The Variation of the Gel Point of Daqing Crude Oil with Pipelining History

Abstract

Daqing crude oil is a typical waxy crude oil with a wax content of 26.6 wt% and a gel point of 32°C. In order to conduct the safety analysis of pipeline operation, the variation of gel point of Daqing crude oil with thermal and shear history was studied through pipelining simulation experiments. The experiments were performed under different annual throughputs and shear rates of pipeline flow. The results suggest that the change of shear action has little effect on the gel point of crude oil. As the final dynamic cooling temperature is increased, the change of gel point experiences two stages, staying constant at first and then decreasing. Through 13 groups of pipelining simulation experiments, the effects of heating temperature ranging from 45°C to 65°C on gel point of Daqing crude oil were studied. It has been indicated that the gel point increases as the heating temperature is increased under the same final dynamic cooling temperature. The relation equation was constructed to be used to predict the gel point under certain heating temperatures and final dynamic cooling temperature. The average absolute deviation of 52 predicted values of gel point is 0.58°C.     

Prediction of Viscosity Variation for Waxy Crude Oils Beneficiated by Pour Point Depressants During Pipelining

Pour-point-depressant (PPD) beneficiation is an emerging technology for facilitating pipelining of waxy crude oils. However, it was found both in lab studies and field tests that the viscosities of PPD-beneficiated waxy crude oils may increase as a result of pump shear or pipe flow shear during pipelining, called the shear effect. Current method for understanding this viscosity variation is to measure the viscosities by experimental simulation, which is quite time- and effort-consuming for long-distance pipelines. A new method for predicting the viscosity variation during pipelining is presented in this article based on a mathematical model for predicting viscosity of PPD-beneficiated waxy crude oils at certain temperature after shear and a model for predicting non-Newtonian viscosity of waxy crude oils as a function of temperature and precipitated wax. As verification, viscosity variations of five PPD-beneficiated waxy crude oils during pipelining are predicted by using the presented method and compared with the measured data from experimental simulations. The predicted viscosities are in good agreement with the measured data, with an average absolute deviation of 14.1% for all 52 data points. Comparisons between the measured or predicted viscosities during pipelining and viscosities obtained from the fast cooling experiment of PPD effectiveness evaluation demonstrate that the evaluation of PPD effectiveness for practical application should take the shear effect into consideration. The presented method provides a new way to evaluate the PPD's effectiveness in pipelining application quickly and effectively.     

A Study on Creep Behavior of Gelled Daqing Crude Oil

Abstract

Gelled waxy crude oils have complex rheological properties. Experimental research and mechanical analogy were performed for the creep behavior of Daqing crude oil. The experienced stages in creep process were partitioned based on the changes of strain and strain rate with the increasing loading time. When the loading stress was lower than the yield stress, there existed three stages; namely, instant elastic deformation, decelerated creep, and constant rate creep. When the applied stress was higher than the yield stress, accelerated creep occurred after the above three stages. By applying mechanical analogy, the elastic element, viscous element, plastic element, and accelerated creep element were employed to reflect the interior strain mechanism of crude oil under creep condition. A creep equation was proposed by incorporation with the analysis on the creep experiment. Based on the creep equation, nonlinear regression was conducted by using the creep experiment data of Daqing crude oil at different temperatures of 28°C, 29°C, 30°C, and 31°C, respectively. The average correlation coefficient was 0.9907. Substituting the calculated equation parameters into the creep equation, the creep behavior under other shear stresses can be predicted at the same temperatures. The average relative deviation is 8.29%. The change of strain of crude oil before yielding can be accurately described by the creep equation.     

