Viscoelasto-thixotropic model for waxy crude

doi:10.3969/j.issn.0438-1157.2013.11.012

Abstract

Abstract: Waxy crude oil shows complex rheological behavior below gelation temperature,such as viscoelasticity,yield stress and thixotropy,owing to the formation of three dimensional sponge-like interlocking network structure.Traditional thixotropic models for waxy crude do not consider the viscoelastic behavior near and/or before yield point.To address this problem,a new thixotropic model with ten independent parameters was proposed.The new model consists of an equation of state and a structural kinetic equation.The total stress is composed of viscous stress and elastic stress.Elastic stress is assumed to be the product of shear modulus and shear strain,and shear modulus varies proportionally to structural parameter and a non-linear damping function.The evolution of structural parameter is described by a new kinetic equation whose breakdown term is assumed to be dependent on energy dissipation rate,to overcome the shortcoming of the kinetic equation whose breakdown term is assumed to be dependent on shear rate.The proposed model was validated by stepwise shear rate tests and hysteresis loop tests of Daqing waxy crude.The proposed model nicely described the viscoelasticity and thixotropy of waxy crude,and for the two kinds of test,the average absolute deviations(AADs)between fitted and measured values were within 2.0% and 5.0%,respectively.The model was further validated by predicting the transient flow curve of hysteresis loop test with the model parameters obtained from the stepwise shear rate measurement,and the results showed that the agreement between the predictions and measurements is acceptable,with AADs about 16%.

Key words: thixotropy, viscoelasticity, thixotropic model, waxy crude

摘要： 在胶凝温度以下,因蜡的结晶析出并形成三维空间网络结构,含蜡原油表现出复杂的非牛顿流体特性,如黏弹性、屈服应力和触变性等。目前,含蜡原油的触变性模型均为黏塑性模型,没有考虑屈服点附近及之前物料所具有的黏弹特性。为此,提出了一个包含状态方程和速率方程、共10个未知参数的黏弹-触变模型。模型的剪应力由弹性应力和黏性应力构成,弹性应力为剪切模量与剪应变的乘积,剪切模量正比于结构参数和损耗函数。描述结构参数随时间变化的速率方程,假设结构破坏项与能量耗散率相关,克服了以往速率方程中结构破坏项与剪切率相关这一假设的不足。模型可描述含蜡原油所具有黏弹-触变特性,并且所预测的剪应力能够从以黏弹性为主的阶段平滑过渡到以触变性为主的阶段。利用大庆原油剪切率阶跃和滞回环的实验数据对模型进行验证。结果表明:两种测试的模型拟合值与实测值的平均相对偏差分别在2.0%和5.0%以内;利用由剪切率阶跃测试数据拟合得到的模型参数对滞回环测试进行预测,预测的平均相对偏差约为16%。

关键词: 触变性, 黏弹性, 黏弹-触变模型, 含蜡原油

CLC Number:

O373

TENG Houxing, ZHANG Jinjun. Viscoelasto-thixotropic model for waxy crude[J]. , 2013, 64(11): 3968-3975.

滕厚兴, 张劲军. 含蜡原油的黏弹-触变模型[J]. CIESC Journal, 2013, 64(11): 3968-3975.

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