Research On Mixing Characteristics And Multiphase Flow Laws In Tubing And Pipeline Of Heay Crude Oil Dilution

With the exploitation of conventional crude oil, the oil is tending to be heavier. Heavy crude oil is characterized by high visocity, high density and easily emulsifying, leading to great challenges during their production and transportaion. Dilution technology is vigorously developed for its technical and economic advantages. However, the uniformity degree of mixing, drop of drag reduction, flow characteristics and stability of mixing fluid along the pipe is still not identified. The prediction of pressure drop of flow before and afer mixing has not yet to be fundamentally solved. Also, theoretical guidances and optimizations are urgent in need for the determination of operation paramer, such as the diluting amount and length of blending section. In view of the problems decribed above, it is necessary to study the mixing characteristics and Multiphase flow characteristics of the heavy oil and thin oil, which of great significance theoretical guidance for security assurance and energy conservation in heavy oil dilution production and transport.In this paper, taking some heavy crude oils collected from typical heavy crude blocks in China, ligh crudes oils, diesel oil and LPG as the research objects, the research methods including rheology, loop flow experimental simulation and theoretical analysis of the combination of methods are adopted to study the mixing characteristics, dispersion characteristics, multiphase flow characteristics, phase reversal characteristics, istribution of boundary layer. Specific research and main achievements are as follows:(1) Based on viscosity measurement principal of stirring and flow regime analysis in in the coaxial cylinder system, heavy oil and thick oil mixing viscosity evaluation methos is puts forward according to the relationship between stirring torque and rotating speed, shear rate and viscosity. The apparent viscosity of diluted oil in stirring process is evaluated by experiment. The key factors influencing the mixing results are revealed through the analysis of the relationship of mixing performance, sheering properties and energy consumption. With the decrease of viscosity of heavy oil and the diluent oil, increase of temperature, stirring speed and diluting ratio, the circulation volume is increased, as a result, the mixing efficiency is improved and the mixing time is shortened. After reaching homogeneous mixing, the torque and viscosity is decreased as well.(2) A constant temperature liquid liquid diffusion coefficient measurement device is designed. The method of refractive index is adopted to determine diffusion coefficient. With the increase of temperature, molecular thermal motion increases, diffusion coefficients showes a trend of increase. At same temperature, with the increase of initial concentration of heavy oil, friction and interaction between the molecules are enhanced. As higher the concentration of heavy oil, the greater the viscosity is which leads to the decrease of diffusion coefficient.(3) From the perspective of binary diffusion, heavy oil and thin oil two-phase mixing motion models of horizontal pipe flow and vertical conduit flow are set up respectively based on the dynamic balance equation by considering diffusion between heavy oil and thin oil. By solved by computer programming, the flow field is characterized to reaveal the mixing process of heavy oil and thin oil and function mechanism. In stratified flow, heavy oil and thin oil diffusion area increases, thin oil-heavy oil mixing interface moves down with the extension of the mixing time. For power law fluid, mixing layer interface of heavy oil is not obvious. Compared with newtonian fluid, the velocity transition zone of maximum flow velocity is thickening. For non-newtonian fluid, the transition section of turbulent flow increases by compared with laminar flow, the mixing speed increase as well. With the extending of mixing time, uniformity of velocity field distribution is improved, flow resistance decreases. In the crude oil production tube, the velocity distribution in the tubing is axial symmetry. Increasing the pressure drop can improve the flow characteristics of heavy oil. But the effective measuresimprove the efficiency is reduce the viscosity of heavy oil by dilution light oil.(4) Filling-mixing-recovery system of LPG is designed and installed to conduct pipe flow simulation experiment research of heavy oil diluted by LPG and diesel oil. Viscosity reduction effect of LPG is evaluated to confirm the feasibility of mixing LPG. Through theoretical analysis, considering the solubility of gas phase in the heavy oil, heavy oil compressibility and transition of flow pattern, LPG gas phase-heavy oil two-phase flow equation is established to analyse the multiphase displacement and movement laws in wellbore by mixing LPG. In the wellbore, the friction coefficient decreases with the increase of mixing amounts of LPG. At the bottom of well, the gas phase slippage velocity is not very high. At the depth of 800m, the gas phase slippage velocity increases rapidly. Along the pipeline, temperature and pressure reduce with the extension of delivery time, as a result, the gas holdup increases gradually. Also, with the the increase of mixing amounts of LPG, pressure drop shows decreaseing tendency.(5) Based on the theory of density waves, the water cut of inverse point judgement mathematical model is established by considering phase interface and virtual mass force, resistance of momentum transfer and other factors, on the basis of the two-fluid model. Combined with the temperature and pressure in the process of mixing thin oil field distribution model, oil/water phase reverse characteristics is analyzes with respesect to different depth, virtual mass force, dilute ratio conditions by using computer programming calculation. With the increase of virtual mass force coefficient, relative motion between dispersed phase and continuous phase strengthens, aggregation of dispersed phase weakens, oil/water phase inverse point is increased. Diluted by diesel oil, the oil/water reverse phase point of heavy oil decreases. Along the well depth, temperature increase, density of dispersed oil phase decrease, inverse point decreases.(6) For different flow pattern in heavy oil-light oil flow, boundary layer theory is divided into three kinds, including laminar boundary layer, turbulent boundary layer and dispersed flow boundary layer. Cosidering density, pipe diameter, surface roughness, wettability and apparent viscosity, heavy oil-light oil two-phase boundary layer model is proposed. In oil-water stratified flow, boundary layer thickness of heavy oil phase is greater than light oil phase. With the increase of heavy oil-light oil ratio, the boundary layer moves down. In stratified turbulent flow, change of boundary layer thickness is very small with the increase of heavy oil-light oil ratio. But heavy oil-light oil boundary layer still appears down phenomenon. In fully dispersed the laminar flow and fully dispersed flow in turbulent flow, heavy oil-light oil evenly dispersed, and distribution law of boundary layer is as same as the single-phase flow. With the increase of heavy oil-light oil mixture viscosity, thickness of heavy oil-light oil boundary layer increases, and the distribution of boundary layer is axial symmetry.The software of mixing characteristics of heavy oil and multiphase flow analysis system compiled by VB.NET by using computer language can be used to calculate the mixing flow characteristics, gas-liquid two-phase flow drag reduction characteristics, phase reverse characteristics and boundary layer properties.The research results of this paper can provide theoretical and practical guidance for hydraulic design and field operation heavy oil dilution transportaiotn and production.