Effects of Thermal and Shear History on the Viscoelasticity of Daqing Crude Oil

Abstract

The relationship between the viscoelasticity and thermal and shear history of waxy crude is important for the safety analysis of waxy crude pipeline operation. A series of oscillatory shear experiments were performed to test the viscoelastic parameters of Daqing crude oil with a controlled-stress rheometer RSH150. The results indicate that the storage modulus and loss modulus increase in an exponential fashion with decreasing testing temperature. There exists a worst heating temperature for the rheological behavior. The strength of waxy structure and gelation temperature increase as the cooling rate is reduced. The shear action is characterized by energy dissipation due to viscous flow. At the same shear temperature, the strength of waxy structure decreases rapidly and finally attains a stable state as the energy dissipation due to viscous flow is increased. Keeping the same energy dissipation due to viscous flow, the closer the shear temperature is to the test temperature, the lower storage modulus, loss modulus, and gelation temperature would emerge. When being close to or higher than the abnormal point, the shear temperature has few influences on the viscoelasticity. The recovery of waxy structure after shear action exhibits an irreversible property.     

Modeling the viscoelasto-plastic behavior of waxy crude

Waxy crude oil exhibits complex rheological behavior below the pour point temperature, such as viscoelasticity, yield stress, and thixotropy, owing to the formation of a three-dimensional spongelike interlock network structure. This viscoelasto-thixotropic behavior is an important rheological behavior of waxy crude oils, determining the flow recovery and safe restart of crude oil pipelines. Up to now, the thixotropic models for waxy crude have been all viscoplastic models, without considering the viscoelastic part before the yield point. In this work, based on analyzing the variation of the elastic stress and viscous stress in the Mujumbar model, a new viscoelasto-plastic model is proposed, whose shear stress is separated into an elastic component and a viscous component. The elastic stress is the product of the shear modulus and elastic strain; the shear modulus is proportional to the structural parameter. For the elastic strain, we followed the line of Zhu and his coauthors and assumed that it may be expressed by an algebraic equation. The model is validated by stepwise shear rate tests and hysteresis loop tests on Daqing and Zhongyuan waxy crude. The results show that the model’s fitting and predictive capability is satisfactory.     

The effect of dynamic cooling on the flow properties of PPD-treated crude oil

Abstract

By means of DSC, test of flow parameters, and microscopic observation, this paper studied the relation between the influence on the low temperature flow properties and that on the waxy crystal of the PPD-treated crude oil due to different final temperature of dynamic cooling. It has been experimental confirmed that the influence caused by the final temperature of dynamic cooling on low temperature flow properties and waxy crystal vary from the different final temperature of dynamic cooling. With this temperature increases, the depressive effects first get worse, then improved and finally kept nearly unchanged at higher temperature.     

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。     

大庆原油的粘弹性实验研究

用RSH150控制应力流变仪对大庆原油进行了振荡剪切实验。结果表明,储能模量和耗能模量随测量温度的降低呈指数规律增加;存在使流变性恶化的“最差加热温度”;降温速率越高,测量温度下蜡晶结构强度和胶凝温度越低。在同样的剪切温度下,粘弹性参数和胶凝温度随粘性流动能量耗散增加而急剧降低直至达到稳定状态;在同样的粘性流动能量耗散下,剪切温度越接近测量温度,储能模量和耗能模量越低,胶凝温度越低;当剪切温度接近或超过反常点时,测量温度下的粘弹性参数基本不随剪切温度变化;原油结构在经历剪切后具有不可逆的恢复特性。     

Effects of Thermal and Shear History on the Viscoelasticity of Daqing Crude Oil

The relationship between the viscoelasticity and thermal and shear history of waxy crude is important for the safety analysis of waxy crude pipeline operation. A series of oscillatory shear experiments were performed to test the viscoelastic parameters of Daqing crude oil with a controlled-stress rheometer RSH150. The results indicate that the storage modulus and loss modulus increase in an exponential fashion with decreasing testing temperature. There exists a worst heating temperature for the rheological behavior. The strength of waxy structure and gelation temperature increase as the cooling rate is reduced. The shear action is characterized by energy dissipation due to viscous flow. At the same shear temperature, the strength of waxy structure decreases rapidly and finally attains a stable state as the energy dissipation due to viscous flow is increased. Keeping the same energy dissipation due to viscous flow, the closer the shear temperature is to the test temperature, the lower storage modulus, loss modulus, and gelation temperature would emerge. When being close to or higher than the abnormal point, the shear temperature has few influences on the viscoelasticity. The recovery of waxy structure after shear action exhibits an irreversible property.     

