Mechanical Response Of Gelled Crude Oil During Initial Structural Deformation

Initial mechanical response of gelled crude oil is the key issue for calculating pipeline restart process.Rotational rheometers of which fundamental is Couette flow are the main tools for rheology measurement of gelled crude oil.But transient Couette flow occurs during the initial shearing stage and do have an effect on rheology measurement.Considering the viscoelastic and thixotropic properties of gelled crude oil,transient Couette flows of different kinds of fluid under various boundary conditions are investigated.It has been found that transient Couette flow has the shortest developed time under constant velocity boundary condition.Then the initial mechanical properties of gelled crude oil under constant shear rates conditions and axial compressing conditions were investigated and compared.The main contents and conclusions are the following.Transient Couette flow of Newtonian fluid under different boundary conditions was researched.Functions of fluid velocity were obtained by solving definite solution problems analytically.Expressions of measured viscosities and dimensionless viscosities under different conditions were derived based on the measuring principle.Comparing the transient Couette flow under different boundary conditions,it has been found that time period needed for Couette flow reaching homogeneous is shortest under constant velocity boundary and longest under linearly increasing shear stress boundary.Transient Couette flow under both step velocity boundary condition and step shear stress boundary condition were discussed.It has been found that the developed time under step shear stress condition is longer than the one under step velocity boundary condition.Considering the thixotropic properties of crude oil,transient Couette flow of Cross fluid was investigated.Based on the momentum equation and constitutive equation,numerical model of transient Couette flow of Cross thixotropic fluid was proposed.Effect of boundary conditions and fluid properties were discussed.It has been found that structure parameter of fluid has little difference in spatial space and the transient stage of Cross thixotropic fluid is quite similar to Newtonian fluid's.But inhomogeneous flow will reappear when fluid structure has been broken down significantly under the constant shear stress boundary condition.Considering the unsteady properties of elastic material,actual shear deformation processes of pure elastic fluid under constant velocity boundary condition and constant shear stress condition were discussed.It has been found that there do have significant oscillations during unsteady shear deformation of elastic material under these two boundary conditions.Then transient Couette flow of Burgers material under different boundary conditions was discussed using numerical method.It has been found that both ???and ???are oscillating with time with gradually decreasing amplitudes.Comparing ???and f(t)? it has been found that ???obtained under constant shear stress condition has a larger amplitude but a lower frequency,and needs more time to become steady.Transient flows of Cross thixotropic fluid and Burgers viscoelastic material in Taylor-Couette system under constant torque condition were investigated numerically.It has been found that actual shear stress on the surface of moving boundary during the transient process is not a constant and has difference with the default value.For the Cross thixotropic fluid,difference between the actual shear stress and the default value become larger when rotational inertia of Taylor-Couette system increases.But for the Burgers viscoelastic material,oscillating period of the actual shear stress increases when the rotational inertia increases.Both numerical and experimental results show that initial measuring strain of Newtonian fluid is nonlinear with time and the initial measuring strain of Burgers viscoelastic material is oscillating significantly.Mechanical properties of gelled crude oil at the initial shear deformation stage were tested under constant shear rate conditions.Influence of relaxation properties of gelled crude oil on initial mechanical response under constant shear rate conditions was considered.Through putting the Maxwell mechanical analog and special dashpot in parallel,a quasi-Jeffreys model was obtained.Using the quasi-Jeffreys model,it has been found that the constitutive equation could fit the mechanical response of gelled crude oil well under a wide range of shear rates.By analyzing change law of the shear stress versus strain,the critical damage softening strain which is irrelevant to the shearing conditions was put forward to describe the yielding behavior of gelled crude oil.Using PP50 measuring system of rheometer,axial compression tests were carried out.Axial stress relaxation properties under constant axial strain were obtained.Relationships between axial compression stress and axial strain were measured under constant axial strain rate conditions.It has been found that the change rules of axial stress vs.strain are similar to the rules under shear conditions,but axial relaxation modulus are much larger than shear relaxation modulus.Under a same specific strain,axial strain could be easier to make the structure of gelled crude oil break down.