the asphaltene precipitation and deposition envelops (APE and ADE) of the oil are developed using solid thermodynamic modeling.     

Modelling asphaltene precipitation with solvent injection using cubic-PR solid model

Cubic equation-of-state solid models are commonly-used to predict asphaltene precipitation behavior. Thermodynamic parameters are needed to model this behavior under different pressures and temperatures, and are usually obtained through fitting the model to multi asphaltene onset experiments. This paper introduces an empirical linear relation (tested on six oil samples) relating Asphaltene Onset Pressure (AOP) with injected solvent amount. In addition, waxes and aromatics correlations are utilized to obtain the thermodynamic parameters within the model. The two modifications decrease the number of tuning parameters of the model, as well as reduce the number of lab measurements needed to apply it. The model is tested on two oil samples, with previously published data, to predict AOPs. Using aromatics correlations provided more rational trends for AOP than waxes correlations. Besides, both correlations create a practical domain inside which the laboratory AOP values lie. The new additions enhance the prediction capabilities of the model in the lack of asphaltene experiments.     

Experimental investigation of dodecylbenzene sulfonic acid and toluene dispersants on asphaltene precipitation of dead and live oil

Asphaltene is the heaviest fraction of oil, and if the thermodynamic conditions of oil change, it can be separated from oil precipitate. Of common methods for preventing asphaltene precipitation, using predictive methods, biological methods and injection of dispersants can be mentioned. In this study, the effect of two dispersants of toluene and dodecylbenzene sulfonic acid on asphaltene precipitation of a dead and a live oil sample has been investigated. According to the results, these dispersants in dead oil create an optimum point for asphaltene precipitation. In live oil, these dispersants reduce asphaltene precipitation down to 70%. In addition, it was observed that as an effect of injecting these dispersants, the average sizes of asphaltene flocculation have reduced.     

Prediction of Asphaltene Precipitation from Empirical Models

Asphaltene precipitation in the reservoir has proved to be a difficult problem to define and study. Field conditions conducive to precipitation include normal depletion, acid stimulation, gas-lift operations, and miscible flooding. Asphaltene precipitation is generally believed to be an irreversible process, which is the main reason it can have a profound impact on production operations. Investigations into asphaltene precipitation have been impeded by a shortage of experimental data and information on precipitation mechanisms. The aim of this work is to generate empirical models with other thermodynamic models to describe and predict precipitate formation models with a variety of reservoir fluids. Experimental asphaltene precipitation data on several live-oil at reservoir conditions were measured to study the effects of temperature, pressure, and composition on precipitate formation and the relationships between critical properties. Data generated by the model can be used to identify operating amount of asphaltene deposited under different conditions.     

Deposition and flocculation of asphaltenes from crude oils

A crude oil has four main constituents: saturates, aromatics, resins, and asphaltenes. The asphaltenes in crude oil are the most complex and heavy organic compounds. The classic definition of asphaltenes is based on the solution properties of petroleum residuum in various solvents. Asphaltenes are a solubility range that is soluble in light aromatics such as benzene and toluene, but are insoluble in lighter paraffins. The particular paraffins, such as n-pentane and n-heptane, are used to precipitate asphaltenes from crude oil. Deposition of asphaltenes in petroleum crude and heavy oil can cause a number of severe problems. The precipitation of asphaltene aggregates can cause such severe problems as reservoir plugging and wettability reversal. Asphaltenes can precipitate on metal surface. Cleaning the precipitation site as well as possible appears to slow reprecipitation. To prevent deposition inside the reservoir, it is necessary to estimate the amount of deposition due to various factors. The processes can be changed to minimize the asphaltene flocculation, and chemical applications can be used effectively to control depositions when process changes are not cost effective. Asphaltene flocculation can be controlled through better knowledge of the mechanisms that cause its flocculation in the first place. The processes can be controlled to minimize the asphaltene flocculation, and chemical applications can be used effectively to control depositions when process changes are not cost effective.     