W/O型含蜡原油乳状液胶凝体系的屈服特性

在恒定剪切应力和剪切应力线性增加2种加载模式下,研究了油包水（W/O）型含蜡原油乳状液胶凝体系在凝点温度附近的屈服行为。以原油乳状液胶凝体系的屈服应变、屈服剪切速率为特征量,分析了剪切应力加载条件、加载量和胶凝体系含水率对含蜡原油乳状液胶凝体系屈服特性的影响规律。引用屈服应变比例系数的概念,分析了不同恒温静置时间后含蜡原油乳状液胶凝体系结构恢复程度及其影响因素。研究发现：屈服应变可作为W/O型含蜡原油乳状液胶凝体系屈服的判据;增大原油乳状液胶凝体系含水体积分数,屈服应变增大,屈服剪切速率减小,体系恢复度增加。增大剪切应力加载量或剪切应力增加速率,原油乳状液胶凝体系屈服剪切速率增大,该体系的结构恢复程度变差。     

储层条件下的含蜡原油流变性研究

为研究含蜡原油在储层条件下的流变性,应用高压拈度计,在储层压力条件下开展了原油流变性实验。结果表明：在较低的温度条件下,含蜡原油的拈度随剪切速率增大而减小,且起初在一定的剪切速率范围内减幅较大,随后减幅越来越小;随着温度的降低,含蜡原油的表观拈度、拈流活化能及稠度系数增大,非牛顿指数减小,流变特性越来越偏离牛顿流体;采用非牛顿指数、稠度系数、拈流活化能和结构拈度指数等4项参数,能较全面地表征储层条件下含蜡原油的流变性。     

Viscosity Prediction of Non-Newtonian Waxy Crude Heated at Various Temperatures

Viscosities are important parameters for design and operation of crude pipelines. The heating temperature is the major factor affecting viscosities of waxy crude below the wax appearance temperature. Below the abnormal point, waxy crude exhibits non-Newtonian flow behavior with the viscosity dependent on the shear rate. Both of these make determination of the non-Newtonian viscosities of waxy crude a very time-consuming job. On the basis of the model for predicting non-Newtonian viscosity of waxy crudes as a function of temperature and precipitated wax, an approach to predict non-Newtonian viscosity of waxy crude heated to various temperatures has been developed only based on a few measurements. The accuracy of prediction by this approach has been verified by 468 viscosity data from the Daqing crude heated at various temperatures. The totally average relative deviation between the measured and predicted viscosity is 9.42%.     

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.     

为管输应用的原油降凝剂效果评价的研究

添加降凝剂处理的含蜡原油的低温流动性受处理后降温过程中的剪切作用及降温速率的影响很大,这种影响因原油及管道运行条件而异,因此为管道设计及运行提供数据的含蜡原油降凝剂效果评价,必须综合模拟管输的降温及剪切条件.现行的测试标准在这方面存在明显缺陷.本文在总结多年研究经验的基础上,讨论了管输过程中泵剪切和管流剪切及其与降温条件的综合模拟方法,提出了评价管输过程中加剂原油动态及静态稳定性的方法,以及为管输提供数据的加剂原油凝点及流变性的评价方法.     

Effects of stress and oscillatory frequency on the structural properties of Daqing gelled crude oil at different temperatures

The structural properties of Daqing gelled waxy crude oil were studied with a controlled-stress rheometer under oscillatory shear mode. Our results demonstrate that the gelled waxy crude oil shows viscoelastic properties. The linear viscoelastic region and yielding of crude oil can be determined not only by the critical linear shear stress (τ_cl) and yield stress (τ_y), but also by the critical linear shear strain (γ_cl) and yield strain (γ_y) respectively. With the decrease of test temperature, τ_cl and τ_y increased greatly while γ_cl and γ_y decreased gradually. The crude oil showed more elastic at lower temperatures. The frequency had little effect on the structure of crude oils in the measured range.     

