A new experimental investigation on upgrading of waxy crude oils by methacrylate polymers

Several methods have been used to reduce problems caused by wax precipitation during the production and/or transportation of waxy crude oil. Polymers are used to improve pour point and rheological behavior of waxy crude oils. In this work, the influence of the polymer inhibitors such as methacrylate polymers, as wax inhibitor, with different range of molecular weight and alkyl side chain carbons on the rheological behavior and pour point of two Iranian waxy crude oils were evaluated. Two Iranian waxy crude oils were selected on the basis of wax and asphaltene contents. The rheological behavior of these crude oils in absence and in presence of methacrylate polymer was studied. The rheological data cover the temperature range of–1 to 12°C. The results indicated that the performance of methacrylate polymer was dependent on the molecular weight, alkyl side chain carbons and the asphaltene content of crude oil. Methacrylate polymers with longer alkyl side chains than 18 carbons would perform best as wax inhibitors in two cases. Also, for crude oil with low asphaltene, higher molecular weight methacrylate polymer is the best flow improver and lower molecular weight methacrylate polymer showed good efficiency for crude oil with high asphaltene content.     

A New Investigation on Asphaltene Precipitation: Experimental and a New Thermodynamic Approach

The process of asphaltene precipitation has a substantial effect on oil production in petroleum industry. In this work, a series of experiments was carried to determine the asphaltene precipitation of crude oil during solvent titration condition with Iranian crude oil sample using experimental set up. The precipitants such as pentane, heptanes, hexane, nonane and decane with varying volumes were used. Also, a developed thermodynamic model based on micellization and PC-SAFT (perturbed chain modification Statistical Associating Fluid Theory) has been proposed to account asphaltene precipitation during solvent titration conditions and the proposed model was verified using experimental data obtained in this work and also with those reported in the literature. In the proposed micellization model the PC-SAFT EOS (cubic equations of state) is applied to predict the onset point and to perform flash calculations. In order to compare the performance of the proposed micellization model, the asphaltene precipitation experimental data were correlated using existing micellization model and the proposed micellization model. The results show that the proposed micellization model predicts more accurately the asphaltene precipitation experimental data and is in agreement with the experimental data of reported in this work and with those reported in the literature with a mean square error of 0.9%. Therefore the predictions obtained from this proposed model, resulted in a good agreement with experimental data which shows a significant improvement in comparison to the previous micellization model in the available literature.     

Prediction of asphaltene precipitation from n-alkane diluted crude oil by using PC-SAFT equation of state

The precipitation tendency of heavy organics such as asphaltene has posed great challenges for petroleum industry, and thus study of asphaltene precipitation amount and formation conditions seems to be necessary. One of the most common approaches for prediction of asphaltene precipitation is using thermodynamic models. In this study a PC-SAFT equation of state (EOS) is used to predict asphaltene precipitation in two Iranian dead oil samples. Asphaltene content is obtained by filtration method of the oil samples diluted with specific concentrations of different normal alkanes. Also liquid-liquid equilibrium is used for characterization of oil sample into one heavy phase (asphaltene) and another light phase (saturates, aromatics, and resin). Calculations show that the developed model is highly sensitive to interaction parameter between oil fractions. Prediction results were improved due to using Chueh-Prausnitz equation. The results indicate good potential of PC-SAFT EOS in the prediction of asphaltene precipitation in crude oil samples diluted with different normal alkanes. The model error is <5% and the model precision is increased by reducing the number of normal alkane carbons.     