PREDICTION OF THE PHASE BEHAVIOR OF ASPHALTENE MICELLE / AROMATIC HYDROCARBON SYSTEMS

Asphaltene particles can be dissolved in an organic molecular system, and they may form micelles in the presence of excess amounts of aromatic molecules. The existent thermodynamic theory of phase separations in micellar solutions, developed for water/ amphiphile solutions, it is generalized and applied to the prediction of the phase separation in micellar solutions of aromatic/ asphaltene at various temperatures and normal pressure, in this paper. Three forms for asphaltene micelle formation are proposed. The calculations performed are indicative of the existence of one new phase in the asphaltene micelle formation region. The results obtained are compared with the available critical micelle concentration data.     

Prediction of Asphaltene Precipitation from Empirical Models

Abstract

Asphaltene precipitation in the reservoir has proved to be a difficult problem to define and study. Field conditions conducive to precipitation include normal depletion, acid stimulation, gas-lift operations, and miscible flooding. Asphaltene precipitation is generally believed to be an irreversible process, which is the main reason it can have a profound impact on production operations. Investigations into asphaltene precipitation have been impeded by a shortage of experimental data and information on precipitation mechanisms. The aim of this work is to generate empirical models with other thermodynamic models to describe and predict precipitate formation models with a variety of reservoir fluids. Experimental asphaltene precipitation data on several live-oil at reservoir conditions were measured to study the effects of temperature, pressure, and composition on precipitate formation and the relationships between critical properties. Data generated by the model can be used to identify operating amount of asphaltene deposited under different conditions.     

PREDICTION OF THE PHASE BEHAVIOR OF ASPHALTENE MICELLE / AROMATIC HYDROCARBON SYSTEMS

ABSTRACT

Asphaltene particles can be dissolved in an organic molecular system, and they may form micelles in the presence of excess amounts of aromatic molecules. The existent thermodynamic theory of phase separations in micellar solutions, developed for water/ amphiphile solutions, it is generalized and applied to the prediction of the phase separation in micellar solutions of aromatic/ asphaltene at various temperatures and normal pressure, in this paper. Three forms for asphaltene micelle formation are proposed. The calculations performed are indicative of the existence of one new phase in the asphaltene micelle formation region. The results obtained are compared with the available critical micelle concentration data.     

道路石油沥青中蜡对沥青性质的影响

测定了大庆和辽河１００号道路石油沥青中蜡的分布,并将其中的饱和蜡,芳香蜡,总蜡分离出来,用其与脱蜡沥青调制成蜡含量不同的系列样品,考察了饱和蜡,芳香蜡对沥青物理性质的影响,总蜡对沥青路用性能的影响,对沥青质量指标中否应设置蜡含量指标进行了讨论．     

Determination of wax content in crude oil

Wax deposition is one of the chronic problems in the petroleum industry. The various crude oils present in the world contain wax contents of up to 32.5%. Paraffin waxes consist of straight chain saturated hydrocarbons with carbons atoms ranging from C18 to C36. Paraffin wax consists mostly with normal paraffin content (80–90%), while, the rest consists of branched paraffins (iso-paraffins) and cycloparaffins. The sources of higher molecular weight waxes in oils have not yet been proven and are under exploration. Waxes may precipitate as the temperature decreases and a solid phase may arise due to their low solubility. For instance, paraffinic waxes can precipitate out when temperature decreases during oil production, transportation through pipelines, and oil storage. The process of solvent dewaxing is used to remove wax from either distillate or residual feedstocks at any stage in the refining process. The solvents used, methyl-ethyl ketone and toluene, can then be separated from dewaxed oil filtrate stream by membrane process and recycled back to be used again in solvent dewaxing process.     

