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.     

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。     

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.     

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。     

利用能量耗散率计算管流的平均剪切率

含蜡原油的流变性与其所经历的剪切历史有关.以往用截面平均剪切率或湍流时的管壁剪切率衡量流体管流的剪切历史,有不当之处.根据能量耗散率与剪切率的关系,提出了按流体的体积平均能量耗散率计算流体平均剪切率的方法,即可根据流体的密度、平均流速、管道直径、流动的摩擦因子或摩阻压降以及流变参数计算牛顿流体和幂律流体管流的平均剪切率.对于湍流,按本方法计算的平均剪切率能合理地度量流体的平均剪切历史.在原油管道正常运行的雷诺数范围内,按截面平均方法计算的湍流剪切率比按体积平均能量耗散率计算的结果约偏低20%～40%.     

含蜡原油特征温度实验研究

用旋转粘度计对原油在温度扫描条件下的粘-温关系进行的实验研究表明,原油在低温时呈胶凝状态。根据原油低温凝胶化理论,用常规方法对实验数据进行了处理,得到了动态降温条件下含蜡原油流动性变化的特征温度,即开始出现最大成胶活化能时所对应的温度。对于同一种原油,特征温度越高其低温流动性能越差。对实验数据的分析表明,特征温度与原油经历剪切过程的粘性流动熵产呈幂函数关系,并据此拟合出相应的经验式。利用该经验式,通过计算管输原油过程中所产生的粘性流动熵产,可确定原油的特征温度。6种空白原油及4种加剂原油的92组特征温度数据的计算值与实测值的最大偏差为0.86℃,平均绝对偏差为0.3℃。该关系式可用于管输含蜡原油流动的安全性评价。     

Shear strengthening viscosity and shear-rate-dependent modulus model for a Shengli crude oil

The steady shear viscosity of a Shengli waxy crude oil at the temperature of 3°C to 4°C, higher than its gel point, was measured by using a parallel-plate rheometer, which shows a shear thickening behavior in the shear rate range of 10^?2–10^?1 s^?1. A spectrum model proposed in our previous work was modified here by considering the modulus change in the spectra, which could be caused by a shear-induced varying internal structure in the crude oil. The shear strengthening viscosity of the Shengli crude oil can be described well by the changing-spectrum viscosity model.     

Rheological behavior of Algerian crude oil: effect of temperature and refined product

Abstract

The rheological behavior and its variation with temperature and refined product concentration of a crude oil sample coming from a quagmire of the separation station of Tin Fouye Tabankort oilfield/southern Algeria were investigated experimentally. The experiments were carried out at various temperatures (20, 30 and 50?°C) over the shear rate range of 0 to 700?s^?1 by using a controlled stress rheometer (AR 2000, TA Instrument). The results showed that the crude oil exhibit non-Newtonian of shear thinning behavior at low shear rate and Newtonian behavior at high shear rate and was adequately described by Casson and Herschel–Bulkley models. The rheological measurements through the steady flow test and viscoelastic behavior, including the storage modulus (G′), loss modulus (G″), and complex modulus (G*), has indicated that the rheological properties of the crude oil were greatly influenced by the temperature and the additive concentration.     

剪切作用对加剂原油凝点影响的数学模型

剪切作用是影响加剂原油流动性的主要因素之一.现场测试数据及实验室数据表明,剪切作用引起的加剂原油凝点上升幅度与剪切前加剂原油的降凝幅度(加剂原油的凝点与空白原油凝点的差值)之间有很好的相关性.基于对加剂原油剪切效应实验规律的分析,建立了对不同原油具有一定通用性并能较好地描述剪切后加剂原油凝点与剪切过程粘性流动熵产关系的数学模型.借助该模型,只须知道空白原油的凝点以及剪切前加剂原油的凝点,即可确定经过一定熵产的剪切后,加剂原油的凝点.对于加剂大庆原油、加剂新疆混合原油以及加剂中原原油的294组数据,模型计算结果与实验测结果偏差小于1℃的占50%,偏差小于2℃的占86.8%,平均偏差仅为1.12℃.该模型的建立为实现对管输过程中加剂原油流动性变化的数值模拟打下了基础.     

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.     

Experimental investigation of the rheological behavior of algerian crude oils from the quagmires

In this work, the rheological behavior for three types of crude oil coming from different quagmires namely Amassak, Tamendjelt and Tin Fouye of the TFT sector (Tin Fouye Tabankort/South Algeria) has been experimentally investigated. A controlled stress rheometer (AR 2000, TA Instrument) was used throughout this investigation. The experimental measurements in terms of flow and dynamic tests were carried out at different temperatures during the shear rate over the range of 0–700?s^?1 and frequency range of 0.1–10?rad/s. The obtained results show that the viscosity and shear stress of the crude oils decreases about 53.30%, 58.80%, and 59% respectively, when the temperature increased from 10 to 20?°C. The yield stress required to flow of crude oils also decreased to 37.06%, 89.78%, and 77.53% respectively. The dynamic analysis of the crude oils by identifying of the storage modulus (G′) and the loss modulus (G″) has indicated that the rheological properties of crude oils were significantly temperature-dependent.     