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.     

Rheological behaviour of crude oil: Effect of temperature and seawater

The effect of temperature and seawater on rheological behavior was investigated in this work for four samples of crude oil from Algerian Sahara. The Newtonian model was used to fit the shear stress dependence on the shear rate for temperature range between 20 and 100°C. The temperature increase leads to a decrease in the dynamic viscosity of crude oil. The temperature dependence of dynamic viscosity was fitted by the Walther equation. For crude oil emulsion, the Herschel-Bulkley model was used to fit the shear stress dependence on the shear rate for volume fraction of seawater between 30 and 70%. The increase in the volume fraction of seawater induces not only an increase in the yield stress and the consistency index of crude oil emulsions but also a decrease in the flow index of crude oil emulsions.     

热处理作用对含蜡原油流变性的影响

热处理技术是改善含蜡原油低温流动性的一种主要方法,以BH油田含蜡原油为例,通过一系列热处理试验对含蜡原油进行了流变性能研究,对实验数据进行了理论分析,并结合现场应用情况验证了试验结论的正确性。分析结果表明：温度是影响热处理效果的主要因素,中温段（50～60℃）的热处理会对含蜡原油的凝点产生恶化效果;高温段（70～95℃）的热处理可以大幅降低含蜡原油的凝点。冷却速度对含蜡原油热处理效果的影响至关重要,当冷却速度较小（0．5～1．0℃／min）时,油样热处理效果较好;冷却速度太快（1．2～1．5％／min）,将使热处理效果变差。剪切速率对热处理效果有一定影响,通常在较低速率的剪切作用下,原油热处理效果较好,随着剪切速率的增加,原油热处理效果变差。热处理后油样的稳定性对热处理效果的影响也不可忽视。     

微观液滴分布对含蜡原油乳状液流变性的影响

通过显微镜观察并拍摄原油乳状液的微观结构图像,研究了乳状液体系分散相液滴大小及分布规律,以及微观液滴分布对乳状液体系流变性的影响机理。W/O型原油乳状液中含水率增加,引起内相液滴个数增多,小液滴所占的比例减小,相对大的液滴所占的比例增大;搅拌转速的增大,使体系内相液滴个数增多,平均液滴直径减小。通过测试在固定搅拌条件下制备的不同含水率的3种含蜡原油乳状液在原油凝点附近温度屈服特性和触变性等流变特性,可以发现随含水率的增大,乳状液体系屈服应力增大、触变性增强,且含水率越高,变化的趋势越明显。通过测试不同搅拌转速下制备的含水率为30% 的原油乳状液在原油凝点附近温度的触变性,可以看出随搅拌转速的增大,体系经受同等剪切速率剪切时对应的剪切应力增大、触变性增强。进一步建立了屈服应力与测试温度、含水率之间的关系式,其平均相对误差为9.83%。     

Rheology of Middle Distillate. I. Role of Composition

Abstract

The rheological behavior of middle distillate (250–375°C) fraction, obtained from the waxy Bombay-High Off-shore crude oil, and its five narrow sub-fractions of 25°C interval each, i.e., Fr 1 (250–275°C), Fr 2 (275–300°C), Fr 3 (300–325°C), Fr 4 (325–350°C), and Fr 5 (350–375°C), have been studied below their ASTM pour point temperatures. The rheograms (the shear stress vs. rate of shear) of the fraction and sub-fractions, at various temperatures below their pour points, are recorded on a Haake RV-12 Co-axial Rota Viscometer fitted with a NV sensor and a temperature programmer (PG-20) and attached with a programmed heating/cooling system. From these rheograms the flow parameters like plastic viscosity, apparent viscosity, and yield stress are obtained and their variations with temperature and shear rate have been studied in terms of compositions of the fraction/sub-fractions. These results might be useful in devising methodology for overcoming the wax separation phenomenon from middle distillates at low temperatures.