Prediction of Asphaltene Precipitation during Pressure Depletion and CO2 Injection for Heavy Crude

Abstract

In this work, a thermodynamic approach is used for modeling the phase behavior of asphaltene precipitation. The precipitated asphaltene phase is represented by an improved solid model, and the oil and gas phases are modeled with an equation of state. The Peng-Robinson equation of state (PR-EOS) was used to perform flash calculations. Then, the onset point and the amount of precipitated asphaltene were predicted. A computer code based on the solid model was developed and used for predicting asphaltene precipitation data reported in the literature as well as the experimental data obtained from high-pressure, high-temperature asphaltene precipitation experiments performed on Sarvak reservoir crude, one of Iranian heavy oil reserves, under pressure depletion and CO₂ injection conditions. The model parameters, obtained from sensitivity analysis, were applied in the thermodynamic model. It has been found that the solid model results describe the experimental data reasonably well under pressure depletion conditions. Also, a significant improvement has been observed in predicting the asphaltene precipitation data under gas injection conditions. In particular, for the maximum value of asphaltene precipitation and for the trend of the curve after the peak point, good agreement was observed, which could not be found in the available literature.     

Aminated Copolymers as Flow Improvers for Super-viscous Crude Oils

The new flow improvers for super-viscous crude oils were developed via esterification of polybasic high carbon alcohol with methacrylate and copolymerizafion of monomers followed by amination of copolymers. The structure of the synthesized polymer flow improver additive was confirmed by IR spectroscopy and the crystal structure of the flow improver additives were determined by X-ray diffrac- tion analysis. The structure of wax crystals was also studied at the same time. The results showed that the wax crystal structure was closely related with the crystal structure of the flow improver, which could change the pour point depression and viscosity reduction behavior of the crude oil. When the wax crystal structure matched well with that of the additive, the wax crystals were dispersed satisfactorily, resulting in favorable effects in terms of pour point depression and viscosity reduction. The new synthe- sized aminated polymer flow improver additive was most efficient for treating super-viscous crude oils. The super-viscous crude oil had a high content of resins and asphaltenes, which might aggregate onto the surface of wax crystals to form blocks to limit the crude oil fluidity. However, amination of copolymers having similar structure with the resins and asphaltenes contained in crude oil could dissolve the huge polar groups to make the deposit formation difficult.     

Characteristic Temperatures of Waxy Crude Oils

Gel point or pour point is widely used to evaluate the low temperature flowability of crude oil. However, it is not adequate to describe the gelling properties of waxy crude oils under flow conditions with gel point or pour point, since the rheological behavior of crude oils is dependent on shear history. Waxy crude oils tend to gel at a low temperature. Based on gelation theory, the characteristic temperature of waxy crude oil was determined by analyzing viscosity-temperature data. Two mathematical models were developed for calculating characteristic temperatures of virgin crude oils and pour point depressants (PPD) beneficiated crude oils, respectively. By using these two models, the characteristic temperatures of crude oils that have experienced shearing and thermal histories can be predicted. The model for predicting the characteristic temperature of virgin crude oils has an average relative deviation of 4.5%, and all predicted values have a deviation within 2 ℃. Tested by 42 sets of data, the prediction model for crude oil treated with PPD has a high accuracy, with an average relative deviation of 4.2%, and 95.2 percent of predicted values have a deviation within 2 ℃. These two models provide useful ways for predicting the flowability of crude oils in pipelines when only wax content, wax appearance point and gel point are available.     

Correlations Between the Pour Point/Gel Point and the Amount of Precipitated Wax for Waxy Crudes

The amount of precipitated wax is one of the key factors that governs the flow properties of waxy crudes. Experimental results of 24 crudes have shown that approximately 2 wt% precipitated wax is sufficient to cause a virgin waxy crude gelling. Accordingly, a correlation between the pour point and the temperature at which 2 wt% of wax has precipitated out from a crude oil; i.e., T_c(2 wt%), and a correlation between the gel point and T_c(2 wt%) have been developed. The proposed correlation of the pour point is in accordance with the correlation developed by Letoffe et al. (1995) on the basis of 14 crudes from eight countries. The development of the gel point correlation and further verification of the pour point correlation indicate that there is a relationship between the gelling of virgin waxy crudes and the amount of precipitated wax. According to these correlations and the amount of precipitated wax, which can be determined only with a little sample by thermodynamic models or Differential Scanning Calorimetry (DSC) experiment, the gel point and pour point of virgin waxy crude can be predicted even if the oil sample is very limited. Heat treatment and chemical treatment can greatly improve flow behavior of waxy crudes, and more precipitated wax is present when the beneficiated (thermally beneficiated or PPD-beneficiated) crude oils gel. Experimental results showed that approximately two or three times the amount of precipitated wax presents at the gelling temperature when the oils were in their beneficiated (thermally beneficiated or PPD-beneficiated) conditions.     

