Research On Asphaltene Deposition Regularity In Oil Wellbores

Asphaltene deposition is a challenging problem in oil and gas production,it may deposit in the wellbore and block the flow channels with changes in temperature,pressure,and composition,causing significant economic losses.It is difficult to determine the location of asphaltene precipitation,and the lack of study on the law of particle deposition seriously hinder the implementation of asphaltene deposition program.The research of wellbore asphaltene deposition and numerical simulation of particle deposition law in tubing can effectively analyze the effect of various parameters on the asphaltene deposition rate,and provide theoretical and technical support for the removal of wellbore asphaltene deposition.The present work is devoted to elaborating the law of asphaltene deposition in wellbore deeply by theoretical and numerical simulation methods.Based on the unstanding of asphaltene element composition,molecular weight,solubility,etc.the wellbore temperature-pressure profile research was carried out.Combining with the physical properties of multiphase flow,the pressure and temperature models of single-phase and multiphase flow in wellbore were established,respectively.The total heat transfer coefficient in wellbore temperature model took into account the steady-state heat transfer of pipe fluid,tubing,annulus fluid,casing and cement ring as well as the unsteady heat transfer of formation.The wellbore temperature-pressure profile could be obtained by solving the temperature and pressure equations simultaneously.Then two prediction methods of asphaltene precipitation were proposed: machine learning classification algorithm and equation of state prediction.Six classification algorithms,such as decision tree,K-nearest neighbor and support vector machine,etc.were used to classify and recognize the stability performance of 186 experimental oil samples,which has the advantages of simple operation and strong operability.The asphaltene precipitate was predicted by the equation of state prediction method,which considered asphaltene geometry and complex phase behavior.Crude oil characterization,PC-SAFT equation of state,phase stability and flash calculation were combined to predict asphaltene precipitate envelope,and asphaltene precipitate envelope prediction software was also written by Matlab language.Two precipitation prediction methods were used to evaluate the stability of the target well oil samples,and the results of near infrared light scattering experiment were compared to verify the accuracy.It was found that the K-nearest neighbor method and the PC-SAFT EOS could accurately predict asphaltene precipitation.Combining the predicted wellbore temperature-pressure profile with the asphaltene precipitation envelope,the initial location and the thickest location of asphaltene precipitation could be determined.Then,asphaltene particles movement in liquid-solid two-phase flow was studied by numerical simulation based on the upper onset location of asphaltene precipitation.The visual mass force,pressure gradient force and saffman lift force were introduced into the force model of asphaltene particle by considering the density of asphaltene particles was close to that of crude oil.The Euler-Lagrange method was used to simulate the deposition rate of asphaltene particles in crude oil.The effects of velocity,particle mass flow,roughness,particle density and particle size on deposition rate were also analyzed.It was found that the average deposition rate of asphaltene increased with the increase of flow velocity,particle mass flow rate,roughness and particle size;it decreased with the increase of particle density.In addition,considering the influence of the asphaltene deposit layer on the pipe wall,the influence of plugging thickness,plugging location and plugging length on the deposition rate was analyzed sensitively,and the main influencing factors of the asphaltene deposition rate were identified by grey relational analysis.The asphaltene particles deposition in the upper part of the thickest part of asphaltene deposition was studied by using the coupled method of multiphase flow model(VOSET),standard k-ε turbulence model and DPM model.The influences of flow pattern(bubble flow,slug flow,agitated flow and annular flow),apparent velocity of gas phase,apparent velocity of liquid phase,particle diameter,interfacial tension of oil and gas and oil viscosity were analyzed,and it showed that the average deposition rate was the largest in bubbly flow,followed by slug flow,agitated flow,and annular flow,which revealed the deposition mechanism of asphaltene particles in gas-liquid two-phase flow.The research results of this paper have important guiding significance to the mechanism of wellbore asphaltene deposition and related prevention work.