降凝剂对原油低温流变性的影响

Crude oil containing more than 10% of wax is defined as a high waxy crude (Liang, 1995), and is widespread across China. Oilfields, such as Daqing, Shengli, Dagang, Huabei, and Zhongyuan oilfield, are the major producers of waxy crude with average wax con…     

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.     

有机凝胶调剖剂强度的剪切恢复性能研究

采用动态剪切实验研究了两种有机凝胶类调剖剂强度的剪切恢复性能,通过测定不同条件下凝胶体系的储能模量\耗能模量和相位角的变化,分析了在聚交比一定的条件下,聚合物浓度、温度以及交联剂类型对凝胶剪切后强度恢复性能的影响。结果表明:在一定的应力下,随着聚合物浓度的增加,凝胶的成交点频率向低频移动;相同聚合物浓度和放置时间下,有机金属交联型调剖剂的强度大于有机酚醛交联型调剖剂;同种凝胶体系,在聚交比一定的条件下,浓度高时体系强度的剪切恢复性能好;对于两种凝胶体系来说,温度升高使得体系的剪切恢复性能下降;有机金属交联的凝胶体系剪切恢复能力高于有机树脂交联的凝胶体系。     

沥青质分子结构与沥青材料性能关系的分子模拟

沥青是一种分子组成多样且介观相态构成复杂的黏弹性材料,其骨架结构为沥青质组成的高贮能模量相,常温环境在外力作用下以弹性响应为主.为了研究分子结构对沥青材料性能的影响,利用分子动力学模拟方法研究不同结构的沥青质在沥青工作温度(250～353 K)区间内的相态,并近似地用剪切模量和杨氏模量分别对各相的切向与正向应力-应变关...     

生物柴油的等温胶凝特性

利用小振幅振荡剪切实验和KWW方程研究了生物柴油凝点附近恒温过程中黏弹性参数随时间的变化。以地沟油生物柴油为例,根据实验数据和KWW方程分析了不同历史条件下生物柴油的等温胶凝过程特性。结果表明,恒温过程中,随着时间的延长,生物柴油的储能模量和损耗模量逐渐增大,损耗角不断减小,表现出明显的等温胶凝特性,KWW方程能较好地描述生物柴油的储能模量随时间的变化。测量温度越高,降温速率越大,恒温剪切时剪切速率越大,特征松弛时间越长,恒温静置过程中生物柴油形成的胶凝结构强度越弱,且由于剪切后生物柴油中晶体的结构特点,导致剪切后的胶凝结构具有不可逆的恢复特性。性。     

驱油用FPS-B剂的流变性和黏弹性及驱油效率研究

利用环境扫描电子显微镜观察了FPS-B剂在溶液中的微观形态。采用稳态剪切和动态剪切实验,研究了不同质量浓度FPS-B剂的流变性和黏弹性,并进行物理模拟驱油实验。结果表明,FPS-B剂比高分子聚合物具有较好的增黏耐盐性、流变性和黏弹性。FPS-B剂质量浓度增大时,溶液的损耗模量和储存模量也相应增加,且两种模量曲线相交;当剪切速率低于该交点处的剪切速率时,溶液的储存模量大于损耗模量;相反,溶液的储存模量小于损耗模量。从分子结构和性质方面分析了产生这种化学现象的原因。岩心驱油实验结果表明,FPS-B剂不仅可以扩大波及体积,还有利于提高原油采收率。     

耐温抗盐聚合物溶液的黏弹性试验研究

通过动态剪切试验,研究了聚合物相对分子质量、矿化度、温度对耐温抗盐聚合物溶液黏弹性的影响。试验结果表明,相同浓度时,相对分子质量越大,动态曲线中相应的储存模量G＇与损耗模量G″也越大;对同一种聚合物,浓度越大,聚合物分子缠结越严重,在一定的频率范围表现出较强的弹性效应;温度升高对黏弹性影响较小;矿化度越高,黏弹性越小;表面活性剂的加入对聚合物溶液黏弹性影响较小。室内驱油试验结果表明,聚合物的高黏弹性对采收率的提高有较大贡献,浓度为1 500 mg/L的PAM2溶液提高采收率为15.4%。     

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%.     

茂金属线性低密度聚乙烯结构-流变性能-加工行为关系的研究

考察了几种茂金属线性低密度聚乙烯和传统线性低密度聚乙烯结构（诸如相对分子质量、分布短支链和长支链）和熔体流变性能间的关系（包括动态粘度、储能与耗能模量、松驰谱与松驰谱分布宽度指数、零切粘度、粘流活化能、以及表观剪切粘度、临界剪切速率、熔体强度等）。研究了这些性能对吹塑薄膜加工行为的影响。聚合物主链上存在长支链改变流变特性。在加工性能差的茂金属线性低密度聚乙烯中加入１０％的线性低密度聚乙烯能显著改善其加工性能，而薄膜物理机械性能降低不多。