含蜡原油添加流动性改进剂的研究

采用自行研制的聚丙烯酸高级混合醋作流动性改进剂,在适宜的条件下,可使胜利油田含蜡原油的流动特性(凝点、粘度和屈服值)和石蜡沉积特性得到明显改善。同时研究了实验条件及聚丙烯酸高级混合酯的化学结构对含蜡原油流动特性和石蜡沉积特性的影响,并对聚丙烯酸高级混合酯的作用机理进行了讨论。文中指出,聚丙烯酸高级混合酯的侧链醋基长度必须与原油中石蜡烃长度相适应才能使二者共晶。     

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.     

Calculation of Wax Precipitation Magnitude in Crude Oil Storage Tanks in Khark Island

During the different seasons of the year, the temperature of the oil tank goes up or down. The present work is composed of two parts, evaluation of the crude oil temperature and determining amount of wax precipitation. The numerical method has been developed for solving the energy equation and a thermodynamic model has been developed to predict wax precipitation based on average crude oil temperature. In the experimental investigation, a crude oil storage tank in Khark Island is selected as the case study and average crude oil temperature have been measured for two days and compared by the numerical result.     

An Experimental Study of Rheological Properties of Nigerian Waxy Crude Oil

Abstract

Nigerian crude oils are known for their good quality (low sulfur, high American Petroleum Institute gravity). However, similar to any other paraffinic-based crudes, most Nigerian crudes contain moderate to high contents of paraffinic waxes. These waxy crudes exhibit non-Newtonian flow behavior at temperatures below the cloud point because of wax crystallization. In order to accurately predict flowing and static temperature profiles, design waxy crude oil pipelines, evaluate flow interruption scenarios, and start up requirements in the handling of waxy crude oils, the effect of temperature and shear rate on the rheology of crude oils must be determined. This work presented some experimental results on the rheology of two waxy crude oils produced from different Nigerian oil fields. A Brookfield DV-III ultraprogrammable rheometer (Brookfield Engineering Laboratories, Middleboro, MA) was utilized. The temperature dependence of rheological properties and thixotropy of these crudes were investigated. The influence of some petroleum-based diluents to depress the wax appearance temperature (WAT), their effect on the thixotropic/yield pseudoplastic behavior of two Nigerian crudes were studied and it was observed that lower wax content crude has higher tendency to regain its cohesive lattice bonding (yield strength) when left undisturbed for some days at its gel point after agitation. The experimental results showed that the addition of petroleum-based diluents to the lower wax content crude oil leads to its lost some of its yield strength regaining capacity.     

含蜡原油石蜡沉积模拟方法研究

对从苏丹Unity油田现场取得有代表性的原油样品进行了原油组分分析和析蜡点测试。并针对油田的生产状况,利用与美国德士古石油公司合作建立的自动化高压石蜡沉积循环管道模拟系统,采用冷却实验步骤测试了苏丹原油从高于原油析蜡点温度到较低环境温度范围内在现场流速、油温和环境温度下的石蜡沉积速率,定量研究了流速和油温对石蜡沉积速率的影响,并采用分子扩散和剪切效应的总效应来描述石蜡沉积机理。利用临界蜡张力作为模拟放大因子,采用半经验的石蜡沉积数学模型,预测了Unity油田21井的生产井筒在现场生产条件下的石蜡沉积情况,模拟了不同产量和不同时间的蜡沉积剖面。结果表明,该油井在目前高产条件下,未发生石蜡沉积问题;在低产时,须考虑清蜡和防蜡措施。     