DEVELOPMENT OF A NEW TYPE OF LOW-TEMPERATURE PATTERN WAX

The effects of several important components on casting waxes, such as low-melting point petroleum waxes, high-melting point waxes, stearic acid and additive 201 were investigated. A new type of LPW low-temperature pattern wax was developed by method of cooling curve. The experimental results showed that stearic acid and No. 56 petroleum wax could lower the casting temperature. No. 80 microcrystalline wax, No. 85 microcrystalline wax and additive 201 all could make the pattern wax crystal construction finer, although additive 201 was more effective than the microcrystalline waxes. The application tests showed that LPM pattern waxes were higher quality pattern waxes such as better gloss, higher rigidity and higher intensity than the original common low casting temperature pattern waxes.     

气-液-固三相相平衡热力学模型预测石蜡沉积

在油气开采过程中,当体系的温度、压力或组成等热力学条件发生改变时,油气体系内的蜡质、胶质、沥青质等有机固相物质将会从体系中析出而沉积,给油气田生产带来严重的危害.采用状态方程和溶液理论相结合而建立了能模拟石蜡沉积的气液固三相相平衡热力学模型.根据正规溶液理论描述固相混合物的非理想性,采用状态方程描述气相和液相的相态.运用该模型对某一含气原油进行了蜡固相沉积模拟计算,计算结果表明,压力对蜡沉积有较大的影响,压力对蜡沉积点温度的影响在低于饱和压力下比高于饱和压力下的影响更为显著.     

DEVELOPMENT OF A NEW TYPE OF LOW-TEMPERATURE PATTERN WAX

The effects of several important components on casting waxes, such as low-melting point petroleum waxes, high-melting point waxes, stearic acid and additive 201 were investigated. A new type of LPW low-temperature pattern wax was developed by method of cooling curve. The experimental results showed that stearic acid and No. 56 petroleum wax could lower the casting temperature. No. 80 microcrystalline wax, No. 85 microcrystalline wax and additive 201 all could make the pattern wax crystal construction finer, although additive 201 was more effective than the microcrystalline waxes. The application tests showed that LPM pattern waxes were higher quality pattern waxes such as better gloss, higher rigidity and higher intensity than the original common low casting temperature pattern waxes.     

Determination of asphaltene precipitation using a CPA equation of state

Asphaltene precipitation during natural depletion and miscible gas injection is a common problem in oilfields throughout the world. Therefore, predicting asphaltene phase behavior through thermodynamic modeling may help to control its precipitation and reduce the associated problems. In this work, a new modified CPA equation of state (EoS) was used to model asphaltene precipitation. This equation is based on a combination of a new physical part and the Wertheim association term.

The results of the new model were compared with the experimental data of five oil samples. The results showed that this modified CPA EoS can predict asphaltene precipitation with good accuracy.     

UNUSUAL RETROGRADE CONDENSATION AND ASPHALTENE PRECIPITATION IN A MODEL HEAVY OIL SYSTEM

A partial phase diagram for the system athabasca bitumen vacuum bottoms (ABVB) (24.6 wt% / 2 mole % ) + dodecane (73.8 wt. % / 47 mole % ) + hydrogen (1.6 wt % / 51 mole % ) was constructed in the temperature range 425 K to 725 K and the pressure range 2 MPa to 7 MPa using an X-ray view cell apparatus. This fluid system is shown to exhibit two phase L₁V and three phase L₁ L₂ V phase behaviour over parts of this P-T region. The shape of the low temperature boundary between the L₁V and L₁ L₂ V zones is characteristic of dilute asymmetric mixtures where a heavy liquid phase, L₂ appears then disappears within the light liquid phase L₁ on isothermal compression. Such phase behaviour is referred to as unusual retrograde condensation and is of both practical and theoretical interest. Transitions between the multiphase regions were found to be reversible at temperatures less than 660 K., in all cases. At higher temperatures irreversible “ asphaltene precipitation” arose within the L₂ phase “ Asphaltene precipitation” did not arise in the absence of the L₂ phase, i.e.: within the L₁ V region, even at temperatures in excess of 700 K. These data provide a strong link between “ asphaltene precipitation” and multiphase behaviour, and demonstrate a physical rather than kinetic basis for asphaltene precipitation at elevated temperatures