Study On The Effect Of CH₄ Dissolution On The Macro Rheological Properties Of Waxy Crude Oil And Its Micro Mechanism

In the process of oil field gathering and transportation,the crude oil in the pipeline usually dissolves some light components.The dissolution of the light components in the crude oil will affect its flow characteristics,and then affect the production and operation of the industrial pipeline.In this paper,the dissolution behavior of CH₄in crude oil and the mechanism of the influence of CH₄dissolution on the rheological properties of crude oil were studied.The dissolution mechanism of CH₄in crude oil was explored by molecular dynamics simulation.The influence of CH₄dissolution on the rheological properties of crude oil was studied by macro rheological test.Combined with microscopic observation and molecular dynamics simulation,the influence mechanism of CH₄dissolution on the wax crystal structure of crude oil and the influence mechanism of CH₄dissolution on the intermolecular interaction of crude oil were expounded,which has important guiding significance for the optimization design and operation of field gathering and transportation system.The specific contents include:The dissolution behavior of CH₄in different crude oil systems at different temperatures and pressures was systematically studied by means of CH₄solubility measurement and molecular dynamics simulation.Taking n-heptane as base oil,wax,resin and asphaltene content as influencing factors,the corresponding crude oil system was constructed according to L16（4³）orthogonal table,and two specific proportion crude oil systems of Shengli crude oil and Nanyang crude oil were constructed,with a total of 18 crude oil systems.Molecular dynamics method was used to simulate the dissolution process of CH₄in different crude oil systems,and the experimental results were used to verify the reliability of the simulation.The results show that:when the dissolved gas pressure is lower than the bubble point pressure,the dissolved amount increases gradually with the increase of the dissolved gas pressure,and CH₄accumulates at the position where there is a large free volume void;when the dissolved gas pressure is higher than the bubble point pressure,the dissolved amount of CH₄basically remains steady The results show that the dissolution of CH₄in crude oil system is a physical dissolution process,and the van der Waals effect is the main mechanical action mode in the process of CH₄dissolution;the larger the free volume fraction of crude oil system is,the larger the dissolution space can be provided for CH₄;the influence degree of different components on CH₄dissolution is wax<resin<asphaltene,and the change of components in crude oil system does not change the mechanical action mode of CH₄dissolution.The influence of CH₄dissolution on the macro rheological properties of waxy crude oil was studied by means of macro rheological test.Firstly,the influence of temperature,static pressure and dissolved gas pressure on the apparent viscosity of crude oil was analyzed systematically,and the relationship between dissolved gas pressure and viscosity was quantified.Secondly,aiming at the torque transmission lag problem in the process of unsteady shear flow in the magnetic coupling rheological testing system,the correction model of torque transmission lag was established,and the real thixotropic characteristic curve of crude oil was obtained.Finally,the influence of static pressure and dissolved gas pressure on the thixotropy of waxy crude oil was studied systematically,and the relationship between static pressure,dissolved gas pressure and thixotropy of waxy crude oil was quantified.The results show that:（1）with the decrease of temperature and the increase of static pressure,the viscosity of crude oil increases gradually;within the test pressure rangeand the increase of dissolved gas pressure,the viscosity of Shengli crude oil decreases gradually,while that of Nanyang crude oil first decreases and then increases;the coefficient of determination of fitting value and measured value of the viscosity model of dissolved gas crude oil based on barus equation and F-K equation is above0.992.The average relative deviation of apparent viscosity of Shengli crude oil and Nanyang crude oil under different static pressure is 0.06%and 0.12%,respectively.（2）Considering the mechanical response characteristics of rheometer motor,load characteristics,friction torque loss of gemstone bearing,inertia of motor and fixture,a correction method for torque transmission delay of magnetic coupling rheological testing system is established according to torque balance.After correction,the flow curve of Newtonian fluid in initial shear stage basically coincides with the steady-state test results,which shows the true viscosity of Newtonian fluid The higher the shear rate is,the more obvious the correction effect is.（3）The thixotropic curves of crude oil under different static pressure and dissolved gas pressure are similar to those under normal pressure.The maximum shear stress and equilibrium shear stress of Shengli crude oil and Nanyang crude oil increase with the increase of static pressure,and the structural fracture drop of gelled crude oil is more significant.In the range of test pressure,the maximum shear stress and equilibrium shear stress of Shengli crude oil decrease with the increase of dissolved gas pressure,while the maximum shear stress and equilibrium shear stress of Nanyang crude oil first decrease and then increase with the increase of dissolved gas pressure.By introducing barus equation and F-K equation to characterize the relationship between rheological parameters and pressure,a pressure type cross thixotropy model is established.The model can better describe the thixotropy curve of crude oil under different static pressure and dissolved gas pressure,and its coefficient of determination is more than0.933.The fitting parameters of Shengli crude oil thixotropy curve under different dissolved gas pressures were used to predict the crude oil thixotropy curve under corresponding static pressure,and the average relative deviation of prediction was9.23%.The influence mechanism of CH₄dissolution on rheological properties of waxy crude oil was studied by means of microscopic observation and molecular dynamics simulation.Firstly,the wax crystal morphology of crude oil under different static pressure and CH₄dissolved gas pressure was observed by using self-made high-pressure microscopic test system,and the influence of static pressure and CH₄dissolved gas pressure on wax crystal morphology was analyzed from the perspective of morphology.Secondly,the molecular dynamics simulation of CH₄/crude oil system model was carried out to investigate the effect of CH₄dissolution on the rheological properties of crude oil system.According to the changes of crude oil intermolecular interaction,radial distribution function,self-diffusion coefficient,volumetric strain and cohesive energy density,the mechanism of CH₄dissolution on the rheological properties of crude oil system was studied.Finally,the effect of CH₄on the intermolecular interaction of crude oil in shear flow field was studied.The results show that:（1）with the increase of static pressure,the wax precipitation temperature of Shengli crude oil and Nanyang crude oil increases gradually,a small amount of wax crystals precipitate from the two kinds of crude oil,the wax crystal particles are connected with each other,and the wax crystal particle size increases gradually;within the test pressure range,with the increase of dissolved gas pressure,the wax precipitation point of Shengli crude oil decreases gradually,but the wax precipitation point of Nanyang crude oil first decreases and then increases.With the dissolution of CH₄,the diameter,area and number of wax crystals decrease gradually.However,with the further increase of dissolved gas pressure,a small amount of wax crystals precipitate and the size of wax crystals increases.At the molecular level,the number of trans configuration molecules in the crude oil system decreases,the molecular chain of crude oil gradually curls,and the crystallinity of crude oil system decreases due to the dissolution of CH₄.（2）CH₄dissolution improves the rheological properties of crude oil system mainly by reducing the interaction between crude oil molecules,reducing the aggregation degree of crude oil molecules,increasing the self-diffusion coefficient of crude oil molecules and the volume of crude oil system,and reducing the cohesive energy density in the system.The effect of pressure on the rheological properties of crude oil system is opposite to that of CH₄.（3）At the beginning of shearing,in order to prevent the change of molecular orientation of crude oil,the molecular chain of crude oil curls gradually,and the shear stress increased.In the process of shear failure,the number of trans configuration molecules in the crude oil system increased significantly,the proportion of pungent and homeotropic configuration molecules decreased,the molecular chain of crude oil extended gradually,the crude oil molecules showed the directional arrangement consistent with the shear direction,and the shear stress decreased.Under the same shear conditions,CH₄dissolution can effectively reduce the degree of aggregation of crude oil molecules,reduce the absolute value of system energy,and improve the rheological properties of crude oil.