大庆原油管输结蜡规律与清管周期的确定

在确定不同流态区管壁处剪切应力、蜡晶溶解度系数、径向温度梯度及管道沿线温降分布的基础上,回归建立了适用于描述大庆油田某两联合站间输油管道蜡沉积的结蜡模型。根据差压法原理,建立了研究原油管输结蜡过程室内模拟试验装置,并覆盖该输油管道的典型工况条件开展了管输原油结蜡模拟试验。相对偏差分析表明,结蜡模型预测结果与试验值的适配性良好。进而在预测运行时间对该输油管道结蜡影响的基础上,结合结蜡层厚度对管道轴向温降及压降的作用,确定了年季节最高与最低土壤温度期的清管作业周期分别为4个月和3个月。     

含蜡原油热处理过程中若干组分的作用

本文研究了大庆、南阳原油中的胶质—沥青质,石蜡—正构烷烃和微晶等组分在含蜡原油热处理过程中的作用,从而指出:(1)只有当原油中含有石蜡和适量的胶质—沥青质时,才具备用热处理方法来改善其低温流动性能的可能性。(2)胶质—沥青质在含蜡原油中具有非常显著的分散作用增粘作用。它们通过吸附、共晶等作用改变原池中石蜡的结晶习性、形态及结构强度,从而取得降凝、降粘等效果。(3)各种原油中胶与正构烷烃之比在0.43～5.2之间,经过最佳条件热处理就可显示出热处理效果。胶—正烷烃比在0.6～3.0之间热处理效果最佳。(4)微晶蜡在含蜡原油热处理过程中,其本身不能显示热处理效果,当加热温度高达微晶蜡能在原油中大量溶解的温度时,在降温重结晶过程中,微晶蜡的析出将恶化热处理效果。     

普适性结蜡模型研究

根据原油在管道内流动特性及析蜡规律,提出了有效析蜡量的计算方法.利用F检验法筛选了原油结蜡的主要影响因素,包括原油黏度、管壁处剪切应力、温度梯度及管壁处蜡晶溶解度系数.利用9种原油室内环道结蜡实验数据,按照逐步线性回归的方法,得到了含蜡原油的普适性结蜡模型.该模型不需进行结蜡模拟实验,只需根据原油的黏度、析蜡特性及密度等物性参数就可预测原油的结蜡规律.在未进行室内结蜡模拟实验的情况下,利用普适性结蜡模型预测了中宁-银川输油管道不同工况下沿线结蜡分布,并和现场运行参数进行了对比,平均误差为6.32%,最大误差为20%,预测结果为现场清管作业提供了依据.     

Synthesis of cyclohexanone-alcohol hemiketals and evaluation as flow improver for waxy crude oil

In this paper, four novel cyclohexanone-alcohol hemiketals (KL-1, KL-2, KL-3, KL-4) as waxy crude oil flow improver have been prepared by cyclohexanone and 1-pentanol, 1-Hexanol, 1-heptanol, 1-octanol. Evaluation results demonstrate the hemiketal compounds have obvious effect on the viscosity and pour point of Henan crude oil. DSC analysis showed that waxing point and waxing peak decreased to a certain degree in 0.05% hemiketal solutions. Wax crystal morphology analysis indicated the four hemiketals played a role of nucleation in the process of waxy crude oil decoagulation, and in that way acts as waxy crude oil flow improver.     

An empirical correlation for estimating the viscosity of non-Newtonian waxy crude oils

Non-Newtonian fluids are the most complex ones when it comes to predicting their flow behavior. In the pre-existing models, rheological behavior in waxy crude oils is mainly affected by shear rate and thermal history of crude oils. In the present work, rheological characteristics of four different crude oils were studied by coaxial cylindrical rheometer at three different temperatures (40°C, 50°C, and 60°C) and a model was proposed keeping into consideration wax content for the first time. This model is prepared to predict the viscosity of four different crude oils with different wax content and at different temperature. The proposed model can efficiently estimate the viscosity as compared to other established models.     

降凝剂对管输原油含蜡的处理作用

本文介绍了含蜡原油在管道输油过程中的结蜡过程,以及利用降凝剂降低含蜡原油析蜡点的机理,通过控制管道中原油的粘度达到有效地利用降凝剂的目的。