胶凝原油初始应力递增过程的力学模型

Mechanical model of shear stress increasing process during initial deformation of gelled crude oil

考虑胶凝原油具有蠕变与松弛特性,将Maxwell线性黏弹性体与幂律流体元件进行并联,得到机械比拟模型。在幂律流体元件应力项中引入与应变相关的损伤因子,Maxwell黏弹性体剪切应力项中引入完整性因子,得到胶凝原油初始应力递增阶段的描述公式。分别对大庆原油、南阳原油进行恒剪切速率试验测试,利用模型公式拟合初始应力递增阶段的试验数据,计算胶凝原油应力松弛过程。结果表明:拟合效果良好,模型参数不随剪切测试条件改变,能够反映胶凝原油的物性特征。该模型可以表述胶凝原油的松弛特性;应变幅值越大,结构损伤越明显,对应的剪切模量数值越小,符合胶凝原油的真实流变特性。

Considering the creep and relaxation properties of gelled crude oil, a mechanical model was established by combi-ning the Maxwell viscoelastic analog and a power-law dashpot in parallel. Constitutive equation of the mechanical model was figured out by introducing strain-dependent damage factor into the power-law dashpot and integrity factor into the Maxwell an-alog. Mechanical responses of Daqing and Nanyang crude oils were tested using rheometer under constant shear rates. The constitutive equation was used to fit the experimental data of shear stress increasing process. Stress relaxation processes under constant shear strain were calculated using the fitted model parameters. It has been found that the constitutive equation fits the data very well, and the parameters in the equation keep unchanged under different shear rates, which could be regarded to represent the actual properties of gelled crude oil. The proposed model can describe the relaxation properties of gelled crude oil theoretically. The relaxation modules decrease and the structure damage becomes greater with the increase of shear strain, which is consistent with the actual rheological properties of gelled crude oils.